Factors that influence the formation of soil structure (Aggregation) 

Factors that influence the formation of soil structure

Organic matter content: organic matter is sticky and brings soil particles together hence increasing stability.

Soil water: moist soils are plastic than dry soils and so are easy to bind together. Too much moisture causes dispersion of the particles.

Living organisms: some living organisms produce substances that cement soil particles together. E.g earthworms.

Compaction: this leads to the formation of a platy structure which comes as a result of destruction of other structures.

Soil texture: soils with large particles e.g. sandy soils are not plastic enough and so their particles do not easily bind together.

Liming:  liming encourages formation of soil aggregates due to the favorable effect  of Ca on the soil.

Ways in which soil structured is destroyed

Continuous tillage: this breaks and separates the soil aggregates.

Mining that breaks the soil particles.

Soil erosion that washes away the top soil.

Water logging: this leads to dispersion of soil particles.

Lack of cover crops: This exposes the soil to erosion.

Poor harvesting practices that do not add organic matter to the soil.

Overgrazing: this creates a bare surface exposed to the effect of soil erosion.

Leaching: this leads to loss of Ca which binds soil particles together.

Pollution: This kills the soil living organisms.

How soil structure is maintained

Minimum tillage

Growing cover crops to reduce rate of erosion.

Application of organic manures to bind soil particles together.

Mulching to control soil erosion and add organic matter.

Bush fallowing involving grass leys to bind soil particles.

Aforestation to control soil erosion and add organic matter.

Draining to remove excess water that causes dispersion.

Liming to bind soil particles together.

Controlled irrigation to provide soil moisture.

Controlled grazing to prevent overgrazing and soil erosion.

ii)      Soil texture

Soil texture refers to the roughness or smoothness of the soil.

OR

It is the measure of the proportion of sand, silt and clay in the soil.

 Methods of determining soil texture

Sedimentation/ mechanical method

Finger feel method

Rolling experiment/  cylinder method

Experiment to show that soil is made up of particles of different sizes/ determination of soil texture by sedimentation method (mechanical method)

Apparatus:

Garden soil, water, Sodium carbonate, measuring cylinder

Procedure

50g of soil is put in a measuring cylinder.

Add 4x its volume of water containing sodium carbonate.

Cover the mouth of the cylinder with your palm.

Shake vigorously for about 2 minutes.

Place the cylinder on a table and allow the contents to settle for about 20 minutes.

NB: Sodium carbonate helps in dispersion of soil particles.

Diagram

Observation:

The soil particles settle according to their sizes in different layers. i.e. heavy coarse gravel, sand, silt, clay, humus/ organic matter in that order.

Conclusion

Soil is made of different sized particles which when subjected to the experiment sediment according to size.

Determination of soil texture by finger feel method

This is mainly a field method where soil is felt by rubbing the soil between the first finger and the thumb when it is dry and when it is moist.

Clay feels smooth when dry and sticky when wet.

Sand feels rough when dry and gritty when wet.

Silt feels floury/ powdery when dry and slightly sticky when wet.

Determination of soil texture by cylinder method/ rolling experiment

Soil samples are wetted with water.

The soil is then rolled between palms into cylinders.

Observation:

Sandy soil doesn’t roll into a cylinder.

Loam soil moulds into a cylinder but cylinder breaks when bent into a ring.

Clay soil moulds easily into a cylinder which is easily bent into a complete ring without breaking.

Importance of soil texture

It influences soil aeration.

It influences the water holding capacity of the soil

It influences soil drainage.

It influences root penetration into the soil.

It affects soil temperature

It influences the workability of the soil i.e. sand soil is easier to till.

It influences soil capillarity.

It influences soil’s susceptibility to erosion i.e. sand is easily eroded.

It influences the ability of the soil to hold soil nutrients i.e. sand soil is easily leached.

iii) Soil temperature

This refers to the measure of the hotness or coldness of the soil.

Importance of soil temperature

It controls the moisture content of the soil by affecting the rate of evaporation.

It affects the aeration of the soil by influencing the moisture content of the soil.

It controls the germination of seeds. All seeds require a certain critical temperature for activation of their enzymes before germination can occur.

It controls root development and expansion. Roots need warmth to grow and indirectly by affecting aeration and moisture content.

Increasing soil temperature increases cell wall permeability.

It affects microbial activity. With in a given temperature range, increase in temperature increases microbial activity.

It indirectly affects the availability of plant nutrients by affecting the rate of breakdown of organic matter.

It affects the rate of weathering by influencing by influencing the rate of chemical reactions and microbial activity.

It indirectly affects soil pH by affecting microbial activity, aeration and break down of organic matter.

It controls uptake of water and mineral salts through its control on root extension, water movement and solubility.

How to maintain soil temperature

Mulching; this keeps the soil cool during hot days.

Shading. This cools the soil surface.

Irrigation; this cools the soil from excessive external heat.

Planting of cover crops.

Application of organic matter.

Drainage to remove excess water.

iv) Soil colour

Soil colour is determined by the colour of the parent material, iron and organic matter in the soil.

Significance of soil colour

It can be used to determine the age of the soil.

It indicates the presence of organic matter in the soil i.e. dark colour indicates the presence of organic matter.

It can be used to reveal the soil profile horizon.

White to grey colour indicates the presence of certain minerals e.g.  White for carbonate lime deposits.

It indicates the extent of mineral loss through leaching.

Effects of soil colour on soil properties

Affects soil temperature: black colour absorbs more heat causing a rise in soil temperature.

Affects soil nutrient levels: black colour absorbs more heat causing moisture loss through evaporation.

Microbial activities: black colour indicates OM content hence improving microbial activities.

v)  Bulk density and Particle density

Bulk density is the ratio of the weight of the soil to the volume of the soil.

Bulk density =       Weight of soil (g)

                             Volume of soil (cm3)

Bulk density is also defined as the mass of dry soil per unit volume.

It is calculated for dry soil and takes into account mass or volume of particles and spaces. It excludes water.

Factors that influence soil bulk density

  • Pore space size: soils that are loose and porous have lower bulk density than compacted ones.
  • Number of pore spaces: the fewer the pore spaces, the greater the bulk density e.g. in sand.
  • Particle sizes: clay soils with tiny particles have lower bulk density than sand with large particles.
  • Organic matter content: increase in organic matter decreases the bulk density of the soil.
  • Soil structure: the more compacted the soil structure, the higher the bulk density.
  • Tillage/ cultivation: Ploughing of a piece of land decreases the bulk density since it increases the pore space.
  • Cropping: cropped soils have generally a higher bulk density than un cropped soil.
  • Machinery movement over the land: use of heavy machines e.g. tractors on a wet land compacts the soil and increases the soil bulk density.

Particle density

Particle density is the ratio of weight of solids to the volume of the solid soil sample.

i.e. particle density = Weight of solid (g)

                             Volume of solids (cm3)

It is a measurement for solid particles and does not include water weight and pore spaces.

Particle density generally increases with soil depth due to decrease in organic matter.