Lime requirement of soil

Determination of the pH Value of Sludge, Soil, Mud and Sediment and the Lime Requirement of Soil (Second Edition) (by Determination of the pH Value of Sludge, Soil, Mud and Sediment or by Determination of the Lime Requirement of Soil), 1992... 

Shoemaker, H. E., E. O. McLean, and P, F. Pratt. 1961. Buffer methods for determining lime requirement of soils with appreciable amounts of extractable aluminum. Soil Sci. Soc. Am. Proc. 25 274-277. 

The lime requirement of soils was at one time defined as the amount of CaCOs (or its equivalent in any other alkaline material) needed to neutralize all exchangeable acidity and bring the soil to 100 percent base saturation. This would raise the soil pH to 7 or higher, and is generally neither economical nor agronomically advisable. Lime requirement is now more rationally based on the quantity of lime required to reduce exchangeable and soluble Al to a nontoxic level for the particular crop to be grown. 

DETERMINATION OF LIME REQUIREMENT OF SOIL (Schoemaker etal. 1961)... 

Shoemaker, H.E., McLean, E.O. and Pratt, P.F. (1961), Buffer Methods for Determining Lime Requirement of Soils with Appreciable Amounts of Extractable Aluminium. Proc. Soil Sci. Soc. Am. 25 274-277. 

Definition. The lime requirement of a mineral soil is the number of tonnes of calcium carbonate calculated to raise the pH of a hectare of soil 200 mm deep, under field conditions, to, and maintain at, 6.5. 

Figure 5.10. The titration curve of soil A, with a low buffer capacity, and soil B, with a high buffer capacity. The lime requirements of these soils are indicated (in units of centimoles of base per kilogram of soil) assuming that they are to be limed to pH 6.0.

The functional groups responsible for the high CEC of humus also buffer soil pH over a wide range. This buffering contributes significantly to the lime requirement of acid soils (Chapter 8). Total acidities of isolated fr actions of humus vary from 3 to 14 mol kg-1. 

The titration process, if carried out so slowly that the reaction is fairly complete following each addition of base, does not distinguish between exchangeable and virtually nonexchangeable components. Hence, titratable acidity is only a measure of the total acidity neutralized during the experimental technique employed. The titratable or total acidity is nonetheless useful for determining the lime requirement of acid soils. 

The most theoretically satisfying way to estimate the lime requirement of acid soils is to measure the quantity of base required to raise soil pH to a specified level. To be realistic the titration must be slow enough for the added base to react completely with the soil. Both exchangeable and titratable acidity will be neutralized during the titration. 

A hydroxyl ion is also consumed during the displacement of adsorbed anions as the soil pH is raised. This effect is not normally a major one, but contributes to field lime requirements of several metric tons per hectare for some highly acid Piedmont soils from the southeastern United States. 

Based on the data of Fig. 10.4, how much effect would variation in titration time from 0.5 to 48 hours have on the lime requirement of pH 5 Greenfield soil, if a final pH of 6.5 were sought and if 5 g of soil were used for the titration shown ... 

The lime requirements of various crops and the loss of calcium and magnesium from the soil are described in sections 10.2.2 and 10.2.3. 

Fitts, J. W., and Nelson, W. L. (1956). The determination of lime and fertilizer requirements of soils through chemical tests. In Advances in Agronomy, vol. 8, ed. A. G. Norman, pp. 241-82. New York Academic Press. 

Munns, D. N., and Fox, R. L. (1977). Comparative lime requirements of tropical and temperate legumes. Plant Soil 46 533-48. 

Forage legumes, of which white clover is the most important, are particularly sensitive to calcium deficiency and will not thrive. Ideally, soil pH should be maintained between 5.8 and 6.5. Heavy-textured soils such as clay require more lime than sandy soils to raise pH, because the higher levels of clay and organic matter act as a buffer against change, and the same is true of peaty soils. 

Pure water in equilibrium with atmospheric CO has a pH of 5.6. If a soil pH is lower or higher than this, it is acting as an acid or base respectively. Several soil components act as buffers (hydroxy aluminium monomers or polymers, soil organic matter and undissolved carbonates), therefore lime requirement tests may also be required. 

Calculation for mineral soils of pH 5.0-6.4. Subtract the indicated pH from 7.00 and multiply by 11.2. The result gives the lime requirement as tonnes ha calcium carbonate. 

Two attributes are required of any liming material 1) a cation capable of displacing soil colloid-adsorbed H+ and Al+ + + ( also a source of soil acidity) and 2) an anion capable of neutralizing the displaced H+ and Al+ + + (see Equation 4). 

The desirable pH increment, ApH, can be decided on the basis of a function similar to that shown in Figure 5.5, which establishes for each soil how high the pH must be to eliminate A1 toxicity for a crop. Of course, the optimum pH varies for different crops because of plant-specific A1 tolerances and Ca requirements. The hme requirement of any soil can be calculated from the slope of the titration curve of that soil, as diagrammed in Figure 5.10. Conversely, the buffer capacity of the soil is the reciprocal of this slope that is, buffer capacity is the quantity of lime (alkali) added to the soil that achieves a unit change in pH. 

The lime requirement is most closely related to total acidity, because lime added to soil neutralizes all forms of acidity. However, lime requirement is adjusted in prac-... 

In this method the soil is equilibrated with a pH 7.5 buffer solution, whereby the reserve H+ is brought into solution, which results in the depression of pH of the buffer solution, a note of which is made and interpreted in terms of lime required to raise the pH to a desired value. 

Lime requirement is determined on the basis of soil-buffer pH ready reckoner given below. 

Probably the most important and distinctive property of soils is that they can retain ions and release them slowly to the soil solution and to plants. The retention prevents concentrations that are too high and too low. The evolution of plants has taken advantage of this buffered range of ion concentrations that soils make available in the soil solution. Over most of the earth s surface, the availability of these ions in the soil solution is adequate, but not necessarily ideal, for plants. Crop and horticultural plants and a desire for maximum yield place greater demands on the soil and may require adjusting the native soil solution. Adjustments by fertilization, liming, and salt removal are usually temporary. The soil and climate tend to return the soil to its native state. 

A major problem of managing acid soils is to estimate the quantity of lime required to raise the soil pH to a certain level. As shown in Table 10.2, plant species vary considerably in their response to soil pH. Such data must be interpreted carefully. In this case, the nonlegumes benefited from nitrogen fixed by legumes in the rotation. Much of the pH response may actually be the pH response of nitrogen fixation by the legume—Rhiz.obium pair. 

The titration of individual soil samples is impractical for soil-testing purposes, because of the time and experimental precision required. Such titration is also highly dependent on the time allowed for each increment of base to react with the soil (Fig. 10.4). The usual procedure is to add a pH buffer solution to the soil, measure the amount of buffer consumed or the resulting pH of the soil-buffer suspension, and calibrate results with field lime requirements for similar soils from the same geographical area. 

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