Biology Form 3 Page 68 Ms. R. Buttigieg Unit 5: Soil - Stages of Soil formation Soil is the layer of material covering parts of the land, the home of many organisms and the area from which land plants obtain their water and minerals. It is produced by the combined action of wind, water, and organic decay. Soils vary widely from place to place. Soil formation is the process by which rocks are broken down into progressively smaller particles and mixed with decaying organic material. 1. Bedrock begins to disintegrate as it is subjected to freezing-thawing cycles, rain, and other environmental forces (I). 2. The rock breaks down into parent material, which in turn breaks into smaller mineral particles (II). The organisms in an area contribute to soil formation by facilitating the disintegration process as they live and adding organic matter to the system when they die. 3. As soil continues to develop, layers called horizons form (III). The A horizon, nearest the surface, is usually richer in organic matter, while the lowest layer, the C horizon, contains more minerals and still looks much like the parent material. 4. The soil will eventually reach a point where it can support a thick cover of vegetation and cycle its resources effectively (IV). At this stage, the soil may feature a B horizon, where leached minerals collect.
Biology Form 3 Page 69 Ms. R. Buttigieg 5.1 Components of different kinds of soil Soil consists mainly of the following components: a) sand, silt and clay particles SOIL PARTICLES b) humus ORGANIC MATTER c) water d) air e) mineral salts f) plant roots g) micro-organisms and small organisms a) SOIL PARTICLES i. Sand particles large particles which have many spaces between them that can be filled by air or water. ii. Silt particles particles are smaller than sand particles. Both sand and silt consist of silicon dioxide (SiO 2 ). iii. Clay Particles consist of small particles of silicon oxide and aluminium oxide. These particles hold positively charged metal ions on their surface. Therefore clay is a valuable source of ions such as K +, Ca ++, and Mg ++. These are important for plants. The sticky particles of clay hold a lot of water and when dry, the clay forms hard clods. B) ORGANIC MATTER This is derived from dead and decaying remains of plants, animals and other organisms. Saprophytic bacteria and fungi in the soil digest the dead remains and produce humus. Humus is a black jelly-like material which coats the sand and clay particles and glues them together forming soil crumbs. Between the crumbs, air and water can penetrate, giving a well-drained and well-aerated soil. The organic matter helps to hold the water in the soil. It also makes the soil texture lighter and easier for the farmer to dig and plough. When organic matter breaks down it releases ions to the soil therefore making the soil more fertile. C) WATER Rainwater falling on the soil carries with it many dissolved salts. How fast water penetrates the soil depends on: The size of the soil particles The number of pore spaces The crumb structure The proportion of clay. Small soil particles e.g. clay and a good crumb structure hold more water molecules.
Biology Form 3 Page 70 Ms. R. Buttigieg D) AIR Soil normally contains pores spaces filled with air. The water forms a thin film around soil particles. Air is important as it provides the oxygen needed for the respiration of the soil organisms and for plant roots. In a waterlogged (water filled) soil the air spaces become filled with water and air is left out. Many organisms therefore suffer due to lack of oxygen. Pore Spaces: location of air and water Soil Particles: Mineral and Organic E) MINERAL SALTS These come from rock particles and from the action of bacteria on organic matter. Negative ions e.g. Sulphate (SO 2-4 ) or Nitrate (NO - 3 ) can be easily washed away by rain, while positive ions can stick to clay particles. These ions are important for plants and they take them up with the water. F) PLANT ROOTS These are always found in the soil. Apart from having the function of anchoring the plant and taking up water and minerals, they play an important part in soil formation, by helping to break the bedrock. G) MICRO-ORGANISMS & SMALL ORGANISMS Micro-organisms include bacteria, fungi, algae and protests. They are mainly found in a film of water in the soil. Small organisms include nematodes (roundworms), arthropods and earthworms. Earthworms also play an important part in improving soil fertility.
Biology Form 3 Page 71 Ms. R. Buttigieg 5.2 Different types of Soil Clay Sand Loam Light Soil Good aeration and drainage Easily worked both in wet and dry conditions Particles do not lump together to form clods A disadvantage is that mineral salts are readily leached (washed away) by heavy rain and water is rapidly lost in dry weather. Heavy soil Poor aeration and drainage When wet will be sticky and difficult to plough Dry clay that has been trodden on when wet will form hard clods that don t break easily Retains water in dry weather and soluble mineral salts aren t washed away Balanced mixture of particle sizes. Good humus content Stable crumb structure A very productive soil A fertile soil is one which: has ideal proportions of sand and clay particles, adequate humus and mineral salts, a stable crumb structure, is well drained and aerated is not too acidic or alkaline. Proportion of Clay & Sand - Soils with a high percentage of sand are not capable of storing sufficient water to provide the best plant growth and lose large amounts of minerals by leaching to the subsoil. By adding organic manure to sandy soil, it will be helped to hold more water and will prevent washing out of salts by rain. Soils containing a larger percentage of finer particles: the clays and loams, are excellent reservoirs for water and contain readily available mineral materials. Heavy clay soils composed largely of clay particles, however, tend to contain water in excess; these soils have a gummy texture rendering them resistant to cultivation and frequently inadequately aerated for normal plant growth. By adding organic manure to it, the crumb structure is improved, pore spaces increase so it is better aerated and is easier to plough.
Biology Form 3 Page 72 Ms. R. Buttigieg Humus and mineral salts The adding of farmyard manure or organic matter increases both the humus and mineral salts from the soil. However due to lack of manure farmers use artificial fertilizers. These increase greatly the ion content of the soil, but they do nothing to improve the crumb structure of the soil. Therefore after years of use the soil tends to become dry and powdery and can be easily be blown away by the wind. Crumb structure, aeration and drainage A good crumb structure helps to keep the soil well drained and aerated. The grass roots help to improve it so crop rotation is important. Earthworms help to improve the drainage by burrowing through the soil. Acid and Alkaline Soils Limestone rich places like Malta, tend to have alkaline soils due to the presence of calcium carbonate. Sandy and clay soils are usually more acidic. Acidic soils are more infertile as acidity dissolves minerals and they are washed away by the rain. Alkaline salts are more fertile with plenty of mineral salts. Acid soil can be made alkaline by adding calcium carbonate. Circle the letter showing the correct answer: 1. Soil is made up of 2. Soil particles are produced by a. rocks and mineral ions a. animals b. humus and mineral ions b. weathering c. organic matter, water and air c. plants d. all of the above d. humans 3. The best soils are 4. Water retention in soil means a. sandy soils a. the water drains quickly b. clay soils b. the water evaporates quickly c. a mixture of both c. the soil can hold water in it d. none of the above d. the soil cannot hold water in it 5. Humus is made up of 6. 2 minerals found in good soils are a. decaying organic matter a. calcium and phosphates b. live plant matter b. nitrates and phosphates c. rocks and minerals c. carbonates and nitrates d. none of the above d. silicates and nitrates
Biology Form 3 Page 73 Ms. R. Buttigieg 5.3 Organisms in the Soil Soil micro-organisms Include bacteria, algae, fungi and protozoa. The photosynthetic micro-organisms are found on the top few millimeters of the soil so as to be able to absorb sunlight as much as possible. Some soil bacteria and fungi are saprophytes. They are important in breaking down dead organic matter and releasing many valuable minerals into the soil. Soil micro-organisms are more abundant near the roots of plants probably because the roots may provide food material e.g. sugars, to these organisms. Larger soil organisms - Include Nematodes, Arthropods and Earthworms Earthworms They burrow through the soil, pushing through the soil particles by swallowing the soil. They digest the organic matter in the soil and they pass the remaining soil to the outside. Some earthworms also pull surface leaves into the soil and they ingest it. Earthworms improve the fertility of the soil The material that passes through the earthworm is grinded and made alkaline. This helps to stick organic material and mineral particles together. The grinding of the soil by their burrows contributes to a good crumb structure. They also improve the drainage and aeration of the soil. Earthworms also form a source of food for many animals, and constitute the principal food of moles and shrews. Harmful soil organisms Some insect larvae can eat the roots of crop plants. They are agricultural pests. Some nematodes can act as endoparasites and ectoparasites of plant roots. Millipedes can be also harmful. The animals live in dark, damp places and feed on decaying plant life, sometimes damaging crops but also enriching the soil. They grow by molting and may live for one to seven years.
Biology Form 3 Page 74 Ms. R. Buttigieg 5.4 Methods of Soil Improvement 1. Use of Fertilizers a. Organic fertilizers (such as farmyard manure and compost) replace both humus and mineral ions. These fertilizers help the crumb structure and the mineral ions they contain are not washed away by heavy rain. b. Inorganic fertilizers replace only mineral ions removed by crops. The most common elements are: nitrogen, phosphorus, potassium, magnesium and cobalt. They are applied in spring just before plants start growing rapidly or they will be lost. They are expensive so need to be used carefully, also not to upset the balance of mineral ions and the ph. They don t improve crumb structure however. c. Lime (calcium carbonate CaCO 3 ) is used to correct overacidity and to improve the texture of heavy clay soils. 2. Cultivation and conservation Ploughing and digging reduce denitrification (loss of nitrates from the soil) by preventing the soil from getting too compact and wet. Digging between seedling reduces competition between the seedlings and weeds, for water, salts and light. Soil erosion (and loss) is brought about by: Deforestation Overgrazing Ploughing on steep slopes or exposed land Soil is conserved by vegetation, trees, terracing and contour ploughing. Plant roots help to hold soil particles together. 3. Crop Rotation Different crops have different demands on the soil, so by changing the crops grown on a particular field from year to year the soil is not depleted of any particular group of minerals. Leguminous plants (e.g. beans and peas) help restore the nitrogen content of the soil. Reduces gradual building of a pest population e.g. fungi, viruses and weeds, which are extremely common in monocultures (fields in which the same crop is grown year after year). Note that the experiments carried out on soil should also be studied well for the exam.