The Impact of Soil pH on Nutrients

Ernie NeffNutrition


In the April Central Florida citrus Extension newsletter, multi-county citrus Extension agent Chris Oswalt discussed the fate of the nutrients nitrogen, phosphorus and potassium in soils that have elevated pH values. Here’s most of the discussion he presented:

Soil pH can affect several reactions involving nitrogen in the soil solution and the efficient use of this nitrogen by plants. Nitrification, the conversion of ammonium to nitrate, is done by soil bacteria and is most rapid in soils with a pH between 7 and 8. Nitrification in the soil is nearly zero at a soil pH of 5. So simply put, if a grower is using ammonium nitrate as a nitrogen source, then at a pH between 7 and 8, the ammonium would be rapidly converted to nitrate. This would result in the nitrogen applied being significantly susceptible to leaching due to the quick conversion of the ammonium nitrogen to the more soluble nitrate. At a lower soil pH, the ammonium in this ammonium nitrate would be more slowly converted to nitrate due to the reduced activity of soil bacteria. This reduction in the rate of ammonium to nitrate conversion would result in more of the nitrogen remaining in the ammonium form that is less susceptible to leaching.

Also, soils with elevated pH values can cause the loss of nitrogen due to the volatilization of ammonia (NH3) in the atmosphere. This volatilization of nitrogen occurs when ammonium sources of nitrogen are applied to a soil surface with pH values greater than 7. In the management of nitrogen applied to soils, when pH values are above 7, there is a greater loss of nitrogen when ammonium forms of nitrogen are used.

Growers can better manage these potential ammoniacal losses of nitrogen by lowering the soil pH to slow the conversion to nitrate and reduce volatilization. Volatilization can also be reduced by incorporating this ammoniacal nitrogen in the soil by cultivation or irrigation. Using irrigation to incorporate ammonium forms of nitrogen in a high pH soil can result in additional leaching due to the faster conversion of this ammonium to nitrate by soil bacteria.

The availability of phosphorus in calcareous soils is also limited. The amount of phosphorus in soil is closely related to the availability of this phosphorus to plants. In higher pH soils, phosphorus reacts with soil calcium resulting in a decreased solubility of phosphorus (a process called phosphorus fixation). In this situation, the availability of phosphorus is determined by the amount of soluble phosphorus applied and any phosphorus that is released from that fixed phosphorus. Application of soluble phosphorus in these soils will only be available to plants for a short time due to the rapid phosphorus fixation at high soil pH. In this situation, lowering the soil pH will decrease the amount of fixed phosphorus, resulting in the availability of additional previously insoluble phosphorus.

Potassium availability in high pH soils is difficult to achieve due to the occupation of the nutrient holding sites of the soil particle surface by excessive calcium. The occupation of these exchangeable soil particle sites will suppress the uptake of potassium by citrus trees due to competition between calcium and potassium for the exchangeable soil particle sites. Although potassium would normally be available for plant uptake at a higher soil pH, this aforementioned competition with soil calcium can negate this availability. Lowering the soil pH in this situation will lead to an overall reduction of the exchangeable sites of the soil particle that could be occupied by potassium (K+) or other basic cations (calcium and magnesium). This would result in the release of potassium and other basic cations, making it more likely leaching could become more of a factor affecting the soil levels of these nutrients.

Learn more about the part pH plays in nutrition from experts at the University of Florida Institute of Food and Agricultural Sciences.

Source: University of Florida Institute of Food and Agricultural Sciences

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