Friday, July 15, 2011
Growing plants take up from the soil certain minerals, which are as essential to their growth as sunlight, air and water. Fortunately, there is a supply of most mineral foods already in the soil. How did they get there?
Soil formation is a gradual process. Rock material, under the influence of weathering, gradually crumbles into fragments of varying size. Temperature changes, the leaching action of rain, oxidation and organic acids produced from the decay of humus, all contribute to the disintegration of the rock fragments into smaller particles. Microorganisms and higher plants gain a foothold in this partially disintegrated material. The rock residues are enriched by the addition of the remains of plants and animals, and the resulting product is then recognized as soil.
It is easy to conceive of soil as a mass of particles of varying degrees of fineness. These particles are separated by spaces filled partly with air and partly with water. Each particle is surrounded by a water film. Within this film soil microorganisms are active. Bacteria are the most numerous constituents; however fungi, protozoa, yeasts and algae are also present in varying numbers. Aside from these, nematodes, earthworms and insects are more or less common and important.
There is a direct relation between the amount and quality of the organic matter in the soil and the numbers of microorganisms. Since a relatively large proportion of the organic matter is present in the upper six inches of the soil, one should find a correspondingly high proportion of microorganisms in the surface soil. There is a gradual increase in the number of microorganisms from early spring to summer and a decrease from late summer to mid-winter.
The average acre of land contains, in the humid regions, about twenty tons of organic matter. This is made up of the remains of plants, microorganisms, insects and animals. The organic matter is constantly under attack by the microbial life of the soil, most of which depend on it for food. Being largely of plant origin, the organic matter is made up of carbohydrates, proteins, fats, organic acids and other compounds found in plants. Ultimately, organic residues in the soil are reduced to carbon dioxide, water and minerals.
Very considerable quantities of nitrogen are found in the surface soil. These nitrogen compounds originated from the air. Assuming that the surface soil of each acre will weigh, when dried, approximately two million pounds, we find an equivalent of one ton of nitrogen per acre. Good loam soils are usually expected to contain about two tons of nitrogen per acre. This is present in organic residues. When the latter are attacked by microorganisms, ammonia and nitrates are formed. The ammonia and nitrates are in part taken up by vegetation and in part leached into the soil water or the atmosphere.
Plants find a large portion of their food in the soil solution. Indeed, it is only in so far as the substances found in soils become water soluble that they are able to contribute to the growth of plants. Materials removed from the soil solution by plant roots are gradually replenished from the soil reserve of inorganic and organic compounds. The rate at which these pass into solution is determined by the effectiveness of the microbiological machinery found in soils. The microorganisms, in their turn, are affected by environmental conditions, such as temperature, the supply of oxygen, and particularly the quantity and quality of the organic matter contained in the soil.
The mineral and organic constituents of the soil, along with air, water and sunshine, are inextricably intertwined in all organic life. There is no demarcation line between mineral and organic substances in the processes that drive plant and animal life on earth. Green plants make no distinction between elements derived from the minerals of the soil and mineral nutrients supplied by man. The application of both organic and inorganic nutrients to the soil is in harmony with the natural processes involved with green plants existence on this planet.Back