Illinois Fertilizer Conference Proceedings

Nitrogen, Phosphorus and Potassium Management for Intensive Wheat and Double-Crop Soybean Rotations

S. A. Ebelhar, C. D. Hart, F. Fernandez and J. D. Hernandez
S. A. Ebelhar is an agronomist and C. D. Hart is a research specialist, Dept. of Crop Sciences, University of Illinois, Dixon Springs. F. Fernandez is an assistant professor, Dept. of Crop Sciences, University of Illinois, Urbana.
J. D. Hernandez is an assistant professor, Southern Illinois University, Carbondale.


With just over one million acres in the state, mostly in southern Illinois, wheat remains the third largest grain crop in Illinois.  The state average wheat yield is approximately 60 bu/acre.  However, recent work at Southern Illinois Univ. and the Univ. of Illinois suggests that the use of intensive wheat management could increase wheat yields by 20-40%.  Little has been done in recent years to determine if intensive wheat management systems require higher soil/fertilizer levels of phosphorus (P) and potassium (K).  Wheat has a very high phosphorus need and certainly one would expect a 100 bu wheat crop would require higher P levels than 60 bu wheat.

With an apparent higher need for fertilizer P, intensive wheat areas that are fertilized with diammonium phosphate (DAP) would receive higher amounts of fall applied N.  There is uncertainty that this higher fall N is used as efficiently as spring applied N. In southern Illinois, double-cropping with soybeans makes the wheat-soybean system economical.  Farmers who fertilize the wheat crop for both wheat and double-crop soybeans would apply even higher amounts of DAP and potash in the fall, increasing the fall N further.  Work needs to be done to help these farmers optimize their N, P and K management of wheat and double-crop soybeans.

The objectives of this study are to

  1. 1) determine optimum nitrogen, phosphorus and potassium fertilization of high yield intensive wheat with double-crop (DC) soybeans,
  2. 2) evaluate fertilizer application to wheat for both wheat and double-crop soybeans compared to applying the fertilizer to wheat and soybeans separately, and
  3. 3) determine if nitrogen from fall applied DAP is available for wheat uptake similar to spring applied nitrogen.

The goal is to optimize fertilizer use in intensive wheat and double-crop soybean rotations in southern Illinois.

Materials and Methods

A field study was conducted at three locations: the University of Illinois (UI) Dixon Springs Agricultural Center (DSAC), the UI Brownstown Agronomy Research Center (BARC) and the Southern Illinois University Belleville Research Center (BRC).  Study details are described in Table 1 below. Wheat was planted at each location on or about the fly-free date. Fertilizer treatments, as describe in Table 2 below, were surface broadcast applied at planting.  Fertilizer P was supplied as DAP (18-46-0) and K as potash (0-0-60).  Spring nitrogen rates were applied at Feekes GS 5.0 and were adjusted for preplant N applied as fall DAP. At wheat flowering, foliar fungicides were applied at each location to limit diseases of the head and flag leaves.

Soybeans were drilled in 7.5" rows shortly after wheat harvest.  Soybean P and K fertilizer treatments were surface applied broadcast at time of soybean planting.

Results and Discussion


High rainfall totals, especially at DSAC, during the spring growing season led to above average N losses and lower yields (Tables 3 and 4).  Even so, yields at the other centers were well above the state average, presumably because of the high N rates and fungicide use.  The linear response to N rates up to the 150 lb N/acre at all three locations (Figure 1) is a clear indication that a significant amount of N was lost during this wet period.  Nitrogen was the most limiting nutrient in this aspect of the study.  Responses to P and K were minimal for wheat as indicated by the N only treatment being as high or higher than the DAP/potash treatments.

There appeared to be significant amounts of nitrogen losses from the fall applied DAP-N. As fall DAP rates increased (which also increased fall N rate), yields tended to decrease due to losses of this N prior to wheat uptake in the spring at most locations (Figures 2-4).  Only at BARC did there appear to be an increase in yield as DAP/potash rates increased.  This would indicate that either the wheat responded to the P/K rates or less of the fall N was lost.

In general, the main effect on wheat yields was the spring N rate.  Because of the balance of fall N with spring N, the more N fall applied meant less N spring applied.  Clearly spring N rate had the most dramatic effect on yields (Figures 5-7).  Only at BARC was there not a highly correlated relationship between spring N rate and yield, again an indication that a significant portion of the fall applied N carried over and was taken up by the wheat in the spring.

Test weights were only minimally affected by treatments.


The no NPK treatment had slightly lower soybean yields at BARC and lower leaf K concentrations, an indication that K may have been in low supply without the fertilizer K application (Tables 5 and 6).  The other two sites did not seem to respond to either DAP or potash treatments.  There were no significant effects of fertilizer application timing or rates on soybean yields (Figure 8).  In these cases the lower rates of DAP and potash appear to be adequate.

At BRC, the plots that received no N on the wheat had soybean plants with lower levels of leaf P and K.  When all of the DAP/potash was applied to the wheat only, the double-crop soybeans at BARC and DSAC had slightly higher trifoliate leaf P levels and slightly lower K levels, but this failed to produce any effects on grain yields.


This is only the first year of the study and results should be interpreted with caution.  It appeared that the optimum P and K rates were different at the different centers, but usually the lower rates were adequate, even for the higher yielding areas.  Spring nitrogen rate was the most critical.  The fall rate usually could not be fully accounted for in the spring which means that the fall N should be adequate enough to support fall growth, usually 20-30 lb, and spring N should be applied at the recommended rates.  The fall N and spring N were not additive due to a large portion of the fall N not carrying over to the spring.

Tables and Figures

Table 1. Site information for each location, 2008. Table 2. N, P and K rates for wheat and double-crop soybeans at each location, 2008. Table 3. N, P and K treatment effects on yield and test weight of wheat, 2008. Table 4. Means for yields and test weights of wheat, 2008. Table 5. N, P and K treatment effects on yield and trifoliate P and K concentrations of DC soybeans, 2008. Table 6. Means for yield and trifoliate P and K concentrations of DC soybeans, 2008.
Figure 1. N rate effects on wheat yields, 2008. Figure 2. DAP rate effects on wheat yields at Brownstown, 2008. Figure 3. DAP rate effects on wheat yields at Dixon Springs, 2008. Figure 4. DAP rate effects on wheat yields at Belleville, 2008. Figure 5. Spring N rate effects on yield at Brownstown, 2008. Figure 6. Spring N rate effects on yield at Dixon Springs, 2008. Figure 7. Spring N rate effects on yield at Belleville, 2008. Figure 8. DAP/potash rate effects on soybean yields, 2008.