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Soil & Fertility


Soil

Deep sandy to loamy soil with a pH of 6.5 to 7.5 is ideal for early brassicas and heavier soils, while more productive, should be used for main season production. Most soils in Utah are suitable for the brassicas provided they are well drained, fertile, and do not have a buildup of salt. A well prepared seed bed encourages uniform planting conditions for seeds or transplants. Several weeks before planting, prepare the field for planting. If the site has drainage issues, consider 6-8 inch raised beds.

The brassica crops may be sensitive to residual herbicides from the previous crop, so pay attention to site selection if residual herbicides have been used in the past. Crop rotation is necessary. Remember, brassicas are all closely related to each other and insects and diseases are common to all. Plant residues should be completely buried to facilitate decomposition and reduce disease incidence.

Fertility

Prior to planting, have the soil tested to determine nutrient needs and deficiencies (Table 4.2). Soil sampling approaches and interpretation can be found on the Utah State University Analytical Laboratories website (www.usual.usu.edu). Organic growers should incorporate composted organic matter before planting to improve soil fertility. If you regularly apply compost or manure, periodic soil testing is critical. Regular additions of organic fertilizers can result in the buildup of salt levels and excess levels of other nutrients. An initial application of 5 tons per acre of high quality compost of known nutrient analysis may be helpful. This can be broadcast over the whole field or banded and then incorporated into the soil.

Generally, ¼ to ⅓ of the nitrogen and all the phosphorous and potassium is applied prior to planting. This fertilizer is either broadcast or banded. In soils with high P and K levels, broadcasting all of the fertilizer is acceptable. Banding is a good method to ensure the fertilizer is near the plant. Fertilizer bands should be 3 inches beside and 3 inches below the seed or transplant to minimize salt injury during establishment.

Transplants are “watered in” with a high P starter fertilizer at one quarter to one half pint per plant. The starter fertilizer is applied in addition to the broadcast of banded fertilizer. Most brassicas need additional magnesium and boron for optimal growth. Be sure you soil test for these minor use nutrients and add them prior to planting

Table 4.2. Phosphorus (P2O5 ) and Potassium (K2O) based on soil test results

Phosphorus Test Results Lb/acre
0-14 150-200
15-29 75-100
30-45 40-70
46-60+ 0-30
Potassium Test Results Lb/acre
0-99 150-200
100-149 75-150
150-199 25-75
200+ none

Use the higher amount when soil test values are in the lower part of the range

Nitrogen (N) – Up to 50 lbs. N/acre can be applied prior to planting. High pre-plant soil N levels cause seed or transplant establishment problems, N leaching, and is wasteful since plants use very little N during the first 4 weeks of field growth. An additional 75-120 lbs. N/acre is often applied in two sidedress applications. Based on the N uptake pattern in brassicas and extensive research, the first side-dressing is at thinning or 4 weeks after transplanting. The second, if needed, is 4-6 weeks later. Research shows that if pre-sidedress soil nitrate-N concentration is below 20 ppm, the brassicas will respond to additional N. Excessive N applications causes loose, soft heads and splitting in cabbage and Brussels sprouts. For broccoli or cauliflower, excess N contributes to non-uniform, rough, leafy heads and hollow stems (see other production problems on pg. 34). Use less N if the soil had additions of manure, compost, or when brassicas are grown after legumes (beans, alfalfa, etc.).