Soil Acidity and Liming

Surface horizons and most subsoils in North Carolina are inherently acid. The rocks and minerals from which the soils are formed contain relatively low amounts of calcium (Ca) and magnesium (Mg). The soils have a low capacity to retain cations and since rainfall in the state exceeds evapotranspiration, Ca and Mg tend to be leached from the soil. In their natural state most North Carolina soils have pH values less than 5. Thus unless soils are limed crop growth is quite limited.

When the first settlers came to North Carolina, forests covered the land. Pine was the predominant species in the coastal plain while oak and hickory were the principal trees in the piedmont. Slash-and-burn was the procedure used to clear the land. This practice added Ca and Mg to the soil in the form of oxides that neutralized acidity and raised the pH of the surface soil. The ash also contained other recycled nutrients. The liming and nutrient effect lasted several years after which time the cleared area was abandoned and a new area cut and burned. For many years this was repeated throughout North Carolina as a means of overcoming soil acidity and adding nutrients in the surface horizons.

As agriculture production became more intensive, farmers started using lime. Along coastal areas there were shell deposits which were burnt to form calcium oxides and then applied to the soil. Marl deposits in the eastern part of the coastal plain also were a source of lime. Farmers observed that where burnt shells and marl were applied crop growth improved.

Initial research on liming by the North Carolina Agricultural Experiment Station was limited primarily to field studies. These studies showed that application of lime and animal manures increased yields. Since soil acidity was a primary factor limiting crop growth in North Carolina, basic research on soil acidity and liming was started in the late 1930s and early 1940s. Data from the Midwest indicated that the ideal pH for crop production was 7, which gave a base saturation of 80 percent for these soils with montmorillonitic (2:1) clays. Soils in North Carolina, however, had mostly kaolinitic (1:1) clays and the question was whether the results from the Midwest applied to North Carolina soils.

The relationship between pH and percent base saturation was studied for pure clay systems and North Carolina soils representative of 1:1 and 2:1 clays. A much higher base saturation was required to raise the pH to 6 with montmorillonite than with kaolinite. White Store soils (fine, mixed, thermic Vertic Hapludults), representative of soils with 2:1 clays, had to be 80 percent base saturated to give the same pH as the Durham soils (fine, loamy, siliceous thermic Typic Hapludult) (1:1) clays at 40 percent base saturation as determined by the sum of cations, pH 8.2 CEC method (Table 1).

¹ Base saturation is of CEC pH 8.2

Growth of cotton and soybeans was maximum at 40 percent base saturation on the Durham soil and 80 percent base saturation on the White Store. Calcium was held much stronger by the montmorilloni-tic clay than by the kaolinitic clay and for this reason a much higher base saturation was required in order to supply adequate Ca in soils with 2:1 clays. Laboratory and greenhouse studies also showed that organic soils (Histosols) only needed to be 40 percent base saturated for optimum growth. The research dealing with pH and percent base saturation established that for mineral soils in North Carolina the lime recommendation was based on adjusting the pH (1:1 H O) to 6.

In the early 1950s researchers challenged the idea that acid soils contained exchangeable hydrogen. Soil chemists had developed the concept that acid soils behaved similar to weak acids rather than strong acids. This, however, raised questions as to whether acid soils actually contained exchangeable H⁺. Soil scientists at NCSU had a major effort in this research which established that acid soils contained exchangeable aluminum (Al³⁺) rather than exchangeable H⁺.

It was found that when H⁺ was added to soil the aluminum silicates clays decomposed releasing aluminum which went on the cation exchange site of the clay. The acid reaction of the soil as measured by pH was due to hydrolysis of the Al ions in the soil solution which formed H⁺ ions. This is illustrated by the reaction Al³⁺ + HOH <—> AlOH²⁺ + H⁺. It was now established that acid soils have exchangeable Al on exchange sites and that a major cause of poor growth in acid soils is due to aluminum toxicity.

Another major finding was that the CEC of Ultisols and Oxisols was pH dependent. The CEC of soils with Fe-oxide-coated clays and 2:1 clays with hydroxy Al interlayers increases as the pH increases. The OH^- ions associated with Fe and Al hydrous oxides and interlayer hydroxy Al release H ions as the pH increases and a negative charge develops. Therefore, in Ultisols and Oxisols, the CEC of the soil is not constant but is variable depending upon the soil pH. Thus measurement of CEC at pH 8.2 did not reflect the chemical environment that roots encountered in our Ultisols. The effective CEC (ECEC) of these soils has been defined as the sum of the exchangeable Ca, Mg, K and the exchangeable Al extracted with an unbuffered salt solution such as KCl. The amount of Al in the soil solution is related to the percent Al saturation of the effective CEC.

When 60 percent or more of the effective CEC is occupied by exchangeable Al, there is a very sharp increase in the Al concentration of the soil solution (Table 2). A marked decrease in plant growth results.

Mineral soils with variable charge mineralogy are 100 percent base saturated at pH 5.8 to 6. However, organic soils above pH 5 were found to have very little aluminum in the soil solution and essentially have their active exchange sites essentially occupied by basic cations. Organic matter holds Al quite strongly and liming organic soils to pH 5 reduces Al in the soil solution to very low concentrations. At pH 5 organic soils contain adequate Ca to overcome the H ion effect on cation uptake.

The knowledge that acid soils contain exchangeable Al brought about a new concept for making lime recommendations. The concept was to apply the amount of lime required to neutralize the Al that was extractable with a neutral unbuffered solution such as KCl. The amount of lime required to neutralize the exchangeable Al is given by the following equation.

The factor of 2 takes into account the pH-dependent charge of the soils, which results in a portion of the nonexchangeable acidity being ionized as the pH increases and reacting with the added lime.

The procedure for measuring exchangeable Al⁺³ is not well suited for routine soil testing procedure. For this reason a buffer solution pH 6.6 is used to measure extractable acidity (Ac) in North Carolina soils. The buffered solution extracts both exchangeable Al³⁺ and the pH dependent acidity (H⁺) which becomes ionized up to pH 6.6. The lime rate to apply is calculated with the following equation.

The desired pH for a soil is the pH at which the activity of Al⁺³ is neutralized. The effect of soil organic matter in decreasing the activity of Al⁺³ has been taken into account by establishing desired pH for each of three classes of soils (mineral, mineral-organic and organic) based on their organic matter content. The desired pH at which exchangeable Al⁺³ is essentially neutralized is 6 for mineral soils, 5.5 for mineral-organic soils, and 5 for organic soils.

Many years of high-input management, including liming, of Ultisols in North Carolina has improved the chemical properties of the subsoil. Increases in exchangeable Ca²⁺ and corresponding decreases in exchangeable Al³⁺ have created a favorable environment for root growth in the subsoil. This has resulted in utilization of subsoil moisture and reduced the detrimental effects of short-term moisture stress.

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