7 Main Importance of Soil Acidity
The major importance of soil acidity includes the following:
1. Availability of Plant Nutrients
Soil acidity affects the availability of most soil nutrients especially phosphorus, nitrogen, sulfur, molybdenum, and other micronutrients such as iron, aluminum, and manganese.
The pH of any soil could be a good guide in predicting which plant nutrients are likely to be deficient.
2. Availability of Phosphorus
Phosphorus compounds in soil have low solubility. The concentration of the various forms of phosphorus taken up by plants, that is, the orthophosphate ions, H2PO4– and HPO4= in soil solution is usually small and intimately related to the pH of the medium. Under low pH, phosphorus is precipitated as iron and aluminum phosphates.
At low pH, below 4.0, A1 and Fe activity in soil is very high and phosphorus is easily fixed up in such a medium. As pH increases above 4.0, P-availability increases as shown by the above equations.
These reactions occur on the surfaces of strengite, Fe (OH)3 or variscite, A1(OH)3. At a high pH medium or solutions of alkaline soils, (pH> 7.5), there is the precipitation of phosphorus by calcium to form calcium phosphates.
(Apatite) that is, the insoluble phosphates of iron and aluminum will be precipitated in acid soils and the insoluble phosphates of calcium and magnesium at pH values greater than 7.5.
Several phosphorus compounds having varying solubilities are normally formed under different soil conditions.
Times, the presence of sodium phosphates, a soluble compound at pH>8.5 makes phosphorus available again. Therefore, the maximum availability of P to most agricultural crops occurs within the soil pH range of 5.5 – 7.5.
3. Availability of Nitrogen and Sulphur
Nitrogen and sulphur are soluble at almost all pH levels in soil solution. The supply of these two elements comes mainly from organic matter and is therefore limited by the rate of decomposition of organic matter.
The fate of mineralization of N and S from soil organic matter is fastest between pH 6 and 8. Much of the available S in soils with a pH higher than 8 comes from mineral sources rather than organic.
Similarly, nitrification processes occur within a pH range of 5.5 – 7.5 with an optimum pH of 6.5. Nitrogen occurs as NO3 under high pH and as NH4 under low pH which it could also be fixed while NO2 could also easily be leached at high pH levels.
4. Potassium
Potassium compounds are soluble at any pH but K+ could easily be removed from soil solution by adsorption, especially into the 2:1 clay layers.
In leached acid soil, K is low. Raising the pH by liming may drive the little amount of K into non-exchangeable positions and further suppress K availability.
5. Calcium and Magnesium
These are the most abundant bases in the soil. They are usually added in soil amendments but not so much as fertilizers.
Read Also: Here’s How Soil Becomes Acidic
6. Iron, manganese, Copper, and Zinc
These are micro-nutrients that form metallic cations that precipitate into low solubility compounds at high pH levels. Therefore they are usually deficient in high lime or alkaline soils.
Under constant oxidation potential, as pH increases from pH2 to pH6, the availability of iron, Fe++, decreases from about 10-2 to 10-6 moles per liter.
Reports of several years of research have also shown that the amount of exchangeable copper and therefore its availability to crops decreased as the soil pH increased.
Similarly, a direct negative correction was obtained between ammonium acetate extractable Mn2+ and soil pH (Tisdale, 1949). The same trend is true of zinc availability which is generally low in alkaline soils.
The greatest zinc uptake both native and applied takes place at lower pH levels with deficiencies occurring within the range of 6.0 to 8.0.
There may be some cases where relationships between zinc uptake and soil pH may not be significant since the uptake of a particular nutrient also depends on other soil conditions.
Curiously enough, these micronutrients, except iron, may be leached and become deficient at very low pH.
7. Boron and Molybdenum
Boron may be leached at a very low pH while solubility is very low at high pH. Boron deficiency symptoms are reportedly associated with high pH values and its uptake by plants is reduced by increasing the soil pH.
In contrast to the behavior of most of the other micronutrients, the availability of molybdenum increases with an increase in soil pH. At high pH levels, there is the conversion of molybdenum oxide to soluble molybdate salts making the element available in alkaline soils.
On the other hand, Mo is precipitated by Fe and A1 as Fe2 (Mo0 )3 and Fe (M.O4)3 A12 (Mo0 ) at low pH values, rendering Mo unavailable to plants.
Read Also: Soil Reaction: Meaning, Acidity, and Liming
