• Agents causing erosion are wind and water.
• Sheet erosion is uniform removal of top soil in thin layer from the field.
• Sheet erosion is least conspicuous and is the first stage of erosion.
• In rill erosion due to runoff, channelisation begins and erosion is no longer uniform.
• In rill erosion incisions are formed on the ground and erosion is more apparent than sheet erosion.
• Rill erosion is the second stage of erosion.
• Gullies are formed when channelised runoff from vast sloping land is sufficient in volume and velocity to cut deep and wide channels.
• Gullies are the most spectacular symptoms of erosion.
• Gullies if unchecked cultivation becomes difficult.
• Ravines are the manifestation of a prolonged process of gully erosion.
• Ravines are typically found in the large expanses of deep alluvial soils.
• Ravines are deep and wide gullies and their formation indicates very advanced stage of gully erosion.
• Sliding down of large chunk of soil due to steep slopes is called land slides.
• Landslides occur in mountain slopes when the slope exceeds 20 percent and width 6m.
• Along with runoff, soil is carried away as fine particles of less than 0.5 mm in diameter are suspended in water.
• Rill erosion starts only when the amount of runoff exceeds 0.3 to 0.7 mm/s.
• Majority of rain drops are between 1 and 4 mm in diameter though the size may vary from tiny droplets to a maximum diameter of 7 mm.
• The rainfall intensity of more than 5 cm/hr is considered as severe.
• Erosivity is the capacity of agents causing erosion.
• Erodibility is the susceptibility of soil to erosion.
• Universal soil loss equation was developed by Wischmeir and Smith.
• Soil loss equation due to water erosion
A = RKLSCP
A = predicted soil loss (t/ac/year)
R = Rainfall and runoff factor
K = soil erodibility
L = Slope length
S = Slope gradient or steepness
C = Soil cover and management
P = erosion control practice
• Soil erodibility factor (K) gives an indication of the soil loss from a unit plot of 22 m long with a 9 percent slope under continuous fallow.
• K value varies from 0 to 0.6.
• K value is low for soils into which water readily infiltrates.
• Soils with intermediate infiltration capacity and moderate soil structural ability have a K factor of 0.2 to 0.3.
• More easily eroded soils with low infiltration capacities have a K value of 0.3 or higher.
• Topographic factor (L.S) reflects influence of length and steepness of slope.
• Topographic factor is the ratio of soil loss from the field in question to that of a unit plot with 9% slope, 22 m long and continuously followed.
• Soil cover and management factor (C) indicates influence of cropping systems and management on soil loss.
• Forests and grass are the best natural soil protective agencies known and are about equal in their effectiveness.
• Forage crops, both legumes and grasses are next in effectiveness because of their relatively dense cover.
• C is the ratio of soil loss under the conditions found in the field in question to that which would occur under clean tilled continuous fallow conditions.
C =1 for bare soil before crop canopy develops
C < 0.1 when large amounts of crop residues are on the land or in areas of dense forests
• Support Practice factor (P) reflects the benefits of contouring, strip cropping and other supporting factors.
• Support practice factor (P) is the ratio of soil loss with a given support practice to the corresponding loss when crop culture is along the slope.
• Annual soil loss in India is 16.35 t/ha.
• Permissible limit of soil loss is 11 t/ha.
• 29% of the total eroded soil is lost permanently to the sea and 10% is deposited in reservoirs.
• About 175 mha constituting 53.3% of India’s geographical area of 328 mha is subject to some kind of degradation.
• Active soil erosion by water and wind is prevalent over 140 mha resulting in the loss of 6000 mt of fertile soil containing 5.53 mt of NPK.
• Based on the capability or limitations, the lands are grouped into eight classes by the U.S. SOIL CONSERVATION SERVICE.
• First four classes are used for agriculture or cultivation of crops.
• Classes from five to eight are not capable of supporting cultivation of crops.
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