Higher prices for fertilizer for the 2006 growing season have stimulated many wheat producers to take a close look at fertilizer costs for next year. The goal is to trim costs without reducing yield and quality potential. With this in mind, now is a good time to review some fertilizer best management practices that growers should consider in their decision making process.

Use a Soil Test for Nitrogen—If wheat planted in 2006 is to follow any crop other than soybeans or some other legume, carryover nitrate-nitrogen (NO₃-N) must be determined. To do this, collect soil from depths of 0 to 6 and 6 to 24 inches. The soil from 0 to 6 inches will be analyzed for NO₃-N as well as the other nutrients (P, K etc.) that are part of the routine analysis. The soil from 6 to 24 inches is analyzed for NO₃-N only.

The amount of nitrogen fertilizer needed for next year’s crop is then based on the expected yield and the carryover soil NO₃-N. This management tool can be very effective for fine-tuning usage of fertilizer N in 2006.

The soil sample collected to determine the carryover NO₃-N should accurately reflect the supply of N in the soil that might be used by the 2006 wheat crop. This objective strongly suggests collecting the soil sample as late in the fall as possible, preferably after the soil temperatures at the 4 inch depth have fallen below 50°F. If soil samples are collected early, NO₃-N may be released (mineralized) from the soil organic matter after sampling and will not be measured. With early sampling this potential change in soil NO₃-N will not be considered in making the fertilizer N recommendation. Early sampling for soil NO₃-N can lead to over application of fertilizer N. The delay in sampling for NO₃-N increases the probability for a more accurate measure of carryover NO₃-N and more accurate fertilizer N recommendations.

Time of Application—There is always a potential for loss of fall applied fertilizer N. The majority of this loss probably occurs as denitrification in early spring but NO₃ leaching below the root zone is also a potential loss. To reduce the potential for N loss by either of these processes, fertilizer N should be applied either just before or at the time of wheat planting. With the use of air seeders, application of fertilizer N at planting should not be a major problem.

Fall application of fertilizer N is a common practice. If application at this time is necessary, the N source should be anhydrous ammonia or urea. Urea should be well incorporated.

It is very important that this fertilizer not be applied until the soil temperatures at the 4 inch depth have fallen below 50°F. Cool temperatures reduce the conversion of ammonium-N to NO₃-N. The efficiency of fertilizer N use is usually maximized and N loss minimized when fertilizer N is applied close to the time of plant need. This takes place when fertilizer N is applied immediately before or during the wheat planting operation.

Time N Application to Soil Texture—If application of fertilizer N with an air seeder at planting is not an option, N application timing should be adjusted according to soil texture. Obviously, soils with a high clay content dry slower in the spring and late fall application of fertilizer N would be the best option. Sandy soils, on the other hand, drain and dry more rapidly with spring weather. As a result, spring application of fertilizer N would be a better option when wheat is grown on sandy soils. Many producers grow wheat in an area where there is a variety of soil textures. If that is the situation, time of fertilizer N application of can be matched to soil texture. Use fall application on the very heavy soils and spring application on the sandy or coarse textured soils.

Consider Changes In Phosphate Placement—For many years, University of Minnesota fertilizer guidelines have stated that the amount of phosphate fertilizer needed can be reduced by ½ if the phosphate is applied in a band near the seed rather than broadcast and incorporated before planting. The arrival of the air seeder, however, has changed the dynamics of phosphate fertilizer placement. With this type of seeding, there are other placement options. Most attachments place the phosphate fertilizer in a band near the seed or mix the fertilizer with the seed in a zone where the width of the zone would vary with the implement.

How is the phosphate use efficiency affected by placement near the seed rather than with the seed? This is a reasonable question that is now on the minds of wheat producers who are taking a serious look at fertilizer costs.

The results of a study conducted near the Northwest Research and Outreach Center in 1997, provide a preliminary answer to this question. The soil test for P as measured by the Olsen test was in the low range (5 ppm).

In this study, phosphate fertilizer as 0-46-0 was applied in one of four ways. "Mixed" (seed and fertilizer mixed together in a 3 inch wide band), "single" (fertilizer placed in a single band near the seed row), "twin row" (fertilizer place in a single band between two seed rows spaced 3 inches apart), and "broadcast" (fertilizer was broadcast and incorporated prior to planting). For each placement, three rates of phosphate (23, 46, 69 lb. P₂O₅ per acre) were compared. Wheat yields are summarized in the table that follows. All treatments received adequate amounts of nitrogen fertilizer.

Wheat yield as influenced by rate and placement of phosphate fertilizer.

control yield (no applied phosphate) = 48.4 bu./acre

In the two treatments when the phosphate was mixed with larger amounts of soil (broadcast, mixed) highest yields were associated with the highest rates of applied phosphate. By contrast, a rate of 46 lb. P₂O₅ per acre was needed if the phosphate was applied in a band close to the seed (single, twin placements). These results from this location show that rate of applied phosphate can be reduced by at least 1/3 if the fertilizer is place near the seed at planting with an air seeder. In times of higher fertilizer prices, reducing the suggested broadcast rate by ½ would also seem to be appropriate. As phosphate fertilizer is mixed with larger volumes of soil, this reduction may not be appropriate.

Don’t Base Fertilizer Recommendations on Crop Removal

Although it might seem to be logical, rates of phosphate and potash needed for the 2006 wheat crop should not be based on crop removal. This would be the most expensive fertilizer program. Results of trials in a wheat/soybean rotation at the Northwest Research and Outreach Center have verified that the concept of crop removal should not be used as a basis for fertilizer recommendations.

The best alternative is to use a soil sampling procedure that can be repeated each year.

Don’t Purchase Nutrients That You Don’t Need

Over the years, there has always been some question about the need for micronutrients in a fertilizer program. Trials were conducted at three locations in 2005 to evaluate the performance of both soil applied and foliar applied micronutrients. Preliminary results of this project supported by the Minnesota Wheat Research and Promotion Council suggest that application of these nutrients had no impact on yield. Except for the use of copper (Cu) on organic soils. It’s doubtful if the use of any of the micronutrients has a positive effect on wheat production on mineral soils in Minnesota.

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