NUTRIENT MANAGEMENT FOR BRASSICA CROPS
By John Howell
for University of Massachusetts Extension Vegetable Program
High Quality Brassica Project
January 2006
Soil pH should be in the range of 6.5 to 7.0. Use ground limestone in fall or spring, or split between spring and fall if the pH needs to be raised a lot. If club root is a concern, the soil pH should be increased to 7.2. Non-organic growers should use about 1,500 lb of hydrated lime per acre along with ground limestone. Organic growers can use 1,500 lb of pulverized lime instead, since hydrated lime is prohibited. Pulverized lime is limestone ground to a mesh size of 200 or smaller and is very fast acting. The liming materials should be thoroughly incorporated and mixed into the soil by harrowing or rototilling. The soil test lab normally makes lime recommendations based on target pH of 6.8. A grower can request that this be based on a target pH of 7.2.
Calcium and magnesium. Tip burn has occasionally been a problem with some Brassicas, including cabbage and kale, but no crop is immune. Internal tipburn may occur in cabbage, in the inner layers of the head. This is caused by an insufficient amount of calcium reaching developing (younger) leaves. To prevent this there should be adequate levels (high) of calcium in the soil and a steady and adequate soil moisture level. Avoid using urea or ammonium sources of nitrogen for side-dressing. The latter is not a problem in organic systems. The base saturation as reported on the soil test should be as follows: potassium (K) 2-5%; magnesium (Mg) 5-15%; and calcium (Ca) 60-85%. If these conditions don=t exist, the possibility of tip burn is increased, but it won=t necessarily happen. A long term goal should be to bring these nutrients within these ranges. Continued use of dolomitic lime leads to a decline in the Ca level of a soil. Alternating use of calcitic lime (if available) is recommended to supply more Ca. Gypsum can supply Ca without changing pH.
Nutrient levels. Phosphorous (P), potassium (K), calcium (Ca) and magnesium Mg) should be maintained in the high or optimum level in most soils. (Those with high cation exchange capacities above 15 can easily handle higher levels). Higher levels are unnecessary, can lead to imbalances and could cause environmental problems (nitrogen and phosphorous). Lower levels may limit crop growth and may result in certain deficiencies. When soils levels of a nutrient are above optimum, avoid adding any more until the level comes down. This may take several years. If levels are below optimum, make an effort to add materials that will increase the nutrient(s) of concern. Once levels are in the high or optimum range small amount can be added regularly to replace what is removed from the field at harvest. If soils are tested every one or two years, you can take action to prevent any potential deficiencies or excesses well before they cause a problem.
Phosphorus. Fall applications of animal manure or unfinished compost will tend to build up P levels (fall application is needed to meet the 120 day limit for certification). Only compost that was made according to NOSB guidelines can be used in the spring if you are certified organic. Compost from animal manure is good long-term strategy to build Phosphorus over time. Rock phosphate is very slow and dollar for dollar, even if you are buying the compost, compost is a better value.
If Phosphorus is in the ‘low’ or ‘medium’ range on a soil test, lack of P may inhibit the crop growth. To help in the short term, put bone meal close to plants as a starter. This can be applied close to roots without risk of burning. For direct seeded crops, use some mechanism for applying a concentrated band near the seeding band, and mix it in to a depth of at least two inches. This will be much more affordable than if the bone meal were broadcast and tilled in; by using a smaller amount concentrated near the plants you can use one fourth or one fifth as much. For beds for transplants, incorporate bone meal with light tillage after the bed is made. Bone meal ranges from 18-34% P; If you estimate 25% P, then you’d need 800 lb bone meal per acre to get 200lbs per acre of PB2BOB5 ,B but if concentrated in a band it would be more like 40 to 50 lb on a per acre basis.
Potassium. Brassicas have a little higher requirement for potassium than other crops; you should aim for well up into the high or very high range. For a good yield of Brassica crop expect it to use about 200 lb of potassium per acre. Remember that plant roots are not going to take all the K that’s in the field. Apply K in the spring after plowing, then harrow it in, at same time as incorporating compost. Don’t plow it under because that might put it too deep and out of reach of the roots. Broadcast, don’t try to sidedress. Remember that K does leach, so it is better not to apply in the fall. Leaching is less likely when the cation exchange capacity is around 10 and more likely when cation exchange capacity gets down below 5.
Potassium sulfate – if from a natural source – is approved for organic production. It is 50% actual K: if you apply 200 lb potassium sulfate then you would get 100 lb of actual K. Sul po mag is another source of readily available K. If you are using compost and it tests high, you can figure that into the total. K, because the Kin compost is readily available. A full compost test is available from the UMass Soil and Tissue Testing Lab for $30.00
(see http://www.umass.edu/plsoils/soiltest/services1.htm)
If K is low to medium, apply 150-125 lb actual K. If Potassium is in the high range, then add more for what the plants will use up (approximately 50 lb.) to keep it in the high or ‘optimum’ range.
Nitrogen (N) is a special case. Crops such as cabbage, broccoli and cauliflower require about 130 to 150 lb/A of N. Short season crops such as greens, turnips and rutabagas require less for a single crop. However these short term crops are usually part of a double or triple cropping system. Taken as an aggregate of crops, the seasonal need for N would be about the same as for a long term crop. Much if not all the needed N (some times too much N) can be supplied by soils with good levels of organic matter. As the OM breaks down, N and other nutrients are released. This is most rapid (and most N is released) when the soil temperature is over 70PoP F, the soil pH is above 6.0, the soil is well aerated and there is adequate soil moisture. Under good conditions N release can be 30 to 40 lb /A for each % soil organic matter. Thus a soil with 4 to 5 % OM can supply the seasonal needs of Brassicas if the above conditions are good. A soil nitrate test (PSNT) can be used in June to check the N level. If the level is 30 ppm, there should be adequate N (unless there is leaching due to heave rain).
Nutrient release is slow in the spring when soil temperature are cool. A small amount (20 lb/A) of soluble starter N may be helpful for early planting. There are many choices of starter fertilizers for non-organic growers. For organic growers blood meal or fish emulsion are readily available sources of N which can be helpful. Compost tea may be useful, but might not provide enough N.
Boron (B). Trace or minor elements are generally not a problem with the exception of boron (B). If soil test levels are not in the normal soil range (as indicated on the test), boron should be applied at 1 to 2 lb/A actual B (5 lb/A for cauliflower) for most Brassicas. There a number of soluble sources of boron, including Solubor and Fertibor, which are OMRI listed. Solubor is 20% B so you’d need 10 lb per acre to achieve 2 lb actual. If you are broadcasting an organic blended fertilizer, ask if your supplier will add boron to the mix. Another way to apply it is to mix it in water, spray it on the soil, and mix it in. A boom sprayer, or a siphon mixer with check valve for garden hose will work. Lack of adequate boron causes hollow stem in broccoli.
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