Cover crop gardening: soil enrichment with green manures

SOIL ENRICHMENT WITH GREEN MANURES

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Cover crop gardening
A Storey Publishing Bulletin, A-5
ISBN 978-0-88266-179-7

Cover Crop Gardening

Green manuring the growing and turning under of crops to fertilize and improve the soil is an age-old practice that is gaining popularity again.

One reason has been the rising costs of chemical fertilizers.

Even more important is the development of equipment that has made the task of turning under crops an easy one. Once gardeners turned these under with spades. It was hard work, and time-consuming, but many gardeners thought it was worth it, even then. Now the gardener can quickly chop up a crop and that speeds decomposition and mix it with the soil, using equipment designed for the job.

But why raise green manures? Why plant cover crops, or catch crops? The answer is simple. All these crops will improve the soil. They add something valuable to it, or prevent something valuable from disappearing.

As you read this bulletin, think of these crops as closely related, and having a single purpose to improve the soil. Green manures add huge amounts of organic material to the soil, cover crops prevent erosion, catch crops prevent the leaching away of nutrients, and all overlap in their contributions.

A Vermont gardener has summed up the benefits he has seen: Gardening is easier now that Im raising green manures. Its easier to get the soil right for planting, easier to keep the weeds out of the garden, easier to get good crops, and easier to clean up the garden and get it ready for winter.

He and others find these green manure crops are simple to raise, and their contribution to the garden, once turned under, is quickly apparent. Like them, you will find the next crop you raise after green manures will be improved.

Lets look more closely at the benefits you can expect from these crops. Turn this page and begin reading about them.

If you are an experienced gardener, you know that you cant have rich, productive soil unless you put something back into it. For many, chemical fertilizers have been the answer, and an increasingly expensive answer.

By growing green manures you can improve the soil and cut your fertilizer needs in half at the same time.

Green manuring is a way to speed up the natural system of creating rich soil. Soil has been built up over the centuries as natural vegetation died and decomposed. By raising crops, then chopping them up and tilling them under, this same process is accelerated, and the benefits to the soil are much more quickly apparent.

The result is that the amount of fertilizer you have to buy and use can be reduced sharply.

Large supplies of nutrients may be locked up in insoluble rocks or minerals. Green manures can help unlock some of this supply.

One way that green manures can make more of these nutrients available is by increasing the activity of microbes in the soil.

These microbes produce weak acids that eat away at soil minerals, causing them to release nutrients for plants to use.

Some green manure crops can extract nutrients from insoluble minerals. Rye, buckwheat, and sweet clover can extract phosphorus from some insoluble minerals. When crops like these are grown in poor soil and turned under, the phosphate they mine from the earth is released to plants grown later in the same area.

Nitrogen is one of the most plentiful elements. Its in the air we breathe. Tons of it hang in the air over each acre of our gardens, free for use if we can get it down into the soil.

At the same time, nitrogen bought through commercial channels has shot up in price, because production of it demands use of expensive petroleum products.

There is a way to get this nitrogen free, and in large quantities. You can plant legumes as green manures. Legumes alfalfa, beans, peas, and vetch are some of them have the ability to host bacteria that can fix nitrogen in the air. The legume seed is inoculated by coating it with a strain of nitrogen-fixing bacteria suited for the particular legume, then planted. This inoculant is widely available in seed stores. When the seeds sprout and grow, the bacteria enter the developing root hairs, take nitrogen from the air and fix it in nodules on those root hairs.

Alfalfa, the best of the nitrogen-fixing crops, can add as much as 200 pounds of nitrogen per acre. Thats the same amount of nitrogen you would get from spreading a ton of 10-10-10 commercial fertilizer on that acre. And think of the extra organic material you are adding to the soil and thus enriching it.

A drop of rain striking bare soil is like an explosion. Small particles of soil splash into the air and scatter in all directions. On bare ground which has poor soil structure, one hard rain can move tons of soil per acre. On a slope, most of this soil will land downhill from its original location, and it can cause even more erosion than flowing water.

Green manures can help this problem in several ways. They shield the soil from the force of a driving rain. The plants absorb the force of the raindrops and the water flows gently into the ground. The roots help hold the soil in place and there is almost no loss to erosion.