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 efficiently without wastage to sustain continuous agricultural production.
• Water loss during the rainy season should be prevented, and excess water conserved for use during periods of scarcity.
• Soil erosion must be controlled at all costs if soil is to be conserved effectively.

Soil Erosion

• Soil erosion is the removal and carrying away of the topsoil 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 its erosive power and 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 become saturated with water.
• Land use:
  • Overstocking leads to bareness of the land and looseness of the soil.
  • Deforestation – indiscriminate removal of trees exposes soil to heavy rainfall and high temperatures.
  • Indiscriminate burning of vegetation exposes the soil to erosive agents.
  • Clean weeding leaves the soil bare and vulnerable to erosion.
  • Ploughing along the slopes increases erosion risk.
  • Monoculture or continuous cultivation reduces soil cover and increases erosion.
• Ground cover:
  • Trees act as windbreakers, reducing wind erosion.
  • Roots of vegetation hold soil particles together, preventing detachment.
  • Leaf fall acts as mulch, reducing erosion by protecting the soil surface.
  • Leaves of vegetation intercept raindrops, reducing their erosive power.

Agents of Erosion

• Water – moving water has strong erosive power.
• Wind – wind carries away loose soil particles.
• Human beings – through activities such as cultivation and mining that disturb the soil.
• Animals – overgrazing and creation of footpaths contribute to soil erosion.

Types of Erosion

• Raindrop (splash) – displacement of soil caused by the impact of raindrops.
• Sheet – uniform removal of soil in thin layers from flat or gently sloping areas.
• Rill – removal of soil from small but well-defined channels or rills.
• Gulley – removal of soil from channels that become progressively deeper and wider.
• Riverbank erosion – removal of soil along river banks by flowing river water.
• Solifluction – gravitational flow of soil saturated with water down a slope.
• Landslides – mass movement of rock debris and soil down a slope:
  • Slip movement of earth or rock masses for a short distance.
  • Debris slide – materials move at a greater speed.
  • Debris fall – movement of materials or debris along a vertical cliff.
  • Rock fall – movement of rock down a very steep slope.
  • Rock slides – mass of rock materials sliding along a bedding plane, joint, or 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 the land slope is between 2–12%.

• Grass strips/filter strips:
  • Narrow uncultivated strips along the contour left between cultivated strips to reduce runoff velocity and trap soil.
• Cover cropping:
  • Establishment of a crop that spreads over the soil surface to provide protective cover.
• Contour farming:
  • Carrying out all land operations along the contour lines to reduce runoff and erosion.
• Mulching:
  • Covering the soil with organic or synthetic materials to protect it from erosion and conserve moisture.
• Proper cropping systems such as:
  • Crop rotation
  • Correct spacing
  • Intercropping
  • Ridging or furrowing
  • Strip cropping
• Controlled grazing:
  • Maintaining proper stocking rates and rotational grazing to prevent overgrazing and soil exposure.
• Strip cropping:
  • Growing crops with little ground cover in alternate strips with crops such as beans that provide good ground cover.
• Afforestation and re-afforestation:
  • Afforestation – planting trees where none existed before.
  • Re-afforestation – planting trees where they have been cut down.
    • Agroforestry – land use involving growing 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 involving mechanical constructions on the land.
• They are used in areas with moderate slopes between 13–55%.

They include:

• Trash or stone lines:
  • Rows of heaped crop residues or stones made along the contours to reduce runoff velocity.
• Filter strips:
  • Growing an open crop on the upper side of the slope followed by a dense crop to reduce water speed and increase infiltration.
• Terraces:
  • Structures constructed across a slope to reduce its length and thus reduce runoff.
• Bench terraces:
  • Constructed where the slope is 35–55%.
  • Tree crops are suitable for such areas.
• Importance of a Bench Terrace:
• Reduces the slope of the land.
• Conserves soil moisture.
• Improves retention of soil fertility.
• Narrow-based terraces – cannot allow cultivation by machines.
• Broad-based terraces – wide enough to allow cultivation by machines.
• Graded terraces:
• Have a drainage channel to lead off excess water to a vegetated area.
• They should be about 100 meters in length.
• Level terraces:
• Have no outlet channels.
• The aim is to allow water infiltration so no water flows from the ends of the terrace.
• Fanya juu:
• A ridge made by digging a channel and throwing the soil uphill.
• Fanya chini:
• Soil is heaped on the lower side of the channel.
• Bunds: heaps of soil (earth) made along the contour to reduce runoff.
• 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, built across slopes and gullies to reduce erosion.
• Dams and reservoirs:
• Dams are barriers built across rivers or waterways to hold and store water, reducing runoff speed.
• Reservoirs are large storage tanks for water.
• Ridging – heaps of soil constructed to reduce the speed of water and retain it for some time.

Water Harvesting Methods

• Water harvesting and storage should be done during the rainy seasons to avoid wastage and ensure availability during dry periods.

This should be done using the following methods:

• Roof catchment – trapping and collecting rainwater from rooftops.
• Rock catchment – harvesting water by constructing a barrier on the lower side of a large impervious rock to trap surface runoff.
• Weirs and dams – structures to hold and manage water flow.
• Dam – a barrier constructed across a river or dry valley to hold water.
• Weirs – barriers constructed across rivers or streams to raise the water level while allowing water to flow over them.
• Ponds – excavations made to hold excess surface water.
• Retention ditches/level terraces – terraces constructed with blocked ends to retain water.

Micro-Catchments

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

Types of Microcatchments:

• Triangular/V-shaped/Negarims:
  • V-shaped bunds measuring 25 cm, built with soil from excavated planting holes to direct runoff water towards the basin area around each plant.
• Semi-circular bunds:
  • Formed around growing plants to hold water near the roots.
• Trapezoidal bunds:
  • Trapezoidal-shaped bunds enclosing a large area where crops are grown.
• Contour bunds/furrows:
  • Furrows made along contours between rows of crops where agroforestry trees are intercropped with annual crops.
• Planting holes/pits:
  • Extra-large planting holes filled with dry plant materials before filling with soil to improve water retention.

Use of Micro-Catchments

• Slow down the speed of surface runoff, reducing erosion.
• Used in landscaping of compounds, parks, and roadside nesting areas.
• Assist in reclamation of land for food crops in dry areas.
• Water collected and stored can be used for irrigation.
• Support afforestation efforts in dry areas.