All plants require nutrients from the soil to grow, as well as water and sunlight. These nutrients are removed when the crop is harvested, and leached from the soil by rain. About 25 nutrients are essential for plants. Some of these nutrients are only needed in minute quantities, and are rarely in short supply. Others are needed in larger quantities and must be added to the soil to maintain its fertility (as fertiliser, manure or compost). In most cases, the most important nutrients are Nitrogen (nitrate or ammonium), Phosphate and Potassium in that order.

Legumes are plants with the ability to obtain their Nitrogen from the air, thanks to bacteria which live inside their roots. This allows legumes to grow on soil with little or no available nitrogen in it (in fact they thrive in this kind of soil, as nothing else can grow there). Legumes will enrich the soil, especially if they are dug into the soil or composted when they are still green.
Legumes include clover, trefoil, vetch, broom, gorse, peas, beans, lupins and many others. Our garden has a laburnum tree which covers the garden with seedlings and leaves every year. This has made our soil extremely fertile.
Garden centres sell weedkillers to remove clover from lawns. I think this is extremely misguided. Clover thrives when soil nitrogen is low. It grows in patches of ground where there isn't enough nitrogen for grass to grow green and healthy. It doesn't usually choking out the grass in the way that say dandylions do. Removing the clover will actually mean that more nitrate fertiliser will need to be added in future to keep the lawn green. Leave the clover to grow freely, and the lawn will always be green. Not only that, but the cuttings, composted or mulched can act as a supply of nitrogen for the rest of the garden. Clover is quite an attractive plant after all, and it feeds bees. White clover is an essential element of most British organic farms

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