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Extension > Agriculture > Nutrient Management > Phosphorus > Agronomic and environmental management of phosphorus

Agronomic and environmental management of phosphorus

George Rehm, John Lamb, Michael Schmitt, Gyles Randall and Lowell Busman

Everyone associated with agriculture is aware of the concerns which revolve around the relationships between nutrient management and environmental quality. In the late 1980s and early 1990s, attention was focused on the environmental impacts of nitrogen applied in fertilizers and manures. More recently, attention has shifted to the effect of phosphorus management on the quality of water in lakes and rivers.

There is ample evidence to show that algal growth in surface waters is directly related to concentrations of nitrogen and phosphorus. Therefore, the amount of phosphorus that enters surface waters has received special attention.

Various research efforts throughout the United States have focused on the development of management practices which can be used to control the loss of phosphorus from the agricultural landscape. This publication will describe those management practices and their effect on phosphorus losses.

In general, phosphorus losses are affected by:

  1. crop grown,
  2. tillage systems,
  3. rate, time, and method of application of inorganic and organic sources of phosphorus, and
  4. soil test level for phosphorus.

Crop selection

The crop grown on the landscape can have a substantial effect on phosphorus loss. Examples of measured losses from various cropped landscapes are listed in Table 1. When measuring losses, both the water and sediment are analyzed for phosphorus. The phosphorus found in the water is described as soluble. The amount attached to the soil particles is referred to as sediment phosphorus. Both components are added together to calculate the total amount of phosphorus lost from the landscape.

Table 1. Phosphorus losses from various landscapes.

----------------------------------------Phosphorus Loss----------------------------------------
Land Use Soluble Sediment Total
----------------------------------------- lb. P/acre ----------------------------------------
grass 0.45 6.60 7.05
no-till corn 0.98 1.90 2.94
conventional corn 0.27 13.48 13.75
Wheat/summer fallow 0.18 1.25 1.43

The losses listed in Table 1 are taken from summaries of a variety of research projects. Losses can be rather small (wheat/summer fallow cropping system) or substantial (conventional corn cropping system). In evaluating these reported losses, it's obvious that most of the phosphorus lost is attached to soil particles. Therefore, any cropping system which reduces soil erosion will reduce the loss of phosphorus from the landscape.

Tillage systems

Several research projects have focused on the effect of tillage system on the loss of phosphorus from the landscape. Because of different procedures used in the research projects, actual measured losses vary with each study. The results summarized in Table 2 are typical of those found in many studies.

Table 2. The effect of tillage system with and without the broadcast application of phosphate fertilizer on phosphorus loss.

---------------------------------------- Phosphorus loss ----------------------------------------
Tillage system Phosphate Use Soluble Sediment Total
---------------------------------------- lb. P/acre ----------------------------------------
ridge-till yes .061 4.09 4.15
no .005 2.96 2.97
conventional yes .003 10.01 10.01
no .001 6.69 6.69

P2O5 Rate = 115 lb./acre. Source: Romkins et al., Purdue

It's obvious that the use of a conservation tillage production system (ridge-till) is important in reducing total phosphorus loss. Since conservation tillage production systems reduce soil loss, major reductions in phosphorus losses are those losses associated with the sediment.

In Minnesota, the ridge-till planting system is an excellent choice for controlling soil erosion. Yields of corn and soybeans grown in this planting system have been comparable to yields of these crops grown in the more conventional moldboard plow system (Table 3).

Table 3. The effect of tillage system on corn and soybean yields in Minnesota.

------------------------------ Tillage system------------------------------
Location Crop Moldboard plow Ridge-till
------------------------------ bu./acre ------------------------------
Morris corn 137.0 138.0
soybeans 43.7 44.2
Lamberton corn 133.0 134.0
soybeans 40.7 41.0
Waseca soybeans 42.4 41.0

Source: Randall et al. (FO-6676), University of Minnesota Extension

The need for planting systems that control soil erosion changes with the varied landscapes of Minnesota. When erosion control is needed, the ridge-till planting system is well-suited for the soils and climate of Minnesota.

Placement of phosphorus sources

If the majority of phosphorus lost from the landscape is attached to soil particles, it's reasonable to expect that placement of phosphate fertilizers and manures can have a substantial impact on phosphorus loss. Research projects have clearly demonstrated that phosphorus loss is strongly affected by the placement of phosphorus sources. Results from a study conducted in Virginia are typical of summaries from several other research projects (Table 4).

Table 4. The effect of placement of phosphate fertilizer in two tillage systems on phosphorus loss.

Tillage system Phosphate* placement Total P loss
lb. P/acre
no-till none applied 0.10
subsurface injection 0.24
surface broadcast 0.53
conventional none applied 1.91
subsurface injection 2.58
surface broadcast 4.71

*P2O5 Rate = 94 lb./acre. Source: Mostaghimi et al., Virginia

In Minnesota planting systems, the use of a deep band applied in the fall before planting, or a starter fertilizer applied at planting, are appropriate practices for the subsurface application of phosphate. When phosphate fertilizers are broadcast on the soil surface and incorporated, some phosphorus is left either at or close to the soil surface. This phosphorus is subject to loss from erosion. The potential for loss is much lower when phosphate fertilizers are applied in a band below the soil surface. The same reasoning applies to manure applications.

The banded application of phosphate fertilizer has not reduced corn yields in Minnesota. Results of several research projects have shown that rates of phosphate needed for optimum production can be reduced if the fertilizer is applied in a band rather than broadcast and incorporated before planting. The results from a study conducted at Lamberton are similar to results from other studies (Table 5).

Table 5. The effect of rate and placement of phosphate fertilizer on corn yield. Lamberton.

P2O5 applied Placement Grain yield
lb./acre bu./acre
0 - 126.1
40 starter 143.6
200 broadcast 143.8

Soil Test P = 6 ppm (low)

In general, recommended rates of phosphate can be reduced by one-half if applied in a band rather than broadcast and incorporated before planting. The benefits of banded applications of phosphate fertilizer have positive economical as well as environmental benefits.

Phosphorus soil test levels

graph

Figure 1. Relationship of soil test levels for phosphorus to relative crop yield and the potential for environmental problems.

Testing for phosphorus in soils is a management practice that is usually used to predict the amount of phosphate needed in a fertilizer or manure program for optimum yield. Crop yields decrease as the soil test level for phosphorus drops into the medium range and below (Figure 1). The potential for environmental problems increases as soil test levels move into the high range. An increase in soil test levels for phosphorus indicates a larger amount of phosphorus in the root zone. With higher amounts present in the soil, there is an increased potential for loss.

In Minnesota, soil test levels are measured in a laboratory by either the Bray or Olsen procedure. The Bray procedure is usually used when soil pH is less than 7.4. The Olsen procedure is used when soil pH is 7.4 or higher. The soil test values for phosphorus which correspond to the relative levels are listed in Table 6.

Table 6. Definition of soil test values for phosphorus as measured by the Bray and Olsen procedures.

------------------------------Procedure------------------------------
Relative level Bray Olsen
------------------------------ ppm ------------------------------
very low 0-5 0-3
low 6-10 4-7
medium 11-15 8-11
high 16-20 12-15
very high 21+ 16+

In general, phosphorus supplied either in fertilizer or manure will not increase crop yields if soil test levels for phosphorus are in the high range and above. A research project conducted at the Morris and Waseca Experiment Stations for several years showed that yields did not increase when the Bray soil test for phosphorus exceeded about 13 ppm at Waseca and 19 ppm at Morris.

graph

Figure 2. Relative grain yields of corn and soybeans in relationship to soil test levels for phosphorus.

This information, gathered over several years, shows that there is no economic justification for building soil test phosphorus levels above 20 ppm as measured by the Bray procedure. When amounts of phosphorus added to the soil as either fertilizer or manure are higher than amounts removed by crops, soil test levels will increase. Therefore, soil testing for phosphorus should be a routine management practice when manure use is a consistent component of the farm enterprise.

Likewise, application of phosphate fertilizer for crop production, when the soil test for phosphorus is 20 ppm or higher, does not increase yield. These additions simply increase the potential for loss of phosphorus from the landscape. There is no economic or environmental justification for adding phosphorus to the soil system when the phosphorus soil test is 20 ppm and higher (Bray test) or 16 ppm and higher (Olsen test).

Summary

Management of phosphorus inputs for crop production systems is a very important task but these management practices do not have to be complicated. In general, management practices that reduce the potential for environmental contamination also provide for optimum economic production. Those important management practices are:

Partial funding for this publication was provided by the Metropolitan Council and the Minnesota Board of Water and Soil Resources.

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