Soil and Water Conservation - Agriculture Form 3 Notes

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Introduction

  • Soil and water are two very important natural resources in farming.
  • They should therefore be well maintained and used without wastage to sustain continuous production.
  • Water loss during the rainy season should be prevented and excess water conserved for use during scarcity.
  • Soil erosion must be controlled at whatever cost if soil is to be conserved.


Soil Erosion

  • It is the removal and carrying away of the top soil by the action of water or wind.

Factors Influencing Soil Erosion

  • Amount and intensity of rainfall.
    • The steeper the land the higher the velocity of surface runof
    • The higher the velocity of surface runoff the greater is its erosive power/effect.
  • Type of soil for example sandy soils are more easily detached and carried away than clayey soils
  • Soil depth;
    • The deeper the soil, the longer it takes to be saturated with
  • Land use:
    • Overstocking leads to bareness of the land and looseness of the soil.
    • Deforestation - indiscriminate removal of trees leads to exposure of soil to      heavy rainfall and high te
    • Indiscriminate burning of vegetation exposes the soil to erosive agents.
    • Clean weeding leaves the soil bare.
    • Ploughing along the slope.
  • Monoculture or continuous cultivation.
  • Ground cover
    • Trees act as windbreakers.
    • Roots of vegetation cover hold the soil particles together.
    • Leaf fall act as mulch which reduces erosion.
    • Leaves of vegetation cover intercepts raindrops reducing their erosive power.

Agents of Erosion

  • Water - moving water has erosive power.
  • Wind - wind carries away soil.
  • Human beings - through man's activities such as cultivation and mining.
  • Animals - through overgrazing and creating footpaths where soil erosion takes place.

Types of Erosion

  • Raindrop (splash) - displacement of the soil caused by raindrops.
  • Sheet - uniform removal of soil in thin layers from flat or gently sloping areas.
  • Rill - removal of soil from small bur well defined channels or rills.
  • Gulley - removal of soil from channels which become progressively deeper and wide
  • Riverbank Erosion - removal of soil along river banks by the river water.
  • Solifluction - gravitational flow of soil saturated with wa
  • Land slides - mass movement of rock debris and soil down a slope,
    For example;
    • Slip movement of earth or rock masses for a short distance.
    • Debris slide - materials move at a greater speed.
    • Debris fall - movement of materials/debris along vertical cliff.
    • Rock fall - movement of rock down a very steep slope.
    • Rock slides - mass of rock materials that slide along a bedding plate, a joint or a fault face.

Soil Erosion Control Measures

- Soil conservation measures can be classified into:

  • Biological or cultural control
  • Physical or structural control

Biological or Cultural Control Measures

- These measures are applicable where land slope is between 2-12%.

  • Grass strips/filter strips;
    • These are narrow uncultivated strips along the contour left between cultivated strips.
  • Cover cropping ;
    • The establishment of a crop that spreads out over the surface of the soil to provide it with a cover.
  • Contour farming ;
    • Carrying out all land operations along the contour.
  • Mulching ;
    • Covering of the soil with either organic or synthetic materials.
  • Proper cropping systems such as:
    • Crop rotation
    • Correct spacing
    • Inter-cropping
    • Ridging/furrowing
    • Strip cropping
  • Controlled grazing;
    • Proper stocking rate, rotational grazing.
  • Strip cropping;
    • Growing crops which give little ground cover in alternate strips with crops such as beans which have a good ground cover.
  • Afforestation/re-afforestation.
    • Afforestation - growing of trees where non-existed.
    • Re-afforestation - growing of trees where they have been cut down.
  • Agroforestry - land use that involves the growing of trees in combination      with crops and pastures on the same piece of land.

Physical or Structural Control Measures

  • These are soil and water conservation measures which involve mechanical constructions on the earth.
  • They are used in areas of moderate slope between 13-55%.

They include:

  • Trash or stone lines;
    • These are rows of heaped crop' residues or stones made along the
  • Filter strips;
    • It involves the growing of an open crop in the upper side of the slope followed by a dense crop to reduce speed of wate
    • This increases infiltration.
  • Terraces;
    • Are structures constructed across a slope to reduce the length of a slope thus reducing run-off.
  • Bench terraces;
    • Are constructed where the slope is 35-55%.
    • Tree crops are suitable for such areas.

    Importance of a Bench Terrace-     
    • Reduces slope of the land.
    • Conserves soil moisture.
    • Better retention of soil fertility.
  • Narrow based terraces Cannot allow cultivation by machines.
  • Broad based terraces - Is wide enough to allow cultivation by machines.
  • Graded terraces:
    • Have a drainage channel to lead off excess water to a vegetated plac
    • They should be about 100m in length.
  • Level terraces:
    • Have no outlet channels,
    • The aim is to have water infiltrating,
    • Hence no water can flow from the ends of the terr
  • Fanya juu:
    • A ridge made by digging a channel and throwing the soil uphill.
  • Fanya chini:
    • In this case the soil is heaped on the lower side of the channel.
  • Bunds: heaps of soil (earth) made along the contour.
  • Cutoff drains:
    • An open trench with an embankment on the lower side into which water from the farm dra
    • Water from the trench should be discharged into;
      • Natural waterways,
      • Artificial waterways,
      • Rocky ground
      • Grassland

  • Gabion/Porous dams:
    • Galvanized wire mesh boxes filled with stones which are built across slopes and gullie
  • Dams and reservoirs ;
    • Dams - barriers built across a river/waterway to hold and store water. It reduces speed of runoff.
    • Reservoirs - these are large storage ta
  • Ridging
    • heaps of soil to reduce the speed of water,
    • They retain the water for some time.


Water Harvesting Methods

  • Water harvesting and storage should be done during the rainy seasons to avoid wastage.
  • This should be done using the following methods:
    • Roof catchment - trapping and collection of rain water from roof tops.
    • Rock catchment - water is harvested by constructing a barrier on the lower side of a large impervious rock to trap surface runoff from the rock.
    • Weirs and dams.
      • Dam - a barrier constructed across a river or a dry valley so that it can hold water.
      • Weirs - barriers constructed across a river or a stream to raise the water level and still allow    water to flow over it.
    • Ponds - water retention excavations' made to hold excess surface water.
    • Retention ditches/level terraces.-These are terraces constructed with blocked ends to retain water.


Micro-Catchments

  • A system of harvesting limited rainfall and storing the water in the ground for use by the planted crops.

Types of Microcatchments

  • Triangular/V-shaped/Negarims;
    • V­-shaped bunds measuring 25cm
    • Are built with soil from the excavated planting holes to direct runoff water towards the basin area around the base of each plant
  • Semi-circular bunds;
    • Formed around the growing plant to hold water around the plant.
  • Trapezoidal bunds;
    • Trapezoidal shaped bunds, which enclose a large area where the crops are grown.
  • Contour bunds/furrows ;
    • These are furrows made along the contours between the rows of crops where agro­forestry trees are intercropped with  annual cro
  • Planting holes/pits ;
    • These are extra large planting holes made and filled with dry plant materials before filling in with soil.

Use of Micro-Catchments

  • Slow down the speed of surface runoff.
  • Used during landscaping of the compound, parks and roadside nest areas.
  • Reclamation of land for food crop in dry areas.
  • Water collected and stored can be used for irrigation
  • Afforestation in dry areas.

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