<P> Many (tropical) plants use toxins to protect themselves . Cassava, one of the most important tropical food crops, produces cyanide upon ingestion unless processed to remove / reduce the cyanide content . Other plants are high in oxalates (the agent that binds calcium to form kidney stones); castor beans are the source of ricin, one of the most powerful poisons in existence; and velvet beans contain 3.1 - 6.1% L - DOPA, which can be toxic in large quantities . The list of toxic plants is long, but toxicity does not always mean a particular plant should be avoided, the knowledge needed to render toxic plants safe to use already exists in most communities . </P> <P> The contents of a bag of commercial fertilizer is described in terms of NPK - nitrogen (N), phosphorus (P) and potassium (K); with nitrogen being the main component of most commercial fertilizers . </P> <P> Oxygen is only a small part of the air; the largest component of air is nitrogen . Nitrogen is the main building block of protein; muscle in mammals and plant tissue in plants . If the level of nitrogen in the soil is increased, plant growth can be significantly increased . Legumes are a group of plants that interact with bacteria (rhizobia) in the soil to fix nitrogen from the air, and deposit the nitrogen into the soil where it is available for other plants to use . The nitrogen deposited by legumes can be readily converted into larger harvests . </P> <P> Green manures are plants grown to improve the soil, suppress weeds, limit erosion, and--when legumes are used--to increase the nitrogen content of the soil . The most common type of green manure used in the tropics is velvet bean . It produces a thick blanket of vines and leaves that in addition to infusing the soil with nitrogen also smother most weeds . In addition it has reasonable tolerance to drought, low soil fertility, and highly acidic soil . Alternatives to the velvet bean include the lablab bean, the jack bean and for use above 500 m altitude, the scarlet runner bean . </P>

Characteristics of tropical agriculture and how they affect the environment