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| - The legume family (leguminosae) are characterised by bearing fruit formed from a single carpel (or pod) that liberates its seeds by splitting open along both sutures into two parts. As well as beans and peas, leguminous plants also include trees such as the Siberian Pea Tree and laburnum, also gorse, broom, clovers, vetches, cowpeas, etc.
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| abstract
| - The legume family (leguminosae) are characterised by bearing fruit formed from a single carpel (or pod) that liberates its seeds by splitting open along both sutures into two parts. As well as beans and peas, leguminous plants also include trees such as the Siberian Pea Tree and laburnum, also gorse, broom, clovers, vetches, cowpeas, etc. Leguminous crops are active nitrogen gatherers, potentially furnishing more nitrogen to the soil than manures and fertilisers combined. This is achieved through the process of nitrogen fixation, wherin rhizobium bacteria form symbiotic relationships with the legumes, and are able to convert nitrogen from the air into a form usable by the roots of the plant. These bacteria infect the plant, colonising and forming visible swollen nodules on the roots. These store nitrogen, an essential protein building constiuent for healthy plant growth, for later release, making it available to following crops. However as Robert Kourik points out, in order to optimally use legumes as part of a soil fertility growing program it is important to understand the cycle of the plant; "Many of the pioneer plants, those benevolent colonisers of disturbed, damaged or infertile soils, are legumes. During the growing season, the nitrogen gathered by the legume's roots is 'banked' in a temporary savings account in the stems and leaves of the entire plant. When the plant flowers, the demand for nitrogen overwhelms the roots, and the plant draws on its savings account to make seed pods. Researchers have found that just before flowering as much as 60% of a legume's nitrogen is in the leaves, only half of it from root nodules. After seed pods are formed a mere 8% remains in the leaves, whilst 70% of the plant's total nitrogen has accumulated in the seeds. The roots and remaining nodules, after the leaves have matured, have even less nitrogen than the leaves- as little as 3 to 6% of the total accumulated by the plant. In short, legumes offer little nitrogen in a form other plants might use- they hoard it for the next generation." ('Designing And Maintaining Your Edible Landscape', Robert Kourik, 1986). Bearing this in mind, an increase in available soil nitrogen is best optimised by cutting back the growing plant just before reaching the flowering stage rather than allowing it to crop and form seeds. Sowing a leguminous green manure such as vetches (Winter Tares), field beans, crimson or white clover and hoeing it into the soil surface at this stage is therefore an efficient way of increasing soil fertility within a rotation scheme. Not only do their root systems aerate the soil, when hoed in they add organic matter, bring minerals to the surface and make nitrogen instantly available without the problems of 'nitrogen robbery'. In regions that have not previously grown leguminous plants the necessary rhizobium bacteria necessary to form the nitrogen fixing root colonies might be lacking. In this case the appropriate strains can be added to the soil, or the legume seed can be coated or 'inoculated' with a culture which can be bought commercially. Inoculation can increase the yields of peas and beans by as much as 67% in areas where rhizobium bacteria were previously low.
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