This advice is based on chemical tests of the soil and the growing plants. The correct use of fertilizer is very important because the cost of the fertilizer must be balanced against the benefit it produces. Guano Natural Fertilizers Legumes, farmyard manure, and other farm wastes were once the only fertilizers used by farmers. They contain much smaller amounts of nutrients than do chemical fertilizers of equal weight, but they help the soil retain moisture and resist erosion. In industrialized nations they now are used more for these purposes than as sources of nutrients.
Manure is by far the most important organic fertilizer.
It is the mixture of solid and liquid animal wastes and the soiled bedding straw that accumulates in stables and animal enclosures. Every year livestock on farms in the United States produce at least 1 billion tons of manure. Some is spread on fields, but most of it is dried, ground, and sold for use in small gardens. Others are made from the wastes from meat-packing houses and from fishes that people find undesirable as food. The Learning Company, Inc.