pH Effects On Plant-Available Phosphorus
When you look at your soil test results, the first thing to look at is the soil pH. What exactly is the soil pH measuring? The answer is simple: soil pH is a measure of the hydrogen (H+) concentration in soil. There is a common misconception that soil pH is used to make lime requirements, however, the soil pH only characterizes the soil as acidic, neutral, or alkaline. If acidic, a buffer solution is typically used to determine lime requirements.
Soil acidification is a naturally occurring process. The decomposition of organic matter, leaching of basic cations (calcium and magnesium), rainfall (which is naturally acid), and plant respiration all contribute to the decrease in soil pH. Any fertilizer that includes ammonium as a nitrogen source will undergo nitrification – the biological oxidation of ammonium to nitrite to nitrate by microorganisms– leading to a decrease in soil pH both localized to application site and eventually widespread.
In general, soil pH affects both the cation exchange capacity (CEC) (due the pH dependent nature of 2:1 clay minerals) of the soil and nutrient availability. Therefore, pH will need to be considered to optimize fertilizer use efficiency and crop yield.
Phosphorus
The chemical nature of phosphorus leads to a high binding ability within the soil, making it unavailable to a plant. Depending on the soil pH, phosphorus solubility can be controlled by aluminum, iron, or calcium.
In acidic soils, phosphorus precipitates with aluminum and/or iron or phosphorus is bound on iron/aluminum oxide and clay mineral surfaces. In neutral and calcareous soils – soil comprised mainly of calcium carbonate –phosphorus precipitates with calcium or is held on the soil surfaces of clay minerals and calcium carbonate.
The important thing to realize is that phosphorus availability can be improved with the adjustment of soil pH.
Increasing the soil pH with lime to at least 6.2 (for most crops) would result in higher phosphorus availability (Mallarino et al., 2013). However, it is a balancing act. As you increase the pH above this point, micronutrients, such as iron, manganese, zinc, etc., begin to decrease in plant availability.
A long-term wheat study conducted in Oklahoma showed that the use of lime with nitrogen, potassium, and phosphorus fertilizer resulted in higher long-term yield and higher phosphorus recovered in the grain compared to using the treatment of fertilizer without lime (Parham et al., 2002). The addition of lime also resulted in increased soil test phosphorus (plant available) compared to the non-lime treatment. This study illustrates that the wheat crop utilized fertilizer phosphorus more efficiently when lime was applied.
Adjusting the soil pH will not replace phosphorus fertilizer applications. Application rates should still be based on either a build-and-maintain or sufficiency approach or by calculating Nutrient Removal. By adjusting the pH, farmers could increase the availability of fertilizer applied and its efficiency of use.
Sources:
Mallarino, A. P., J. E. Sawyer, and S. K. Barnhart. 2013 revised. Crop Nutrient and Limestone Recommendations in Iowa. PM 1688.
Parham, J. A., S. P. Deng, W. R. Raun, and G. V. Johnson. 2002. Long-term cattle manure application in soil. I. Effect on soil phosphorus levels, microbial biomass C, and dehydrogenase and phosphatase activities. Biol Fertil Soils. 35:328-337.
