• Aggregates of B horizon are bigger due to weight of the top-layers and less shrinkage and expansion activity as there are less fluctuations of soil moisture.
• Profiles in semi-arid region contain granulated A horizon with a prismatic B horizon.
• In humid temperate regions, A horizon contains granulated aggregates while B horizons have platy or blocky aggregates.
• Water is held around soil particles in thin layers due to adhesion and cohesion.
• Water is held with greater force (i.e., 10,000 bars) near the particle surface.
• As the thickness of water layer increases, the force with which it is held is reduced.
• When the soil particles are holding water with less than 0.3 to 0.1 bar force, water is lost into deeper layers due to gravitational pull.
• When pure water is held in hypothetical place where no force is acting on it, its capacity to do work is zero.
• When water is held by soil particles or if it contains salts or due to gravitational force, its ability to do work or its potential is restricted. Therefore, water potential in soils is always negative.
• Water potential of soil is expressed in bars or megapascals.
• Soil water, held between -0.3 to -15 bars, is considered as available water.
• Soil water moves from higher potential to lower potential.
• Specific heat is the heat in calories required to raise the temperature of one gram of a substance to 1oC.
• Specific heat of water is 1.0 cal/g.
• Specific heat of organic matter is 0.5 cal/g.
• Specific heat of soil is 0.2 cal/g.
• If one calorie of heat raises the temperature of water by 1oC, it increases organic matter temperature by 2oC and soil temperature by 5oC.
• Specific heat of soil increases with increase in moisture content.
• Flow of temperature in the soil is expressed as thermal conductivity.
• Thermal conductivity increases with water content and decreases with porosity.
• In moist soils, with higher thermal conductivity, heat flows into lower layers and surface temperatures are therefore low.
• Tillage improves porosity and decreases thermal conductivity resulting in higher surface temperature.
• Latent heat of evaporation for water is 580 calories.
• Dark coloured soils get warmed up quicker than light coloured soils, since dark coloured soils reflect back less radiation to atmosphere and thereby absorb more heat.
• Sandy soils warm up quicker than clay http://healthsavy.com soils, since clay soils have more water holding capacity.
• Optimum soil temperature for germination of different crops are
o Winter cereals : 15-18oC
o Mustard : 18-23oC
o Maize: 32oC
o Sorghum, sugarcane and cotton: 30oC
• Crops such as maize or sorghum need high soil temperature for active seedling growth.
• No microbial activity is noticed below 5o or above 50oC with optimum between 25oC and 35oC.
• Nutrient availability is higher at optimum temperature.
• Field air capacity is the fractional volume of air in a soil at field capacity.
• In sandy soils field air capacity is 25% or more, loamy soils 15 to 20%, clay soils < 10%.
• Carbon dioxide content and relative humidity are higher and oxygen is less in soil air compared to atmospheric air.
• CO2 content in atmospheric air is 0.03%.
• CO2 content in soil air is 0.25%.
• Growth of most crops is affected and stops when soil oxygen content reaches below 2%.
• Root growth is decreased due to decrease in oxygen and increase in concentration of CO2.
• Germination is inhibited in the absence of oxygen.
• Potato, tobacco, cotton, linseed, tea and legumes need higher level of oxygen in soil air compared to other crops.
• Cereals except rice are intermediate in oxygen requirement.
• Rice can tolerate very low level or even complete absence of oxygen in the soil.
• Nutrient and water uptake are reduced when the oxygen content of soil is less as it affects activity and permeability of roots.
• Volume of soil not occupied by soil particles is known as pore space.
• Pore space is occupied by air or water or both.
• Plant roots exist and grow in pore space.
• Pore space directly controls the amount of water and air in the soil and thus indirectly controls plant growth and crop production.
• Diameter of micropores is 2 to 20 µm.
• Diameter of macropores is 200mm to 0.3mm.
• Amount of pore space depends on particle size, texture, structure and biological activity.
• Clay soils have less macropores compared to sandy soils.
• Total pore space in clay soils is 50-60%, loamy soils is 30-50%, sandy soils is 20-30%.
• Sandy soils contain 25,000 pores/m2.
• Clay soils contain 25 million pores/m2.
• Vertical section of the soil to expose the layers is called soil profile.
• Upper layer of soil is usually higher in organic matter and darker in colour than the layers below.
• Upper layer of soil is called A horizon or top soil.
• Middle part of the profile usually contains more clay. This layer is called B horizon or subsoil.
• A and B horizons together are referred to as solum or true soil.
• Crops feed mostly in the top soil and to a very less extent in subsoil.
• C horizon commonly referred to as parent material, occurs beneath the solum and extends downward up to bedrock.
• All crops suffer due to shallow water table except rice.
• Soil strength or soil mechanical resistance indicates the resistance offered by the soil to root penetration.
.• Soil strength or soil mechanical resistance is measured with cone penetrometer and expressed as kg/cm2 or bars.
• Factors responsible for soil colour are parent material, soil organic matter, presence of minerals.
• Red sand stone gives rise to red soil.
• Soil organic matter in surface soil imparts dark brown to black colour.
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