You want cheap fertilizer. Here is where to get it for free!  

Green manure is the best free fertilizer that you might need to raise your farm’s productivity.

This is because traditional methods of replenishing soil fertility, such as fallowing or rotations, are no longer possible due to the diminishing size of land holdings.

A way out of this sad development is the use of green manure legumes to improve soil fertility. Legumes such as lablab, desmodium and purple vetch have many benefits, including soil fertility replenishment, soil and water conservation and weed and pest control, not to forget their use in human and animal nutrition.

Research done by the Kenya Agricultural Research and Livestock Organisation (KARLO) as well as by many other institutes  worldwide is pinning much hope on the use of green manure legumes and their addition of nitrogen to the soil  (the most important nutrient for crop production). However, the change is slow and is not seen for three to four seasons.

According to KARLO studies in the Kenyan highlands, legumes such as mucuna, lablab, crotalaria and canavalia, planted as a monoculture during one season, added 2– 15 tonnes of organic matter per hectare and were contributing 35 – 150 kg nitrogen under the same area.

Research by scientists and farmers in Kisii discovered the benefit of these legumes in intercropping with maize: Before the use of green manure, the yields were 12 bags of maize per acre, and afterwards, the yield rose to 20 bags per acre.

Thus, instead of spending money on commercial nitrogen fertilizers, farmers can use green manure legumes to improve soil quality and get a better harvest. However, using up space in a small shamba for plants that cannot be eaten is a very strange idea to many farmers.

The green manure method is a pillar of organic and sustainable farming. Green manures are plants grown to accumulate nutrients for the main crop. When they have built up the maximum biomass, they are cut and incorporated into the soil. This fresh plant material will decompose within a short period of time and release nutrients quickly. This is different from crop residues, which decompose at a slower rate and will therefore contribute more to the build-up of soil organic matter than to fertilizing the crop.

Increased yields for the farmer

Green manures penetrate the soil with their roots, bind nutrients and support the infiltration of water into the soil. The contribution of organic matter to the soil from a green manure crop is comparable to the addition of 9 to 13 tonnes per acre (0.4 ha) of farmyard manure.

Leguminous plants fix nitrogen from the air into the soil. This nitrogen enriches the soil and feeds all plants in the area. The portion of green manure-nitrogen available to a crop planted later is usually about 40 to 60 percent of the total amount of nitrogen contained in the legume.

For example, a purple vetch crop that accumulated 90 kg of nitrogen per acre prior to plowing down will contribute approximately 45 kg of nitrogen per acre to the succeeding grain or vegetable crop. Lesser amounts are available for the second or third crop following a legume, but increased yields are apparent for two to three growing seasons. Other advantages of green manure are as follows:

  • Some green manures can be used as fodder plants, or even to provide food for human consumption (e.g. beans and peas).
  • By decomposing, green manures release all kinds of nutrients, including phosphorus, potassium, calcium, magnesium, sulphur, and others.

They are accumulated by cover crops during a growing season. When the green manure is incorporated or laid down as mulch, these plant-essential nutrients become available during decomposition.

– Green manures suppress weeds and protect the soil from erosion by wind and water and direct sunlight. Soil cover reduces soil crusting and surface water runoff during rainy periods.

Important points to consider

Green manuring is an inexpensive way to improve soil fertility and the nutrition of the main crops grown.

There are some aspects which must be considered:

  • Green manuring requires some additional labour.
  • If green manures are intercropped with the main crops they compete for nutrients, water and light, but the benefit of having a source of high protein food (e.g. beans) may outweigh this.
  • If there is not enough food available in a farmer’s family and little space on the shamba, it may be more appropriate to grow a food crop rather than a green manure and recycle the crop residues, or to intercrop a green manure crop with the main crop.

Legumes as Nitrogen Factories

All plants need nitrogen to grow well. In theory, there is no lack of nitrogen.

Air consists mainly of nitrogen (78 percent), but most plants are not able to take nitrogen directly from the air; they need it in modified form. Some plants, especially from the legume family, are capable of fixing nitrogen directly from the air with their roots, and changing it into a soluble form as nutrients. Some of this nitrogen is spread in the soil and can be used by neighbouring plants. That is why intercropping is important.

2). The effect is even better when legumes are dug into the soil. They enrich the soil with nitrogen, and plants growing next on the same field benefit from this readily available nitrogen.

 

How to use green manures

  • If you grow green manures in crop rotation, the time of sowing must be chosen such that the green manures can be cut down and worked into the  soil before the next crop is sown.
  • Green manures need water for ger- mination and growth!
  • If legumes and green manure are grown in a field for the first time, inoculation of the seeds with the  specific rhizobia (soil bacteria) may  be necessary to benefit from nitrogen  fixation by the legume; you can find  the inoculation material in Kenya  Seed Company stores and shops.
  • In the undersown method, the green manure is sown at the same time as the main crop or a little bit later, if the green manure grows faster than the main crop.
  • The time gap between digging in the green manure into the soil and planting the next crop should not be longer than 2 to 3 weeks, so as to  prevent nutrient losses from decomposing green manure.
  • Green manures are worked in easily when the plants are still young; if they are taller or too bulky, it is preferabe to chop the plants. The best time to dig in green manures is just before flowering.
  • Green manures should be incorporated near the surface of the soil, not too deep: In heavy soils, dig in 5 to 15 cm; in light soils, 10 cm to a maximum of 20 cm!

 

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