Contents preface (VII) introduction 1—37

3.2.2. Soil Structure
Volume of space (i.e., the pores space) between the soil particles depends on the shape and size distribution of the particles. The pore space in irrigated soils may vary from 35 to 55 per cent. The term porosity is used to measure the pore space and is defined as the ratio of the volume of voids (i.e., air and water-filled space) to the total volume of soil (including water and air). The pore space directly affects the soil fertility (i.e., the productive value of soil) due to its influence upon the water-holding capacity and also on the movement of air, water, and roots through the soil.
Soils of uniform particle size have large spaces between the particles, whereas soils of varying particle sizes are closely packed and the space between the particles is less. The particles of a coarse-grained soil function separately but those of fine-grained soils function as granules. Each granule consists of many soil particles. Fine-textured soils offer a favourable soil structure permitting retention of water, proper movement of air and penetration of roots which is essential for the growth of a crop.
The granules are broken due to excessive irrigation, ploughing or working under too wet (puddling) or too dry conditions. Such working affects the soil structure adversely. The structure of the irrigated soil can be maintained and improved by proper irrigation practices some of which are as follows (1):
(i) Ploughing up to below the compacted layers,
(ii) After ploughing, allowing sufficient time for soil and air to interact before preparing the seed bed or giving pre-planting irrigation,
(iii) The organic matter spent by the soil for previous crops should be returned in the form of fertilisers, manures, etc.,
(iv) Keeping cultivation and tillage operations to a minimum, and (v) Adopting a good crop rotation.

Green manures keep the soil fertility high. Crops like hamp, gwar, moong etc. are grown on the fields. When these plants start flowering, ploughing is carried out on the fields so that these plants are buried below the ground surface. Their decomposition makes up for the soil deficiencies.

The tendency of cultivators to grow only one type of crop (due to better returns) should be stopped as this cultivation practice leads to the deficiency in the soil of those nutrients which are needed by the crop. If the land is not used for cultivation for some season, the soil recoups its fertility. Alternatively, green manures can be used. Rotation of crops (which means growing different crops on a field by rotation) is also useful in maintaining soil fertility at a satisfactory level.

SOIL-WATER RELATIONS AND IRRIGATION METHODS