The regular application of lime will increase soil pH by lowering the soil’s acidity. Several options are available such as; adding calcium (Ca), or magnesium (Mg), to reduce the solubility of Al and Mn below the toxic level. Liming materials vary in their effectiveness. Calcium or magnesium carbonate are traditionally used and react with soil acidity to neutralize it. Liming materials have very slow movement into the soil without proper mixing. Field practices such as tillage increases the effectiveness of all lime materials by incorporating them into the rooting zone (Anderson et al., 2013).
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Soil acidification is neutralized by the addition of hydroxides, carbonates, and silicates of Ca and/or Mg. The base anions in liming materials react with soil acidity H+
to neutralize it. The most commonly used liming material that provides carbonate as the base is calcium carbonate. Calcium itself does not raise soil pH. For example, calcium sulfate (gypsum) and other additives contain Ca but do not contain a basic anion (carbonate, hydroxide, oxide, or silicate). Thus, they do not neutralize soil acidity (Spies and Harms, 1988).
8.2 Lowering soil pH
Reducing soil pH or soil acidification is a natural process that is enhanced by some field cultural practices, mainly application of sulfur, nitrogen (N) fertilizers in form of urea or ammonium sulfate or other soil amendments that contain ammonium-N (Adams, 1984). As soil acidification occurs, several chemical and biological properties of the soil also change. An important chemical change occurs in the acid soil, is the increase of aluminum (Al) and manganese Mn solubility which causes phytotoxicity to plants (Everhart, 1994). Plants vary in their tolerance and response to Al and Mn toxicity, indicating a crop-specific soil pH requirement. While soil acidification involves a purely chemical reaction, biological association in the form of soil microorganism must metabolize those fertilizers before effectively lowering the soil pH. Therefore, lowering soil pH requires the proper soil properties and conditions suitable for microorganisms to assess the biological reaction. (McCauley et al., 2009)
8.3 Soil pH buffering capacity.
One of the fundamental soil properties is its pH buffering capacity (pHBC) (Bloom, 2000). Soil pHBC has been used to estimate the change in the soil pH after acidic or alkaline elements are added to the soil. Quick and accurate determination of
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soil pHBC if achieved at low cost can be used to assess the agricultural liming or sulfur recommendations (Liu et al., 2004). It can also serve as a long-term predictor for the rate of soil acidification by knowing previous external acidity sources.
Nitrogen simulation models such as mineralization, nitrification, urea hydrolysis, NH3 retention, and volatilization require knowledge of soil pHBC (Kissel et al., 2012).
These N cycle reaction rates and chemical speciation depend on soil pH and consequentially depend on soil’s pHBC along with other environmental factors. The typical determination of soil pHBC is done using multiple doses of the base in a titration procedure to construct a pH buffer curve (Nelson and Su, 2010). The titration curve of the topsoil is usually linear in the pH range of 4.5 -6.5 (Magdoff and Bartlett, 1985). The inverse of the slope of pH to the amount of base added to the soil is defined as pHBC and is expressed in millimoles H+ per kilogram per pH unite (Kissel et al., 2012).
From the titration curve, the soil pH increase can be estimated from the millimoles of H+ used to make a change in the pH per kilogram of soil. The titration process can also
be used to estimate the soil pH decrease from the millimoles of H+ added per kilogram
of soil. Then the change in the soil pH (ΔpH) can be predicted from either the addition or removal of H+ (ΔH+, mmol H+ kg−1) from the soil as
ΔpH = ΔH + ΔH + soil pHBC
The titration requires a lengthy procedure of many days of incubation to reach a pH equilibrium. It requires a wide range of solution /soil ratios and different solution ionic strengths. Atken and Moody suggested that the soil pHBC is a fixed soil property but it is still unclear when soil pHBC values form the same soil are different due to
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different bases like; Ca(OH)2 vs. NaOH or acids HCl or H
2SO4 (Kissel et al., 2012).
This discrepancy is due to the sensitivity of soil pH measurements to the changes in ionic strength of a solution with electrical conductivity (EC) less than 2dSm -1 (Miller
and Kissel, 2010). However, this condition can be avoided by using a uniform ionic strength for titration, organic ionic strength or to process the titration in a low salt solution (< 0.01 molL-1). Nevertheless, Calcium chloride (CaCl
2) is the best option in
most cases because Ca+2 is normally the dominant exchangeable cation in arable surface
soils (Kissel et al., 2012). Thompson et al. (2010) compared the time required to raised several soils pH using CaCl2 and found that it took 4 incubation days to increase average
pH by .023 (). Another study reported that the incubation of acid soil with base required substantial time to reach equilibrium pH (Aitken and Moody (1994)).
8.4 Pot-in-pot for fruit tree plantation
Planting fruit trees directly in a nonhomogenous soil is not the best option for growing plants is the cheapest option (Erez et al., 1989). Even with long historical use of soil analysis, the soil is still a mystery in term of all physical, chemical and biological complexity that might occur. Thus, container growing present not only an aesthetic and luxurious way of growing plants but also a tool for researches to evaluate certain aspects that might influence plant growth and productivity. It relieves the plants from two major constraints, the unpredicted climate and the unknown soil dynamics (Burdett et al., 1983).
The large number of quantitative studies designed to determine the effect of a certain factor affecting fruit tree have been done using potting methods (Marsal et al., 2000). This approach maintains almost full control of other environmental conditions
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that might act as confounding factors by providing uniform soil variables. Those studies included the investigation effect of irrigation on pears fruit (Marsal et al., 2000), root restriction of apple and peach trees (Myers, 1992), effect of root pruning on apple trees
(Hsu et al., 1996), nitrate absorption by orange trees (Chapman and Parker, 1942) and evaluation of chemical control of pathogenic disease in apple (Kirby and Frick, 1963).
Hoestra studied the effect of soil pH on apple seedlings in pot experiments and found a good growth of apple seedlings at pH 3.8 (Hoestra, 1968). When using this approach, it is essential to pre-adjust soil pH and to maintain consistency throughout the experiment. Root temperature can cause a dramatic change in the root growth and ultimately in overall tree growth. Direct sunlight can be absorbed on the exposed surface of the container leading to significant temperature fluctuation and extreme temperature during the summer (Martin and Ingram, 1992) and the winter (Mathers, 2003). To avoid this problem, the trenched pot-in-pot system can be used. where pots are stacked and placed in 80cm wide x 70cm deep trench.