P, K Cycling
One of the main issues related to natural processes is that P and K cycling from the crop to the soil is an important mechanism of soil nutrient replenishment.
Corn and soybean return approximately one-fourth to one-third of the total P taken up, and about two-thirds of the total K taken up, to the soil. Potassium cycling has the greatest impact on fall soil sampling because this nutrient remains in an inorganic form in plant tissues and readily leaches out of the plant with precipitation once the crop reaches maturity.
“We should not be as concerned about P because P is in organic form in plant tissues and those materials need to be decomposed before P is released back to the soil,” Fernández says.
Fernández warns that this year it is not safe to assume that lower removal rates and lower yields will result in greater P and K levels in the soil because drought limits the equilibration of nutrients in the soil. During the season, plants have extracted P and K from the more easily available nutrient pools. Because of the lack of moisture, the soil has, to a large extent, not been able to replenish those pools from less available nutrient pools.
“While some of the moisture we are beginning to receive now will help the replenishment process from the nutrient reserves in the soil, those processes take time,” he says.
Fernández recommends waiting until as late as possible in the fall to collect samples. The later the sample is taken, the more reliable the K test values will be.
“My prediction is that soil K test levels will end up being lower than actual levels, and P test levels will be about normal to slightly lower than actual levels,” Fernández says.
Finally, Fernández has been asked about the effect of drought on soil pH. Studies looking at pH variability over years and seasons encompassing a variety of wet and dry conditions have shown no consistent trend. Thus, there is no reason to assume that dry conditions will always affect soil pH.
However, dry conditions may cause lower pH in soils that are neutral to slightly acidic. Differences of as much as 0.3 pH units are possible because lack of rain can induce salt accumulation in the soil solution, increasing its hydrogen ion concentration and resulting in lower (more acidic) pH.
“However, buffer pH values used to determine the amount of lime applications are usually unaffected by these small amounts of salt,” Fernández says. “Thus, even if there is an over-prediction of acidity that calls for a lime application, the amount of lime to be applied should not be affected by the dry conditions.”