Porespace

Soil component

1. Pore space The amounts of water and air present in pore spaces vary and depend upon their relative amounts. If the amount of water is more, air decreases and vice versa. The amount of pore space depends upon the arrangement of solid particles . Pore spaces directly control the amount of water and air in the soil and indirectly influence the plant growth and crop production. The quantity of pores in a soil and their size distribution are useful general indicators of the physical condition of soils. Apart from quantity and distribution, the tortuosity and continuity of pores are important features influencing aeration

2. Types of pores Macro (Coarse) pores : Macropores have diameters greater than 100 μm or 0.1 mm , and their main function is aeration and drainage by gravity flow. They are also the pores in which root proliferates. They are visible to the naked eye . Rapid water movement through soil and aeration at field capacity takes place through soil pores. The coarse porosity are measured by estimating total porosity of soil at field capacity . Coarse porosity equals the total porosity less the volumetric water content at field capacity . Additional measurements of hydraulic conductivity are necessary for characterizing the spatial distribution of coarse pores as channels for drainage.

3. Medium ( meso ) pores : Medium pores have diameters from 30 to 100 μm and their main function is conduction of water by rapid capillary flow ; they are visible at times 10 magnification.   Fine (micro) pores : Micro pores have diameters less than 30 μm and their main function is water retention and slow capillary flow . Micro pores are not visible, but their presence can be inferred from observation of the face of aggregates: when the aggregates have a rough surface, there are many micropores .

4. In general, soils should contain an adequate number of pores of > 250 diameter to allow good root penetration.; at least 10 % by volume of the soil in the rooting depth should be composed of interconnected pores > 50 μm in diameter to allow free drainage. At least 10% by volume of the soil should consist of pores with equivalent cylindrical diameter between 0.5 and 50 μm to allow for storage of available water .

5. Porosity and total porosity Porosity refers to the percentage of soil volume occupied by pore spaces . Size of individual pores, rather than total pore space in a soil, is more significant in its plant growth relationship. For optimum growth of the plant, the existence of approximately equal amount of macro and micro pores which influence aeration, permeability, drainage and water retention favourably . Porosity in a soil can be easily changed. Total porosity of soil is calculated from the dry bulk density and particle density as shown below Dry bulk density Total Porosity (% volume) = 1 - --------------------- x 100 Particle density Air filled porosity : Pores that are not filled with water are filled with air, and an estimate of their volume can give an indication of the aeration and drainage status of a soil. Generally, the higher the air filled porosity, the better the drainage and aeration of a soil .

6. Factors affecting porosity of soil  Soil structure : A soil having granular and crumb structure contains more pore spaces than that of prismatic and platy soil structure. So well aggregated soil structure has greater pore space as compared to structureless or single grain size. Soil texture : In sandy soils, the total pore space is small whereas in fine textured clay or clayey loam soils total pore space is high and there is a possibility of more granulation in clay soils. Arrangement of soil particles : When the sphere like particles are arranged in a columnar form ( i.e one after another on the surface forming column like shape) it gives the most open packing system resulting very low amount of pore spaces . When such particles are arranged in the pyramidal form it gives the most close packing system resulting high amount of pore spaces.