Soil organic matter accumulation often taken as cause of soil acidity in most books and also literature. The reason given is associated with the dissociation of H ion from functional groups. But as to me this also increases CEC and buffering capacity.
Dear Bekale, you are absolutely right. Soil organic matter act as a reservoir for plant nutrients and prevents leaching of elements, which are vital to plant growth. This is achieved by nutrient adsorption due to the high cation exchange capacity of more than 300 cmol (p+) kg -1. Moreover, formation of clay-humus complexes through the interaction of humic substances and clays increases the buffering and exchange capacity of soil.
Highly acidic but organic carbon rich ( >> 1%) Alfisols, Inceptisols and Ultisols of humid tropical climate of India generally do not show soil CEC > 25 cmol(+)/kg.
Dear Bekele, please consider that during decomposition of soil organic matter, which is an oxidation ration, protons are released causing soil acidification. Soil organic matter can also buffer soil pH over a wide pH range (...3 to 8) depending on sorbed or releasable cations (e.g. Ca, Mg, K,..). Best Nils
Of course,soil OM adds acidity but if we see this with a wholistic approach then this does not means that such kind of acidity goes on accumulating and may be toxic over the years.Fact is that in soil there is a complex system which is always active and depending upor rainfall,weather /temperature conditions,soil flora/fauna many micro level things actually dictate the overall reaction.However the effect of adding organic matter never leads to any acidity level that might be toxic to any of the field crops.
If you look at the composition of organic matter it includes plant tissues, remains of harvested crops, soil organisms, animals and waste product of animals. The organic compound in the plant tissues are mainly cellulose, hemicellulose, protein, lignin etc. The elemental composition of these compounds are carbon, oxygen, hydrogen and ash. Thus decomposition of organic compounds in soil results in production of CO2, water, biomass, energy, mineralisable organic N, P, S etc and formation of compounds which are resistant to microbial action. Organic matter content about 20% non-humic substances like lipids, carbohydrates, protein/.amino acids etc. and 80% humic substances. Generally, non-humic substances are low molecular weight compounds, which can be utilized as substrates by soil microbes. Main humic substances are humic substances. Therefore, it is hardly any scope of organic matter incorporation to build the soil acidity.
I am also in agreement with Dr J C Tarafdar . However there is a possibility of soil acidification, when addition of organic matter into soil exists , i.e
Many forms of organic matter can also be acidifying, depending on the plant from which the organic matter is derived. Some plants contain significant quantities of organic acids. As their residues are decomposed, the organic acids naturally affect the soil acidity. Other plants are acidifying simply because of the low concentrations of bases they contain. If the plant does not contain enough bases to satisfy microbial needs, the decomposition of the plant debris will not only give off carbon dioxide, but will also remove base nutrients such as calcium and magnesium from the soil, which will in turn increase the acidity. But the quantum of this acidity will be very minor in range, when compared to other causes of soil acidification such as high intense rainfall, leaching of basic cations, application of acid forming fertilizers etc.
According to the previous contributions, organic matter may contribute to soil acidification as well as to cation exchange capacity (CEC) and buffer capacity. Acidification occurs mainly through organic matter mineralisation. So we can say that labile organic matter contributes more to acidification, while stable or recalcitrant organic matter (humic substances, OM bound to inorganic components, biochar) contributes more to buffering and CEC. We must also remind that organic matter has variable charge, i.e. CEC of soil organic matter is pH-dependent, that is CEC is low at acid pH and moderate to high at pH close to neutrality.
Soil organic matter and acidity of soil depends on the kind of organic matter and the conditions under which the same degrades. In general, degradation of organic material under wet and low temperature conditions leads to formation of more organic acids, which causes increase in acidity. E.g. Podsole in Taiga; But in tropics, even under humid conditions, organic degradation at high temperature and aerobic conditions do not contributes to any significant change in acidity. Degradation of organic waste rich of protein, fat and starch contribute to more acids than the usual garbage rich of cellulose, lignin and other compounds. Therefore, generalization on organic matter and acidity needs careful analysis of the situation.
i agreed with Joseph, SOM impact on SA largely depends on specific site conditions especially on temperature.
I dont know for sure. I have observed that addition of dairy manure resulted in increase of organic matter, CEC and pH. In a very acid oxisol.
Im sure OM accumulation will increase the CEC and the buffering capacity but the SA will depend on other factors
This is too complex a phenomenon to generalize because it is a function of temperature and the dominant type of organic matter in the soil matrix.
I agree with Maria Luisa Fernandez-Marcos , where all of the charges on the organic colloid are considered pH dependent charge, that is CEC is low at acid pH and moderate to high at pH close to neutrality.
I agree with Joseph Ray sir kind of composting material also contribute its effect on soil. No doubt it increases acidity in soil but under which environmental condition it affects most is also important.
Perhaps depends in the C:N ratio of the soil organic matter. Higher N and higher pH.
The decomposition of the organic matter results in the formation of carbon dioxide, which is soon reacted with water. The component of the carbonic acid, which is a concern, soon dissolves into the bicarbonate and hydrogen, and then the bacterium secrete either enzymes or acids to dissolve the organic matter to be decomposed Acidic.
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