Much like the fertility-building clovers and vetches long-used by farmers, nitrogen fixing trees and shrubs such as alder trees and gorse bushes work with bacteria in the soil to convert nitrogen into a form useful to plants. Though nitrogen is fixed in the roots or taken up from soil, it accumulates in the leaves which are then harvested and added to cropland to fertilise the soil. Unlike in the production of manufactured fertiliser, supplying nitrogen via PGMs does not cause carbon dioxide emissions, and unlike traditional green manures grown in rotation, PGMs can be grown on marginal land e.g. steep slopes or flood-prone areas, making efficient use of farm resources. PGMs provide organic matter so are good for soil health. They can be added at any time (fresh, dried or pelleted) to match with the crop nutrient needs, so increasing efficiency and reducing nitrogen pollution.