High soil test phosphorus and potassium levels on a long-term organic farm:

A trend observed on many organic vegetable farms that rely on imported manures and manure-based composts and fertilizers is an increase in soil test phosphorus (P) and potassium (K) levels above sufficiency (the level at which additional nutrient input is unlikely to increase yield or product quality). High soil test P levels are not detrimental to crops, but they do increase the possibility of off-farm P movement, primarily through sediment loss in runoff. Phosphorus that enters waterways may contribute to an increase in algal growth (eutrophication), which can ultimately result in death of aquatic organisms. High soil test K levels are not harmful to the environment, but can reduce plant calcium (Ca) and magnesium (Mg) uptake. If forage grown on high soil test K soils is fed to ruminant livestock (beef and dairy cattle and sheep), there is an increased risk of magnesium deficiency, which can result in the metabolic disease called grass tetany. A challenge for organic vegetable farmers over the long term is to develop a nutrient and soil management plan that does not generate high soil P and K levels.

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