How much variation can be expected across seasons, within a year?
Soil pH—can vary as much as 0.5 to 1 pH units on poorly buffered soils, especially on the coarser textured soils. Soil pH is usually lower in dry periods and higher in wet conditions. Extractable soil phosphorus—-may be more stable than soil pH and extractable potassium in the majority of soils. Under prolonged flooding, phosphorus associated with iron complexes can be released. When soils dry, phosphorus can be bound tightly in iron and aluminum complexes, which lowers availability to plants. Seasonal variation can be as much as 10 to 20 pounds per acre or 5 to 10 parts per million (ppm). Extractable soil potassium—-can be affected by soil freezing and thawing, and wide variations in soil moisture. Under very dry conditions, and upon freezing, certain clay minerals can release potassium from their mineral structure. Upon re-wetting, the potassium may be bound in the clay structure. In some high clay soils, the seasonal variation can be as much as 20 to 50 pounds per acre, or 10 to 25
