Did you know that when you apply a fertilizer containing ammonium you are acidifying your soil? Additionally, the soil pH may influence the conversion rate of ammonium to nitrate and ultimately affect nitrogen losses from the soil profile.

Ammonium is made up of nitrogen and hydrogen (NH3). Shortly after application, the ammonium binds to soil or organic matter. Eventually it is converted to nitrate by bacteria in the soil. In the conversion process from ammonia to nitrate, three hydrogen ions are released into the soil solution. They are then free to react with other substances in the soil. Free hydrogen is very active and is the "acid" in the soil. The H+ can react with lime in the soil forming water and CO2 and be neutralized or it can be tied up on soil particles.

Soils have differing abilities to deal with the added H+ ions. Lime already present in the soil may neutralize the H+ ions or the ions may temporarily bind to the negatively charged organic matter and clay in the soil. All of these processes depend on the content of lime, organic matter and clay already in the soil. This is generally termed as the soils buffering ability, or its ability to handle additional H+ ions without the pH of the soil changing. If the H+ ions are not neutralized or bound to a soil particle, they float in the soil water solution and create an acid soil environment.

The pH of a soil is its measure of relative acidity or alkalinity. The acidity-alkalinity scale ranges from 0 to 14. Within this range soils are classified as being acid (pH < 7), neutral (pH = 7) and alkaline (pH > 7). Most mineral soils have a pH range from 5.5 to 7.5. In addition to the application of ammonium fertilizers, soil pH is affected by rainfall, soil parent material, soil organic matter, soil texture, and soil microorganisms. The application of fertilizer and water normally changes the soil pH most rapidly. Agricultural limestone is most commonly used to increase the soil's pH. Sulfur is normally used to lower the soil pH.

The availability of nutrients is directly affected by soil pH. If the soil's pH is too high or too low, some nutrients become insoluble, limiting the availability of these nutrients to the plant root system.

Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils. A pH range of approximately 6 to 7 promotes the most ready availability of plant nutrients. The diagram illustrates nutrient availability under different pH scenarios.

The soil pH can also influence the activity of beneficial microorganisms. Bacteria that decompose soil organic matter are hindered in strong acid soils. This prevents organic matter from breaking down, resulting in an accumulation of organic matter and the tie up of nutrients, particularly nitrogen, that are held in the organic matter.

Soil temperature has been the primary criterion for evaluating risks associated with losses of fall-applied nitrogen (N). Recent studies indicate that soil pH also deserves attention. Current results suggest that soil pH has important effects on recovery of fall-applied N in the spring when conditions are favorable for leaching and denitrification, but soil pH does not have important effects when conditions are unfavorable for N loss by these mechanisms. Soil pH, therefore, deserves attention as an important factor determining the amount of risk associated with applying N in the fall. The most likely explanation of the results is that conversion of ammonium to nitrate occurs more rapidly in soils having high pH than in soils having low pH. Rapid conversion makes the N vulnerable to loss by these mechanisms.

This page last updated on 01/26/05

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