Ag Talk

February 2003

Phosphorus Plays Many Roles

Phosphorus is a nutrient needed for plant growth. Most of our native soils are deficient and so additions need to be made for optimal plant growth.

Phosphorus is important in energy transfer in the plant, root growth, flowering and fruiting including seed formation, strength of straw in cereals which helps prevent lodging and crop maturation. You have probably seen small corn in a cool, wet spring turn purple which is attributed to lack of phosphorus uptake. Plants usually grow out of this as soon as the temperature warms.

A role of phosphorus discovered by University of Missouri researchers in recent years is that phosphorus may help prevent grass tetany. Grass tetany occurs in the spring when grass plants are growing quickly and magnesium content of the plants is low. Research at UMC found that the addition of phosphorus on low test level soils increased magnesium content in fescue plants in early spring. The mechanism of this magnesium is still not completely understood, but the response seems to work. So in addition to increasing plant growth and yield, you can reduce the potential for grass tetany with phosphorus fertilizer.

Table 1. Phosphorus removal by selected Missouri crops
Crop	Yield	Phosphorus fertilizer equivalent removal (lbs. P2O5)
Alfalfa hay	4 tons/acre	40
Red clover/grass hay	3 tons/acre	25
Fescue	3 tons/acre	27
Corn (grain)	120 bu/acre	54
Sorghum (grain)	6000 lbs/acre	56
Soybean	45 bu/acre	38
Wheat	50 bu/acre	30

Missouri soils contain 800-2000 lbs of phosphorus per acre, but most of this is held by the soil and released slowly. Only about 0.25 lb/acre of phosphorus is actually available in soil solution at one time. As this phosphorus is used by plants it is replaced by phosphorus in mineral and organic forms.

In this area soils range in soil test phosphorus levels of 1 lb/acre available P to 700-800 lbs/acre or more. High soil test levels are typically associated with livestock operations where the addition of manure to fields adds phosphorus. Any time feed is brought onto the farm or grains are fed on the farm the resulting manure retains a large part of the nutrients originally in the feed and those nutrients have to go somewhere. Soil testing can help you find where your nutrient levels are. Desired soil test phosphorus levels for alfalfa and most grain crops is 45 lbs./acre. For forages the goal is 30-40 lbs./acre.

Phosphorus fertilizer recommendations include recommendations for buildup and maintenance. The buildup part provides phosphorus to build up soil test phosphorus levels. The maintenance part of the recommendation provides for crop removal. Phosphorus removal by crops are listed in Table 1.

If you are operating with limited dollars for fertilizer apply fertilizer first where soil test levels are the lowest. The probability of a crop response to fertilizer nutrients is higher on a low testing soil than on a high testing soil.

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		Matt Herring - Agronomy/Natural Resources Ken Bolte - Farm Management/Livestock Franklin County University Extension Center Last revised: 11/28/05		Go to Franklin County University Extension Center page