Using green manures for fertilization instead of synthetic fertilizers is a classical organic method. Green manure crops are grown before a main crop. They are usually slashed two to three weeks before sowing the main crop and ideally before or at the flowering stage, when they have accumulated a maximum of nitrogen. They are worked into the topsoil to decay and to feed a subsequent crop with their nutrients. Alternatively, fresh plant material from shrubs and trees or grass cuttings can be worked into the soil one to two weeks before planting.