Relationship between Soil Types and Water Holding Effects of Soil on Vegetation

Soil is the uppermost layer of the earth’s crust which provides support and nutrient for plants growth and habitat for some animals. The soil is a complete mixture of mineral matter, humus, air and living organisms. Soil is classified on the basis of the size of the particles present in it. Soil particles vary in size and chemical composition, depending on the types of rock from which they were formed and how they were weathered. Those soils with a high proportion of sand are known as sandy soils; those with a high proportion of clay and silt are called clayey soils and those with nearly equal amount of sand, clay and silt are known as loamy soils. The proportions of these particles in the soil have an important effect on their properties and on the types of plants found on them.

Types of Soil

1. Sandy Soil: This contains 80% sand and gravel and 20% of the other types of particles taken together. Large coarse particles of sand and gravel predominate.
2. Clay Soil: This contains more fine clay (60%) and silt particles.
3. Loamy Soil:This contains a mixture of both clay and sand with some humus in roughly equal proportion. Loamy soils are the most fertile and the humus in it gives it a mellow tilth i.e. the size of the soil particles and the air spaces between the particles are the most suitable for cultivation.

Effects of Soil On Vegetation

Soil factors play an important role in determining the vegetation of a region. Soils account for the variation in type of plants that are found in regions with similar climates. To support a rich growth of plants, soil must have the following characteristic;

(i) A rich humus content

(ii) A rich mineral content

(iii) A good water-holding capacity; this is determined by the amount of humus and clay in it.

(iv) Good soil porosity; determined by the humus, sand content and soil texture.

Sandy soil is low in plant nutrient and so it supports scanty vegetation or grassland. Clay soil has a little more amount of plants nutrients than sandy soils and thus can support light vegetation such as shrubs. Loamy soil is very fertile and can support luxuriant vegetations such as a forest.

Water Holding Capacity of Soil

Water holding capacity of soil refers to the ability of the soil to retain water. The amount of water retained by any soil depends or the size of the particles, the humus content, aeration, temperature and presence of microbes. Clay and humus retain a higher amount of the water than sand. In clay soil, most of the water is held firmly to the surface of the soil particles (hygroscopic water) and this is not usually available to plants. In sandy soil very little amount of water is retained as most of it drains off. Loamy soil is able to retain more water within its particles. This is called capillary water and is available for plants use.

Practical Guide on Soil

1. Experiment to Determine The Water Retaining Capacity of Soil Types

Title of experiment: To compare the porosity and water holding capacity of three soil types

Materials required: Three measuring cylinders of 100cm³, cotton wool, three funnels, water, dry sand, dry clay, dry loam, stop clock, balance.

Method: Stand the three funnels in the three measuring cylinders and block the funnels with cotton wool.

1. Place an equal weight of dry sand, dry clay and dry loam in the three funnels respectively.
2. Pour 50ml of water onto each sample at the same time and allow to drain.
3. Allow the set up to stand for an hour or until the water has stopped dripping through each funnel.
4. Read the level of water in the measuring cylinder.

Calculations:

(a) Rate of drainage/porosity is calculated from the amount of water collected in the measuring cylinder. The more the water, the more porous the soil sample. Usually porosity is highest in sandy soil because it has large pore spaces and large particle sizes, followed by loamy soil and then clay soil which has the least drainage because of its tiny pore spaces and fine particles .

(b) Water holding/retaining capacity is calculated as follows;

Volume of water added to soil =50ml

Volume of water collected in cylinder =xml

Volume of water retained in the soil =(50−x)ml

The percentage of water retained in each of the soil samples will be

50−x50×100=Y%

Observation: It is observed that water drained out from the sandy soil faster than the loamy soil and finally the clay soil. It was also observed that clayey soil retained more water than loamy soil and least retained by the sandy soil.

Conclusion: Sandy soil is more porous than loam which is more porous than clay. Clayey soil retained more water than the loamy soil and the sandy soil retained the least amount of water.

2. Experiment To Compare The Capillary Action Of Soil Types
3. Take three wide glass tubes and plug each at one end with cotton wool.
4. Nearly fill the tubes with the three soil samples separately.
5. Clamp the tubes upright in a trough of water,
6. Allow the set-up to remain for 3-6 hours
7. Observe every 30mins.

Observation: It will be observed that at the early stage of the rise of water in the three tubes, it was faster in sandy soil than the clay and loamy soil samples, however by the end of the experiment the water had risen to the highest levels in loam, followed by clay but remained at a low level in the sandy soil.

Conclusion: Loamy and clayey soils have greater capillary actions due to their tiny pore spaces. The presence of organic matter in loam also enhanced its capillary. The sandy soil had poor capillary action because of its large pore spaces and large particles.