Organic soil fertility in your organic garden is based on the principle "feed the soil not the plant." This approach is carried out through practices that increase organic matter, microorganism (biological) activity, and soil nutrients. Your ultimate goal is healthy soil, enhanced biological activity, great soil structure, and readily available soil nutrients.
Organic Matter and Humus
Increasing the organic matter in your soil is an essential part of organic gardening. A healthy soil has lots of biological life and these organisms need organic matter to survive and thrive. Organic matter increases fertility by feeding the biological life that in turn helps with decomposition of organic matter, replenishment of soil nutrients, humus formation, promotion of root growth, and nutrient uptake. (see Organic Gardening Compost Benefits)
Humus is an important by-product of organic matter. Humus improves fertility of the soil and plant growth. Humus results from decomposition of all the organic matter and is the glue that holds all the soil particles together, and helps prevent erosion and increases a soil’s moisture holding ability.
Organic Matter and Soil Nutrients
As the organic matter is forming in your soil (or compost), important "macronutrients" are forming in your soil such as nitrogen (N) and phosphorus (P). Another important macronutrient is Potassium (K). Potassium is made more available with a rich amount of organic matter.
Nitrogen is important for vigorous vegetative growth and the plant’s dark green color. Unless it is their natural color, when your plants become pale green or yellow, this is a sign of nitrogen deficiency.
Phosphorus provides overall plant quality, stem strength, root growth, and normal plant maturity.
While potassium is not a component of any part of the plant compounds, it plays an important role in many aspects of plant life and soil fertility. For example, potassium is needed to sustain plant growth and reproduction. It also increases root growth and improves drought tolerance.