Northern Idaho Fertilizer Guide Current Information Series No. 785

Winter Rapeseed
R. L. Mahler and Glen A. Murray

These guidelines were developed from research at the University of Idaho. (For the actual data, see University of Idaho Bulletin 634 and University of Idaho Progress Report 226). The suggested fertilizer rates are based on research results from fallow fields and are designed to produce above-average yields unless other factors are limiting production. Thus, the fertilizer guide assumes the use of good 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 soil sampling instructions, refer to University of Idaho Extension Bulletin 704, Soil Sampling. Soil sampling should be done within one month of planting.

Optimum production and returns from rapeseed are achieved when the crop is managed properly. Low yields are most often caused by poor stands, inadequate fertilization and poor control of cabbage seedpod weevil. Managing summer fallow for effective water conservation and erosion, avoiding soil compaction and not planting in fields that become waterlogged during winter and early spring are essential for economic returns from proper fertilization.

Nitrogen
Total nitrogen need based on potential yield -- The total nitrogen (N) requirement can be estimated from the field's potential yield (Table 1).

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

Fertilizer N needed = total N need based on potential yield - [mineralizable N (Table 2) + Soil test N (Table 3)]

Mineralizable nitrogen -- Northern Idaho soils release mineralizable N (N contained in organic matter) at four levels based on their organic matter contents (Table 2). Low levels of mineralizable N are released from 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 inorganic N in the soil can be evaluated most effectively with a soil test. The soil samples should represent the crop's entire rooting depth because nitrate-nitrogen (NO₃-N) is mobile in soil. Winter rapeseed is capable of efficiently removing N to a depth of 3 feet or more unless its roots are blocked by a restricting layer.

Soil test values include both NO₃-N and ammonium-nitrogen (NH₄-N.) To convert soil test NO₃-N and NH₄-N values to pounds N per acre, add the N values (ppm) for each foot increment of sampling depth and multiply by 4 (Table 3).

Fertilizer nitrogen -- 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)		__________

Example: With a potential yield of 2,200 pounds per acre, 2.5 percent organic matter and soil test levels of inorganic N from soil test values in Table 3, the calculation for fertilizer N needed is:

Total N needed (Table 1)		200
Minus mineralizable N (Table 2)	-	35
Minus soil test N (lb/acre) (Table 3)	-	92
Equals N fertilizer required (lb/acre)		73

The calculation assumes rapeseed is planted into fallow ground. If stubble is left standing through the winter and summer and incoporated into the soil before planting, extra N will be needed for rapeseed residue breakdown. Add 15 pounds available N for each ton of straw or nonlegume residue up to 50 pounds N per acre. Remember, 1 ton of residue is produced for each 20 bushels of wheat or 1,400 pounds of barley grain. Subtract approximately 25 percent of the cereal residue production in this calculation to allow for residue decomposition prior to planting rapeseed.

Fertilizer nitrogen need based on the previous crop -- You may also estimate the amount of N fertilizer required for winter rapeseed based on the previous crop. The values in Table 4 are generalized recommendations based on field experiments and observations of production after the various crops. Note that N recommendations based on the previous crop are not as accurate as recommendation based on good soil tests.

Phosphorus
Winter rapeseed has a moderate requirement for phosphorus (P) (Table 5). Phosphorus deficiencies in rapeseed are difficult to diagnose visually. Usually the plants remain dark green, but growth is stunted. Because phosphorus is not mobile in soils, it must be banded or incorporated into the soil for efficient utilization by rapeseed. Commonly, P is broadcast incorporated or drill banded.

Potassium
Potassium (K) levels are normally sufficient for rapeseed production, but K should be applied when soils test low (Table 6). Fertilizer can be effectively surface broadcast incorporated or drill banded. Fertilizer can be placed with the seed, below the seed or to the side. When applied with the seed, the total N and K (as K₂O) should not exceed 25 pounds of nutrient per acre. Use whichever application method is most convenient.

Sulfur
Adequate levels of sulfur (S) are necessary for maximum production of winter rapeseed. Without adequate S the rapeseed will appear light green to yellow. Plants require S to use N efficiently. Because S is mobile in soils, it is prone to leaching during winter and early spring. consequently, soil testing for S is important. Sulfur needs based on soil test results are in Table 7.

Sulfur can be surface applied and will move into the soil with precipitation. Elemental S is not recommended because it becomes available to plants slowly.

Micronutrients
Boron -- Winter rapeseed requires high levels of boron (B). On deficient soils -- soils testing at less than 0.5 ppm B -- apply 1 to 2 pounds of B in a uniform broadcast application. Never band B. For information on B and availability of specific fertilizer materials, see University of Idaho CIS 1085 (formerly CIS 608), Essential Plant Micronutrients: Boron in Idaho.

Zinc -- Zinc (Zn) deficiencies are rare, occurring only in severely eroded soils. If soils are severely eroded and a soil test for Zn shows less than 0.6 ppm of Zn, see University of Idaho CIS 1088 (formerly CIS 617), Essential Plant Micronutrients: Zinc in Idaho. Rapeseed growers in the Kootenai River Valley of Boundary County should watch for Zn deficiencies.

Other micronutrients -- Winter rapeseed should not respond to applications of chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn) or molybdenum (Mo). Extensive field experiments on micronutrients have not been conducted. Still, micronutrient applications often are more likely to create toxicity problems thatn to correct deficiencies. Avoid applications of these materials in northern Idaho. However, growers in the Kootenai River Valley of Boundary County should watch for manganese deficiencies.

General Comments

1. Nitrogen fertilizer applications should be split between spring and fall in areas receiving more than 18 inches of precipitation. Research has shown that heavy fall applications can reduce rapeseed's winter hardiness. Fall-applied N is also susceptible to leaching. Consequently, no more than 50 percent of the required N should be applied in fall. In areas receiving less than 18 inches of precipitation, including traditionally summer-fallowed areas, all N may be applied in fall.
2. Phosphorus can either be banded below the seed or applied before planting and incorporated. Banding below the seed appears to be the most efficient method.
3. Potassium can be surface broadcast, broadcast incorporated or banded below the seed. Banding below the seed appears to be the most efficient method.
4. Sulfur can either be incorporated or surface applied in fall. Sulfur may also be applied with N in spring.
5. Contact the Extension agricultural agent in your county if you need more information.

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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