Source of Hydroxyl Ions (Alkalinity)

When adsorbed hydrogen and aluminum are replaced from acid soils by cations such as calcium, magnesium, and potassium. The hydrogen concentration in the soil solution decreases the concentration of hydroxyl ions and will simultaneously increase since there is an inverse relationship between the hydrogen and hydroxyl ions. These "base forming" cations become sources of hydroxyl ions merely by replacing the adsorbed hydrogen. ¹

The metallic cations such as calcium, magnesium, and potassium also have a more direct effect on the hydroxyl ion concentration of the soil solution. A definite alkaline reaction results from the hydrolysis of colloids saturated with these cations. Under natural conditions the reactions to furnish hydrogen and hydroxyl ions to the soil solution occur at the same time, that is, hydrogen and aluminum ions and the basic cations are held at one time. ¹ The pH of the soil solution will depend upon the relative amounts of absorbed hydrogen and aluminum compared to adsorbed metallic cations. Where the effect of hydrogen and aluminum are dominant, acidity results. Excess bases yield alkalinity, and at just the right balance, the pH of the soil solution will be 7 (neutral).

As exchangeable calcium and magnesium are lost by leaching, the acidity of the soil gradually increases. In humid regions, there is a direct correlation between the pH and the amounts of the two elements. When the pH of a mineral soil is low, large amounts of aluminum, iron, and manganese are soluble. In fact, they may become toxic to certain plants due to their high solubility. However, as pH is increased, precipitation takes place and the amounts of these ions in solution becomes less and less. At neutrality plants may suffer from lack of available manganese and iron.

Copper and zinc are affected in the same way by a rise in pH, the critical point being near pH 7, above which their availability declines. Boron is different and more complicated. Neither the soil untreated nor lime alone, appreciably precipitate boron, but the two in combination fix it markedly. ¹ Too much calcium in plant cells might interfere with boron metabolism, even if plenty is present.

Molybdenum availability is dependent on pH. It is quite unavailable in strongly acid soils. It becomes available at pH 6 and above.

The kind of phosphorus ion present varies with soil pH. Where soil is alkaline, the HPO₄ ion is most active. ¹ As pH is lowered and the soil becomes slightly to moderately acid, both the HPO₄ ion and the H₂PO₄ ion prevail. At still higher acidities, the H₂PO₄ ion tends to dominate. Under conditions where soil acidity increases and there is a rise in iron, aluminum, and manganese, phosphates are strongly fixed as complex and insoluble compounds.

If the pH of a soil is raised much above 7, the solubility of both the native and applied phosphorus may be impaired. Additionally, a pH above 7, the excess calcium that forms may hinder phosphorus absorption and utilization by plants.

Between pH 6 to 7, phosphorus fixation is at a minimum; conversely, phosphorus availability is at a maximum for most plants. ¹ It is therefore important to keep pH near the 6 to 7 pH for the best nutrition for crops. ²

Nitrification and nitrogen fixation take place vigorously in mineral soils only at pH values well above 5.5. ¹ However, mineralization, although reduced, will still proceed at lower pH because fungi are able to effect these enzymatic transfers at high acidities. ²

Biologically, soil in the intermediate pH range presents the most satisfactory regime. Nutrient conditions are favorable without being extreme and phosphorus availability is at a maximum.

Soil pH and how it affects nutrient availability through direct and indirect means, can be very complex. Understanding how certain pH factors affect nutrient availability plays an important role in the proper management of crop and forage production. Overall, based on my findings, it would seem very important to attempt to keep soil pH at or near neutral for best overall nutrient availability and plant nutrition.

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