(2) LIME, SOILS AND SOIL TESTS |
| Soil pH is a measure of the H+ ion activity in soil solution.
In most soils, the soil pH is buffered by several components of the solid phase, including
hydroxyaluminium monomers and polymers, the soil organic matter, and (in alkaline soils)
undissolved carbonate compounds (Coulter, 1969). An equilibrium condition exists between
these components and the soil solution such that when acid or base is added to the
solution, the buffering agents may absorb the addition and/or release acid or base to
maintain the initial equilibrium. Thus, a change in soil pH due to the addition of a base
(lime) is usually much less than predicted by neutralisation, based only on the quantity
of acid or base present in the soil solution (as given by the soil pH). Lime requirement
tests for soils, which generate recommendations for effecting relatively long term changes
in soil pH, are designed to take account of the buffering power of soils which is usually
related to soil texture and organic matter content. |
| pH is useful as a measure of the degree of acidity/alkalinity of a soil
while the lime requirement is an estimate of the amount of lime required to alter the pH
in the soil to a target pH. |
| The pH of the soil can be taken by pH indicators or by pH meters. pH
indicators are organic dyes in solution in ethyl alcohol. They change colour depending on
pH. A wide pH range can be determined by using a mixture of dyes. These are useful tools
for analysis in the field but are restricted in application. They can be helpful in
diagnosing the cause of crop failure by checking near the roots of diseased and healthy
plants for acidity. They may also be used to decide quickly whether a soil needs lime or
not. If the soil is acid a sample should be send to the laboratory for a precise estimate
of lime requirement. They can be used to determine the uniformity of an area before taking
soil samples. |
| The disadvantages of the
indicator are :- |
| 1. |
A very small sample must be used and it can not therefore be
representative. |
| 2. |
The indicators are not affected by the buffering capacity of the soil so
lime requirement based on pH will be inaccurate. |
| 3. |
Indicators are colour tests and yet four per cent of the population are
colour blind. |
| 4. |
Soil colour can mask the colour of the indicator. |
| 5. |
It is not possible to accurately estimate L.R. from the pH. pH does not
measure the buffering capacity of the soil. Using pH to determine how much lime to use can
lead to overliming in some soils and underliming in other soils. |
|
| pH can also be determined potentiometrically in a slurry system using an
electronic pH meter (McLean, 1982). There are many different types of these around the
world (McLean 1982) and specific instructions are provided with individual units. When
used carefully, they are more reliable then indicators as the sample is larger and
therefore more representative. They measure the pH and should not be used for making lime
recommendations as they do not measure the buffering capacity of the soil. |
| The lime requirement of soils in Ireland is estimated using the
Shoemaker-McLean-Pratt Method (SMP) buffer method. Over the years, it has been found to be
reasonably accurate for most Irish soils and has been calibrated in experiments in which
lime incubated with different soils over long periods. Some studies have suggested that it
is less accurate in poorly buffered, sandy soils, in soils with organic matter greater
than 10% or in soils with a predominance of Kaolinite and aluminium (McLean 1966).
Nonetheless, this buffer method has worked well in Ireland for many years. The lime
requirement for peats will be discussed in the recommendations section. |
| To know the actual quantity of ground limestone to recommend involves the
following laboratory procedures on each soil sample, 1. Drying; 2. Sieving; 3. Mixing; 4.
Use of buffer test. You can with confidence take responsibility for lime recommendations
where each of these steps has been followed carefully on correctly taken soil samples. |
| Other Methods
A rapid ‘Quick
Test’ method was suggested by Bailey et al. (1989), in which titratable acidity was
shown to relate closely to soil organic matter and the latter to soil density; a measure
of bulk density by a simple scoop method and of the initial pH was combined in a
regression model to calculate the lime requirement. This method has not gained widespread
acceptance. Currently, in Britain a series of look-up tables to determine lime requirement
(ADAS, 1986). This followed the analysis of many years data that showed a linear
relationship between soil pH and lime requirement, with a different relationship for each
textural class. However, the use of this method involves assessment of soil texture and
organic matter content. Rothamsted have been working on a lime requirement model that uses
empirical lime loss under arable and grassland system. It is eventually hoped to extend
this model into other soil types. Curtin and Smillie (1995) have suggested that a simple
method of predicting cation ratios in the soil solution in itself may have diagnostic
value in plant nutritional problems. At the moment, in Ireland there are no plans to
change from the buffer method currently being used. |
| All lime requirement estimations are based on soil samples. It is critical
that the soil sample be representative of the area being sampled. The sampling procedures
for one similar to those required for sampling for P and K. |
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