Importance of soil air :- it is used for the respiration by the root Decomposition of the organic matter by the microorganisms. Most desired condition is well aerated soil – where oxygen exchange between soil air and athmospheric air is rapid. Factors which influence rate of gases exchange pore spores Temperature` Depth of soil Wetting and drying covering
PORE SPACE Called void – have a portion of the soil volume not occupied by solid. It made by: Irregular shapes of primary aggregates and their aggregation. Forces of penetrating roots Microorganism Expanding gases entrapped by water. Pores occupied by water and air %PS depends on :- Soil texture Aggregate arrangement Structure
Sandy soils : 35-50% Medium to fine textured soil : 40-60% Good for plant growth : 50% Compacted soil : 25-30%
SIZE OF PORESMacro pore space Micro pore spaceDiameter > 0.06 mm Diameter < 0.06 mmPrimaly found between aggregates Occur within structural aggregatesAllow ready movement of air and •Mostly filled with water in moist soilperlocating water •Not permit much air movement into/ out of soil and water movement also slow
SOIL AERATION Process- air in soil replaced by air from atmosphere. In well – aerated soils, the soil is similar to the atmosphere, while poor- aerated soils usually contain < o2 and > co2 than the atmosphere. Vital process – largely controls the soil levels of o2 and co2.
Poor soil aeration under 2 conditions : excess moisture Very slow gaseous exchange 2 mechanisms: Mass flow diffusion
MASS FLOW due to pressure differences between atmosphere and soil air. Achieved by fluctuation in soil moisture content. Modified by temperature, wind, barometric pressure.
DIFFUSION More important than mass flow. Involve great bulk of gaseous exchange. Each gas moves in a direction determined by its own partial pressure. Diffusion allows extensive movement of air from one area to another even though there is no overall pressure gradient.
MEANS OF CHARACTERIZING SOIL AERATION Content of O2 Atmospheric air – O2 = 21%, CO2 = 0.03%, N2 = 79% Soil air – O2 = 20.9%, CO2 = 0.035%, N2 => 78% O2 diffusion rate ODR determines the rate at which oxygen can be replenished when it is used by roots/microorganisms or when it forced out by water. If the rate of the replenishment is high, ODR is good. ODR decrease with soil depth Top growth : ODR = 30-40x 10-8 g/cm2
Oxidation and reduction potential Measure of tendency of a substance to accept or donate electrons. If a oxidation and reduction potential is high, the substance is called oxidizing agent.
INFLUENCES OF SOIL AERATION ON Soil Plants Through its effects on the Activities of higher plantsredox potential, the level of are adversely effected in Soil microorganisms soil O2 largely determines three ways by poor aeration Organisms influences many the forms of several : soil reaction and inturn, soil inorganic elements. The - Growth of plants, properties. oxidised forms of these especially roots. elements are readily Most obvious is microbial utilizable by plants. The - Absorption of nutrients breakdown of organic reduced forms of these and water. matter.elements are so soluble that - Formation of inorganic toxicities may occur. compounds toxic to plants