Plants require food, water and air to survive and thrive. Through the process of photosynthesis, plants convert water and air into sugars and starches, two of their food requirements. Plants also require a host of other nutrients. A source of these nutrients is soil. As plant roots burrow into the soil, they seek out and take up the nutrients within the soil.
Plants require macronutrients and micronutrients. Nitrogen, phosphorous and potassium are macronutrients and plants need these in abundance. Micronutrients are so named because plants require smaller amounts of these minerals. Boron, copper, iron, zinc, chloride and manganese are examples of micronutrients.
Each macronutrient fulfills specific needs. Phosphorous is directed to cellular growth, allowing the plant to grow flowers, produce seeds and establish a healthy root system. Potassium aids in resistance to pest invasion and disease. Nitrogen promotes leafy growth, necessary to protect the flowers and fruits set by the plant from overexposure to sunlight and other natural elements.
Healthy soil contains decomposing organic matter, microbial life forms and oxygen. Soil formation may take hundreds of thousands of years as rock is weathered and organic matter, such as felled trees, decompose naturally. Microbes aid in the decomposition, both as catalysts and compost.
Within a backyard garden, soil is already present. The soil, however, may have an insufficient supply of nutrients or the nutrients are still in organic form and unavailable to plants. Inorganic fertilizers added to soil allow for immediate uptake of nutrients by the plants.
Both macro and micro nutrients need to be in an inorganic state for uptake through root systems. Inorganic does not, in this instance, mean synthetic. It means the nutrients need to be in the form of mineral ions, or a non-living state. In an organically structured soil, the microbes will work to convert the nutrients into an inorganic state, thus allowing the plant roots to access them.
The presence of organic matter, both in the form of microbial life forms and decomposing plant and mineral matter, aid in the development of nutrients and form a foundation for a healthy soil. The presence of these elements creates in soil the ability referred to as "Cation Exchange Capacity" or CEC. This ability allows the soil to prevent nutrients from being washed away during watering or rainfalls. A loamy soil, with a roughly equal mix of sand, silt and clay, would have a high degree of CEC. This results in plant roots having a consistent access to necessary nutrients.