Soil Conservation Point Wise Notes for Competitive exam Part 2

Soil Conservation

• Measures to prevent erosion grouped as agronomic, mechanical, forestry and agrostological measures.

• When these are used in combination, erosion can be reduced even if the slope is more than 2 per cent.

• Soil erosion can be controlled by agronomic methods when the slope is gentle i.e., less than 2 %.

Agronomic measures include contour cultivation, tillage, mulching, strip cropping and other improved dryland practices.

Guatemala grass (Tripsacum laxum) can be used for mulching.

Strip-cropping is a system of crop production in which long and narrow strips of erosion resisting crops (close growing crops) are alternated with strips of erosion permitting crops (erect growing crops).

• Erosion resistant crops are groundnut, mothbean, horsegram (pulses).

• Erosion permitting crops are sorghum, maize and millet.

• Aggregate stability can be increased by spraying chemicals like polyvinyl alcohol at 480 kg/ha.

• Soils treated with bitumen increase water stable aggregates and infiltration capacity of the soil.

Dead furrows (with closed ends) formed at 3.6 m interval after emergence of the crop sown across slope, reduce the length of the run of rain water, hold water and increase opportune time for infiltration.

Mechanical measures to control soil erosion include contour bunding, graded bunding, bench terracing, gully control etc.

Contour bunding = upto 6%

slope Graded bunds = 6 – 10%

slope Graded trenches = 10 – 16%

slope Bench terracing = 16 – 33% slope

VI = S/2+3

S = percent slope

VI = vertical interval between bunds

Height of contour bund depends on the spacing between bunds, soil conditions and maximum intensity of rainfall.

D = FR/6

D = Depth of water to be impounded in feet Or Theoretical height of the bund to be put up

R = Total runoff in inches

F = vertical fall between bunds in feet

• Contour bunds are usually laid in areas with less than 1500 mm rainfall and up to 6 per cent slope of land.

Graded bunding is recommended in situations where the rain water is not readily absorbed due to high rainfall or low intake of the soil.

• In graded bunds soil from the excavation of the channel is formed into a bund on the downstream to guide the water into a grassed waterway.

• In deep black soils with high clay content develop deep cracks in summer and bunds in these soils breach extensively during rainy season, especially where rains are of high intensity.

• In deep black soils broad based terraces are constructed.

• A terrace is a combination of ridge and channel built across the slope on a controlled grade.

• In broad base terrace excess rain water is led at non-erosive velocity into grassed water ways.

• On steeply sloping and undulated land, intensive farming is possible only with bench terracing.

Bench terracing consists of principally transforming relatively steep land into a series of level strips or platforms across the slope of the land.

• Bench terracing reduces the slope length and consequently erosion.

Type of terraces for different soil and rainfall conditions

Type : Suitability

Level and table top: Area receiving medium rainfall (750 mm) of even distribution with highly permeable deep soils

Sloping outwards: Low rainfall (< 750 mm) area with permeable soil of medium depth

Sloping inwards: Heavy rainfall areas (> 750mm) with soil of poor infiltration rate

• Zing terracing is adopted in lands with 3 – 10% slope.

Zing terraces are constructed in medium to deep soils in moderate to high rainfall areas.

• In zing terracing length of the field is divided into donor area and receiving area in the ratio of 2:1 to 5:1, but usually 2:1.

• Vegetative barrier = Khus khus grass (Vetivaria zeylanica).

Check dams are constructed across gullies to reduce the velocity of runoff, heal the gully, store water for use by livestock and recharge groundwater in wells lower down.

• Grasses are used for stabilizing the surfaces of waterways, contour bunds and front faces of bench-terraces.

Cenchrus ciliaris + Clitoria ternate is the best mixture for eroded soils of UP.

Cenchrus ciliaris + Stylosanthes hamata combination is the best for Andhra Pradesh

. • Growing a mixture of grasses instead of any single grass proved to be better to stabilize newly formed bunds or terraces.

Wind erosion is a natural phenomenon in arid and semi-arid zones.

• Minimum wind velocity necessary for initiating the movement of most erodible soil particles is about 16 km hr-1 at a height of 30.5 cm.

• Movement of soil particles through wind erosion takes place in three stages i.e., saltation, surface creep and suspension.

Saltation is the first stage of movement of soil particles in series of jumps.

• Soil particles moved by saltation are between 0.1 to 0.5 mm in diameter (fine sand).

• In saltation soil particles jump up vertically into air and rise to a height of 30 to 60 cm.

50 to 75% of the weight of soil lost by wind erosion is carried in saltation.

• Rolling of coarse grains, larger than 0.5 to 3 mm in diameter and too heavy to be lifted, by wind along the surface of the ground is called surface creep.

5 to 25 % weight of the soil lost by wind erosion is carried in surface creep.

• Floating of fine dust particles smaller than 0.1 mm diameter through the air is known as suspension.

3 to 4 % of the weight of soil lost by wind erosion is carried in suspension.

• Equation to predict soil loss due to wind erosion

E = I.R.K.F.C.W.D.B

E = soil loss by wind erosion

I = Soil cloddiness factor

R = Surface cover factor

K = Surface roughness factor

C = local wind factor

W = Field width factor

D = Wind direction factor

B = Wind barrier factor

• Trees selected for agroforestry should have quick growth and less crown.

Casuarina, cashew and coconut are useful for coastal sands and date palms are suitable for deserts.

• Among the pasture grasses Cenchrus ciliaris is drought resistant and persistent grass.

• Among the fodder trees, subabul is best suited because of its fast growth, adaptability and multiplicity of uses.

• For establishment of pastures in wastelands, it is better to grow leguminous plants initially.

• During subsequent years i.e., after improvement of soil fertility, grasses can be included to increase fodder production.

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