Demonstrations show P fertilizer
applications reduce profits
by Chad Ingels, nutrient and manure management specialist, Maquoketa Watershed Project
Phosphorus (P) management demonstrations in the Maquoketa Headwaters Watershed this crop year show that yields are not increased and profits are actually reduced when crop removal rates of phosphate, P2O5, fertilizer are applied to soils on three farms in the watershed. Soils on two of the demonstration sites were in the very high soil test range, while one site had a low P soil test prior to the demonstrations.
To field test the economic benefits of reducing P applications, the Watershed Council enlisted Maquoketa Watershed Project staff to conduct on-farm demonstrations with area producers. Jule Brown, Gary Soules and Joe Wingert were cooperators during crop year 2000. These field demonstrations compared three rates of commercial phosphate and 120 pounds of nitrogen (N) per acre applied to corn following the previous soybean crop.
Soil tests were taken at each site in fall 1999. Replicated plots were measured and triple phosphate fertilizer was applied to three different treatments at the time of soil testing.
The compared treatments were 0
P, 46 pounds of P per acre and 92 pounds of P per acre. The 46 pounds of P and
92 pounds of P per acre rates are one-year crop removal and two-year crop
removal respectively. In the spring of this year, immediately following
planting, N at the rate of 120 pounds per acre was applied as ammonium nitrate
to each treatment.
At harvest, yields were measured for the three treatments. Figure 1 shows that there was no yield benefit by adding phosphorus to the soil. Soil tests of the three treatments were also taken at harvest. The addition of P at the one-year crop removal rate did not change the soil test results, compared to no additional P; both showed 47 parts per million (ppm). The soil test increased only slightly at the two-year crop removal rate, to 58 ppm.
An economic comparison between the treatments was made by assuming P2O5 at $.23 per pound and corn at $2.40 per bushel (black bar) and $2.00 per bushel (gray bar), shown in figure 2. With the similar yields, the addition of P at the one-year crop removal rate actually reduced profit by $15 per acre and $14 per acre. When adding the two-year crop removal rate the losses increased to $26 per acre and $25 per acre.
Soil tests taken from within
the watershed in the fall of 1999 show that 88 percent of fields are testing
high or very high for soil P.
This information supports results from a January 1999 survey of producers in the watershed. The survey showed average annual commercial P2O5 applications of 35 pounds per acre on corn following soybean rotations and 41 pounds per acre on continuous corn.
In addition, the Headwaters Watershed Council estimates that 90 percent of row-crop land in the watershed received an annual crop-removal application of commercial phosphorus.
These demonstrations reinforce Iowa State University recommendations that suggest when soils are testing high or very high for P, adding commercial broadcast phosphorus fertilizer does not increase profits.
Eliminating unnecessary fall-applied fertilizer would reduce the amount of P left on the soil surface susceptible to run-off events through the winter months. The elimination of unnecessary fall-applied P is the first step in the long-term reduction of soil P levels that affect surface-water P levels and help achieve the Watershed Council’s goal of 50 percent reduction of P leaving the watershed.
Look for more demonstrations comparing three rates of P application to be conducted within the headwaters watershed during the following crop seasons.
For more details about these demonstrations, contact the MWP staff in Fayette at (319) 425-3233.
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