Development of the Press Extraction Method for Plug Substrate Analysis

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Conventional testing methods for soilless substrates are either unsuited for plug

production orýLQWHUSUHWLYHýVWDQGDUGVýKDYHýQRWýEHHQýSXEOLVKHGýIRUýS+ñýVROXEOHýVDOWVñýRU

VSHFLILFýQXWULHQWV.ý:LWKýWKHý3(0ñýWKHýURRWLQJýVXEVWUDWHýLVýLUULJDWHGýWRýFRQWDLQHUýFDSDFLW\

DQGýDIWHUýDýSHULRGýRIýRQHýKRXUýWKHýSOXJýVXUIDFHýLVýSUHVVHGýZLWKýDýILQJHUýRUýWKXPEýWRýH[SHO WKHýVROXWLRQïýý7KHýILUVWýVHULHVýRIýH[SHULPHQWVýH[DPLQHGýSRWHQWLDOýYDULDWLRQýLQýS+ñýHOHFWULFDO FRQGXFWLYLW\ýõ(&ôñýDQGýH[WUDFWDEOHýQXWULHQWVý(NO₃-N, NH₄+_{-N, P, K, Ca, Mg, Na, B, Cu,}

Fe, Mn, and Zn)ýDVVRFLDWHGýZLWKýGLIIHULQJýH[WUDFWLRQýIRUFHVïýý$ýSUHVVýPRGLILHGýIURPýD

IUXLWýSHQHWURPHWHUýZDVýGHVLJQHGýWRýDSSO\ýDýUDQJHýRIýIRUFHýõVLPLODUýWRýZKDWýFRXOGýEH H[HUWHGýPDQXDOO\ôýto a 35 g sample of soilless substrate. Testing done on a peat-based

substrate fertilized with a single fertilizer rate (20N-4.3P-16.6K at 75 mg._L-1 _{N) and then}

fertilized with a range of rates (75, 125, and 175 mg._L-1 _{N). A coir-based substrate was}

also tested at one rate (125 ppm mg.L-1 N). For all experiments, the range of extraction forces within a fertilizer rate did not affect chemical properties. When testing included a

range of fertilizer rates, the method’s ability to detect changes in pH and EC was

demonstrated.

The second study compared the PEM to the saturated media extract (SME) and

the 2:1 water : substrate (v/v) suspension method (2:1). The extraction methods were used

150, and 200 mg.L-1 N. Sample sizes of 20 and 60 plugs were used to determine if similar results could be obtained with the smaller, less destructive sample size.. Values

for pH were similar among the three methods. The PEM method resulted in the highest

EC, NH₄+-N, NO₃--N, K, Ca, and Mg. Testing 20 plugs was sufficient since sample size accounted for little variation in the results. In a second experiment comparing peat- and

coir-based substrates, the coir extract pH averaged 1.5 units higher with much higher

levels of K and Na regardless of extraction method when compared to the peat. Solution

from the peat substrate was higher in NH₄+-N, NO₃--N, P, Ca, and Mg. Within each substrate, pH, EC, and nutrients tested were similar between the PEM and SME. The

similarity between the PEM and SME nutrient analysis in the second experiment

suggested existing SME interpretation standards may be used for the PEM but further

investigation was needed.

The third study was designed to provide the necessary correlations between the

methods to allow for development of pH, EC, and nutrient interpretive ranges for actively

growing plugs. First, a wide range of bedding plant plugs grown at various commercial

greenhouses were collected and the substrate solution extracted with the PEM, SME, and

2:1 methods. This quickly and inexpensively provided the varied population necessary to

establish relative comparisons and correlations between the methods when performed on

actively growing plugs. A second experiment provided comparisons of the three

extraction methods performed on plugs grown at low, medium, and high fertility rates

with a variety of bedding plant species; celosia (Celosia argentea L. var. cristata (L.)

Kuntze Plumosa Group), impatiens (Impatiens wallerana Hook. f.), marigold (Tagetes

Sellow ex Roem.& Schult.) Thirty days after planting (DAP), shoots were harvested and

the solution extraction methods were performed on each flat. For both experiments, the

PEM EC was consistently higher and the pH equal to or lower than the SME. The pH

with the 2:1 method was similar to that for the PEM, however EC was consistently low

ith this method due to the dilution effect. Solution NO3--N, P, and K were well-correlated between the PEM and SME. However, between the two experiments, there were

anomalous results for most nutrients extracted by the PEM and SME methods. As found

in previous studies, Ca and Mg levels were consistently higher in the PEM extracted

solution.

The pH and EC relationships between the three extraction methods remained

consistent throughout the study. As a result, quantitative interpretation ranges for these

two analyses were calculated between the PEM and SME. Plug growers can now use the

PEM for on-site pH and EC testing and compare their results with the previously

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Table 2.2 Comparison of chemical properties of peat-based and coir-based substrates

subjected to same fertilizer. Force treatments were not significant, data are pooled means

(expt. 3 & 4)... 30

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Table 4.4 Comparison of nutrients extracted with the SME

and PEM: includes all spp. and rates ... 72

Table 4.5 Interpretive scale for pH and EC by testing method;

(PEM values from expt. 2 predictive equations) ...ýæê

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Table Appendix.2 Analysis of variance for comparison of macronutrients

between PEM and SME (chapter 4, expt. 2) ... 78

Table Appendix.3 Analysis of variance for comparison of micronutrients

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"The prime objective in testing a growth medium, whether soil or soilless, is to characterize the nutrient environment as closely as possible to the way the plant root system experiences it." (Warncke, 1990).

Greenhouse crops are grown using intense cultural regimes with frequent

fertilization and watering. Substrate testing for physical and chemical properties is

invaluable for predictive analyses % the most valuable tool the grower has when designing a fertilizer program. Diagnostic analyses also are necessary when problems occur during

the growing cycle. Seedling or vegetative plug production presents additional challenges

to the grower. Of primary concern is the small volume of the substrate in the container in

which the pH can fluctuate, there is a low buffering capacity, nutrients can leach out

quickly, or soluble salts can build up rapidly (Bailey et al; 1995; Fonteno, 1996),

Though the time in the plug stage (sowing to transplanting) for many annual bedding

plants ranges from 6-10 weeks, that for perennial crop plugs averages 8-12 weeks, more

if plugs are pre-cooled (vernalized) by the grower (Styer and Koranski, 1997). Thus,

more time spent in a plug flat cell increases the probability for salt buildup or nutritional

problems to occur.

The three ways to detect nutritional problems in a growing crop are 1) visually;

2) foliar or tissue analysis; and 3) substrate testing (Nelson, 1991). Imparting a visual

ë

short production cycle. Foliar analysis standards are available for most bedding plants

(Mills and Jones, 1996) but not specifically for plug crops. This leaves the plug grower

with nutritional monitoring through substrate analysis. Regardless of the crop grown,

testing the levels of essential and non-essential nutrients, substrate pH, and electrical

conductivity (EC) (soluble salts) provides the grower with important knowledge of

nutrient status below the substrate surface % in the root zone of the crop. There are many variables that can confound the results. Consistency in testing methods, sampling

techniques, and timing after fertilizer application are all crucial to producing reliable

results (Compton and Nelson, 1997). Testing procedures have changed over the years as

growers have moved away from soil-based mixes to completely soilless growth media.

The Evolution of Testing Procedures

Soil testing began as a logical extension of the emerging field of soil chemistry.

By the 1940's, soil testing was a legitimate tool for the development of agronomic liming

and fertilizer programs. (Peck and Soltanpour, 1990). Soil scientists such as Bray,

Morgan and Spurway looked to the soil solution and exchangeable nutrient pool to

provide nutrient availability information instead of the total mineral content of a field

soil. Soil testing has moved from the realm of solely determining nutrient deficiencies.

The maintenance of adequate nutrient supply and the limiting of financial and

environmental costs associated with over-fertilization are also factors to consider.

The horticulture industry also benefitted from soil testing for both field and

ê

media evolved from straight field soils to soil-based substrate amended with organic or

manufactured components, and finally to the soilless substrate used today (Warncke,

1986). The concurrent change in substrate physical and chemical properties necessitated

a change in testing methodology.

Soil-based substrates are still used in the greenhouse industry, particularly for

raised-bed production of cut flowers. The chemical extraction tests remain useful for

testing these substrates for exchange capacities and nutrient levels. Though testing

soil-based substrate is moving toward standardization among laboratories, the fact remains

that no single soil test is appropriate for all soils. The tremendous variation in procedures

renders results that can confuse both growers and grower advisers (Peck and Soltanpour,

1990). Interpretation and recommendation protocol can be even more variable.

Spaghnum peat moss, perlite, pine bark and vermiculite are the standard

components in today’s soilless mix (Fonteno, 1996). Combinations of two or more of

these components, at varying ratios, comprise the substrate. The nutrient holding and

exchange capacity is relative small when compared to that of mineral soils (Warncke,

1986). Testing methods previously used for mineral soils that hold elements tightly were

not necessary or appropriate for these substrates (Bunt, 1986). However, soilless media is

not an inert substrate. Peat, vermiculite, and pine bark all have a good cation exchange

é

Interpretation of test results

Field or agronomic soil testing has traditionally relied on two criteria that are key

to interpretation: first, the correlation between the amount of a nutrient extracted and the

amount taken up by the plant and secondly, a calibration curve relating the amount of the

nutrient extracted to some kind of marketable yield (Corey, 1987). This information

assists growers in predicting the total need or uptake of nutrients by a specific crop

(usually annual - the life cycle is completed in one growing season). This system is

difficult to implement in a horticultural situation. There is insufficient data regarding

nutrient extraction vs. uptake for the seemingly endless variety of greenhouse and nursery

crops. "Yield" of ornamental crops is often difficult to quantify. Therefore, nutrient

management of greenhouse crops has been limited to general guidelines. These

parameters indicate analytical ranges and sufficiency levels for essential nutrients and

other properties such as pH and soluble salts. An additional complication is that these

analytical guidelines are specific to the nutrient extraction method used.

The use of chemical reagents for nutrient extraction

Nutrient extraction employs a chemical reagent solution (or water) to separate all

or a fraction of nutrients available to plants growing in the substrate (Peck and

Soltanpour, 1990). Chemical reagent solutions contain acids, neutral salts, or a

combination of the two (Tan, 1996). Extractants displace ions from colloidal exchange

sites (K, Ca, and Mg); complex nutrients in a process similar to natural chelation (Fe, Mn,

è

comparable to those available to a plant in situ depends on several mechanisms:

cation/anion exchange complexes, degree of solubility/insolubility, ionic chelation,

biological tie-up through uptake and mineralization, or salt interactions and adsorption

(Peck and Soltanpour, 1990)

There are several specific extractants cited for each and every macro- and

micronutrient, but it is beyond the scope of this paper to describe each one. However,

universal extraction reagents that extract most of the essential nutrients with variable

results (Tan, 1996) will be discussed.

Mehlich. Three acid extraction methods for soil-based substrates were developed

by Dr. Adolph Mehlich of the North Carolina Department of Agriculture. The Mehlich-1

(0.05N HCl and 0.025N H₂SO₄) is widely used throughout the southeastern U.S. and Latin America. The Mehlich-3, a modification of the Mehlich-2, has been used world-wide on

a variety of soils since the mid-1980's (Tucker et al., 1996). A combination of several

reagents, the Mehlich-3 is considered the most universally applicable.

Spurway. The Spurway weak acid extraction method (0.018 N HOAc, pH of 3.3) has been used on both soil-based and soilless substrates since the 1940's (Markus, 1986).

Nutrients assayed are P, K, Ca, Mg, and NO₃-N. The micronutrients Cu, Mn, Fe, and Zn can also be measured with the addition of DTPA to the extractant. Warncke (1990) used

the Spurway procedure as the standard against which to compare the saturated medium

ç

Other. Many other extractants can be used to assess levels of nutrients in

soil-based and soilless substrates. A few of these include "neutral" ammonium acetate (P,K,

Ca, Mg, NO₃-N, and Na); the Bray P_{1 -} ammonium fluoride with HCl (P, K, Ca, Mg, NO₃ -N), sodium chloride (P, K, Ca, Mg, NO₃-N and NH₄-N); and boric acid (P, K, NO₃-N and NH₄-N) (Markus, 1986). Other tests on soil-based media include ion exchange capacity tests (to identify the substrates ability to store nutrients on the particles) and buffer pH to

determine lime requirements (Van Lierop, 1990).

The pH of a soil-based substrate provides information on acidity or alkalinity but

not on lime requirement of the substrate. This is determined by the addition of a buffer

reagent (there are several - the SMP buffer, Adams-Evans buffer, and the Mehlich buffer)

and measurement of the buffer pH drop (van Lierop, 1990). The buffer method is not

necessary for determining the lime requirement of soilless media.

The shift to water extraction for soilless substrates

Most greenhouse and container nursery potting mixes are soilless, comprised

instead of organic and inert components such as pine bark, peat, sand, vermiculite and

perlite. When compared to testing soil-based substrate, the importance has shifted in the

testing of soilless substrate from the extracting solution to the extraction method. The

argument is whether plants grown in soilless substrate are limited to the nutrients

available in the directly soluble form or can also utilize indigenous or acid-extractable

æ

Distilled water as the extracting removes the soluble fraction vs. an exchange

solution such as NH₄OAc which removes both the soluble and exchangeable fraction of cations (Warncke, 1990). Most greenhouse growing conditions - with frequent watering

and fertilizing - renders the exchangeable fraction of the nutrient supply less important,

eliminating the relevance of acid or some other chemical extractant. When correlating

acid vs. water extraction procedures on soilless substrate, the most significant differences

(lowest r-values) are encountered with the divalent cations such as Ca and Mg (Markus,

1986). However, water-extractable nutrient levels have been shown to stay in proportion

with exchangeable solutions levels (Warncke, 1986).

Another point of discussion is whether the substrate sample should be air-dried

prior to extraction or left moist. Moist samples better reflect the substrate-nutrient

environment the root encounters, plus the drying process takes extra time and labor

(Markus, 1986). The concentration of nutrients in the root zone is inversely correlated to

the moisture content of the media (Warncke and Krauskopf, 1983). Determined by the

moisture holding capacity of the media, this property is dependent on the bulk density (wt

: vol) of the mix. The wide variation in composition of the soilless mixes also

complicates efforts to produce testing and fertilization guidelines.

Current procedures: a comparison

Bunt (1986) separates the water extraction methods used for soilless substrate

å

suspensions performed on a weight or volume basis. The water-extracted supernatant can

then be analyzed for any or all of the essential nutrients.

Saturation Extraction Method (Saturated Media Extract or SME). The SME was

developed at Michigan State University by Lucas and Rieke (Warncke, 1986). The SME

had a moderate to high degree of correlation with the Spurway test procedure (especially

soluble salts, NO₃--N, P, and K). Procedures for the SME starts with moist substrate (about 500 cc) to which enough distilled or deionized water is stirred in until the media is

saturated to the point of glistening but with little or no standing water on the surface

(Warncke and Krauskopf, 1983). After about 30 minutes equilibration time, the substrate

solution is extracted with a vacuum filter. EC and nutrient analysis is then performed on

the extracted supernatant. The saturated substrate pH can be read from the slurry before

the filtering occurs, though values may be slightly lower (Lang, 1996).

Greenhouse growers can measure the pH and EC on their own, sending the rest of

the sample to an analytical lab for nutrient analysis, or simply send in a substrate sample

for testing. One of the benefits of the SME procedure is that any slow-release fertilizer in

the sample is subjected to minimal mixing and abrasion. Disadvantages include the fact

that it is labor-intensive, even when compared to other methods that do not require prior

drying and handling; and the point of saturation of the substrate is subjective, though

variation decreases if one person performs the mixing and saturation procedure

ä

Comparative studies have shown sample analysis results and interpretative ranges

for SME results can vary greatly among labs (Kirven, 1986). For example, optimum or

desired levels of Mg are published as: >70 ppm at Michigan State University, 80-125

ppm at the Ohio State University, and 60-90 ppm at the University of Georgia (Go

Dawgs!) (Kirven, 1986). Differences between sample analyses are often attributable to

judgement of saturation (Warncke, 1986). Despite the subjectivity and labor involved in

the SME procedure, it is currently the soilless media nutrient extraction method of choice

at most state and commercial analytical laboratories (Kirven, 1986). There are

interpretive ranges published for plug pH and EC. Bailey et al. (1995) propose ranges

for plug crops that grow best at a relatively high pH (6.0-6.8), low pH (5.4-5.8), or are pH

tolerant (5.4-6.8). Plug recommendations for EC range from initial substrate levels of

0.4-1.0 dS m-1 _{(Styer and Koranski, 1997) to levels for seedlings or plugs of 0.75-1.99}

(Fonteno et al, 1995) or 1.0-2.0 dS m-1_{(Lang, 1996).}

Water Suspension Extraction. Water extraction can be done with various

substrate to water ratios - 1:1.5, 1:2, 1:5 (Sonneveld, 1990). This is usually done on a

volume basis (v/v). Sampling by volume instead of weight helps overcome the wide

range of bulk densities of different soilless substrates (Bunt, 1986).ý Sources differ as whether the substrate should be tested moist and freshly collected (Fonteno, 1996) or

dried (Lang, 1996). At least 100 ml of substrate should be used. The substrate is then

mixed with the appropriate ratio of distilled or de-ionized water - 1.5 parts, 2 parts, 5

ìí

slurry can be poured through some kind of filter (coffee filters are often used) and the

solution collected for nutrient analysis (Lang, 1996). However, the dilution inherent in

the water suspension method in conjunction with a low-level fertility regime (such as

with plugs) can render micronutrient levels undetectable (W.C. Fonteno, personal

communication).

Most commercial labs do not have interpretive standards for the use of the 1:2

method (often referred to in industry literature as the 2:1; 2 parts water : 1 part substrate)

(Lang, 1996). Fonteno (1996) recommended the 2:1 method to greenhouse growers for

both electrical conductivity and pH analyses performed in-house. Though the level of

dilution can decrease sensitivity to variation in the samples, the method is less

equipment- and labor-intensive than the SME. Plug EC standards are available: the level

of soluble salts needs to be below 0.75 dS m-1 _{to avoid injury to the roots (Styer and}

Koranski, 1997). Recommended pH values of 5.4% 6.2 are slightly lower than those for the SME and only for greenhouse soilless substrate in general (Lang, 1996). Indicative of

the level of dilution with the 2:1 method, EC values for seedlings of 0.25-0.75 dS m-1 (Fonteno, et al., 1995) and 0.5-0.9 dS m-1(Lang, 1996) are over 50% lower than values expected with the SME.

Pour-through Nutrient Extraction Procedure. This modification of the soil

solution displacement procedure is also known as the Virginia Tech Extraction Method or

VTEM (Wright, 1986). The pour-through is gaining popularity as a quick and easy

ìì

over a collection dish and enough distilled water is added to the surface of the substrate to

collect approximately 50 ml over several minutes (best results are achieved with

containers sized 6" and up). The leachate can then be analyzed on-site for EC and pH,

and if addition nutrient analysis is desired, shipped to a lab (refrigerate until analysis)

(Wright, 1986). This procedure is now standard practice in the container nursery industry

for pine bark-based substrates, but work has also been done with greenhouse pot plant

production (Wright et al., 1990). However, nutrient standards for peat-based substrates

have not been formally developed, and pH/EC interpretive scales are primarily based on

grower observations (Lang, 1996).

The big advantage to the pour-through method is that it’s not a destructive

sampling procedure, and no special equipment is needed for the extraction procedure.

The elimination of substrate handling, stirring, etc. keeps any slow-release fertilizer prills

from breaking (Wright, 1986). The time required for the extraction procedure is greatly

reduced. This also eliminates the labor necessary to perform extractions at the analytical

laboratory.

Pour-through nutrient sufficiency standards have yet to be developed for

greenhouse crops. Nutrient levels are higher in pour-through samples than in those

extracted with the SME and established interpretive values. The established

recommended values for the SME are not suitable for pour-through testing (Wright et al.,

1990). The minimum container volume to create sufficient leachate without undue

ìë

smaller containers and cell packs - grower experimentation is necessary to determine the

volume of water used (Lang, 1996). Preliminary work has been done to assess the use of

the Pour-through on plug trays but this information has not be conveyed to potential users

(Schweizer, 1993).

Press Extraction Method. Variations on this method have been used by Danish

pot rose growers for several years. The protocol for this method is being refined by the

plug research group at NC State University. Also referred to as the "squeeze", this is a

simple extraction method that can be performed right at the greenhouse bench on plugs

(Fonteno et al., 1995). The substrate is irrigated with fertilizer solution to container

capacity, then allowed to equilibrate for about an hour. Depending on the size of the

container, a portion of the substrate can then be scooped out or the cell popped out,

wrapped in cheesecloth, and gently squeezed, collecting the supernatant. Plugs still in the

tray can simply be pressed with the finger from above, again collecting the solution as it

runs out. The solution pH and EC can be measured immediately, and the extractant can

be forwarded to an analytical laboratory for nutrient testing. Test timing is important as it

must be accomplished within a few hours of fertilizer application (Compton and Nelson,

1997) before the plant takes up the available nutrients. This is a relatively new method,

and interpretive tables have yet to be completed.

pH and electrical conductivity testing for plug substrate

Substrate pH can affect nutrient availability in soilless substrates (Bailey et al.,

ìê

available pens and meters. Consistent soil/solution ratios and uniformity of procedures

are necessary to produce comparable pH and EC values.

It can be advantageous for a grower to perform pH and EC analysis on-site. A

laboratory’s handling of the substrate prior to analysis can affect the results. Air or oven

drying can decrease pH values up to 1.0 unit (T.E. Bilderback, personal communication).

Grinding of samples ruptures slow-release fertilizer particles, which increases soluble

salts and lowers pH.

Over nine extraction procedures for measuring soluble salts have been recorded in

the literature. There are a different set of interpretative ranges for each one (Kirven,

1986). Other water extraction methods such as the pour-through and the NCSU

"squeeze" (both described previously) are also suitable, and the extracted solution can be

sent on for nutrient analysis by an analytical lab. The recommended seedling or plug pH

and EC interpretation guidelines for SME and 2:1 are pH of 5.5-6.5 for either method

(Lang, 1996); EC of 0.75-1.99 dS m-1 for SME and 0.25-0.75 dS m-1 for 2:1 (Styer and Koranski, 1997).

Nutrient analysis for plug production

Fertilizer recommendations specific to plug growth stage and crop are not based

on individual nutrients but on EC levels obtained from the 2:1 extraction method (Styer

and Koranski, 1997). Acceptable pre-plant substrate nutrient levels, pH, and EC values

are based on the SME (Styer, 1996) but no recommendations are made for the growing

ìé

diversity of crops, substrates, and fertilizer regimes, growers are seldom sure what they

are looking for. The best technology currently available consists of individual ion (NO₃ --N and K+ meters) with little research done on their accuracy (Rosen et al., 1996).

Sending in substrate (or leachate) for lab analysis can take several days while in the

meantime, nutritional disorders worsen or compound. The best-case scenario for the

future would be that each grower would have an in-house method of testing for the entire

range of essential nutrients within hours of solution extraction and a reasonably complete

ìè

Literature Cited

Bailey, D.A., P.V. Nelson, W.C. Fonteno, J.W. Lee, and J.S. Huang. 1995. Plug pH % the make or break factor in nutrition. GrowerTalks 59(9):6,8,10.12-13.

Bunt, A.C. 1986. Problems in the analysis of organic and lightweight potting substrates.

Hortscience 21:229-331.

Compton, A.J. and P.V. Nelson. 1997. Timing is crucial for plug seedling substrate

testing. HortTechnology 7:63-68.

Corey, R.B. 1987. Soil test procedures: correlation, p. 15-22. In: J.R. Brown (ed.). Soil

testing: sampling, correlation, calibration, and interpretation. Soil Science Society

of America, Madison, WI.

)RQWHQRñý:ï&ïýìääçïýý*URZLQJýPHGLDãýý7\SHVýDQGýSK\VLFDOîFKHPLFDOýSURSHUWLHVñýSïýäêð ìëëïýý,Qãýý'ï:ïý5HHGýõHGïôïý:DWHUñýPHGLDñýDQGýQXWULWLRQýIRUýJUHHQKRXVHýFURSVïý %DOOñý%DWDYLDñý,OOï

Fonteno, W.C., D.A. Bailey, and P.V. Nelson. 1995. Squeeze your plugs for simple,

accurate nutrient monitoring. GrowerTalks. 59(9):22,24,26-27.

Kirven, D.M. 1986. An industry viewpoint: horticultural testing - is our language

ìç

Lang, H.J. 1996. Grower media testing and interpretation. In: D.W. Reed (ed.) Water,

media and nutrition for greenhouse crops. Ball Publishing, Batavia, IL.

Markus, D.K., 1986. Spurway/acid extraction procedures. HortScience 21:217-222.

Mills, H.A., and J.B. Jones, Jr. 1996. Plant analysis handbook II. MicroMacro, Athens,

Ga.

Nelson, P.V. 1991. Greenhouse operation and management. 4th ed. Prentice-Hall, Englewood Cliffs, N.J.

Peck, T.R. and P.N. Soltanpour. 1990. The principles of soil testing. In: Soil Testing

and Plant Analysis, 3rd ed. Soil Science Society of America, Madison, WI.

Rosen, C.J., M. Errebhi, and W. Wang. 1996. Testing petiole sap for nitrate and

potassium: A comparison of several analytical procedures. HortScience

31:1173-1176.

Schweizer, A. L. 1993. Determination and assessment of procedures of the pour-through

nutrient extraction procedure for bedding flats and plug trays. M.S. Diss., Dept. of

Horticulture, Virginia Polytechnic Inst. and State Univ., Blacksburg.

Sonneveld, C. 1990. Estimating quantities of water-soluble nutrients in soils using a

ìæ

Styer, R.C. 1996. Key factors of water media and nutrition [for plug production].

GrowerTalks. 60(9):22-30.

Styer, R.C. and D.S. Koranski. 1997. Plug & transplant production: a grower’s guide.

Ball Publishing, Batavia, IL.

Tan, Kim H. 1996. Soil sampling, preparation, and analysis. Marcel Dekker, Inc., New

York.

Tucker, M. R., J.K. Messick, and C.C. Carter. 1996. Crop fertilization based on North

Carolina soil tests. Circ. No. 1. NC Dept. of Ag., Agronomic Div. Raleigh, NC.

Van Lierop, W. 1990. Soil pH and lime requirement determination. In: Soil testing and

plant analysis, 3rd ed. Soil Science Society of America, Madison, WI.

Warncke, D. D. 1990. Testing artifical growth media and interpreting the results. pp

337-357. In: Westerman, R.L. (ed.) 1990. Soil testing and plant analysis, 3rd

Edition. Soil Science Society of America, Inc. Madison WI

Warncke, D. D. 1986. Analyzing greenhouse growth media by the saturation extraction

method. HortScience 21:223-225.

Warncke, D.D. and D.M. Krauskopf. 1983. Greenhouse growth media: testing and

nutrition guidelines. Michigan State University, MSU Ag Facts Extension

ìå

Wright, R.D. 1986. The pour-through nutrient extraction procedure. HortScience

21:227-229.

Wright, R.D., K.L. Grueber, and C. Leda. 1990. Medium nutrient extraction with the

pour-through and saturated medium extraction procedures for poinsettia.

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Substrate solution testing is an essential management tool for greenhouse plug

production. Current methods of plug solution extraction and testing can be confounded

by subjective aspects of their techniques. The press extraction method (PEM) developed

at N.C. State University offers a convenient and timely method of solution extraction.

The root substrate is brought to container capacity and after a period of one hour, the plug

surface is pressed with a finger or thumb to expel the solution. This series of experiments

serves to quantify possible variation that may occur in pH, electrical conductivity (EC),

and nutrient analysis from differing manual extraction forces. A modified press was

designed to apply a range of force (53, 71, 89, 106, and 124 Newtons), and sampling

protocol consistency was verified. For all three experiments, the range of extraction

forces within a single fertilizer rate did not significantly affect solution pH or EC. When

testing included a range of fertilizer rates, results were significantly different among the

fertilizer rates, demonstrating the method’s ability to detect changes in pH and EC

resulting from increases in fertility levels. Nutrient analysis (NO₃-, NH₄+, P, K, Ca, Mg, Na, B, Cu, Fe, Mn, and Zn) of solution extracted from two different rooting substrates

(peat-based and coir-based) showed no differences within substrates for the range of force

ëí

Plug production is a rapidly growing and significant sector of the floriculture

industry (Styer and Koranski, 1997). On-site testing is crucial for successful plug

production (Bailey et al., 1995). The relatively small volume of substrate results in a

smaller nutrient reservoir, or conversely, a reduced buffering capacity. Due to this small

margin of error, the recommendations are to test substrate pH and soluble salts or

electrical conductivity (EC) frequently (Fonteno et al., 1995). Nearly 80% of nutritional

problems are due to fluctuations in substrate pH and soluble salts levels that should be

monitored on a regular basis (Styer, 1996). However, the rapid production cycle for most

plugs of three to five weeks (from seed to transplant) negates the usefulness of analysis if

a lengthy turn-around is involved, as is often the case with analytical laboratories. Thus,

there is a need to develop a dependable, yet quick, method of substrate solution extraction

(Bunt, 1986) that can be implemented on a regular basis by plug growers.

Analysis of the true, displaced solution best assesses the nutrient status of the

growth substrate (Bunt, 1986). The press extraction method (PEM) is a variation of the

"squeeze" method introduced by Fonteno et al. (1995). The substrate is brought to

container capacity and not further diluted. Pressing the top of the plug and collecting the

expelled solution provides an extract that closely resembles the root zone solution.

Solution pH and EC can be determined within minutes, and the extracted solution can be

sent on to an analytical laboratory for nutrient testing.

The objective of these experiments was to quantify how extraction force may

ëì

solution pH and EC initially with one fertilizer concentration, then over a range of

concentrations. We also examined the effect of variable force on the chemical analysis

of coir, an alternative substrate component.

Materials and methods

Common to all experiments. Though pressure from a finger or thumb is all that is

required to extract the solution, an apparatus was created to apply and quantify the

variations in force used to extract the solution from the substrate. Modified manual drill

presses (penetrometers or firmness testers) have been used to measure force exerted on

the skin of fruit in postharvest experiments (Reid, 1992). For our purposes, a drill press

was modified to hold a 150 cm3 (9.15 inch3 ) volume of substrate in a Plexiglas cylinder while a plate 4.45 cm (1.75 in) in diameter compressed the sample of substrate as force

was applied. The expelled solution was collected at the base. Treatments were fixed at

53, 71, 89, 106, and 124 Newtons (N) [12, 16, 20, 24, 28 pound-force (lbf) (pounds of

force per surface area of the pressure plate)]. This range was determined by measuring

the volume of solution that could be expelled manually (by squeezing or pressing) from

the same volume of substrate. These volumes were then correlated to the amounts

expelled by the apparatus at these forces.

Plug flats [288 cells, cell volume 6 cm3 (1.22 inch3)] were filled with screened substrate consisting of 3 peat : 1 perlite (by volume) (unless otherwise specified),

amended with dolomitic lime at a rate of 6 g._L-1 _{(10.1 lb/yd}3 _{), and placed on a}

ëë

irrigation event as recommended by Compton and Nelson (1997). This timing allows for

nutrient equilibrium within the substrate to produce an adequate amount of solution for

analysis. Data were subjected to analysis of variance using the general linear model (SAS

Institute, Cary N.C.). When model analysis showed a significant force (treatment) or

fertilizer rate effect, means were subjected to trend analysis.

Effect of extraction force on pH and EC (expt. 1). Three plug flats were filled

with substrate, placed in trays, subirrigated with 20N-4.3P-16.6K (Peter’s Peat Lite

Special, Scotts, Marysville, Oh.) at 75 ppm (75 mg.L-1) N, and allowed to equilibrate for 24 h. Flats were subirrigated with water to obtain container capacity 1 h prior to testing,

then removed from trays to allow drainage of excess moisture. The contents of the flats

were removed and gently combined to help insure uniformity of the samples. Each

sample weighed 35 g (1.23 oz) ( 5%), predetermined as the sample size necessary to

produce at least 20 ml (0.67 fl oz) of expelled solution.

Measurements included pH and EC of the extracted solution (Twin pH meter

B-213 and conductivity meter B-173, Horiba, Inc., Kyoto, Japan). To verify consistency of

our samples and cylinder packing technique, substrate sample wet weight (WW) and dry

weight (DW) were measured to calculate percentage recovery of moisture

([WW%DW] WW) 100) (Fonteno, 1996). Samples were weighed at the time of testing

(filling the press cylinder) and again after drying at 1052F (412C) for 24 h. The five

ëê

Effect of extraction force on pH and EC at three N rates (expt. 2). The same

methods were used as established in the previous experiment but the plug flats were

subirrigated with 20N-4.3P-16.6K at 75, 125, and 175 ppm (mg.L-1) N (three flats per rate) and allowed to equilibrate for 24 h. Flats were subirrigated with water one h prior to

testing. Contents of the three flats for each rate were carefully combined to reduce

sample variability. The five treatments 53, 71, 89, 106, and 124 N, were replicated six

times for each of the three fertilizer rates for a total of 90 experimental units. pH and EC

of the extracted solution were measured.

Effect of extraction force on nutrient analysis of peat substrate (expt. 3). The

same methods were used as in the prior experiments but extracted solution was also

analyzed for NH₄+ , NO₃- (LACHAT QuickChem, Milwaukee, Wis.); P, K, Ca, Mg, B, Cu, Fe, Mn, Na, and Zn (Jarrell Ash ICAP 9000, Franklin, Mass.) content. Flats were

filled with a commercial soilless germination mix (2-P; Fafard, Anderson, S.C.) and

subirrigated twice (within 48 h) with 20N-4.3P-16.6K at 125 ppm (mg.L-1) N to insure measurable levels of nutrients. Flats were allowed to equilibrate for 24 h and then were

subirrigated with water 1 h prior to extraction. The contents were combined to reduce

sample variability. The five treatments (force) were replicated five times for a total of 25

experimental units for the peat-based germination mix.

Effect of extraction force on nutrient analysis of coir substrate (expt. 4). The

substrate tested was 3 coir : 1 perlite (by volume) (screened coconut husk fiber; Crystal

20N-4.3P-ëé

16.6K at 125 ppm (mg.L-1) N. The procedures used were the same as in the previous experiment, with one exception. The lowest-force treatment of 53 N was not used on the

coir (insufficient amount of solution for testing was extracted), leaving four treatments

with five replications each for a total of 20 experimental units. After pH and EC were

measured, the extracted solution was analyzed for NH₄+ , NO₃- P, K, Ca, Mg, B, Cu, Fe, Mn, Na, and Zn content.

Results and discussion

Effect of extraction force on pH and EC (expt. 1&2). For plug flats subirrigated

with 75 ppm (mg.L-1) N, the pH, 5.4 0.06 (mean SD), and EC, 0.65± 0.04 dS·m-1 (mmho·cm-1), of the substrate solution were unaffected by differing extraction forces. After extraction, the remaining percent moisture content (87.1 0.5%), of the substrate

samples was not significantly different, verifying consistency in our sampling techniques.

The pH and EC of solution extracted from substrate treated with three N rates was

not affected by the range of extraction forces within each fertilizer rate, therefore, data

were pooled within N rates. The pH and EC were significantly different among the

fertilizer rates as presented in Table 1. These data help demonstrate the method’s ability

to detect changes in pH and EC resulting from increases of at least 50 ppm (mg.L-1) N in fertility levels.

Effect of extraction force on nutrient analysis (expt 3 & 4). Within each substrate,

ëè

previous experiments, solution pH and EC were unaffected by extraction force for either

peat or coir substrates. Therefore, data were pooled within each substrate. EC levels were

relatively high (with associated lower pH levels) due to two applications of the fertilizer

without leaching. Though not an initial objective, comparisons of the nutrient analysis

between the peat-based and coir substrates revealed differences that may be inherent in

the nutrient holding and exchange capacities as well as product source (Table 2).

Elevated levels of K and Na are common due to the source of coir (W.C. Fonteno,

personal communication). Other analyses of coir versus peat substrates have yielded

significant differences in pH (Handreck, 1993), EC, and nutrient content (Evans et al.,

1996; Evans and Iles, 1997; Meerow, 1994). Determination of substrate chemical

properties is advised and amendments and fertilizer regimes should be adjusted

accordingly.

There are other methods of substrate analysis suitable for production floriculture.

However, there has been little documentation of these methods (or accompanying

standards) for their use in plug production. These methods include suspension analysis of

various water to substrate ratios (e.g. 2:1, 6:1) (v:v), and the saturated media extract

method (SME). For both methods, the substrate is removed from the container and water

is added, resulting in significant and variable dilution that has to be considered when

comparing to standards for pH and EC (Wright, 1986). In the case of the suspension

analysis technique, the substrate packing density is subjective, while in the SME

ëç

analysis technique has an additional disadvantage in that the diluted extract can only be

used for measuring pH and EC % the degree of dilution can reduce micronutrients to undetectable levels, hence labs in the United States have not developed standards for

interpreting this method. The lack of significant dilution and uniformity in results

regardless of extraction force dispels two potential sources of subjectivity for the PEM.

Another method of extraction is the pour-through or Virginia Tech Extraction

Method (VTEM) wherein a known volume of water is added to the substrate surface and

the displaced solution is collected at the base (Wright, 1986). The pour-through method

has become the method of choice for the container nursery industry and shows potential

for floriculture pot crops, but the nature of plug flats limits its suitability for plug

production. The short substrate column limits the gravitational flow which is necessary to

displace the root-zone solution (Fonteno, 1996). This results in a perched water table that

does not drain rapidly enough for easy on-site solution collection.

The PEM is appealing since it is extremely easy, may be used at any stage of

plug growth, and requires no special equipment (Fonteno et al., 1995). We have

demonstrated that variability in extraction force did not affect the solution chemical

analysis for either peat-based or coir soilless substrates. This method is especially suited

for on-site testing of root-zone solution pH and EC. The use of specific ion meters for

on-site nutrient testing could also be of value to growers. Portable specific ion electrodes

have been developed for NO₃- _{and K}+ _{testing, though little work has been done to assess}

ëæ

further development of this simple, alternative, on-site technique for grower monitoring

of the root-zone status of plug crops.

Literature Cited

Bailey, D.A., P.V. Nelson, W.C. Fonteno, J.W. Lee, and J.S. Huang. 1995. Plug pH % the make or break factor in nutrition. GrowerTalks 59(9):6-13.

Bunt, A.C. 1986. Problems in the analysis of organic and lightweight potting substrates.

Hortscience 21:229-331.

Compton, A.J. and P.V. Nelson. 1997. Timing is crucial for plug seedling substrate

testing. HortTechnology 7:63-68.

Evans, M.R. and J.K. Iles. 1997. Growth of Viburnum dentatum and Syringa ×

prestoniae ‘Donald Wyman’ in Sphagnum peat and coir dust-based substrates. J.

Environ. Hort. 15:156-159.

Evans, M.R., S. Knoduru, and R.H. Stamps. 1996. Source variation in physical and

chemical properties of coconut coir dust. HortScience 31:965-967.

Fonteno, W.C., D.A. Bailey, and P.V. Nelson. 1995. Squeeze your plugs for simple,

accurate nutrient monitoring. GrowerTalks 59(9):22-27.

Fonteno, W.C. 1996. Growing media: Types and physical/chemical properties, p.

93-122. In: D.W. Reed (ed.). Water, media, and nutrition for greenhouse crops.

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Handreck, K.A. 1993. Properties of coir dust, and its use in the formulation of soilless

potting media. Commun. Soil Sci. Plant Anal. 24:349-363.

Meerow, A.W. 1994. Growth of two subtropical ornamentals using coir (coconut

mesocarp pith) as a peat substitute. HortScience 29:1484-1486.

Reid, M.S. 1992. Maturation and maturity indices, p. 21-28. In: A.A. Kader (ed.).

Postharvest technology of horticultural crops, 2nd ed. Univ. of Calif. Div. Agr. and Nat. Resources. Pub. 3311. Oakland, Calif.

Rosen, C.J., M. Errebhi, and W. Wang. 1996. Testing petiole sap for nitrate and

potassium: A comparison of several analytical procedures. HortScience

31:1173-1176.

Styer, R.C. 1996. Key factors of water media and nutrition [for plug production].

GrowerTalks 60(9):22-30.

Styer, R.C. and D.S. Koranski. 1997. Plug & transplant production: A grower’s guide.

Ball , Batavia, Ill.

Wright, R.D. 1986. The pour- through nutrient extraction procedure. HortScience

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Table 1. Effect of rate of fertilizer application on pH and electrical conductivity of

extracted solution. Extraction force treatments were not significant (data pooled within

fertilizer rates) (expt.2).

Fertilizer

rate EC

mg.L-1 N pH (dS·m-1)

75 5.97 0.1z 0.93 0.2

125 5.77 0.1 1.84 0.1

175 5.63 0.1 1.98 0.2

Significance

L** Q**

r2 0.58 0.82

z

Mean standard deviation n = 25

**

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Table 2. Comparison of chemical properties of peat-based and coir-based substrates

subjected to same fertilizer. Force treatments were not significant, data are pooled means

(expt. 3 & 4).

Substrate

Variable Peat-based Coir-based

pH 4.8 0.05z 5.0 0.06y

(dS·m-1)

EC 3.2 0.2 3.2 0.1

ppm (mg#L-1)

NO₃- 295 26.4 234 7.3

NH₄+ 69.5 5.0 31.0 2.2

P 39.2 5.3 40.3 1.2

K 303 39.5 644 19.5

Ca 165 25.2 26.3 2.8

Mg 125 17.2 21.7 1.2

Na 47.6 5.4 195 6.2

Fe 2.3 0.3 0.7 0.02

Mn 1.1 0.2 0.5 0.03

Cu 0.06 0.02 0.2 0.02

Zn 0.3 0.05 0.3 0.01

B 0.2 0.02 0.4 0.01

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Abstract

Substrate electrical conductivity (EC), pH, and nutrient content should be

monitored frequently during seedling plug production. Current testing methods are either

complicated, unsuited to plug production, or interpretation standards do not exist. This

study compares the press extraction method (PEM) developed at North Carolina State

University to the saturated media extract (SME) and the 2:1 water : substrate suspension

method (2:1). These solution extraction methods were applied to plug trays containing

peat-based germination mix treated with four levels of fertilizer. Two sample sizes of 20

and 60 plugs were used to determine if the smaller, less destructive sample size would

produce satisfactory results. Resulting pH values varied within 0.3 units among methods

but variability in EC and nutrient content was greater. The PEM method resulted in the

highest values for EC, NH₄+-N, NO₃- -N, K, Ca, and Mg while sample size had little effect on analyses. The three extraction methods were then compared on both peat- and

coir-based substrates. Within substrates, pH, EC, and nutrients tested were similar

between the PEM and SME. Between substrates, the coir extract had a higher pH and

much higher levels of K and Na. Solution from the peat was higher in N, P, Ca, and Mg.

The similarity between the PEM and SME analysis in the second experiment suggests

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Introduction

Maintaining optimum fertility levels is difficult for plugs because 1) cell volumes are

small resulting in little buffering of nutrients by substrates; and 2) the large seedling to

substrate volume ratio results in either rapid uptake of nutrients or dilution/leaching

effects from watering (Compton and Nelson, 1997). Substrate pH and soluble salts

should be monitored weekly as seedlings are especially sensitive to extreme levels of

soluble salts (Fonteno et al., 1995). Substrate electrical conductivity (EC) is a reliable

indicator of the soluble salts concentration (Poole and Chase, 1987). Substrate pH, EC,

and nutrient concentrations can be monitored by analyzing samples in the greenhouse or

sending samples to a lab. However, consistent procedures at both the sampling and

analysis stage are necessary to produce consistent and reliable results (Lang, 1996).

Current greenhouse (peat-based) substrate pH and soluble salts interpretation ranges are

based on 2 water : 1 substrate (v/v) suspension (2:1), the saturated media extract (SME)

method, or the pour-through. Interpretive standards for nutrient content are only

available for the SME.

The press extraction method (PEM) is a viable alternative to conventional testing

methods. Variations on this method have been introduced as the "squeeze" extraction by

researchers at N.C. State University (Compton and Nelson, 1997; Fonteno et al., 1995).

The PEM is very simple and quick: as soon as one hour after fertigation, while the

substrate is still at or close to container capacity, the grower picks up a tray and presses

several plugs, collecting the expelled solution for immediate analysis. Precedent for

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hydraulic press as the "standard closest to the actual root zone solution" with which to

compare the 1:1.5 and 1:25 suspension methods. In previous experiments with our own

press, we found that varying the force applied within a range likely to be manually

applied by growers did not affect the pH, EC, or nutrient analysis of the extracted

solution (Scoggins et al., 1999).

The saturated media extract (SME) is most commonly used by commercial

laboratories (Warncke, 1986). Greenhouse growers can test for pH and EC on-site by the

2:1 (Sonneveld, 1990). Neither of these methods is well suited to plug production. Both

the SME and the 2:1 method require removal of the substrate from the tray and the

subjective addition of water (Fonteno et al., 1995). Also, as plugs reach transplant size

(especially for trays numbering 288 cells), roots comprise the greater volume of the

cell; making it difficult to remove enough substrate to perform the SME or 2:1. The

pour-through method (Wright et al., 1990) of root-zone solution displacement is rapidly

gaining acceptance for bedding and pot plant testing. However, this method is also

difficult for plug systems. The short substrate column limits the effectiveness of using

gravity as the solution extraction force (Fonteno, 1996). Vigorous shoot growth in the

later stages of plug production often covers the tray, leaving little or no surface area for

applying the displacing water (personal observation).

Depending on the extraction method used, alternative substrates such as coir may

yield differing analyses. Properties of peat and coir have been compared (Evans et al.,

1996; Handreck, 1993; Meerow, 1995) but not the impact of different extraction methods

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The objectives of this study were to (1) compare the SME and 2:1 methods to the

PEM for use in the analysis of a greenhouse substrate; (2) examine the relationships

between these methods if a different substrate component (coir) is used; and (3)

determine if sample size (number of plugs per analysis procedure) affects the results.

Regarding sample size, protocol for the SME method suggests using 400 cm3 of substrate per sample (Warncke, 1986) % this would require nearly 70 plugs from a 288-cell tray. A reduction in the number of plugs necessary for a sample would limit the grower’s losses

associated with testing.

Materials and Methods

Comparisons among methods % peat-based substrate (expt.1). Plug flats (288

cells, cell volume 5.7 cm3_{) were filled with commercial peat-based germination mix}

containing limestone and a preplant nutrient charge ( 4-P, Fafard,Anderson, S.C.) and

placed on a greenhouse bench. Fertilizer was applied via subirrigation with

20N-4.3P-16.6K (Peter’s Peat-Lite Special, Scotts, Marysville, Oh.) at four rates: 50, 100, 150, and

200 mg.L-1 N. After 24 h of equilibration, flats were again subirrigated with these rates one hour prior to testing, then removed from trays to allow drainage of excess moisture.

There were five reps (plug flat = rep) for each fertilizer rate. Samples for the three

substrate solution extraction methods (SME, 2:1, and PEM) were collected from each

flat. The experimental units consisted of the 20 or 60 plugs used for each method.

Extracted solution was analyzed for pH and EC (EXTECH 695 pH/Conductivity Meter,

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from the SME and PEM was then tested for NH₄+-N, NO₃- -N (Lachat Quik Chem 8000, Zellweger Analytics, Milwaukee, WI), and P, K, Ca, and Mg (Plasma 2000, Perkin Elmer

Corp, Norwalk, CT).

Solution extraction methods:

SME. Samples of 20 or 60 plugs were chosen randomly, removed from the flat,

and placed in a beaker. Deionized water was added while gently mixing until the sample

was saturated to the point of "glistening", then allowed to equilibrate for 30 minutes. The

sample was transferred to a Büchner funnel lined with filter paper (Whatman #1), placed

over a vacuum flask, and the solution removed under vacuum.

2:1. Again, 20 or 60 plugs were randomly removed from the flat and placed in a

beaker. The plugs were gently stirred (broken up) to determine volume (around 110 ml

for 20 plugs and 330 ml for 60 plugs). Deionized water was added at a rate of twice the

substrate volume. The sample was stirred thoroughly and allowed to equilibrate for 30

minutes. The pH and EC electrodes were placed directly into the slurry. Nutrient

analysis was not performed on the 2:1 samples as most commercial labs do not have

interpretive standards for this method.

PEM. The substrate remaining in the plug trays was rehydrated to replace any

moisture that might have been lost during the time required for the SME and 2:1

extractions. This was accomplished by placing the plug tray in a container with enough

deionized water to bring the remaining plugs to container capacity. They were placed on

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beaker and 20 or 60 plugs were pressed to expel the solution. pH, EC, and nutrient

analysis were conducted as with the SME samples.

Data were analyzed as a split plot design. The plug tray treatments of different

fertilizer rates, the main plot factor, were arranged in a RCBD. Subplot factors were

factorial combinations of sample size and solution extraction methods. Data were

subjected to an analysis of variance using the general linear model (SAS Institute, Cary

NC). When model analysis showed a significant main effect or interaction, means of the

qualitative factors, extraction method and sample size, were compared with a mean

separation procedure (Fisher’s protected LSD). Means of the quantitative factor, fertilizer

rate, were subjected to trend analysis.

Comparisons among methods % coir- vs. peat-based substrates (expt. 2).

Plug tray sections of 72 cells were filled with 3 sphagnum peat: 1 perlite (v/v) or 3 coir: 1

perlite (v/v) substrates. No lime or nutrient amendments were added. Four fertilizer

treatments of 0 (water), 100, 200, and 300 mg.L-1 N were randomly assigned within each block. Increments between fertilizer rates were increased relative to expt. 1, to ensure

that changes in EC and nutrient content could be detected. The fertilizer source was

15N-2.15P-12.45K ( Miracle Gro Excel, Scott’s, Marysville, Ohio). Trays were subirrigated

with the appropriate fertilizer treatments until the substrate was saturated, drained to

container capacity, and allowed one hour to equilibrate before testing. Extraction

methods were performed nearly simultaneously, eliminating the need for further