the bare necessities

Winter is coming, and with it heavy winter rains. The nutrients we have spent so long trying to build up in our soils are in danger. They may disappear through our bare, winter plots, like dry sand through our opened fingers.

For organic growers, loss of fertility can be a real problem. We do not use chemical fertilisers for a ‘quick fix’. We treasure the natural fertility that mulching, manuring and composting give us. Our aim is to build fertility over years by careful rotation of crops and use of organic matter.

Winter rains can wash nutrients away and compact the soil. To prevent this, organic gardeners use ‘green manures’ so that the soil is not left bare and unprotected.

Typically, we plant green manures in early autumn after our main crops have been taken. They are allowed to grow through the autumn and on into spring. They are usually chopped down before flowering when their stems may become too woody for easy incorporation into the soil. They are either chopped down or hoed off to act as a mulch, to be dug into the soil or they are removed for composting.

Their job over the winter is to:
hold soil structure together and reduce leaching of nutrients
provide leaf cover to dissipate the compacting effects of heavy winter rains
store nutrients in stems, roots and leaves that can be released back into the soil when needed.

There is a wide range of green manures to call on. The Organic Gardening Catalogue has a great choice and your own allotment association may sell them.

We have had success including the following in our four year organic rotation:

When potatoes are lifted, field beans are sown. Beans, peas, squash and corn follow potatoes for us. We have, therefore, found field beans to fit into our rotation and they are highly vigorous and reliable. They produce great quantities of leaf and stems that are ideal for bulking up the compost or for leaving on the soil surface as a mulch. Broadcast the seed then rake over or push the small beans in with your thumb. Many plotholders have seen our field beans and complimented us on the vigour of our broad beans. I wish my broad beans were as reliable!

We follow beans with the cabbage family (brassicas). So, mustard is sown after the beans, peas, squash and corn have finished because mustard is a brassica. Not as vigorous or as frost hardy as field beans, mustard is still a reliable green manure that once again produces lots of compostable material. Sow in drills or broadcast.

Following brassicas in our organic cycle are roots and the onion family. The winter ground is usually pretty full with overwintering leeks, red and white onions, garlic and then shallots in the New Year. If we have any gaps we sow grazing rye. Very vigorous, it is also popular with pests like rabbits and wood pigeons,. It does sometimes need protection to get it going unless you are that rarity – a rabbit and pigeon loving gardener! As with the other green manures, grazing rye can be hoed or chopped down in the spring and used as a mulch or composted if you don’t want the bother of digging it in. Sow in drills or broadcast.

After the summer onions have been lifted, once again we sow grazing rye to cover any unused ground as the bed prepares itself for potatoes in the spring.

This year we will also be sowing shade tolerant trefoil beneath Brussels Sprouts and Early Purple Sprouting Broccoli as well. I’m sure the organic police will scream that this is a legume. I know, I know. But it’s good for bees and is said to deter pests because the green of the undersown trefoil makes less of a contrast than the bare soil does against the juicy big leaves of brussels and broccoli. It is said that the butterflies can’t find the leaves quite as easily. We’ll see. But even if this doesn’t work, we’ll be getting an extra green manure fertility boost beneath those hungry brassicas.

Those of use who are passionate about encouraging wildlife to our gardens are keen to use green manures too. They provide excellent invertebrate cover overwinter and the improved soil quality is welcomed by earthworms.

A word of caution when sowing your food crops in the spring. When organic matter has been dug into the soil, this tends to inhibit germination of small seeds. So, remove green manures to the compost bin if you are planning to sow seeds – or transplant young plants into the bed if you have dug in your green manures or are planting beneath mulches.

Text of article published in Allotment and Leisure gardener Autumn 2009