It's hard to imagine that a cubic metre of earth can contain more species and more individuals than a rain forest, but that's the nature of healthy soil. Every acre is inhabited by about 2 tons of living organisms, while the number of individual organisms in that space is estimated to being far greater than the world's entire human population.
Since everything produced on a farm or in a garden ultimately derives from the soil, good soil management is the cornerstone of sustainable organic farming. Good management includes understanding what soil is, how it functions, how it stays fertile and healthy in ecosystems that are without human interference. Good crops become by-products of healthy soil and crop yields are maintained by sustaining the health of the soil.
Fertile soil is about 25% air and 25% water. The remaining 50% is made up of 3 components: tiny rock particles, organic matter and living organisms. Soils are technically described by their dominant rock particle size, and soils that are a good mix of 30 - 50% sand, 25-50% silt and 10-25 % clay are called loam, which has the best texture for plant growth.
A measure of alkalinity or acidity of the soil is the soil's pH rating, which has a huge effect on plant growth and nutrient availability. Crops with significantly high or low pH will have significantly lower yields and problems with nutrient deficiencies. A pH of less than 7.2 is acidic, while a pH of higher than 7.2 is alkaline. Each number represents a tenfold difference i.e. soil with a pH of 5 is 10 times more acidic than that of a pH of 6, and a 100 times more acidic than soil with a pH of 7. Most plants and soil life perform best at a slightly acidic or near-neutral pH (between 6.5 and 7.2). Ph of the soil can be fixed over a period of time with the use of calcium (more alkaline), magnesium and sodium (more acidic).
Organic matter (nutrient rich material derived from decaying matter) not only holds the mineral components together, but also provides nutrition for the plants. During their lifetime all organisms take up nutrients, and after death these organisms decay and release these nutrients (nitrogen, phosphorus and sulphur) in forms that can be absorbed by plants. Organic matter is slow to break down and therefore provides nutrition over a long period of time. It also acts as a sponge, holding 6 times its weight in water, which improves the soil's ability to hold water and nutrients. And because organic matter absorbs so much water, it reduces water run-off and erosion in all soils.
Sixteen chemical elements are known to be important to a plant's growth and survival. The sixteen chemical elements are divided into two main groups: non-mineral and mineral. Of these 16 nutrients that plants need, they extract 3 Non-Mineral Nutrients - carbon, oxygen and hydrogen - directly from the air. The remaining 13 Mineral Nutrients need to come from the soil.
A soil's parent rock is important, since it determines which of the 13 necessary plant nutrients might be naturally present in the rock particle component of the soil. Igneous rocks such as granite and basalt may contain as much as 12 of the 13 nutrients, while sedimentary rocks vary : sandstone-derived soils tend to be dry and infertile, while limestone-derived soils contain a lot of calcium and magnesium. If there is a pH imbalance or a chemical lock (eg phosphorus) on the nutrients, the plants will not be able to absorb them.
The Non-Mineral Nutrients are hydrogen (H), oxygen (O), & carbon (C). These nutrients are found in the air and water.
In a process called photosynthesis, plants use energy from the sun to change carbon dioxide (CO2 - carbon and oxygen) and water (H2O- hydrogen and oxygen) into starches and sugars.
These starches and sugars are the plant's food and since plants get carbon, hydrogen, and oxygen from the air and water, there is little farmers and gardeners can do to control how much of these nutrients a plant can use.
Mineral nutrients are dissolved in water in the soil and are absorbed by the plant through its root structure.
Depending on the environment and surrounding vegetation, the soil at any specific location could lack in these nutrients and inhibit the healthy growth of a plant. The use of specific fertilisers will add the necessary mineral nutrients back into the soil for absorption.
The mineral nutrients are divided into two groups: macronutrients (primary and secondary) and micronutrients.
The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients are usually the first nutrients to be lacking in the soil, as plants use large amounts for their growth and survival.
The secondary nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). There are usually enough of these nutrients in the soil so fertilisation is not always needed. Large amounts of Calcium and Magnesium will naturally be added with the application of lime to acidic soils. Sulfur is usually found in sufficient amounts from the slow decomposition of soil organic matter (an important reason for not throwing out grass clippings and leaves).
Micronutrients are those elements essential for plant growth but are only needed in very small (micro) quantities (sometimes called minor elements or trace elements). The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). Recycling organic matter such as grass clippings and tree leaves is an excellent way of providing micronutrients (as well as macronutrients) to growing plants.
Of the three major nutrients (Nitrogen, Phosphorus and Potassium), plants require nitrogen in the largest amounts. Nitrogen promotes rapid growth, increases leaf size and quality, hastens crop maturity, and promotes fruit and seed development.
Because nitrogen is a constituent of amino acids, which are required to synthesise proteins and other related compounds, it plays a role in almost all plant metabolic processes. Nitrogen is an integral part of chlorophyll manufacture through photosynthesis. (Photosynthesis is the process through which plants utilise light energy to convert atmospheric carbon dioxide into carbohydrates. Carbohydrates (sugars) provide energy required for growth and development).
Nitrogen-deficient plants exhibit slow stunted growth, and their foliage is pale green. Deficiency symptoms generally appear on the bottom leaves first. In severe cases, the lower leaves have a 'fired' appearance on the tips, turn brown, usually disintegrate and fall off. In contrast, too much nitrogen causes excessive vegetative growth, delays maturity, increases lodging, fosters disease and poses an environmental threat to surface and ground water.
Nitrogen deficiency generally stems from inadequate fertiliser application, denitrification by soil microbes, or leaching loss due to excessive rainfall. Leaching occurs most commonly in sandy-textured coastal plain soils during periods of excessive rainfall. Nitrogen is also lost through volatilisation from surface applications during periods of hot, dry weather.
Nitrogen deficiency can be corrected with an application of nitrogen fertiliser. Crop response to fertilisation with nitrogen is generally very prompt, depending on the source of nitrogen, stage of plant growth, rainfall and temperature.
Normal plant growth cannot be achieved without phosphorus. It is a constituent of nucleic acids, phospholipids, the coenzymes DNA and NADP, and most importantly ATP. It activates coenzymes for amino acid production used in protein synthesis; it decomposes carbohydrates produced in photosynthesis; and it is involved in many other metabolic processes required for normal growth, such as photosynthesis, glycolysis, respiration, and fatty 2 acid synthesis. It enhances seed germination and early growth, stimulates blooming, enhances bud set, aids in seed formation, hastens maturity and provides winter hardiness.
Phosphorus deficient plants are characterised by stunted growth, dark green leaves with a leathery texture, and reddish purple leaf tips and margins. Deficiency symptoms may appear when soil phosphorus levels are adequate. When soil is cooler than 15 degrees C, less phosphorus is available for plant uptake, whether or not an adequate amount is present. Symptoms related to cool weather generally disappear as soil temperature increases.
Since phosphorus does not leach in mineral soils, any problems associated with surface water contamination can be attributed to soil erosion.
Phosphorus deficiency symptoms generally occur in soils with a low phosphorus content. An application of phosphate fertiliser based on rates recommended by a soil test will correct this problem. Phosphorus occurs in organic fertilisers (manures); inorganic blended fertilisers; and high phosphate materials such as mono-and diammonium phosphate (11-48-0 & 18-46-0), triple superphosphate (0-46-0), and liquid mixes such as 10-34-0.
Potassium has many functions in plant growth. It :
Although not an integral part of cell structure, potassium regulates many metabolic processes required for growth, fruit and seed development. Many vegetable and fruit crops are high in potassium, which is vital for animal and human nutrition. Indeed, the health and survival of man and beast is dependent on potassium.
The lowest amount of potassium is found in sandy soils where it is subject to leaching. The higher concentrations are found in the clay. High potassium is also found in areas where animal and poultry wastes have been applied.
Potassium-deficient plants exhibit chlorosis (loss of green color) along the leaf margins or tips starting with the bottom leaves and progressing up the plant. In severe cases, the whole plant turns yellow, and the lower leaves fall off.
As with other nutrients, lack of potassium causes stunted plants with small branches and little vigor. An application of potassium fertiliser will correct a deficiency.
Potassium can be obtained from fertilisers such as potassium nitrate (13-0-44), muriate of potash (0-0-60), potassium sulfate (0-0-50), or a mixture of potassium and magnesium sulfate (22% K2O).