Soil aggregations

Soil aggregations

Soil aggregations:- soil aggregations are ‘clumps; of oil particles that are held together by moist clay, organic matter(such as roots), by organic compounds (from bacteria and fungi) and by fungal hyphae. Aggregates vary in size from about 2 thousandths of a millimeter across up to about up to about 2 millimeters across, and are made up of particles of varying sizes. Some of these particles fit closely together and some do not and this creates spaces of many different sizes in the soil. These spaces, or pores, within and between soil aggregates are essential for storing air and water, microbes, nutrients and organic matter. Soil with many aggregates are called ‘’well-aggregated’’. Such soils are more stable and less susceptible to erosion.

There are two ways that bacteria could be involved in soil aggregation. One eay is by producing organic compounds called polysaccharides. Bacterial polysaccharides are more stable than plant polysaccharides, resisting decomposition long enough to be involved in holding soil particles together in aggregates . The other way bacteria are involved in soil aggregation is by developing a small electrostatic charge that attracts the electrostatic charge on day surfaces, bringing small aggregates of soil.

Fungi grow in long, threadlike structures, called hyphae. The amount of aggregation in the soil has been found to relate to the length of fungal hyphae in the soil. Fungi help to from aggregates in the soil by enmeshing soil particles with their hyphae and forming cross-links between soil particles. Mycorrhizal fungi and fungi that colonies fresh organic matter are believed to be the most important for assisting with stabilization of soil particles into aggregates.

Significance of soil aggregation:- crop growth is often constrained by poor root development, by slow water infiltration and water movement through the soil, and by poor soil aeration. These constraints are often associated with poor soil porosity. Soil aggregation is important to developing and maintaining good soil porosity and hence to good root growth and to movement of soil water and gases, with more soil in water stable aggregates, it is expected that:

·        The rate of water infiltration and percolation will increase.

·       Soil crushing will be less- which improved root penetration and access to soil moisture and nutrients and emergence of seedlings.

·       Resistance to the splash effect of raindrops will increase and soil erodibility will decrease.

·       Runoff will decrease, making more water available to the crop.

The importance of soil physical properties to crop growth, including soil aggregation properties, is often under- estimated by producers, and practical exercises can be useful in conveying information on the importance of these properties.

Soil consistence

Soil consistence is defined as ‘’the resistance of a soil at various moisture contents to mechanical stresses or manipulation’’. it combines both the cohesive and adhesive forces, which determine the ease with which a soil can be reshaped or ruptures

Adhesion Molecular attraction that holds the surfaces of two substances in contact eg. Water and soil particles.

Cohesion Holding together: force holding a solid or liquid together, owing to attraction between like molecules. Decreases with rise in temperature

Soil consistence is described at three moisture levels namely ‘wet’, moist and dry.

1.    Wet- consistency is denoted by terms stickiness and plasticity

Stickiness is grouped into four categories namely non-sticky, slightly sticky, sticky and very sticky.

Plasticity of a soil is its capacity to be moulded (to change its shape depending on stress) and to retain the shape ever when the stress is removed. Soils containing more than about 15% clay exhibit plasticity pliability and the capacity of being moulded. There are four degree in plasticity namely non plastic, slightly plastic, plastic and very plastic.

2.    Moist soil -moist soil with least coherence adheres very strongly and resists crushing between the thumb and forefinger. The different categories are:-

·       Loose-non coherent

·       Very friable- coherent, but very easily crushed

·       Friable- easily crushed

·       Firm- crushable with moderate pressure

·       Very firm – completely resistant to crushing (type and amount of clay and humus influence this consistency)

3.    Dry soil – in the absence of moisture, the degree of resistance is related to the attraction of particles for each other. The different categories are :-

·       Loose- non coherent

·       Soft- breaks with slight pressure and becomes powder

·       Slightly hard- break under moderate pressure

·       Hard- breaks with difficulty with pressure

·       Very hard- very resistant to pressure

·       Extremely hard- extreme resistance and cannot be broken