Agriculture Form 3 – 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 runoff.
  • 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 water.
  • 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 temperatures.
      • 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 wider.
• Riverbank Erosion
  – removal of soil along river banks by the river water.
  • Solifluction – gravitational flow of soil saturated with water.
    
• 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
    contours.
• 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 water.
  • 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 terrace.
• 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 drains.

  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 gullies.
• 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 tanks.
• 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 crops.
• 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.