SOIL AND WATER CONSERVATION

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 soil

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 place
  • 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 terrain

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/Vshaped/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

Semicircular 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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