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Soil PH and Nutrient Availability
By Robert Mullen, Ohio State University Extension
When discussing nutrient management, the analogy of crop production and vehicle fluid maintenance have often been used. Nitrogen and phosphorus are the gas and oil of crop production, respectively. The amount of gas (N) necessary is directly related to the length of the trip each season (yield potential). If we know how far we are going, we know how much to put in the tank (our tank is never empty, but unfortunately we do not know how much gas will become available nor do we know for sure how long the trip will be - due to weather). The amount of oil necessary is not related to the length of the trip. We just have to make sure the crankcase is full in order to complete the journey. If nitrogen and phosphorus are the gas and oil, then soil pH is the air pressure of the tires. If the tires are flat, you are not going to get far.
Understanding that soil pH controls the availability of most nutrients is important for proper nutrient management. Maintaining the proper pH for crop production is a must for efficient use of other crop inputs.
Nitrogen - While soil pH does not directly control N availability per se, it does affect soil microbial activity. Acid soil conditions can limit microbial activity and slow mineralization of N (from organic matter) as well as nitrification. High soil pH can result in significant loss of N by volatilization, especially when urea-based fertilizers are surface applied.
Phosphorus - P availability is strongly influenced by soil pH. Availability of P is maximized when soil pH is between 5.5 and 7.5. Acid soil conditions (pH < 5.5) cause dissolution of aluminum and iron minerals which precipitates with solution P effectively "tying" it up. Basic soil conditions (pH > 7.5) cause excessive calcium to be present in soil solution which can precipitate with P decreasing P availability.
Magnesium - Mg availability is affected by soil pH. Highly weathered, low pH soils can be deficient in Mg. Mg is leached out of the soil due to excess hydrogen, aluminum, and iron which compete for cation exchange sites.
Micronutrients
Most micronutrient metals are directly affected by soil pH. As the pH decreases, the availability of iron, manganese, zinc, boron, and copper all increase. Molybdenum availability, on the other hand, decreases as soil pH decreases. This fact is especially important for legume production. The nodules of leguminous crops contain the enzyme nitrogenase which is rich in molybdenum. Thus if soil pH is low and available molybdenum is low, legumes will appear N stressed and production will suffer.
Understanding soil pH should also help in diagnosing crop production issues on specific soils. From the example above, if alfalfa is being grown in a soil with a pH less than 6.5 and the stand is poor and the leaves are yellows, the first thought should be molybdenum might be deficient. The best method to remedy the situation would be to lime the soil and increase soil pH, which is why it is important to soil test prior to planting. Surface application of lime to increase soil pH would be slow and would not impact molybdenum availability for quite a while. Similarly, if the soil pH is high (> 7.5) and the new leaves are exhibiting interveinal chlorosis, manganese and iron deficiency should be the first consideration.
Due to the influence of soil pH on nutrient availability (not too mention aluminum toxicity), maintaining proper soil pH is vital to a good fertility program.
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