Soil profile. 

Soil profile. 

This is a vertical section through the soil showing the different layers from the top to the parent rock.

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Classification of soil.

Soil may be classified based on;

  • Size of the particles/texture.
  • Soil colour.

Based on the size of the particles, the soil is divided into;

  • Sand soil.
  • Clay soil.
  • Loam soil.

Relative size of soil particles.

Soil Size in diameter (in mm).
Gravels(small stones)20-2.0
Coarse sand.2.0 – 0.2
Fine sand.0.2-0.02
Silt.0.02-0.002
Clay Less than 0.002
Characteristics/properties of the different types of soils.
Clay soils.
  • Very small particles which are closely packed together, hence limited air spaces in between the particles.
  • Poorly drained/waterlogged/high water retention.
  • Fertile due to high nutrient retention capacity and low leaching.
  • Difficult to work on.
Silt.
  • Good drainage.
  • Easy to cultivate.
  • Very high fertility.
  • Very small quartz particles mainly silicon dioxide.
  • Particles are smooth and powdery.
  • Very high water-holding ability.
Sand soil.
  • Excellent drainage due to large air spaces in between the particles.
  • Poor water holding capacity.
  • Easy to work on.
  • Poor fertility due to rapid leaching out of mineral salts.
  • Not good for cropping unless fertilized.
  • High rate of leaching.
  • Good aeration due to large air spaces between the particles.
Loam soil.
  • Good drainage.
  • Easy to cultivate.
  • Fertile soil with sufficient supplies of soil moisture, air, and plant nutrients.
  • Best arable soil.
Importance of soil texture.
  • It influences the water-holding capacity of the soil.
  • Influences root penetration.
  • Affects soil temperature, e.g. clay soils are colder than sandy soils.
  • Determines the ability of the soil to retain water and nutrients.
  • Influences soil aeration.
  • Influences the movement of water in the soil.
Experiment to determine the percentage of water in a soil sample.
Apparatus.

Wet soil sample, evaporating dish, stirring rod, source of heat, thermometer.

Procedure.
  • Weigh the evaporating dish when empty. Record its weight.
  • Place wet soil sample in the evaporating dish and weigh it. Record the weight of the dish and wet soil.
  • Heat the soil while stirring it at 100oC to dry. Cool and reweigh. 
  • Repeat heating, cooling, and reweighing until when a constant weight is obtained. Record the final weight of the dish and the dry soil.
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Results.

Mass of empty dish =  a gm.

Mass of wet soil +dish = b gm.

Mass of dish + dry soil = c gm.

Mass of water in soil sample = (b-c) gm.

Mass of soil + water = (b-a) gm.

Therefore percentage of water in the soil 

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Experiment to determine soil porosity or drainage and water retention of soil samples A, B and C.

Apparatus. 

3 Measuring cylinders, cotton wool, 3 funnels, water, soil samples A (sand soil), B (clay soil) and C (loam soil).

Procedure.

Obtain dry soil samples A (sand), B (clay) and C (loam). Obtain equal volumes of soil samples and measure them separately. Plug the three funnels with cotton wool. Place the funnels in the mouth of three measuring cylinders. Pour sample A into the funnel 1, sample B into funnel 2 and sample C into funnel 3. Tap the funnels gently so that the soil particles settle completely.

Pour quickly a known volume of water, e.g. 50cm3, into each funnel. Start timing. Note the time taken for the first drop of water to drain into the measuring cylinder in each set.

Allow the water to drain through the soil samples until no more water drains through into the cylinder. Record the volume of water collected in the cylinders for the different soil samples.

Observation.

The Shortest time for the first drop of water to drain through occurred in sample A, followed by sample C and the the longest time in sample B.

Results.

  • The volume of water poured into each funnel = V1 cm3.
  • The volume of water collected in cylinder = V2 cm3.
  • The volume of water retained by each soil sample = V1-V2 cm3

Explanation.

Loam soil retains the highest amount of water because it contains organic matter that holds water firmly since it is sticky. Sand soil does not contain organic matter and has a coarse structure hence a lot of air spaces between the particles. This allows water to drain through it easily. Small size particles in clay soils help to hold water which may lead to water logging.

Conclusion.

Sample A (sand soil) is the most porous, followed by sample C (loam soil) and then B (clay soil).

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