Brassica vegetables like a very specific type of soil, they need a certain balance of nutrients, and the soil must be a specific PH so that they can access those nutrients.
To create the perfect bed for the this group you need to provide a good balance of Nitrogen, Phosphate and Potassium, and the soil needs to be slightly alkaline, as well as being firm enough to support large plants that could be in the ground from late spring to the following early spring.
Most people will tend to think that a good feed of well-rotted manure will provide enough for most plants to thrive, and in theory, if a vegetable plant puts a high demand on the soil, that is, if it produces a lot of green leaves and grows to a fair size then manure will normally be the perfect soil food.
However, manure tends to be acid, and although all the elements needed by the plant may be contained in the manure, the brassica plant cannot access those nutrients in an acid environment.
To overcome this many gardeners will recommend that you dig in a barrow of well-rotted manure in the autumn and then dress the soil with lime in the spring, the theory is that the manure will provide the nutrients but makes the soil to acid, so the lime reduces the acidity ready for these alkaline loving vegetables.
The trouble with this method is that although manure provides around 140 grams of nitrogen per square meter, over ninety percent leaches out over the winter leaving only 14 grams of nitrogen in the soil. And as a large cabbage needs over 20 grams per square meter, manure won’t provide the nutrients you need.
The other downside to adding manure is that you would have to dig the bed to mix the manure in, which we would not recommend. In fact, the brassica bed is the one part of the garden that needs no digging at all.
The soil needs to be very firm to allow plants to have a firm footing as any wind will cause the plants to rock and damage the root system.
When we look at our rotation plan we start with the roots, which we manure heavily, we also pack the ground with seaweed which increases the level of rhizobia bacteria that fix nitrogen from the atmosphere into the soil.
We then plant legumes which take up the bacteria in its roots, we also dust the soil with bone meal which increase root formation. The rhizobia bacteria thrive in the roots of the legumes storing nitrates in root nodes.
When the legume crop has finished we cut of the plants at the root neck leaving the roots in the ground to rot and release the nitrates in the soil.
The legumes fix up to 30 grams of nitrogen in the soil ready for the brassicas, we also add bone meal to improve the levels of phosphorus and wood ash to improve the levels of potassium and make the soil more alkaline.
If you follow this rotation plan, because the acidification that the manure causes is two seasons before, and you have also added pot ash for two seasons, the soil should be the perfect PH. And as you have improved the health of the nitrogen fixing bacteria in the soil before growing legumes, you should have plenty of nitrogen in the soil, and end up with the perfect bed.
However, not all things are equal, and because you are dealing with an ever changing environment I would never assume that the nitrogen, that is the most important part of the brassica development, is exactly at the levels you expect, so adding a nitrogen rich food as a supplement, sometime later in the season would be good practise.
We make up a nitrogen rich liquid feed which we apply to the base of the plants on a weekly basis, using a liquid feed makes the nitrogen available to the plant immediately.
To make a nitrogen feed cut stingy nettle and half fill a plastic bin, (wear gloves of course), cover the nettles with water and after a month start to use the tea, diluting by approximately 10:1, as you need it. Throughout the season add more nettles and more water.
Once the season has finished, instead of throwing this feed away you could bottle the liquid that is left and add the remaining solids to your compost bin, then make a fresh cut and soak it ready for next year.
If you don’t fancy making up this feed you could use blood meal or pelleted chicken manure.