3.2 Experimental Design and Treatments
3.2.2 EXPERIMENT TWO: Soil Strength, Aggregate Stability and Water Conservation after
Treatments and experimental design
The WRCC were first planted in winter of 2009 and subsequently in 2010 in rotation with maize (Zea mays L) in summer. The WRCC were planted as bicultures at different seeding percentages. The treatments were thus the different biculture ratios i.e. 90% oat plus 10%
grazing vetch; 70% oat plus 30% grazing vetch; 50% oat plus 50% grazing vetch and control, which was left weedy without WRCC. The percentage refers to the percentage of the recommended seeding rate that is used in a monoculture that is 100 kg/ha for oats and 50 kg/ha for grazing vetch. These treatments were arranged in a randomized complete block design with three blocks where slope constituted the blocks. Supplementary overhead sprinkler irrigation was done whenever deemed necessary. After reaching the flowering stage, the cover crops were killed by applying glyphosate (360 g/L) at a rate of 5 L/ha. Maize (cv. SC 701) was planted every year in summer in all plots with the fertilizer applied at 60 N kg/ha in which a third of the N was applied at planting as a compound (6.7 % N; 10 % P; 13.3% K) and the remainder applied at 6 weeks after planting as LAN (28% N) by banding. The gross plot sizes were 5.4 m × 6 m and during sampling, 0.5 m was discarded on all plot borders to give a net plot size of 4.4 m × 5 m.
27 3.3 Field Determinations
Infiltration and soil strength measured as penetration resistance and bulk density were determined just before cover crop destruction at the end of the flowering stage. This corresponded with the end of the fourth and second rotations for the monoculture and biculture experiments respectively.
3.3.1 Penetration resistance
Soil penetration resistance (PR) was determined using a hammer-type dynamic cone penetrometer (Herrick and Jones, 2002). The penetrometer had a 30° hardened steel cone with a 20.3 mm diameter shaft. The shaft had a striking plate attached to it. The shaft was used to guide a 2 kg hammer onto the striking plate forcing the steel cone into the soil. An adjustable collar was used to fix the dropping height of the hammer to ensure repeatability. The cone of the penetrometer was placed on the soil surface with the shaft oriented vertically. The cone was pressed into the soil to ensure that the base of the cone was level with the soil surface, and this minimized starting depth variability. The 2 kg hammer was raised until it touched the collar and then released to drop through a 65 cm height to hit the striking plate. The total number of blows required penetrating a depth of 0 to 15 cm and 15 to 30 cm was recorded. Each strike contained the equivalent kinetic energy of 12.74 Joules. Gravimetric water content was also simultaneously determined and later used as a covariate in the statistical analysis of PR. Five measurements per plot were done.
28 3.3.2 Infiltration rate
Infiltration rate was determined using a double ring infiltrometer (Eijkelkamp Agrisearch Equipment, 6987 ZG, Giiesbeek, Netherlands). Plant debris was carefully removed from the surface ensuring the soil was not disturbed. Plants were carefully cut making sure that within the rings, no plant material were more than 3 cm in height. The cylinders were inserted into the soil using a drop hammer to a depth of 3 to 5 cm. Short circuit flow or leakage was prevented by lightly tapping the contact between the soil and inside surface of the cylinder. Water would be poured inside both rings to a height of 10 to 15 cm ensuring that the same depth of water was used for both rings. The rate at which water was infiltrating into the soil was measured using the measuring rod at the center of the cylinder (Reynolds et al., 2002). Three measurements were done at each plot.
3.3.3 Bulk Density
Undisturbed samples for bulk density determination were collected using a core sampler with a diameter of 6.96 cm and height of 8.63 and a volume of 328.3 cm 3. The soil cores were
29 3.4 Laboratory Determinations
Soil aggregate stability, polysaccharides and water retention were determined in the laboratory. Debris and trash was removed from the soil surface prior to sample collection as described by Okalebo et al. (2002). Sampling was done at three positions in each plot for each specific depth using a spade. After collection, the soil samples were taken to the laboratory, spread on a bench and air dried for 48 h and large clods broken by hand.
3.4.1 Aggregate stability
The fast wetting method described by Le Bissonnais (1996) was used to measure aggregate stability. Calibrated aggregates with a size of 3 to 5 cm were used. The aggregates were oven dried at 40° C for 24 h to ensure that all samples were at a constant matric potential. A 5 g sample of aggregates was gently immersed in a 250 cm3 beaker filled with 50 cm3 of deionized water for 10 min. The water was then sucked off with a pipette leaving behind slaked aggregates. The slaked aggregates were then gently transferred onto a 0.053 mm sieve previously immersed in ethanol, to avoid re-aggregation during drying. The sieve was gently moved up and down in ethanol five times to separate the aggregates < 0.053 mm from those > 0.053 mm. The remaining > 0.053 mm fraction was oven dried at 40 °C for 24 h and its size distribution was measured by gently dry sieving by hand on a column of 2 mm, 1 mm, 0.5 mm, 0.2 mm, 0.1 mm and 0.053 mm sieves. The weight of each fraction was measured and the < 0.053 mm fraction was calculated as the difference between the initial weight and the sum of weights of the other six fractions. Aggregate stability was expressed as mean weight diameter (MWD) as shown in Equation 2 and as percentage aggregate size fractions of the seven classes.
30
Disturbed air-dried soil samples ground and passed firstly through a 2 mm sieve and then through a 0.15 mm sieve to remove plant remains, were used. The method described by Martens and Frankenberger Jr (1990) was used for extraction of the soil polysaccharides, after which the carbohydrate concentration was measured by the phenol-sulphuric method (Dubois et al., 1956).
Total acid extractable polysaccharides
One gram (1 g) of soil was mixed with 10 mL of 0.25 M H2SO4 and shaken in a rotary shaking machine for 16 h. The extracts were treated with 0.1 M EDTA to prevent co-precipitation of saccharides with di- and trivalent cations. The soil suspension was then centrifuged at 10 000 rpm for 10 min and then 1 mL of the supernatant solution was used to determine polysaccharide concentration using the phenol-sulphuric method.
Hot water extractable polysaccharides
One gram (1 g) of soil was mixed with 10 mL of hot distilled water at 85°C and heated in a water bath for 16 h. The extracts were treated with 0.1 M EDTA to prevent co-precipitation of
31
saccharides with di- and trivalent cations. The soil suspension was centrifuged at 10 000 rpm for 10 min and then 1 mL of the supernatant solution was used to determine polysaccharide concentration using the phenol-sulphuric method .
A standard curve was prepared by dispensing varied volumes of 100 mg/L solution of standard glucose solution into labeled test tubes. The varied volume of each tube was then filled up to 50 mL by adding distilled water to create varied concentrations of 0, 2, 5, 10, 20, 40 and 60 mg/L.
One mL from each varied concentration was then transferred to another test tube in which 0.5 mL phenol reagent (4% w/v) and 2.5 mL of concentrated sulphuric acid was added, and the mixture vigorously shaken. After allowing the mixture to stand at room temperature for 30 min, absorbance was then measured using a Helios Delta Thermo Spectrometer (Thermo Fisher Scientific, England) at 490 nm. The same procedure was then done for the samples with unknown concentrations.
3.4.2 Water Retention and Release
The pressure plate apparatus (Soil Moisture Equipment Corp., Goleta, CA) was used to determine water retention and release properties of the soil samples. Disturbed soil samples passed through a 2 mm sieve were used for the water retention and release measurements (Dane and Hopmans, 2002; Cresswell et al., 2008). The ceramic porous plates were first saturated for 24 h in 0.005 M CaSO4 solution. The disturbed soil samples were contained in polyvinyl rings 2 cm in height and 4.5 cm diameter. The disturbed soil samples were packed in the polyvinyl rings at a bulk density of 1.6 g/cm, the average bulk density of the soils in the field. The soil samples were then directly placed onto the ceramic plate surface and saturated with 0.005 M CaSO4 for
32
24 h. The ceramic plate with the soil samples would then be transferred into the pressure chamber and air forced in at 33, 75, 100, 200 800 and 1500 kPa. Equilibrium would be determined once outflow of water from the pressure chamber had ceased and this took from 24 h to 17 days depending on the pressure. At equilibrium the samples gravimetric water content was determined (Dane and Hopmans, 2002). The samples from the pressure chamber at equilibrium were immediately weighed and dried at 105 °C for 48 h and reweighed again. The gravimetric water content ω (kg/kg), of the soil samples was calculated using Equation 3.
ms mw
ω [Eq. 3]
where mw was the mass of water and ms the mass of dry soil. The gravimetric water content was converted to the volumetric water content by multiplying the gravimetric water content values with the bulk density value. The volumetric water content was plotted against the suction values to produce the soil-water characteristic curves.
3.5 Statistical analysis
Analysis of variance was performed using JMP® Release 9.0 statistical package (JMP®, 2010). Mean separations was done using Fishers protected Least Significant Difference (LSD) at P ≤ 0.05. Correlation analysis was done to determine the association between the aggregate stability and extractable polysaccharides.
33
CHAPTER FOUR 4.0 RESULTS
4.1 EXPERIMENT ONE: Soil Strength, Aggregate Stability and Water Conservation after Four Years and Four Rotations with Winter Cover Crops Grown as Monoculture in a Maize Based Conservation Agriculture System.
4.1.1 SOIL STRENGTH
There were no significant differences (p ≤ 0.05) observed in the bulk density values with the use of WRCC grown as monocultures within the two depths after the four rotations compared to the weedy fallow control (Figure 4.1). However, within the 0 to 15 cm soil layer, WRCC vetch, lupin and oats reduced bulk density by 3%, 4.3% and 3% respectively, compared to the control. Within the 15 to 30 cm depth, WRCC vetch and oats slightly increased bulk density by 4.9% and 1.2% respectively, compared to the control.
After four years and four rotations with WRCC there was a significant (P = 0.043) interaction between cover crop and depth (Table 4.1). Whilst PR increased with an increase in depth (0 to 15 and 15 to 30 cm) for vetch and lupin, oats and the weedy fallow control showed no significant (P ≤ 0.05) increase in PR with an increase in depth. Within the 0 to 15 cm and 15 to 30 cm soil layers, oats significantly (P ≤ 0.05) reduced PR by 104.2% and 83.7% respectively, compared to the control. Within the 0 to 15 and 15 to 30 cm soil layers, vetch consistently resulted in increased PR by 11.8% and 42.5% respectively, compared to the control.
34 1.4
1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8
0 to 15 cm 15 to 30 cm
bulk density (g/cm3)
Soil depth
vetch
Oats
Lupin
Weedy fallow (control)
Figure 4.1. Bulk density at two depths as affected by winter rotational cover crops grown as monocultures after four rotations in a maize based conservation agriculture system in a hardsetting soil.(Bars indicate standard deviation).
35
Table 4.1. Penetration resistance (Joules) at two depths as affected by winter rotational cover crops grown as monocultures after four rotations in a maize based conservation agriculture system on a hardsetting soil.
Treatments Depth vetch Lupin Oats
PR (Joules)
Weedy control
0 to 15 cm 130.6aA† 109.9aA 57.2bA 116.8aA
15 to 30 cm 175.1aB 161.3abB 66.9cA 122.9bA
ANOVA Main effects
WRCC < 0.001***
Depth 0.007**
WRCC x Depth 0.043*
By Depth
0 to 15 cm 0.018* (19.3)
15 to 30 cm 0.029* (15.9)
† Within depths means followed by the same lowercase letter are not significantly different at P ≤ 0.05. In each column, means followed by the same uppercase are not significantly different at P ≤ 0.05.
* significance at P ≤ 0.05; ** significance at P ≤ 0.01; *** significance at P ≤ 0.001.
Values in parenthesis are coefficient of variation, CV%.
36 4.1.2 AGGREGATE STABILITY
4.1.2.1 Mean weight diameter and aggregate size fractions.
After the four rotations, WRCC significantly (P ≤ 0.05) increased mean weight diameter (MWD) within the 0 to 15 cm and 15 to 30 cm soil layers compared to the weedy fallow control.
Vetch, lupin and oats significantly (P ≤ 0.05) increased MWD by 41.4%; 20.4% and 15.7%
respectively within the 0 to 15 cm soil layer compared to the control (Table 4.2). A similar significant increase (P ≤ 0.01) was also observed under WRCC within the 15 to 30 cm soil layer where vetch, lupin and oats increased MWD by 47.2%; 44.2% and 39.7% compared to the weedy fallow control. A similar trend of increase was also observed across the 0 to 15 cm and 15 to 30 cm soil layers for the macro aggregates, the > 1 mm and the 1 to 0.250 mm aggregate size fractions, where WRCC significantly (P ≤ 0.05) increased the percentage of aggregates within these classes compared to the control. Within 0 to 15 cm soil layer, vetch, oats and lupin increased the macro-aggregate fraction by 29.5%; 28.4% and 27.3% respectively, compare to the control. The effects were more pronounced within the 15 to 30 cm soil layer where vetch, oats and lupin resulted in a highly significant increase in the macro aggregate size fraction (P ≤ 0.001). The 0.250 to 0.106 mm size constituted the greatest fraction among the aggregate size fractions across all treatments within the two soil layers.
37
Table 4.2. Effects of winter rotational cover crops grown as monocultures on soil aggregate size fractions, macro aggregates, mean weight diameter (MWD) under conservation agriculture.
Aggregate size fractions (mm)
*significant at P ≤ 0.05; ** significant at P ≤ 0.01; *** significant at P ≤ 0.001; NS = not significant
‡No letters within a column indicates no significant differences between treatments.
† Soil aggregates ≥ 0.250mm in diameter; Values in parentheses are coefficients of variation, CV%.
38 4.1.2.2 Soil polysaccharides
Generally there were higher values observed for the WRCC monocultures compared to the weedy fallow control on both the dilute acid extractable polysaccharides (DAEPS) and hot water extractable polysaccharides (HWEPS) across the two soil layers (Figure 4.2 and Figure 4.3). The highest concentrations of soil polysaccharides were observed within the dilute acid extracts for the WRCC monocultures. After four rotations, a significant difference (P ≤ 0.05) was observed between WRCC monocultures and the weedy fallow control on DAEPS within the 0 to 15 cm and the 15 to 30 cm soil layer. Vetch, oats and lupin increased the DAEPS within the 0 to 15 cm soil layer by 22.4%; 23% and 39.2% respectively. Within the 15 to 30 cm soil layer, a similar increase was also observed under WRCC. Across depths, a slight increase was observed across all treatments with an increase in depth for the DAEPS. There were no significant differences were observed between WRCC monocultures and the weedy fallow control for the HWEPS across the two depths. Oats and lupin showed consistently higher contents for the HWEPS fraction compared to the weedy control within the 0 to 15 cm and 15 to 30 cm soil layers.
39 0
100 200 300 400 500 600 700 800 900
0 to 15 cm 15 to 30 cm
polysaccharide content (mg/Kg)
Soil depth
vetch Oats Lupin
Weedy fallow (control)
Figure 4.2. Influence of winter rotational cover crops on dilute acid extractable polysaccharides (DAEPS) at two depths after four rotations involving monocultures in a maize based conservation agriculture system on a hardsetting soil. (Bars indicate standard deviation).
40 0
50 100 150 200 250 300 350 400
0 to 15 cm 15 to 30 cm
polysaccharide content (mg/Kg)
Soil depth
vetch
Oats
Lupin
Weedy fallow (control)
Figure 4.3. Influence of winter rotational cover crops on hot water extractable polysaccharides (HWEPS) two depths after four rotations involving monocultures in a maize based conservation agriculture system on a hardsetting soil. (Bars indicate standard deviation).
41
4.1.2.3 Correlation of soil polysaccharides with aggregate stability
Mean weight diameter was correlated with the acid extractable and hot water extractable polysaccharides within the 0 to 15 cm and 15 to 30 cm soil layers to assess the roles of the polysaccharide fractions in stabilizing soil aggregates. There were no significant correlations observed between the DAEPS and MWD across the 0 to 15 cm and the 15 to 30 cm soil layers after four rotations with WRCC monocultures (Table 4.3). Hot water extractable polysaccharides correlated negatively but significantly (P ≤ 0.05) with MWD across the two depths (Table 4.3).
Table 4.3. Correlation of soil polysaccharides with aggregate stability.
Extraction
* significant at P ≤ 0.05; ns = not significant
‡ DAEPS, dilute acid- extractable polysaccharides; HWEPS, hot water – extractable polysaccharides
4.1.3 WATER CONSERVATION
42 4.1.3.1 Infiltration
Oats showed a significant difference (P ≤ 0.05) in total infiltrated water compared to the control after 2 hours (Fig 4.4). Within the first 30 minutes, cumulative infiltration was significantly higher in soils under WRCC compared to the weedy fallow control. After 30 minutes, oats continued to show a consistent significant increase in cumulative infiltration.
However, vetch and lupin showed a decrease in infiltration, consequently resulting in a decrease in cumulative infiltration showing no significant differences from the control. There were no significant differences were observed between all treatments in final infiltration rates (FIR) after the four rotations involving WRCC monocultures (Fig 4.4). Oats resulted in a 7.8% increase in FIR whilst vetch and lupin decreased FIR by 9% and 16.7% compared to the control.
Cover crop FIR mm/h
oats 10.38a
vetch 9.00a
lupin 8.25a
weedy fallow (control) 9.63a
43
0 10 20 30 40 50 60 70
0 0.5 1 1.5 2 2.5
Cumulative infiltration (mm)
Time (h)
oats vetch lupin
weedy fallow (control)
Figure 4.4. Cumulative infiltration as a function of time for a hardsetting soil under different winter rotational cover crops in a maize based conservation agriculture system. (Bars indicate standard deviation and same letters preceding the final infiltration rate (FIR) indicate no significant (P ≤ 0.05) difference between treatments).
44 4.1.3.2 Water retention characteristics
After four rotations, WRCC resulted in significant (P ≤ 0.05) increase in plant available water (PAW) compared to the weedy fallow control (Table 4.4). Vetch, lupin and oats resulted in a 28.3%, 23.9% and 22% increase in PAW respectively, compared to the control. At a water potential of -33 kPa and -100 kPa, a significant (P ≤ 0.05) increase was observed on water retention between WRCC and the control, with the control retaining the least amount of water (Figure 4.5). This trend was consistent across the plant available water range (-33 kPa to -1500 kPa) where the weedy fallow control retained the least amount of water compared to the WRCC.
Within the –33 kPa matric potential, vetch, oats and lupin increased water retention by 20%, 14.7% and 15.1% respectively, compared to the control. Most of the water retained in the soil was released between -33 kPa and -200 kPa matric potential across all treatments.
45
Table 4.4. Water retention characteristics of a hardsetting soil after growing winter rotational cover crops as monocultures under CA in a maize based system.
Matric potential of water (kPa)
-33 -1500 PAW†
Treatment
Soil water ( cm3 cm-3) 0 to 30 cm depth
Vetch 0.342a†† 0.138 0.204a
Oats 0.327a 0.133 0.194a
Lupin 0.328a 0.132 0.197a
Weedy fallow (control)
0.285b 0.126 0.159b
ANOVA P value
0.0189* (4.8) NS (4.5) 0.0257* (7.1)
* = significant at (P ≤ 0.05), NS = not significant at P ≤ 0.05.
† plant available water between field capacity (-33kPa) and permanent wilting point (-1500 kPa).
†† different letters within a column indicate significant difference among treatments at P ≤ 0.05.
Values in parentheses are coefficients of variation, CV %.
46 0.1
0.15 0.2 0.25 0.3 0.35 0.4
10 100 1000 10000
Volumetric water content cm3/cm3
Matric potential (-kPa)
lupin oats vetch
weedy fallow (control)
Figure 4.5. Effects of winter rotational cover crops grown as monocultures after 4 rotations on water retention characteristics of a hardsetting soil. (Bars indicate standard deviation).
47
4.2 EXPERIMENT TWO: Soil Strength, Aggregate Stability and Water Conservation after Two Years and Two Rotations with Winter Cover Crops Grown as Biculture in a Maize Based Conservation Agriculture System.
4.2.1 SOIL STRENGTH
There were no significant differences observed between WRCC grown as bicultures and the control of weedy fallow across the two soil layers. Bulk density also did not significantly change with an increase in depth for the control, with a similar trend being observed under cover crop bicultures (Figure 4.6). Within the 0 to 15 cm soil layer, cover crop bicultures resulted in a slightly increased bulk density relative to the control, with both treatments O70V30 and O90V10 resulting in a 1.9% increase in bulk density.
Within the 0 to 15 cm soil layer, no significant differences (P ≤ 0.05) were observed in penetration resistance (PR) between treatments (Table 4.5). The treatments O50V50 and O70V30 resulted in a respective 6.6% and 4.6% increase in PR within the 0 to 15 cm soil layer.
However, significant differences (P = 0.028) were observed across treatments within the 15 to 30 cm soil layer with the treatments O50V50 and O70V30 consistently increasing PR by 11% and 1.7% respectively, compared to the control. The treatment O90V10 resulted in a decrease in PR compared to the control by 14.8% within the 15 to 30 cm soil layer. A significant increase (P ≤ 0.001) in PR was observed with an increase in depth for all the treatments.
48 1.3
1.35 1.4 1.45 1.5 1.55 1.6 1.65
0 to 15 cm 15 to 30 cm
bulk density (g/cm3)
Soil depth
O50V50 O70V30 O90V10
Weedy fallow (control)
Figure 4.6. Bulk density at two depths as affected by winter rotational cover crops grown as bicultures after two rotations in a maize based conservation agriculture system in a hardsetting
Figure 4.6. Bulk density at two depths as affected by winter rotational cover crops grown as bicultures after two rotations in a maize based conservation agriculture system in a hardsetting