Besides free air, the soil contains air held by colloids or that dissilved in soil solution. This air contains about 90% carbon dioxide, about 10% nitrogen and a trace of oxygen.

Soil water.Water has perhaps the greatest influence on the growth and yield of a crop. It is needed in much larger quantity than that of any other substance that contributes to growth or yield. Water serves the following functions in relation to plant life.

(i)It is an essential part of plant food. it constitutes nearly 90% of plant tissues.
(ii)It serves as a solvent and carrier of plant nutrients.
(iii)It maintains cell turgidity and regulates temperature.

Water is held in soil in the following forms:
1.HYGROSCOPIC WATER. It occurs as a thin film (4-5 millimicron) and is held tenaciously with a tension of 31 atmospheres or more. It is not available to plants.
2.CAPILLARY WATER. It forms a continuous film around soil particles(outside the film of hygroscopic water) and in the micropore spaces. It is held by surface tension. Capillary water is held at a twnsion ranging from 1/3 to 31 atmospheres.
3.GRAVITATONAL WATER. It is free water held at a tension below 1/3 atmospheres. It saturates the soil and percolates downwards under the influence of gravity.

The maximum amount of capillary water remaining in the soil after the removal of gravitational water is called its field capacity. In most cases it represents the water held at a tension of 1/3 atmospheres. It is generally recognised that capillary wwater held a tension greater than 15 atm is not available to plants. At this point of soil moisture, the plant wilts permanently and hence the percentage moisture at 15 atmospheres is called its wilting point or wilting percentage. It follows then that the moisture held at tensions between 1/3 atm(field capacity) to 15 atm (wilting point) is the water available to plants to the maximum extent. Thus capillary water is the major source of water used by plants.

Capillary water is capable of movment upwards, downwards or laterally, the movment taking place from the thicker part of the film to the thinner part. Similiarly, as the capillary-water zone moves farther from the watertable below the rate of movment becomes less and the suction needed to draw up water increases. Hence in general, a water table lying more than 2-3 metres below the root zone is not of much use to crops.

Crops differ greatly in their water requirement according to growth characteristics, climate and water-supply. Water requirement of a crop includes the evapo-transpiration needs, the water needed for metabolic activities for leaching and other unavoidable losses. It is the water needed for raising a crop in a given period and is expressed as depth of water in millimetres. Generally the water requirement of a crop is related to the potential evaporation during the growth of thecrop.

Soil Structure. Soil stricture refers to the arrangement of soil particles, both primary and secondary. Soil structure is one of the most important properties of soil mass, since it influences aeration, permeability, water capacity etc. In the field the structure is described in terms of
(i)type-referring to shape and arrangement
(ii)class-referring to size
(iii)grade-referrint to the degree of aggregation.

Types.There are 4 types of primary structures.:
1.Platy With particles arranged around a plane, generally horizontal.
2.Prism-like. With particles arranged around a vertical axis and bounded by relatively flat vertical surfaces which are the casts or the molds formed by the faces of the surrounding peds. This type includes prismatic and columnar types.
3.Block-like. With particles arranged around a point and bounded by flat or rounded surfaces which are the casts or the molds formed by the faces of the surrounding peds. This type includes angular and subangular types.
4.Spheroidal Particles arranged around a point and bounded by curved or very irregular surfaces that are not accomodated in the adjoining aggregates. This type includes granular and crumb types.

Classes. Five size classes are recognized in each of the primary types. They are fine, very fine, medium, coarse and very coarse. The actual size for classes ineach type varies. ,/p>

Grade. The grade of structure representing the degree of aggregation is determined in the field mainly by noting the durability of the aggregates and the proportions between aggregated and unaggregated material that results when the aggregates are disturbed or gently crushed. Grades are termed structureless (massive if coherent, and single-grained if non-coherent), weak, moderate, strong and very strong, depending on the stability of aggregates when disturbed. ,/p>

Factors affecting structure. Structure is primarily influenced by texture. The beneficial effects of organic matter on the structure and working of the soils are well known. Soil structure is also influenced by:

1.SOIL MANAGEMENT. The cutting action of ploughs or other tillage implements breaks up the soil mass and may have favourable or adverse effects on the structure, depending on whether the soil is worked under optimum moisture conditions or not. A good soil management, with proper system of crop rotation has the effect of maintaining the soil in a good state of aggregation.
2.ABSORBED CATIONS. Sodium and potassium ions on the clay complex have a tendency to disperse the soil; calcium has favourable effects on the aggregation. Similiarly the presence of soluble salts favours floculation.
3.MICRO-ORGANISMS. The filamentous growth of soil fungi and the microbial decomposition products of organic matter have a binding effect on soil particles thus favouring aggregation. Similiarly the excrements of earthworms and the burrowing activities of insects and other creatures cause appreciable changes in the soil structure.
4.VARIATIONS OF SOIL MOISTURE. Variations of soil moisture due to drying and wetting influnce the structure . The drying of soil forms cracks and big clods. Poorly drained soils with excess of moisture usually have an unfavourable structure.

The structure of cultivated soils have a very important bearing on their drainage, ease of tillage, root penetration and resistance to erosion and ultimately their general productivity. From this point of view the crumb and granular structure(spheroidal) is considered very favourable to plant growth.

The structure and physical properties of soil can be improved by adopting a suitable system of soil management, including legumes in the rotation system, green manuring, and regularly supplying the soil with organic manure.

Mineral composition of the soil seperates. Owing to a varying rate if wearing down of different minerals, the more easily decomposing feldspars predomonate in the relatively fine seperates, whereas the resistant minerals, e.g. are more abundant in coarser seperates. The seperates therefore show differences in chemical composition also. As indicated earlier, the clay which has a high specific surface is more reactive than silt or sand.

The clay fraction. In the process of decomposition relatively stable new elements are formed from the products of weathering which constitute largely the clay fraction. The clay particles formed in this manner are crystalline and as shown by X-ray diffraction and ptrographic techniques are composed of sheets of hydrated alumina and silica linked by oxygen atoms. Because of their being very small, they are highly reactive and form the seat of ion exchange in the soil and this controls and regulates adsorption , retention, and release of many plant nutrients , such as potassium, calcium, magnesium and phosphorous. The precise nature of clay properties depends on the type of minerals that predominantly compose the clay. These fall into three main groups:

1.KAOLINITE. A unit kaolinite crystal lattice consists of one sheet of silica and one sheet of alumina and is hence called 1:1-layer silicate. The two sheets are held together by mutually shared oxygen atoms. These units in turn are tenaciously held together by oxygen-hydroxyl linkages and cosequently no expansion occurs when the mineral is wetted. It has a low specific surface and a low caton exchange capacity. Similiarly plasticity, cohesion, shrinkage and the swelling properties of kaolinite are low.
2.MONTMORILLONITE. A unit montmorillonite crystal lattice consists of 2 sheests of silica and 1 sheet of alumina (2:1-layer silicate) held together by mutually shared oxygen atoms. Such units however are held together by weak oxygen-oxygen linkages. There is some isomorphic substitution of iron or magnesium in the alumina sheet; mont morillonite crystals can expand and owing to this expansion, cations and water molecules are able to move in between the crystal units. Montmorillonite has a high specific surface and caton-exchange capacity. Similiarly, plasticity, cohesion, shrinkage and sweeling properties are high.
3.ILLITE. Illite has the same general structure organisaton as montmorillonite except in respect of the linkages between the crystal units. About 15% of silica in the silica sheet is replaced by aluminium and potassium atoms and they supply the additional connecting linkages between the crystal units, due to which illite shows a lower expansion capacity. it has properties intermediate between kaolinite and montmorillonite.

Soil colloids. The most active portions of the soil are those which are in the colloidal state. The colloidal state implies a two\phase system in which the material called the dispersed phase (fine clay and humus) is dispersed in the dispersed medium(water). In soils, the mineral and organic colloids exist in intimate and heterogenous admixture. The mineral colloid is present almost exclusively as the clay of various kinds, whereas the organic colloid is present as humus. It is generally supposed that clay particles less than one micron in diameter possoss colloidal properties and these properties increase with a decrease in the size of the particles. The most distinctive colloidal properties are:

(i)the large specific surface or interface, and
(ii)the capacity to hold solids, gases, salts and ions.

Depending on their nature, the soil colloids determine many of the physical and chemical properties.The colloidal material may occur as a thin gelatinous film around coarser particles, or it may occupy a considerable part of the space between the larger particles, thus serving as a binding material.

Soil colloids especially the mineral colloids, exhibit cohesive and adhesive properties. Soils with a high amount of colloidal clay compete with plant roots for water and mineral nutrients, especially at lower levels of their availability. Generally the soil colloids have a high exchange capacity, which increases with silica sesquioxide ratio.