Fall 2003

Northcentral

THE MOST RECENT SURVEY of soil tests conducted by the Potash & Phosphate Institute demonstrated, at a state and province scale, the varying phosphorus (P) needs across North America (Figure 1). Phosphorus levels also vary among and within fields, making more intensive soil sampling necessary in many situations.

Does a field area have enough P already or is more needed? The only way to tell is to take a good soil sample and have it tested by a reputable laboratory.

Each state’s Cooperative Extension Service, as well as many laboratories, provide guidance on how to take and submit a soil sample and interpret lab results.

At lower soil test levels, crops generally yield at a reduced percentage of what is possible when levels are higher. Figure 2 shows an example of how relative yield of wheat (percent of yield attainable when P is sufficient) increases with higher soil test levels. An important part of this figure is the critical soil test P level. This is the level (or range of levels) beyond which crops are not likely to respond to P additions in the year of application.

The concept of tilling narrow strips in the fall is attractive because:
• It requires only one-third to one-half the time and fuel of a fall moldboard plowspring secondary tillage system.
• It provides a zone of bare soil that warms more quickly in the spring.
• It retains residue cover on the untilled land, protecting against erosion and maintaining infiltration.

Equipment is available that uses air delivery to combine fall application of granular fertilizer with fall zone tillage. This could be an economical way University of Guelphto supply some of the crop’s nutrient requirements,particularly for nutrients like phosphorus (P) and potassium (K), that are not easily lost from the soil.Getting some of the nutrient application job done in the fall helps streamline spring field operations,resulting in better chances of timely planting.

With the high cost and time associated with forage stand termination and reestablishment, farmers are anxious to identify all options for sustaining a forage stand. Fertilization of mature forage stands rejuvenates the stand and improves forage quality. Fall provides an opportune time to soil test and apply nutrients like phosphorus (P) and potassium (K) to forage stands.

The nutrient requirements of top yielding forage crops are high, as shown in the estimates of crop removal in Table 1. Given that the entire crop biomass is removed in the fodder, growing forages is one of the fastest means of drawing down the soil’s nutrient supply. In the absence of nutrient replacement, low forage yield ultimately leads to a decision to terminate a forage stand.

Fall planting of winter wheat and other crops is just around the corner. The time for fall fertilization for spring planted crops is also coming soon. Now is a good time to start seriously considering fertilization plans for this fall.

Winter Wheat
Adequate N must be available to the wheat plant at all phases of development. Shortages of N will ultimately result in reduced tillering, reduction in head size, poor grain fill, reduced yields, and low protein content. Optimum rate of N will depend on factors such as residual soil nitrate (NO 3–) level and the yield potential of the environment. Splitting applications of N between preplant and early spring is a best management practice (BMP) that often results in greater N use efficiency and reduced economic, environmental, and agronomic risks.

That seems simple until we try to repeat the same test on a different field and realize that the level of potassium (K) in the soil has a great effect on not only the yield level attained, but also on the N level at which the optimum rate is achieved. Figure 1 illustrates this interaction effect.

At the higher soil test K, the optimum yield was reached with much lower N, and it was at a much higher yield level. This relationship points out the importance of knowing the levels of each nutrient in studies where the best rate for any individual nutrient is being investigated. It also is important to try to manage all nutrients at their optimum levels to ensure that the most efficient use of all of the others will be realized.

Many fields that were rutted during a wet harvest last fall and winter have been smoothed and are ready to receive any fertilizers that were not applied last fall. If wet weather continues, farmers will be facing a conflict of time for planting and fertilization operations.

No firm planting intentions by farmers are known at this time. Last year's acreage of major crops is expected to be maintained, largely due to the absence of information to the contrary.

Forest management and forest fertilization does not look as attractive this year as a result of poor pulpwood prices, which detract from thinning operations in pine plantations that should precede fertilization operations. Some southern public tree nurseries are reporting a surplus of pine seedlings because of reduced pulpwood and timber harvests, which result in a lower replanting demand.

In the east, winter grain crops are rated as fair to good, with as much as 20 to 25% rated in poor condition. Wheat fertilization has begun in much of the region. Aerial application is being relied on in much of the midsouth area because fields are too wet to travel with ground applicators. Many of these midsouth fields may be receiving diammonium phosphate (18-46-0) with the first nitrogen (N) application to help offset the potential for phosphorus (P) deficiency on cool, wet soils that have marginally sufficient soil test P levels.

Nitrogen price and N fertilizer availability is a concern among farmers and many fertilizer dealers. Cotton, corn, rice, sugarcane, and forage producers are deliberating over their N purchases this spring. Since cotton is a major crop common to the majority of the Southeastern states, PPI developed N rate response graphs, based on university research across the Cotton Belt. These graphs and a comparison of economic optimum N rates for cotton, across a range in N prices and cotton prices, have been posted to the PPI-PPIC website and will be made available to farm press editors and others. (website: www.ppi-ppic.org.) The cotton N response data indicate that the economic optimum N rate is affected little by considerable ranges in fertilizer N price or a range in cotton prices. This information will help to guide farmers and their crop advisers in making N rate decisions this spring, and remind them that significant reduction in N rates could negatively impact farm profit potential.

Since forages typically have an extensive root system, they can effectively use water and nutrients in the soil when they are present. However, harvesting large amounts of forage can deplete the nutrients from the soil more quickly than other crops. Virtually all of the top growth of forage is removed from the field, along with large amounts of nutrients. Since forage harvest leaves almost no crop residue in the field, there is little nutrient recycling that occurs. This situation is different from many annual crops where much of the vegetation remains in the field after harvest.

Table 1 gives examples of nutrient removal in various perennial forages. As an example, an alfalfa crop of 8 t/A annually removes 120 lb P2O5/A and 480 lb K2O/A. If nutrients are not replaced at a rate that keeps up with crop harvest, the soil fertility will decline over time and productivity will drop. Fall is an excellent time to replace these nutrients wherever soil tests indicate a need.