Plants and soil are symbiotic, i.e. they are interdependent and require a good balance of give and take. Soil nutrients are absorbed by plants and then recycled back into the soil in the form of plant residue. When more nutrients are removed from the soil than can be replenished by the application of organic and inorganic matter, the soil gradually becomes poorer and unable to support the growth of healthy plants.
Plant residues must decompose in order to release plant nutrients. The microorganisms, worms, fungi and bacteria found in soil are responsible for decomposing organic matter and producing humus (the fertile part of the soil). Most of the humus is found in topsoil. Subsoils contain less organic matter and are therefore less fertile.
Some soil nutrients react slowly and are stable (e.g. phosphorus). Others are absorbed quickly and used up by plants (e.g. nitrogen). These must be replaced continually in order to keep a good balance between supply and demand.
Excessive use of chemical fertilizers is discouraged. Home garden managers are instead encouraged to use natural soil additives, such as different types of manure, in order to maintain the natural balance between soil and plants. Natural fertilizers such as green manure, compost and animal manure provide plant nutrients but also improve the structure of the soil.
Allowing soil to rest and restore its natural fertility is called fallowing. Fallowing is practical, however, only if land is available. When land is left fallow, it is important to prevent weeds from seeding on it. In this case, the use of green manure or cover crops during fallowing is recommended.
In addition, crop rotation of deep- and shallow-rooted crops, and crops with different nutrient requirements, is also recommended.
