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/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.