Study outline:
Soil definition and the factors of plant growth.
Plant roots and soil relations.
Soil fertility and soil productivity.
Soil texture.
Soil is a substance that has a lot of meaning for humanity. Ancient civilizations had a close bond with the soil. It is stated in the Genesis: “and the Lord God formed Man of dust from the ground”.
Even today, most of the world’s people are tillers of the soil and use simple tools to produce their food and fiber. The concept of soil as a medium of plant growth, though born in antiquity, still remains one of the most important concepts of soil today. Dokuchaev V.V. (1846-1903) defined soil as the surface (“Daily”) horizons of geologic rocks transformed by the joint action of air, water, and different organisms, living and dead and as the function of five natural processes of soil formation: climate, organisms, parent material, landforms, and time.
1. Soil definition and the factors of plant growth.
It does not require professional training to view the soil as a mixture of mineral and organic particles of varying size and composition. The particles occupy about 50% of the soil’s volume. The remaining 50% is pore space. The pores are of varying shapes and sizes. They contain air and water. Pore spaces are avenues for the extension and growth of roots. Ukrainian soil scientist O.N.Sokolowsky depicted the soil factors of plant growth as a hexagon:
The three essential things that plants absorb from the soil and use are (1) water that is mainly evaporated from plant leaves, (2) nutrients and (3) oxygen for root respiration. One of the most obvious functions of soils is to provide support for plants. Another hexagon represents the factors controlling the growth of higher plants:
Water Nutrients
Mechanical
support Air
Light Heat
The soil is an agent in supplying these factors either wholly or in part. Plant growth is dependable upon a favorable combination of these factors. But the growth and fruition of plants can be no better than that allowed by the most limiting of the factors.
Plants need certain essential nutrient elements to complete their life cycle. At least 16 elements are currently essential for the growth of vascular plants. C, H and O are obtained from air and water and combined in photosynthetic reactions. The remaining 13 elements are obtained largely from the soil. These are macro- (N, P, K, Ca, Mg, S) and micro- (B, Cl, Cu, Fe, Mn, Mo and Zn) nutrients. Co is a micronutrient that is needed by only some plants.
Water requirement of plants. A few hundred to a few thousand grams of water are required to produce 1 gram of dry plant material. Approx. 1% of this water becomes an integral part of the plant. The growth of most economic crops will be curtailed when a shortage of water occurs, even though it may be temporary. But! When soils become water saturated, the water excludes air from the pore space and creates an oxygen deficiency. The need for oxygen may require the removal of excess water from soils.
Oxygen requirement of plants. Roots have openings that permit gas exchange. Oxygen from the atmosphere diffuses into the soil and is used by root cells for respiration. The carbon dioxide produced by the respiration of roots and microbes diffuses through the soil pore space and exits into the atmosphere. Respiration releases energy that plant cells need. Some plants, such as water lilies and rice can grow in water-saturated soil. They have morphological structures that permit the diffusion of atmospheric oxygen down to the roots. Plants in water culture require aeration of the solution. Aerobic microorganisms require O2 from the soil atmosphere to decompose organic matter and convert unavailable nutrients in organic matter into ionic forms that plants can reuse.