Nitrogen fixers & Green Manure

Dear Margaret, When acacia branches are cut & broken up in a mulcher do the leaves provide more nitrogen than the mulching of any other tree branches? Do casuarinas fix nitrogen? And is it better ( in terms of the nitrogen being available to the plant) to put the mulch straight on the ground in the vegetable garden, or to compost it first?

Like the majority of the Legume family (carob trees are an important exception) acacias develop nodules on their roots which, in the presence of suitable bacteria, 'fix' nitrogen - it needs fixing, because it starts out as a gas & is very volatile - & store it, thus ensuring a steady supply of this element essential for the manufacture of proteins. Casuarinas (She-oaks & Swamp-oaks) also have this useful talent. But this does not mean that the foliage of such plants is necessarily any richer in proteins, or nitrogenous material, than other green foliage. Fresh lawn clippings, for instance are very high in nitrogen, because people fertilise their lawns so heavily. So the nitrogen content of green foliage depends on the amount of nitrogen that is available to the plant while it is growing.

As soon as the foliage is broken up & starts to dry out, nitrogen begins to escape back to the atmosphere. (About 80% of air at sea level consists of nitrogen). So we need to extract as much value from this nitrogen as possible before it gets away. Trap it by incorporating the chopped foliage into the top layer of soil immediately, or compost it as soon as possible. Or place the fresh green material directly on the soil, then cover thickly with mulch. Another strategy is to 'drown' fresh green material in a well-covered drum of water for about 6 weeks, then strain & use as liquid manure.

The amount of nitrogen applied to any plant needs to be carefully tailored to its requirements. As you are no doubt aware, fresh lawn clippings, or urine, will burn quite strong vegetation. So, while mulching with fresh green stuff is fine for established trees & shrubs, composting it first is safer for seedlings & delicate plants. The liquid fertiliser should be diluted on a common sense basis before watering into the root area - the colour is a fair guide. Say about 1 part to four of water for most things, but for seedlings & as a foliar spray, the colour of weak tea is just about right.

When legumes die, the nitrogen stored in the nodules is released into the soil. This is why green manure crops contain a high percentage of annual legumes, such as field peas or tick beans, usually mixed with field mustard, rape or cress, which provide sulphur. This is an excellent way of enriching & improving soil . Simply sow the green manure crop thickly where required - over a whole bed, or between the rows of trees or perennials, in early autumn. When the plants have made good growth, but before they flower, mow or chop them down & incorporate directly into the soil where they stand, then cover with a layer of mulch. The roots, which have opened up the soil, remain there as humus, nitrogen is released from the roots of the legumes & the rich green foliage supplies a well-balanced mix of nutrients & organic matter, which actually warms the soil as it decomposes. This gives the garden a wonderful boost, is less work than composting, & much less expensive than buying in manure or fertiliser.


Note: In researching & answering questions about earthwise living, Margaret offers information, opinion, & personal experience, but no quick fixes! Readers should evaluate these offerings in the context of their own situations; they are suggestions, *not* recommendations. Any responsibility for their implementation rests *solely* with the reader.

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