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Forms of Water in Soil; Function of Soil for Plant Growth
[13th century. Via Anglo-Norman, “piece of land” < Latin solium “seat,” by association with solum “ground, soil”]
- The top layer of most of the Earth’s land surface, consisting of the unconsolidated products of rock erosion and organic decay, along with bacteria and fungi. – (LoF)
- (from Latin “Solum” which means “ground/ soil”) A natural body on surface of earth, in which plants grow, composed of organic and inorganic matter. It consists of fragmented and partly weathered rocks and minerals, organic matter, water and air in varying proportions. It has more or less distinct layers due to influence of climate and living organisms. – (LoF)
FUNCTIONS OF THE SOIL FOR PLANT GROWTH:
The soil serves three main functions in plant growth: to anchor the plant; to serve as a reservoir and supply of mineral nutrients and to provide a water supply and reservoir.
FACTORS AFFECTING THE ABILITY OF THE SOIL TO ANCHOR PLANTS:
- Sandy and gravelly soils with little internal cohesion may not provide a good anchoring medium. Soils on steep slopes that are subject to plastic flow may disrupt rooting and also provide poor anchorage.
- Shallow soils underlain by a hard pan or rock, or effectively shallow because of poor root growth may provide poor anchorage. Root growth may be restricted to surface layers by poor aeration caused by water saturation.
- Genetic. Species differ in their rooting patterns, and this affects how well anchored they are some species develop shallow, spreading roots, others taproots, others deep spreading roots, and still others have little root development.
- Environment. The environment in which a plant develops affects rooting. Trees that develop in the shade generally have less root development than tress that develops in open sunlight. Trees that develop as members of an even age stand have less root development than those that develop
These environmental factors become apparent after a forest is clear cut when wind throw of trees at the boundary of the cutting block may occur because such trees are often shallow rooted having developed as members of an even aged stand or in the shade.
THE SOIL AS A MINERAL NUTRIENT RESERVOIR AND SUPPLY:
- The ability of the soil to supply minerals depends in part upon their parent material. Residual soils that developed in place from granitic rocks tend to be less fertile because of low base ion (e.g. Ca, Mg) content of granitic rocks. Soils that develop from calcareous rocks and basalt tend to be more fertile because of the high base ion content of these rocks.
- Most soils have not developed in place, but on parent materials that have been transported by wind (loess, dunes), gravity (colluvial), water (alluvial), ice (moraine, till), lake bottom (lacustrine), or ocean bottom (marine). The mineral supplying power of these soils is a composite of the mixture of transported parent materials.
- The actual or effective (e.g. limited by water table, etc.) soil depth and root penetration throughout the soil determines the volume of soil available to the plant. The greater the soil volume occupied by roots the greater the minerals available.
- Cations such as K+, Ca**, and Mg++, are prevented from being leached from the soil by being attracted to clay particles and organic matter. Clay, and to a lesser extent organic matter, possess a cation exchange capacity. That is, they can bind one ion, but this ion can be exchanged for another, e.g., a H+ can replace a K+ bound on the exchange complex. Although the chemistry of cation exchange is beyond the content of these lessons, it should be recognized that the greater the clay content the greater the ability of the soil to store mineral nutrients by preventing them from being leached, and the more fertile the soil. Thus, loam soils are more fertile than sandy soils.
- The organic matter in the soil functions as a reservoir of minerals that is made available for absorption after organic matter decomposition.
- The rate of organic matter decomposition varies greatly with climate and vegetation type. In grasslands decomposition of organic matter to humus is rapid, but the decomposition of humus less so. The dark color of grassland soils is due to their high humus content.
- Organic matter decomposition in humid tropical forest soils is so rapid, and leaching so intense, that the nutrient cycle requires minerals be transferred back into twigs efficiently before the leaf fall. Mycorrhizae fungi at the soil surface absorb the minerals released by decomposition within a few centimeters of the surface before the minerals can be leached.
- The litter of conifers tends to be acidic and contain tannin, terpene, resins, and other substances that resist bacterial decomposition. The cool climate and short growing season where some of these species are found in also discourages bacterial activity, so much of the decomposition is by less efficient fungi. As a result minerals remain in the undecomposed litter. This is a transfer of the mineral capital of the forest soil to soil litter, where it no longer is available for nutrient cycling, and the forest soil fertility is reduced as a consequence.
- A common way in which these minerals are leased is by fire. Burning removes the acidity and antibiotic chemicals, encouraging bacterial activity on the remaining organic matter, and releasing “pot ash” by fire. Ash contains inorganic minerals that are available for absorption by roots.
FORMS OF WATER IN THE SOIL:
- The water content of a soil is measured as the amount of water lost when the soil is dried at 105 degree. Water may occur in soils in three forms mainly:
- Gravitational water moves down through the soil in response to gravity. If a soil is flooded with 3 cm of water, a wetting front will move through the soil in large pores to a depth of about 20 cm, filling capillary spaces as the front moves. There are enough capillary spaces in a common soil to absorb 3 cm of water in 20 cm of soil, so when the wetting front reaches the 20 cm level, no further downward movement occurs because all of the gravitational water has been used to fill capillary spaces.
- Capillary water is held in the soil at tensions greater than -0.03MPa, the approximate force of gravity.
- Unavailable water occurs as a film on colloidal and larger particle surfaces (hygroscopic water), or in capillary pores so small that it is held at forces greater than -l.5MPa.
- Some plants, especially some that occur in arid regions, can extract water unavailable to crop species, but as each component is removed at one.force,e.g. -2.0MPa, the water remaining is held at nearly exponentially increasing force.
- Available water is that portion of soil water held against gravity at a force greater than -0.03MPa, but less than -l.5MPa, the generally accepted permanent wilting percentage for crop species.
- Soils differ in Water Holding Capacity (WHC). Most of the difference can be accounted for by soil texture and the organic matter content of the soil. Soil texture determines the distribution of pore sizes. Sandy soils contain large pore spaces, and most of the water that falls on sandy soils percolates through it as gravitational water. Clay soils, at the opposite extreme, contain a large portion of capillary spaces, and thus have a high water holding capacity. Clay soils also have a large portion of capillary spaces so small that the water is unavailable (i.e. held at tensions greater than -l.5MPa), and a large amount of water still remains in such soils after plant roots have extracted all that-they can. Soil organic matter also contains a large amount of capillary space which can act as a sponge in soaking up and maintaining water for future plant absorption.
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