Northern Idaho Fertilizer Guide Current Information Series No. 453

Winter Wheat
Robert L. Mahler

These fertilizer guidelines have been developed through research conducted by the University of Idaho and Washington State University. They are based on relationships between soil tests and yield responses. The fertilizer rates are designed to produce above-average yields if other factors are not limiting production. Thus, the fertilizer guidelines assume the use of sound management practices.

The suggested fertilizer rates will be accurate for a given field provided (1) soil samples are properly taken and represent the area to be fertilized and (2) crop history information is complete and accurate.

Optimum production and economical returns from wheat are achieved when the crop is managed properly. Inadequate fertilization, poor stands and poor pest control are major contributors to low yields.

Nitrogen and sulfur are the major plant nutrients needed for wheat production. Phosphorus and potassium may also be needed. The need for these nutrients can best be determined by a soil test.

1. The wheat variety and its potential yield in your location. The suggested rates in this publication were developed over many years based on several varieties. Modern varieties require high levels of available soil nutrients and superior management to achieve their high yield potentials. Recent varieties such as Stephens, Daws and Hill 81 have very high yield potentials.
2. The potential yield or the average yield obtained from the field in past years. Research in northern Idaho and eastern Washington has shown that 2.5 pounds available N per acre are needed to produce 1 bushel of wheat in the 16- to 21-inch annual precipitation zone. In areas receiving more than 21 inches of annual precipitation, 2.7 pounds N per acre are needed to produce 1 bushel of wheat.
3. 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).
4. Actual total annual precipitation and other climatic factors.
5. The density of the plant stand.

Total N need based on potential yield
Estimates of N needed to produce a crop of winter wheat in a particular field should be based on potential yield -- the field's long-term average yield. Multiply the potential yield in bushels per acre by 2.5 or 2.7 pounds N per bushel, depending on annual precipitation, to arrive at total N needed (Table 1). If, for example, annual precipitation is 19 inches and potential yield is 100 bushels per acre, then 2.5 x 100 or 250 pounds per acre are needed.

Once the total amount of N needed to produce a winter wheat crop is known, a simple equation can be used to determine the amount of fertilizer N to apply to meet this need:

Total N needed (Table 1) - [Mineralizable N (Table 2) + Soil test N (Table 3)] = N fertilizer needed

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, soil erosion, available soil moisture and soil temperature during the growing season.

Five different mineralizable N release rates are used for northern Idaho soils (Table 2). Low N release rates are found on severely eroded clay knobs and hilltops, cutover timberland soils, soils in areas of low precipitation, soils with low water-holding capacities and 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. Winter wheat is capable of removing N to a depth of 5 feet or more.

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 3).

Nitrogen fertilizer -- The calculation for N fertilizer needed is:

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

For example, with a potential yield of 100 bushels per acre, annual precipitation of 23 inches, a medium level of mineralizable N and soil test values from the example in Table 3, 133 pounds N per acre are needed:

Total N needed (Table 1)		270
Minus mineralizable N (Table 2)	-	45
Minus soil test N (lb/acre) (Table 3)	-	92
Equals N fertilizer required (lb/acre)		133

Add 15 pounds available N for each ton of straw or nonlegume residue incorporated into the soil up to 50 pounds N per acre. Remember that 1 ton of residue is produced for each 20 bushels of wheat or 1,400 pounds of barley grain produced.

Estimate of N fertilizer based on previous crop
You also can estimate the N fertilizer requirement on the basis of the previous crop. The values in Table 4 are generalized recommendations based on field experiments and observations of wheat production following the production of various crops. N recommendations based on the previous crop are not as accurate as recommendations based on good soil tests.

Broadcast plowdown, broadcast seedbed incorporated and drill banding are commonly used methods of application. Drill banding P is usually the most efficient application method. It allows fertilizer placement with, below or to the side of the seed. The choice of application method usually depends on convenience to the grower.

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. When applied with the seed, the total of N plus K (as K₂O) should not exceed 25 pounds per acre. Choose the most convenient application method.

Lime applications on strongly acid soils (pH less than 5.2) should be tried on an experimental basis to determine if an economical response is likely. When needed, apply 1 to 2 tons per acre (based on the lime requirement test available from soil testing labs) and mix it into the surface 6 inches of soil.

1. Poor N management can result in excessive nitrate leaching and groundwater pollution under certain conditions. Poor management practices can cause excessive erosion and contamination of surface waters with P.
2. In areas receiving less than 22 inches of precipitation use split or fall applications of N. In areas receiving more than 22 inches of annual precipitation, apply N as a split (fall-spring) application. N applied too early in fall (when soil temperatures are above 50 F) may leach after heavy winter precipitation.
3. In areas of heavy winter precipitation (annual precipitation more than 24 inches) or sandy soils, spring applications of N may be more desirable than fall applications.
4. In areas receiving more than 21 inches of annual precipitation, test the top 2 feet of soil for NO₃-N in spring and topdress with additional N, if needed.
5. Use caution in topdressing with N in spring because N applied after the boot stage or at excessive rates can result in undesirably high protein levels in soft white wheats. Excessive N rates also increase the lodging hazard.
6. Banding fertilizer improves both N and P use efficiency. Consequently, if applying N and/or P in a band, cut the recommended N fertilizer application rate by up to 10 percent and the recommended P rate by up to 20 percent.
7. The ammoniacal forms of N (ammonium and ammonia) do not leach as readily as NO₃. When temperature and moisture are favorable for plant growth, however, ammoniacal N and urea are quickly converted to the NO₃ form. Thus, N applied in early fall, regardless of its form, is subject to leaching in areas of heavy winter precipitation.
8. N-Serve and other N stabilizers block conversion of NH₄-N to NO₃-N. Results obtained from N stabilizers are inconsistent although reduced N fertilizer losses have been obtained in some areas. N stabilizers have not been effective in deep, dark-colored soils that have high organic matter contents. For more details refer to University of Idaho CIS 313, N-Serve and Its Potential Use in Northern Idaho.
9. Cutover timberlands (which usually have clayey subsoils) used for wheat production are not as susceptible to leaching losses because of the low permeability of the subsoil. Low permeability, however, makes these soils subject to wetness or waterlogging that can result in N loss by denitrification (conversion of NO₃-N into gaseous forms of N that dissipate into the atmosphere).

The author -- Robert L. Mahler is Extension soil fertility specialist in the University of Idaho Department of Plant, Soil and Entomological Sciences at 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.

1,000, July 1991 (revised) 35 cents per copy

Comments to author: karenl@uidaho.edu

All contents copyright © 1996-2002.
College of Agricultural and Life Sciences, University of Idaho.
All rights reserved.

Revised: October 10, 1998

URL: http://www.uidaho.edu/wq/wqfert/cis453.html