What are Green Manures?

Posted by admin on July 28th, 2007 at 04:31pm

Find out how using green manures in your organic vegetable garden can reduce or eliminate any need for alternative fertilizers.

We’ve all heard of animal manures as a soil amendment for vegetable gardening. Farmers have been using manure for centuries in their soil preparation as a way to add beneficial elements like nitrogen, potassium, and calcium back into the soil to be used as essential food for their vegetable crops. However, by incorporating a system of green manures into your garden, you can provide your land with an green alternative to using animal products.

What is a green manure?
Green manures are crops that are grown by farmers on their vegetable plots or beds which aren’t meant for harvesting or consumption. They are simply grown in order to be tilled into the soil to add their nitrogen and other elements back into the earth to make the piece of land more productive. While many traditional farmers grow these plants as feed for their animals, they don’t need to be used as such. They can go directly back into your soil to make it more fertile. These crops include plants such as alfalfa, clover, rye, vetch, soybeans, and oats. Green manures can be grown in a couple of different ways. Let’s take a look at the benefits of green manures, as well as a couple of different methods for incorporating a green manure into the crop rotation of your organic vegetable garden.

The Benefits of Green Manures
First, let’s take a look at why we would want to bother with growing plants in our garden that we’re not going to harvest. We can always add fertilizers and other amendments to the soil, so why bother? Keep in mind that we are practicing these methods of farming in order to create a sustainable garden. If we can give the soil everything that it needs to be good, rich earth, we can grow better vegetables that are healthier for you with methods that are better for the environment. Industrial manufactured fertilizers are expensive and can be full of detrimental or even toxic substances. You don’t know where this stuff is coming from and you certainly don’t want to put it into your soil. Your land is your own, so be proud and aware of what you’re putting into it, just as you should be aware of the food that you are taking into your own body. The economics are simply better, too. Seed is quite cheap in comparison to buying outside fertilizer. The costs for supplying an adequate amount of fertilizer to even one acre of land are substantial, whereas the price of seeds needed to cover the same area is much less expensive. Here is a breakdown of some of the benefits of growing green manures:

Protects your Soil from Erosion:
Plants constantly growing in the ground keep the ground stable. The root systems of plants give integrity to the soil, strengthening it and binding it together. Soil that is left barren can be adversely affected by rain, sun, and wind.

Adds Nitrogen back into your Soil:
Cover crops, especially legumes like soybeans, add beneficial nitrogen back into the soil. The three big elements that you need to be aware of that plants need to grow well and to produce food are nitrogen, potassium, and phosphorus. While adding supplementary sources of other elements to your soil often come from outside your garden, a free source of nitrogen from established plants is a sensible practice.

Bring Nutrients from Deeper Soil to the Topsoil:
Your deep-root vegetables like carrots are naturally adept at bringing elements from further down in the soil closer to the surface for future, shallower-rooted crops to benefit from. They do this by digging deep with a substantial root structure. Green manure crops do the same thing. Plants like clover have a very deep root structure and can bring these elements up from the deep places where they will be needed by your other crops.

Protects against Weeds:
Weed control is one of the biggest factors that an organic vegetable gardener needs to be aware of. Once weeds get started, they can become difficult to manage and be incredibly invasive to your crops. They compete for space, and can overwhelm or kill your food crops. The best way to go about getting rid of weeds is not giving them the space to let them germinate in the first place. With a cover crop taking up your available soil space, there is less room for weeds to grow.

Now that we are familiar with some of the major benefits of growing a green manure, let’s look into the different ways that the organic farmer can go about this.

Growing a Cover Crop
For many farmers, their garden is divided into two different schedules for the entire year - the season for growing their crop, and the season for growing their cover crop. When the harvest is finished, the cover crop is sown into the soil and grows until ready to be tilled under for the next season of food crops. For instance, once you’ve finished harvesting your crop of tomatoes in September or October, a crop of oats can occupy the same plot for a few months until being tilled into the soil for your planting of peas or beans in the spring. The remains of the tomato plants from the previous season coupled with that burst of fresh nitrogen from the oats makes the soil rich and ready for your legume crop.

Undersow your Green Manure
You can also grow your green manure crops alongside your food crops at the same time. Why would you want to grow both kinds of crops at once? Wouldn’t the green manure crops crowd out or leave the food crops at a disadvantage, or vice versa? The answers to these questions revolve around timing your planting, and if done correctly, this method can be even more beneficial than simply growing a cover crop after you harvest the food crop.

For example, if you plant clover next to and a few weeks after your squash plants, the squash has had adequate time to grow and secure it’s place on the land before the smaller crops come up. Once you’ve harvested your squash a few months later, the clover is firmly established in the bed, efficiently protecting the soil and improving the soil structure. There is no empty field that has to then be sown with the green manure only to wait for weeks before the crop gets going, leaving the plot susceptible to erosion and other soil damage. The cover crop is already there once you harvest. It can then continue to grow for months before you till it and plant your next crop. This method of undersowing your green manures is not only a good idea, it may simply be necessary in colder climates where the planting of cover crops like soybeans or vetch simply isn’t possible after harvesting vegetables in late fall.

How to Sow your Green Manure Crop
Sowing your green manures is a strong and effective way to manage your organic garden, and taking the time to plant these cover crops properly and with as much attention to detail as your food crops will yield the best results. This isn’t simply about scattering some seeds that will come up alongside your crops for harvest. Sowing green manures correctly and effectively means taking the time to consider some essential factors:

Sow Into a Weed-Free Bed:
If there are scattered weeds throughout the plot of land you’re going to be gardening in, you present a big problem for the plants you are hoping to grow there. Established weeds will suppress the smaller crops, overcrowding them and killing them. This is true for your green manure crops as well. Plant your seeds in clean, rich soil that is free of competing weeds.

Plant Your Green Manure Seeds with as Much Attention as Your Food Crops:
You may find that the most efficient way to plant cover crop seeds is to use the same kind of drill that you would use to plant root vegetables like carrots. Your green manure seeds need to be planted in a specific way, between your plantings of your food crops, in order to be the most effective.

Different Food Crops Require Different Green Manure Spacing:
Not all of your crops for harvest are the same size. From melons to peas, squash to tomatoes, potatoes to carrots, different crops require different soil nutrients and different amounts of space between the plants. This will have a direct effect on how much of the green manure crop you can plant between them. Between rows of corn, for example, five rows of a crop like vetch or soybeans can grow. Between carrots (which are planted closer together than corn), you’ll have room to grow only one row of clover. Crops that require more space like tomatoes should give you a good amount of room to grow as much as 20 rows of oats.

What Green Manures should I grow?
This all depends on what crops you are going to be growing. As shown in our discussion on crop rotation, there are some guidelines that you can follow, but you may wish to experiment for yourself and decide which system works best for you. You are in charge of your own garden, and your job is to discover how to make it the most effective piece of land that it can be for growing quality organic vegetables. However, here are some suggestions to get you started on specific crops and the undersown green manures that work well when grown the season before you plant them:

Tomatoes - Vetch is a suitable crop preceding your tomatoes.
Corn - Clover is a good choice for the plot that will next host your corn plants.
Potatoes - Soybeans have been found to be one of the best preceding crops for potatoes, as they help reduce the possibility of many potato diseases.
Squash - Try planting rye before your next crop of squash.
Peas - Peas respond well to having oats planted and tilled before their turn in the crop rotation.

We’re all learning better ways to become more self-sufficient, and green manures are a great start for getting your organic garden to better sustain itself. The benefits to improving your soil from the use of green manures are well tested and shown to yield better quality crops that have a better chance of surviving. And as with many green improvements to our lives, it has been ultimately shown to be a most cost-effective and natural solution to enhancing the way we live and grow.

Under Organic Gardening