Northern Idaho Fertilizer Guide Current Information Series No. 921

Spring wheat
R. L. Mahler and S. O. Guy

The following fertilizer guidelines were developed through research conducted by the University of Idaho and Washington State University. The guidelines are based on relationships between soil test data and yields of spring wheat. The suggested fertilizer rates are designed to produce above-average yields if other factors such as pests, soil moisture, planting date, and stand are not limiting production. Thus, the fertilizer guidelines assume the use of sound management practices.

The suggested fertilizer rates will be accurate for your field if (1) soil samples are properly taken and represent the area to be fertilized and (2) the crop history you supply is complete and accurate. For assistance in obtaining a good soil sample, refer to EXT 704, Soil Sampling.

• The spring wheat variety, its market class, and its potential yield in the field. The suggested rates in this publication were developed over many years based on several varieties of soft white spring wheat. Modern varieties require high levels of available soil nutrients and superior management to achieve their high yield potentials.
• The potential yield of the selected variety based on its historical yield in the particular location and good management. Research in northern Idaho and eastern Washington has shown that 2.3 pounds available N per acre are needed to produce 1 bushel of soft white spring wheat in the 16- to 21-inch annual precipitation zone. In areas receiving more than 21 inches of annual precipitation, 2.4 pounds N per acre are needed to produce 1 bushel of soft white spring wheat.
• The amount of usable N in the soil profile. This includes mineralizable N (N released by decomposing organic matter during the growing season) and inorganic N in the forms of nitrate (NO₃) and ammonium (NH₄).
• Actual total annual precipitation and other climatic factors.
• The density and vigor of the plant stand.
• The type and yield of the previous crop.

Nitrogen fertilizer based on soil testing
Use the following equation to determine the amount of fertilizer N to apply to meet your crop's need:
Fertilizer N needed = [N needed based on potential yield (Table 1) + N needed for residue breakdown (Table 2)] - [Mineralizable N (Table 3) + Soil test N (Table 4)]
Note: The amounts of N needed to attain the desirable high-protein hard red spring wheats are about 40 percent greater than those needed for soft white spring wheats. The values in this guide are for soft white spring wheats. Increase N fertilizer rates by 40 percent for hard red spring wheat production.

Nitrogen needed based on potential yield -- Estimates of N needed to produce a crop of soft white spring wheat should be based on potential yield -- the field's long-term average yield. Multiply the potential yield in bushels per acre by 2.3 or 2.4 pounds N per bushel, depending on annual precipitation, to arrive at total N needed (Table 1). If, for example, annual precipitation in your area is 19 inches and potential yield is 60 bushels per acre, then you would need 138 pounds per acre (2.3 x 60).

Nitrogen needed for residue breakdown -- Nitrogen is needed to break down straw from the previous cereal crop. Apply 15 pounds available N for each ton of straw incorporated into the soil up to 50 pounds N per acre (Table 2). Remember, 1 ton of residue is produced for each 20 bushels of wheat or 1,400 pounds of barley grain produced.

Mineralizable nitrogen -- Soils vary in their capacities to release N from organic matter during the growing season. The rate or amount of N released depends on factors such as the amount of soil organic matter, past soil erosion, available soil moisture, and soil temperature during the growing season.

Four different mineralizable N release rates are used for northern Idaho soils (Table 3). Low N release rates are found on severely eroded clay knobs and hilltops, in cutover timberland soils, in soils in areas of low precipitation, in soils with low water-holding capacities, and in soils with low organic matter contents.

Soil test nitrogen -- The amount of available N in the soil can be evaluated most effectively with a soil test. The soil samples should represent the rooting depth of the crop because nitrate-nitrogen (NO₃-N) is mobile in soil. Spring wheat is capable of removing N to a depth of 3 feet.

Soil test values include both NO₃-N and ammonium-nitrogen (NH₄-N.) To convert soil test NO₃-N and NH₄-N values in parts per million (ppm) to pounds per acre, add the N values (ppm) for each foot of sampling depth and multiply by 4 (Table 4).

Nitrogen fertilizer -- Again, the calculation for N fertilizer needed is:

Total N needed (lb/acre) (Table 1 + Table 2)		__________
Minus mineralizable N (lb/acre) (Table 3)	-	__________
Minus soil test N (lb/acre) (Table 4)	-	__________
Equals N fertilizer required (lb/acre)	=	__________

For example, with a potential yield of 60 bushels per acre, annual precipitation of 23 inches, 2.5 percent organic matter, no straw residue, and soil test values from the example in Table 4, you would need 63 pounds N per acre:

Total N needed (Table 1 + Table 2)	(144 + 0)	144
Minus mineralizable N (Table 3)	-	45
Minus soil test N (Table 4)	-	36
Equals N fertilizer needed	=	63

Nitrogen fertilizer based on the previous crop
You also can estimate the N fertilizer requirement on the basis of the previous crop. The values in Table 5 are generalized recommendations based on field experiments and observations of soft white spring wheat production following the production of various crops. Note: Fertility recommendations based only on the previous crop are not as accurate as recommendations based on good soil tests.

Phosphorus should be either banded or incorporated into the seedbed before or at planting. Broadcast-plowdown, broadcast-seedbed incorporated, and drill-banding are commonly used methods of application. Drill-banding is usually the most efficient application method, allowing placement with, below, or to the side of the seed. Choose whichever application method is most convenient. Note: If the P material banded with the seed contains N, do not apply more than 20 pounds N per acre.

K should be incorporated into the seedbed before or at planting. Broadcast-plowdown, broadcast-seedbed incorporated, and drill-banding are effective methods of application. Drill-banded fertilizer can be placed with, below, or to the side of the seed. Choose whichever application method is most convenient. The total of N plus K (as K₂O) applied with the seed should not exceed 20 pounds per acre due to potential harm to the seed.

Try experimental lime applications on highly acid soils (pH less than 5.3) to determine whether the crop gives an economical response. Apply needed lime at a rate of 1 to 2 tons per acre and mix it well into the soil. For additional information see CIS 811, The Relationship of Soil pH and Crop Yields in Northern Idaho.

• Weeds, insects, diseases, and environmental stress can influence the effectiveness of a fertilizer program and reduce yields.
• Early planting of spring wheat has been shown to result in the highest yields.
• Starter or pop-up fertilizers have been tried with limited success. Starter fertilizers as a management tool have been most effective when soils were cold and root growth could be stimulated by a readily available supply of both P and N.
• Take care when banding fertilizer to prevent placing excessive amounts close to the seed. Excessive amounts of N and K can result in salt damage during germination. Wheat is especially sensitive to excess salts during germination.
• Banding fertilizer improves N and P use efficiency. Consequently, if applying N, P, or both in a band, cut the recommended fertilizer application rates by 10 to 15 percent.

The Authors -- Robert L. Mahler, professor of soil fertility, and Stephen O. Guy, Extension crop management specialist, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, gender, age, disability, or status as a Vietnam-era veteran, as required by state and federal laws.

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Revised: January 3, 2002

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