Ideas for community gardens < gardening

GROW YOUR OWN FERTILISER WITH NITROGENUS SHRUBS

Legumes, whether peas and beans or shrubs such as Pigeon Pea (Cajanus cajan), are living fertiliser factories that help our vegetables, herbs and fruit grow strong and healthy. They take nitrogen from air trapped in the soil and convert it into fertiliser.

There are two methods of using legumes in the community garden. In crop rotation, leguminous annual vegetables such as beans or peas (bush or climbing varieties that go through their life cycle of growth from seed to seed within a year) - are planted first. After fruiting, gardeners turn the leaves and stems into the soil while others compost it instead, returning the compost to the garden. Others use the residue as a mulch, leaving it on the surface of the soil.

Leafy greens follow the legumes in the vegetable beds, being in turn replaced by fruiting crops such as capsicum and tomato, then by root crops such as potato or carrot. This is the sequential rotatipn of crops of different types through the garden bed to take advantage of different nutrients and to discourage soil-borne disease.

A second approach to using legumes, known as slash and mulch, is employed in Northey Street City Farm's market garden and has been adopted by developing country farmers to avoid the high cost of imported fertilisers.

In this method, perennial (lasting longer than two years) leguminous shrubs such as Pigeon Pea or Croatalaria are planted around the perimeter of the garden or across it in alleys, with the vegetable crop grown between the alleys. This is known as alley cropping.

Periodically, according to the rate of growth of the legume, the foliage is slashed and placed on the garden where it decomposes, releasing nitrogen to the root systems of the main crop. When the shrub has regrown it is again slashed to a metre or less above the ground.

In the Solomon Islands, Gliricidia sepium, which grows into a large tree if not slashed, is the preferred legume. The rate of regrowth of leguminous shrubs between slashing may be too low in cool temperate regions and crop rotation may be a more effective technique.

At Northey Street, the leguminous shrub Pigeon Pea is used as a slash and mulch crop and has been planted around the edge of the market garden. It is a shrub growing to around three metres that produces a hard, brown seed is in India to make a food known as dhal.

At the city farm, the shrub is grown to near its full height before its woody growth is slashed to about a metre. The foliage is stripped from the cut branches and used as a nitrogen-rich mulch on the garden. Pigeon pea grows back, coppicing from the cut branches to form a bushy shrub.

Leguminous shrubs and trees are 'pioneer' species that colonise cleared land, so they grow readily and rapidly. Community gardeners should take care in the selection of legumes to avoid introducing rampant species that could displace native vegetation or crops. Species selection depends on climate and soil.

PAGE UPDATED... Thursday, 29 September 2005