By Blake Jackson
Phosphorus (P), a vital plant nutrient second only to nitrogen and potassium in crop requirements, poses an environmental risk to surface waters when soil test levels are high.
Phosphorus can be transported via eroding soil particles or dissolved in runoff, leading to increased algae blooms in lakes and ponds. The subsequent decomposition of algae depletes dissolved oxygen, harming aquatic life.
Effective phosphorus management is a continuous, long-term endeavor. Implementing multiple strategies is crucial for minimizing phosphorus loss from fields with high soil test P.
Regular soil testing is the most reliable method for monitoring phosphorus levels and identifying fields with elevated concentrations. Each new soil test should prompt a reevaluation of individual fields to track changes in P levels.
Soil test reports typically categorize phosphorus results as very low (VL), low (L), optimal (O), high (H), and excessively high (EH).
When soil test phosphorus falls into the high (H) or excessively high (EH) categories, the potential for environmental phosphorus loss increases significantly. To mitigate this, phosphorus fertilizer application should be eliminated or reduced in such fields.
For most common crops, additional phosphorus from fertilizer or manure is unlikely to improve yields once soil test P exceeds 30 ppm.
Generally, if soil tests indicate excessively high phosphorus levels, applying phosphorus from manure or fertilizer should cease until soil test levels decline.
However, if a specific crop with high P demands exhibits deficiency symptoms, phosphorus application may be necessary, aligning with nutrient recommendations. Remember to account for phosphorus contributions from manure and other organic amendments by subtracting their amount from the recommended rate.
Research by Daniel Kaiser of University of Minnesota Extension suggests that soil test phosphorus levels in high-testing soils typically decrease by about 2 ppm annually without additional P fertilizer.
This decline can be more pronounced in soils testing above 50 ppm. Consequently, soils with P levels of 50 ppm or higher may contain sufficient phosphorus to meet crop needs for 15 to 20 years before requiring fertilizer applications.
Since phosphorus is immobile in soil, applications should be placed below the soil surface, close to plant roots (ideally into living cover) with minimal soil disturbance to prevent runoff.
Methods like in-furrow placement with the seed (2x2 below the seed) or subsurface banding before planting enhance root uptake and prevent phosphorus accumulation on the soil surface, where it is more susceptible to runoff.
Photo Credit: gettyimages-mvburling
Categories: Wisconsin, Crops