Soils that are biologically active require the least addition of fertilizer and soil amendments. Biologically active soils provide the optimum conditions for growing plants. The living organisms in soil convert raw plant and animal residue materials into nutrients that feed plants. Soil needs a full range of micro and macronutrients for optimum health. Organic fertilizers and soil amendments can be used to correct imbalances that soil testing has identified.
“Good garden loam” is the soil recommendation most often heard from gardeners. Garden loam is the soil that requires the least use of fertilizer. It is soft and crumbly, drains easily, holds water easily, has few clods and does not form a crust, does not erode and has a high population of soil organisms such as worms, beneficial bacteria and insects. The organism population eats soil materials and produces nutrients, which reduces the need for fertilizer. Naturally healthy soils such as forests and grasslands continue their productivity without fertilizers through the decay process of plants and animals that grow on them.
Heavy clay-like soil has high mineral nutrient content but the texture can be too dense to allow plant root expansion. Clay soil has too little oxygen content. Organic matter is needed more than fertilizer. Fertilizer increases the nutrient content of soil and organic matter increases its ability to create an ongoing nutrient cycle. Organic matter added to clay soil also improves soil texture so roots can grow and expand. The organisms in organic matter (compost) bring oxygen in and help create space between soil particles. Adding sand also improves the productivity of clay soil.
Sand is the coarsest textured soil with the lowest organic content. Plant roots spread easily but there are not enough soil particles to create strong root structures. Sandy soil does not retain water well. It needs amendments such as fertilizers and compost. Compost provides needed soil structure as well as nutrient content. Compost may be thought of as a fertilizer and a soil amendment. Healthy soil has 2 to 5 percent organic content. Sandy soil has little erosion resistance and cannot be tilled easily.