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by Paul Sachs
Brassica is a Latin word meaning cabbage. Mustard, rutabaga, kale, collard, cauliflower, Brussels sprouts broccoli and turnip join this genus by their similar leaf and flower structure. These plumbs are also referred to as cole crops. The word cole is Middle English, derived from the Anglo Saxon word camel meaning cool or cold. This reference to temperature gives us a clue as to the type of growing conditions Brassicas like. Most Cole crops do well in a cooler environment, especially broccoli, whose heads quickly bolt into flowers when the temperatures rise.
Unfortunately, during cooler times of the year the biological activity in the soil is naturally suppressed, inhibiting bacterial mineralization of nitrogen and availability of other nutrients. To make matters even more challenging, Brassicas are relatively heavy feeders and have a moderately low tolerance for soluble sales. What this means is that somehow we have to provide good fertility in cold soil with only a minimal increase in the soil's conductivity (a measure of salt concentration) from the use of soluble nitrogen.
Brassicas also require certain trace elements in moderate to high amounts. Their sensitivity to boron manganese molybdenum and iron deficiencies is significant. Keeping these micro-nutrients available is also a challenge because Cole crops are sensitive to soils with a low pH and with the exception of molybdenum, most trace elements become less available as the pH of a soil rises.
Moisture is another important consideration when growing Brassicas. They are sensitive to both dry and excessively wet conditions. Coarse sandy soils cannot hold adequate water and heavy clay soils hold too much. Fortunately, all of these challenges can be addressed with one remedy: Organic Matter. Soils with adequate levels of organic matter hold sufficient moisture but have ample porosity for good drainage. This same humid material holds a multitude of trace elements in a chelated (organically bound) form that become available through biological activity. Humus can also buffer pH changes and the immense population of organisms it supports can control soil solution salinity (try saying that three times fast) by immobilizing certain soluble soil compounds.
The dark color of soil organic matter helps warm the soil faster in the spring by absorbing the sun's heat and, surprisingly, keeps it cooler in the summer. The warming effect in the span" increases biological activity which mineralizes more soil nitrogen earlier. Cooling in the summer occurs from surface evaporation and is enhanced by organic matter's greater capacity to hold water.
This is not to say that if there is adequate soil organic matter then no fertilization is needed. On the contrary. It is important always to replace nutrients that are harvested from the soil. A soil test is recommended to determine pH and to see if any nutrients are deficient. Brassicas, in general, grow best in soils with a pH of 6.0 - 7.0 with the exception of collards and mustard that prefer a slightly lower range and cabbage that tolerates a pH of up to 7.5.
Organic fertilizers are best but some soluble inorganic nitrogen may be necessary for cold New England soils. The only mineral nitrogen allowed by most organic certification groups is Chilean nitrate and its use is restricted to approximately 3/4 pound of nitrogen per 1000 square feet (3/4#N/MSF) per year. This would be excessive to use as a starter and not recommended as the sole source of nitrogen for either starting or sidedressing.
As a starter fertilizer I recommend a N-P-K ratio of approximately 1-1-1, applying around ½#N/MSF. If Brassicas are being planted in rows, consider MSF equivalent to 1000 linear feet of row. When heads begin to form, use a N-P-K ratio of approximately 3-2-2, applying around l#N/MSF. Try to kind fertilizers that have approximately 2/3 of their total nitrogen in a water insoluble form for both starting and sidedressing. If your base soil fertility is low, these application rates may need to be increased. The ratios of N-P-K mentioned above are not critical, especially when dealing with natural organic fertilizers. A 2-3-3 analysis as a starter, for example, would serve just as well as a 3-3-3 so long as the same amount of nitrogen/MSF is applied.
It is important that fertilizers especially organic ones make contact with soil particles, so work them into the soil as well as possible. But beware, Brassicas are shallow rooted for the most part, so take care not to till your fertilizer in too deep. They work best in the top 2-3 inches of the soil.
Manure is okay to use but don't apply too much. Excessive amounts of manure can cause problems - such as locking up boron causing hollow hearts in broccoli. Too much manure can also cause excessive stalk and leaf growth with less than desirable head development. Working manure into the soil immediately after applying will save nitrogen and promote better biological breakdown of its nutrients.
Manures from different animals vary in their nutrient and moisture content As moisture content increases the nutrient content is diluted and vice versa. Poultry, horse and sheep manures, for example, are typically dryer and consequently have a greater concentration of nutrients. Apply high moisture manures such as cow or swine at 300-700#/MSF. Dryer manures should be applied at half that amount. Old, well rotted manure has lost some of its punch over time and can be applied more liberally.
Compost is à far better additive than raw manure because most of the nitrogen content has been stabilized by bacteria and, if made properly, has no significant amount of weed seed. Applications of composes should range between 400-800#/MSF depending on base soil fertility and the quality of the compost being utilized. Up to 1 500#/MSF can be used where base fertility is extremely poor. Some soluble nitrogen may still be necessary in the Spring for starting.
Important points to remember:
1) Pay attention to organic matter content in your soil. Most soil test labs can measure the percent of organic matter in your sample. In the Northeast five percent is great, two percent or less needs help.
2) Watch your salt. It doesn't pay to use excessive quantities of soluble nitrogen.
3) Watch your pH. Brassicas do best in soils with a pH of 6.0 to 7.0.
4) Periodically test for trace element deficiencies. Brassicas need trace elements.
5) George Bush still does not like broccoli.
Copyright © 1997 Ecological Agriculture Project BRASSICAS AND SOIL. Reprinted with permission. All rights reserved.
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