Soil constituents refer to the composition of the soil or the components of the soil.
Soil is composed of the following
Phases of soil
Soil consists of three phases i.e.
Solid phase: this consists of solid particles e.g. small stones, organic matter.
Liquid phase: this consists of soil water
Gaseous phase: this consists of air i.e. oxygen and carbon dioxide.
The different components of soil vary from one another in percentage, composition and volume as seen below:
Mineral matter: this forms the inorganic portion of the soil. It comprises fragments of rock materials that are broken down.
The mineral particles form 45% of the volume of the soil. It includes minerals like iron, aluminum, zinc, copper etc.
Importance of mineral matter
They supply the necessary plant nutrients for nutrition.
They hold plant roots in place.
The elements provide surfaces on which soil water is held.
They form the air spaces for both air and water.
The make the skeleton of the soil.
They influence the physical and chemical properties of the soil.
Soil water: this is found in the micro and macro pores of the soil.
Importance of soil water
Necessary for seed germination.
It is an essential solvent for soil solution.
Dissolves plant nutrients for plant absorption.
Prevents desiccation of plant roots and micro organisms.
Controls the amount of soil air present in the soil.
Cools the plants through transpiration.
Controls soil temperature i.e. soils with a lot of water are cool.
Important in chemical weathering of rocks.
Helps to remove toxic substances from the soil.
Keeps the soil soft for easy cultivation.
Necessary for photosynthesis in plants.
Types of soil water
- Gravitational water: This is excess water that can easily flood out of the soil. It is not available to crops.
- Hygroscopic water: This is the water that is held so tightly on the soil particles that it cannot be absorbed by the roots. It cannot be absorbed by plants.
- Capillarity water: This is the water that is held in the spaces between soil particles and can easily be absorbed by the plant roots.
Experiment to determine amount of water in a soil sample
Source of heat
Weigh a clean, dry empty evaporating dish(a g)
Half fill the dish with soil and weigh again (b g)
Find the weight of the soil alone by subtracting (a-b)=C g
Gently heat the evaporatingfrom time to time
Allow the dish and soil to cool in a dessicator so that it does not pick up moisture from the air.
Continue heating till a constant weight is obtained
Record the final weight of the dry soil sample and
calculate the amount and percentage of water in the soil as below:
Weight of dry evaporating dish= a g
Weight of evaporating dish + soil before heating = b g
Weight of evaporating dish + soil after heating = c g
Weight of soil alone before heating = (b – a) = e g
Weight of dry soil alone after heating = (c – a) = f g
Amount of water in the soil = ( e- f) g
- Percentage of water = Weight of water in soil sample
Weight of soil before heating
= (e – f) * 100
Soil air: soil air is found in the pores of the soil which are not occupied by water.