Durum Wheat Response to Pre-plant Phosphorus
at Safford Agricultural Center, 1998

L.J. Clark and E.W. Carpenter



The economic effect of applying phosphorus at planting of durum wheat is directly correlated to the phosphorus that is available to the plants from the soil. For bicarbonate soluble phosphorus levels less than 6 ppm, there will likely be a positive economic effect from applying some phosphorus. At this site the soil phosphorus level was 4.8 ppm and the application of 200 pounds of 16-20-0 caused an increase in yield of 2101 pounds per acre. A cost of $23 per acre with a return of $168 per acre.


Pre-plant (or at planting) phosphorus is normally applied to winter crops in the high desert soils in southeastern Arizona. The University of Arizona has phosphorus fertilizer recommendations for small grains grown in Arizona based on the bicarbonate soluble P extractions from soils. These guidelines were tested recently in a study by Ottman, et.al. (1) in an effort to encourage growers to test their soils to see if the cost of the phosphorus was justified. This study was established to find the efficacy of pre-plant phosphorus applications in the Safford Valley, particularly at our location. 

Materials and Methods

The phosphorus study on durum wheat was established on the Safford Agricultural Center in December of 1997. Seed was planted with an International 12 foot grain drill with a fertilizer attachment over four 3-foot beds. The crop history follows:

Crop History:

Elevation: 2954 feet above sea level
Soil type: Pima clay loam variant
Planting date: 17 December 1997 Seeding rate: 200 pounds per acre
Herbicide: 2,4-D on 3-20-98 to control broad-leafed weeds
Fertilizer: 400 lbs/ac elemental sulfur, starter fertilizer containing phosphorus at planting according to treatment plans, 244 lbs/ac urea on 3-13-98
Insecticides: None
Irrigation: Furrow, watered up plus 9 irrigations for a total of ca. 40 acre inches
Rainfall during the growing season: 2.8 inches
Plot size: 12 feet by 210 feet
Replicates: Three 

Plots were harvested with a Gleaner Combine with a 13 foot header. Individual plots were weighed using electronic weigh scales and samples were taken to determine moisture, bushel weight, percent protein, and hard vitreous amber count (HVAC). 

A soil sample was taken at planting time and submitted to chemical analysis. The results of the soil test are found in Table 1.

Phosphorous treatments applied at planting

Treatment 1. Untreated check (0 lbs/ac P2O5)
Treatment 2. 100 lbs/ac 16-20-0 applied (20 lbs/ac P2O5
Treatment 3. 200 lbs/ac 16-20-0 applied (40 lbs/ac P2O5)
Treatment 4. 400 lbs/ac 16-20-0 applied (80 lbs/ac P2O5)

Results and Discussion

The single most important factor to get a response to applied phosphorus (P) is having a low soil test for phosphorus . The test results shown in Table 1 indicate 4.8 parts per million (ppm) of bicarbonate extractable P, which is classified as low in Table 2 (borrowed from Ottman, reference 1) and very low (VL) by IAS Laboratory. Following the line in Table 2 indicates that a response to applied phosphorus is likely and that 50 to 100 pounds of P2O5 would be the recommended fertilizer rate. Phosphorus applied to the plots varied from 0 to 80 lbs/ac P2O5.

Table 3 shows the results of the study with statistically significant differences in yield between the check and all the other treatments. Treatment 3 (200 pound rate) was also statistically different from the other treatments. The increase in yield from 200 pounds to 400 pounds was slight and not statistically different. Figure 1 shows the yields and income (excluding the cost of phosphorus) graphically and make it easy to determine that 200 pounds of 16-20-0 per acre would be the optimal fertilization level. (For this study a value of $8/cwt for durum wheat and $230/T for 16-20-0 were used.) There were a couple of differences in bushel weights between treatments, the most significant being that the check bushel weight was lower than the rest. This correlates well with the yields and would indicate that the plants in the check plots were not as healthy as those that received phosphorus. The HVAC (hard vitreous amber count) is normally affected by nitrogen more than phosphorus. Nitrogen was added along with the phosphorus in the 16-20-0 at a rate of 0, 16, 32, and 64 pounds of N per acre from treatments 1 - 4, respectively. This could have affected the yields as well as the HVAC. The other theory is that phosphorus increased the root growth so the plants were better able to forage for the nitrogen that was available in the soil. 

One year╠s data is not sufficient to cause massive changes in fertilization practices but is sufficient to encourage area farmers to seriously consider a soil testing program to see if they have a potential need for more phosphorus at planting. Test results might also indicate that there is sufficient phosphorus available to the crop that no application is needed for a given year. 


1. Ottman, M.J., S.H. Husman and B.R. Tickes. 1997. Barley and Durum response to phosphorus at Buckeye, Maricopa and Yuma, 1997. Forage and Grain, A College of Agriculture Report, The University of Arizona, Tucson, AZ. Series P-110, pp. 125-129.

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This is a part of publication AZ1059: "1998 Forage and Grain Agriculture Report," College of Agriculture, The University of Arizona, Tucson, Arizona, 85721. 
This document located at http://ag.arizona.edu/pubs/crops/az1059/az105912.html
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