Crop Rotation, Green manures and Cover Crops

A green manure is a crop grown specifically for fertility enhancement. Quite often, green manure is confused with cover cropping. The latter is specifically grown for weed suppression and prevention of soil erosion when the ground would otherwise remain bare for a period of three months, or more. Of course, a crop can be grown for both purposes, but the type of crop grown is determined by the most important purpose.

European organic farmers have brought green manuring to a state approaching perfection. They almost invariably sow a mixture of a legume, a cereal and a crucifer. The legume fixes nitrogen. The cereal straw produces soil binding materials when it decomposes, called mucins. They bind the soil in the crumb structure that is essential for good drainage, aeration and ease of tillage. The sulphur compounds in the crucifers are believed to enhance the health of the soil.

Typical legumes include lupins, tick beans, field peas and tares (vetch). Typical cereals include oats, rye and barley. Typical crucifers include oilseed radish, rape and mustard.

The optimum time to plough a green manure under is when the flowers are just starting to form. At this point, the protein content is at a peak. Afterward, the fibre content increases, necessitating the application of additional protein, or a longer wait for complete decomposition. The effectiveness of a green manure is enhanced when the carbon to nitrogen ratio is between 25 and 35 to 1. That is, the protein content complements the fibre. Looked at from the point of view of using pelletised poultry manure, its effects too are greatly enhanced when used in conjunction with a fibrous green manure due to the low fibre content of the poultry manure. The remarks above applying to ploughing in pasture also apply to green manures. Wilting the green manure by rolling or mowing prior to turning under promotes an increase in decomposition rate.

Cover cropping requirements are rapid establishment to get ahead of the weeds, and for plenty of fibrous roots that will hold the soil together. Usually this is a cereal, or annual ryegrass. One suspects that a mixture of species would perform better from the point of view of the soil biology vastly preferring a mixed diet, but this may not be as economic as using a single species.

Crop Area per kilogram Comments
Lupins 70 sq metres Sow late summer, or early autumn. Grows O.K. in acid soils.
Ryecorn 100 sq metres Slow to decompose. Grows very tall (up to 2 metres). Germinates in cold soil. Sow autumn to spring.
Barley 100 sq metres Needs warmish soil to germinate. Prone to fungal disease in very wet winters. Sow autumn or spring.
Oats 100 sq metres Does not mind cold, wet conditions. Algerian is the most common variety used for green manure. Sow autumn.
Tick Beans 60 sq metres Also called Fava beans, or horse beans. Prefers neutral soil and need plenty of potassium to remain disease free. Sow autumn.
Field Peas 100 sq metres Grow better in cold weather than tick beans. Tick beans are preferred where amount of nitrogen required is high. Sow autumn.
Buckwheat 130 sq metres Very rapid growth. Matures in less than two months. Needs warm soil to germinate.
Vetch 60 sq metres Called tares in British books. This is a legume. Grows over winter. Sow autumn.
Red Clover 450 sq metres Sow autumn. Good nitrogen fixer, but hard to sow accurately due to fineness of seed.
Weeds n.a. If your weeds are predominantly annuals, such as chickweed, or fat hen (lamb's quarters), they are the cheapest of green manures. Do not allow them to seed as they are erratic germinators.


Crop Rotation

Crop rotation is the succession of crops grown over time. If plants from the same family are grown too frequently in the same soil, there is a build up of soil borne disease. Also, some plants leave residues in the soil that affect the following crop. Peas and beans, for instance, do not grow well following the onion tribe. Reversing the order creates above average results. The shortest period between related crops should be three years.

Brassicas are prone to the disease club root, also known as finger and toe in turnips and swedes. The brassica family includes cauliflower, broccoli, brussels sprouts, cabbage, kale, and many of the newly introduced Asian leaf vegetables, senposai, mizuna and bok choi for instance. Even though radish is a member of the tribe, it is in the ground for too short a time to develop the disease.

The Solanums include tomatoes, potatoes capsicums and egg plant. Tomatoes are said to benefit from being grown in the same ground for up to five years. I would not risk it myself.

The legumes are peas and beans. They all have the property of fixing atmospheric nitrogen in a form plants can use via bacteria in nodules on their roots. If these nodules are absent on your mature plants, you can purchase the bacterial inoculant, or cadge some garden soil from a friend that does not have your problem.

The umbellifers include parsley, parsnips and carrots.

The Chenopodiceae include silver beet, spinach and beetroot.

The cucurbits include pumpkins, squash, cucumbers and melons.

The alliums include onions, garlic, chives and leeks.

The Compositae include lettuce, Jerusalem artichoke and salsify.

Sweet corn is on its own in the grass family.

In the average garden it is not too difficult to arrange a rotation that prevents relatives being grown too close together in time. A typical rotation is potato tribe/brassicas/peas and beans/everything else and back to the potatoes again. This takes account of brassicas and potatoes being great staples in our diet. The brassicas are generally transplanted as seedlings, a necessity in most areas if they are to follow potatoes. Most rotations published in books from overseas are based on the assumption that nothing is grown in the winter, which is far from the case here in Tasmania. My own rotation is peas and beans/onion tribe/brassicas/everything else and back to peas and beans, taking three years to complete. This is because I grow rather a lot of garlic and leeks, which I sell.

If you plan your garden well, you can have something growing to eat all year round. There will be gaps between crops from time to time. If the period is to exceed a couple of months, grow a green manure crop to protect the soil. It will also "rest" the soil, which is said to be beneficial.

Some rotations are based on the feeding needs of the plants. Peas and beans generally use few nutrients from the soil and do not mind fresh lime. Leaf vegetables need lots of compost and are heavy feeders, as are the fruiting vegetables and potatoes. The roots, carrots, onions (not really roots, but gardeners call them roots), salsify, swedes, turnips, and beetroot are light feeders, and like the peas and beans, require no manure, or compost if the application for the heavy feeders was great enough.

To make your rotation workable, divide your garden into as many sections as there are groups in your rotation. This can create monoculture type pest problems in a large garden. If you grow on narrow raised beds in the French-intensive system, as many gardeners do these days, rotation is more easily accomplished.

Some gardeners do away with rotations by so thoroughly mixing up their plants that disease has no chance of being encouraged. This has the snag of being difficult to plan how closely yield is going to match consumption needs. Where the garden is supplemented by purchased vegetables this is not so much of a worry. One of the problems of organic growing is finding quality produce to match your own, so this may not be such an attractive option.

Where garden space is limited, crops can be chosen under three different sets of criteria, flavour, cost saving and yield per unit area. You cannot buy asparagus or sweet corn that tastes as good as you can grow. This is because they deteriorate so rapidly following harvest. Unfortunately, both yield very poorly for the area they take up. Organically grown carrots have astoundingly better flavour than conventionally grown and also yield well. Organic potatoes taste better, but take up a lot of room and are bought so cheaply, there is no financial incentive to grow one's own. Commercial tomatoes are so totally lacking in flavour that tomatoes are the single most common garden vegetable in Australia. They also yield well. The following table will act as a guide. Value is problematic as cost varies according to seasonal availability. Lettuces can cost between 30 cents and two dollars, for instance. The * means do not know.

Crop Yield Value Flavour Difference

Artichoke, Jerusalem high medium marked

Artichoke, globe low high *

Asparagus low high marked

Beans, broad high low some

Beans, French low medium little

Beetroot medium high little

Broccoli medium high marked

Brussels sprouts high high some

Cabbage high low marked

Carrots high low marked

Cauliflower medium medium marked

Capsicums low medium little

Celery high medium marked

Cucumbers high medium some

Egg plant medium medium little

Garlic medium high little

Kale medium low little

Kohl rabi medium low some

Leeks high high little

Lettuce medium-high low some

Melons medium medium little

Onions high low little

Parsnips high low marked

Peas low low marked

Potatoes high low marked

Pumpkin medium low some

Silver beet high medium little

Swede high low marked

Sweet corn low low marked

Tomatoes high low marked

Turnips high low some


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Jonathan Sturm 2003 - 2011

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