Let’s Review: Phosphorus

December 6, 2011

Phosphorus is a naturally occurring nutrient and is one of the three primary essential nutrients for soil fertility and crop growth.  Soil phosphorus is found in organic and inorganic forms, but is only available to plants in the orthophosphate form, also referred to as dissolved or soluble phosphorus.  Sources of phosphorus include phosphate rock, precipitation, plant residues, manure, biosolids, and fertilizers.  Each of these sources can break down into available and unavailable forms.  Whether naturally occurring or applied, phosphorus availability to crops is controlled by interactions in the soil. Phosphorus is removed from the soil through erosion, runoff, plant uptake, harvest and removal of crops, and to a lesser extent, leaching. 

The loss of phosphorus to freshwater systems can come from a variety of sources: urban/suburban runoff, stream bank erosion, agricultural runoff, municipal and industrial waste water and natural sources.   Phosphorus in these sources is found in two forms: dissolved (orthophosphate) and particulate.  Phosphorus in freshwater systems is a concern because it causes eutrophication.    Agricultural producers can minimize phosphorus loss by reducing soil loss and managing fertilizer and manure appropriately. Best management practices, such as grassed water ways, cover crops, reduced tillage practices, and proper management of critical areas and during critical times, can help control phosphorus loss. All farms are different because of the physical setting or the production potential of the soil.  Proper nutrient management planning can aid in matching field inputs to crop requirements for growth. 

In the overall scheme of nutrient management, producers should use a phosphorus based strategy when applying fertilizer, especially manure, instead of a nitrogen based strategy.  Producers should apply phosphorus according to soil test results.  If soil has low phosphorus levels, additional phosphorus (manure or fertilizer) should be added to meet the crops’ needs plus extra to build up soil reserves.  Soil test phosphorus levels above the optimum range mean that yields will not increase significantly with additional phosphorus. Thus, if soil tests show high or excessive phosphorus levels, producers should reduce phosphorus to levels less than the crop needs, or apply no phosphorus at all.   Over application of phosphorus increases the potential for loss to freshwater systems.      

Phosphorus in soil takes a long time to draw down.  So start with a good soil test and utilize nutrient management planning to optimize the use of phosphorus and other nutrients.

Additional information can be found at: http://www.soils.wisc.edu/extension/index.php and UW-Extension publication A3771 Understanding Soil Phosphorus.

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