Soils – continued H
ELLO, STUDENTS, how are you this week? Do you know what my wish is for you? It is for you to have a very productive and stress-free day! Impossible, you say; of course it isn’t! Everything is possible; you just need to work hard to achieve it! Some of you are probably asking, ‘What was I thinking when I was longing to get to fourth form?’ Others of you are probably thinking, ‘Am I going to survive the months that I have left in fifth form?’ Guess what, you will come to enjoy fourth form and you will definitely survive fifth form. You’ll see!
This week, we will continue our study of soils as we look at soil and its components. Do you remember what these components are? You don’t? You got them in last week’s lesson! Let’s refresh our memories. The components can be obtained from the definition which is:
Soils are complex mixtures of different-sized rock particles, humus, air, water and dissolved minerals, and millions of organisms, most of which are microscopic. What are the components of soil? These are: Mineral/rock particles Air Humus Water and minerals Living organisms We will be looking at each of these in turn.
MINERAL PARTICLES
These, as we have already learnt, can be large (0.02 -2.0mm), as in sandy soil, medium, as in silt (0.002 - 0.02mm), and small, as in clay (<0.002mm). The rest of the information on the mineral particles can be found in the previous lesson. The mineral particles provide anchorage support for plant roots and shelter for soil organisms.
SOIL AIR
Air is held in the soil spaces that are not completely filled with water. The organisms that are found in the soil need oxygen for respiration and that oxygen is in the air found in these spaces. Plant roots also need oxygen for respiration, in order to be able to grow and to take up the minerals
HUMUS
Humus is formed from the combining of the dead remains of animals (and their faeces) and plants (mostly leaf litter) and any other organic matter that enters the soil. The decomposition of these is brought about by bacteria and fungi that are found in the soil. These organisms are known as decomposers.
Decomposition of this material is slow and, hence, the humus (during decomposition) provides storage of some nutrients such as nitrogen. It also provides food for living organisms such as earthworms (detritivore). When the decomposition is complete, it forms a dark, sticky material which coats the soil particles. Its stickiness causes soil particles to come together in clumps called crumbs. These crumbs improve the texture of the soil and hold water and minerals. They also improve the soil’s drainage properties.
WATER AND MINERALS
found in the soil. Without respiration, the roots, and eventually the plants, will die. Nitrogen-fixing bacteria also need the nitrogen that the soil air contains to satisfy the nitrogen cycle. This is why plants cannot survive in waterlogged soils because, in these soils, all the spaces are completely filled with water and, hence, the space to contain air is no longer available. Soil water exists in 4 forms:
Free water – This collects in large pores between the particles and drains easily from the soil. This means that this water is only available to plants while it is moving through the soil.
Capillary water – This is held in medium-size pores by capillary action. It is held against the force of gravity, so it will not drain.
The other two types of water are closely bound with the soil particle and are not available to the plant. These are hygroscopic water and combined water.
Soil water contains dissolved chemicals produced from the weathering of the rocks and from the decomposition of organic materials. It also provides the plant with water for photosynthesis.
LIVING ORGANISMS
We have already mentioned some of these: the decomposers, bacteria and fungi, and the detritivore, the earthworm. Others include insects such as ants, cockroaches and termites, molluscs such as snails and slugs, myriapods such as millipedes and centipedes, crustaceans such as woodlice, as well as nematodes which live in the roots of many plants. These:
Improve aeration of the soil by burrowing and also serve to bring minerals up to the upper layers of the soil – ants, earthworms, termites.
Improve soil fertility by producing waste matter, and by feeding on leaf litter and other decaying material – earthworms, woodlice, snails and slugs (these also feed on living plant material and can be quite destructive) and millipedes.
Improve soil texture – earthworm. These animals take in soil and grind it in their gizzards; they then pass out ‘worm casts’ which are made up of finer soil with minerals.
SOIL PH
The pH of the soil also plays a part in the type of plants that it supports because it has a major effect on which soil nutrients will become available to the plants. The pH of the soils describes the acidity or alkalinity of the soil. Most of the nutrients that plants in general require are found in slightly acidic soils. Bet you did not know that there were so many interesting things to know about soils! See you next week!