soil solution. These essential nutrient elements are present in the form of ions: cations, anions ► cations which are positively charged (K', Ca" and Mg") and anions which are negatively charged (Cl-, SO4 and NO3-). The uptake of ions by the roots is not
passive but requires energy from aerobic respiration.
Soil particles, both mineral and organic, are reservoirs of soil nutrient elements. These elements may be held in combinations not readily available for plant nutrition and are released by physical, chemical and biological processes. The rate of release is affected by environmental conditions in the soil. Table 8.6 summarises some soil minerals and their associated nutrient elements.
Group of Soil Soil minerals Soil nutrient
elements
soil particles particles
Organic Humus Nitrogen, sulphur,
phosphorus, copper. Inorganic Sand Quartz, feldspars, micas, Potassium, calcium, iron,
hornblende. magnesium, sodium. Inorganic Silt Feldspars, micas, Calcium, iron, sodium,
haematite, limonite. potassium, magnesium. Inorganic Clay Kaolinite, illite, Magnesium, iron,
montmorillonite, manganese, zinc. vermiculite, chlorite.
Table 8.6 Soil minerals and their associated soil nutrient elements.
Leaching
Once nutrients have been released into the soil, they maybe lost through leaching— in this process soluble substances are removed by water. The nature and size of the soil particles are directly related to this loss of nutrients. For example, leaching is greater in soils made up of coarse sand particles than in soils with finer particles, such as silt and clay. Potassium, a nutrient found in most soils, is generally low in sandy soils due to leaching.
Similarly, chemical reactions and the exchange of soil nutrient elements are associated with the nature and size of clay and humus particles. Clay and humus particles are very small but they possess large surface areas and negative charges which attract positive nutrient ions and water.
Each particle is referred to as a micelle or micro-cell and has a great capacity for attracting positively charged nutrient ions. This attraction of nutrient ions to the surfaces of the clay and humus particles enables nutrients to be held in the soil so that they are not removed by leaching.
Soil pH
The pH of the soil is a measure of the hydrogen ion concentration of the soil water. pH is measured on a scale from 1 to 14. A value of 7 is neutral: values below 7 are acidic and those from 8 to 14 are alkaline. The pH range for soils is from 3 to 10, but most tropical soils have a pH value between 5 and 7.
Soils in limestone areas are slightly alkaline due to particles of calcium carbonate. Sandy soils tend to be slightly acidic because the rain causes leaching of soluble ions which would otherwise neutralise the acidity.
Acid soils are less fertile than alkaline soils because the acidity causes the mineral salts to be more soluble and therefore more easily washed away by rain. When rainfall is greater than evaporation, calcium, magnesium and potassium ions are leached away from the topsoil as the water moves downwards. The soil becomes more acidic because hydrogen ions replace the calcium, magnesium and potassium ions. In tropical regions, minerals that are less soluble in water, such as aluminium, kaolinite and quartz, are left in the top layers of soil. Soil acidity can be reduced by liming.
soil aeration ► Soil aeration is dependent on soil porosity and also on the amount of the pore space which is occupied at any one time by soil water. Soil air and soil water occupy the same pore space and the amounts of each will vary according to the What is meant by 'soil porosity'? conditions. A well-drained soil contains more air than a waterlogged soil.
Practical activities:
1. Using a sandy soil, a clay soil and a loam, set up an experiment to find out which soil drains more quickly and which soil holds most water. Make your experiment quantitative by using measured volumes of soil and water, and by allowing a specified time for the water to drain through.
2. Soil contains organic and mineral matter. The organic matter consists of dead and decaying remains of plants and animals. The mineral matter consists of the weathered rock particles. Follow the instructions to determine the quantity of each in the soil:
a. Weigh about 10 g of soil (M,) in a heatproof container.
b. Weigh the sample again (M2)before heating it at a high temperature to burn off
the organic matter. Use a propane torch for this. c. When the soil has cooled, re-weigh the sample (M3).
d. The quantity of organic matter can be calculated by subtracting M3 from M2.
The quantity of water in the sample is given by subtracting M, from M, and the quantity of mineral matter is given by M3.
Soil temperature and soil organisms
In the Caribbean, temperature on the soil surface ranges between 23°C and 30°C. Within the top 15 cm of soil (the furrow slice), temperatures between 28°C and 30°C are the most favourable for the soil organisms, biochemical processes and soil formation.
Soil temperature is influenced by sunlight, vegetation cover, soil cover (both natural and artificial), soil moisture and organic matter content. All these factors, with the exception of direct sunlight, lower the soil temperature. Soils that lack vegetation cover and which have little organic matter, lose moisture rapidly when exposed to direct sunlight. Consequently, the soil temperature will rise.
Soil temperature affects:
macro-organisms ►
•
soil macro-organisms, such as earthworms, which are more actively burrowing when it is warm• soil microbial activity which increases when warm and decreases when cold • roots of seedlings which are destroyed by high soil temperatures as plant cells State THREE major factors which affect soil dehydrate due to evapo-transpiration
temperature. • germination of seeds which is more rapid under warm temperatures • soil caking and crusting which occurs as a result of high soil temperatures,
direct sunlight and rapid loss of moisture. State TWO beneficial effects of warm soil
temperatures on crop growth.
1
Farmers can lower soil temperature by mulching, cover cropping, intercropping, irrigation, improving soil cover and incorporating organic matter into the soil.
Chemical properties of soil
Soil nutrients
Plants require 17 essential nutrient elements for their growth and development. These are shown in Figure 8.9, overleaf. Fourteen of these are supplied by the soil. The others (carbon, hydrogen and oxygen) come from air or water.
Section B: Crop Production
macro-nutrients ► micro-nutrients ►
primary elements ►
secondary elements ►
I ESSENTIAL NUTRIENT ELEMENTS
Macronutrients (major) I I Micronutrients (minor/trace) • From the soil: • From the soil:
nitrogen iron
phosphorus primary elements copper
potassium ) zinc
calcium manganese required in
magnesium secondary elements cobalt ((( small amounts
sulphur I molybdenum
• From air and water: chlorine carbon, hydrogen, oxygen boron Figure 8.9 The essential nutrient elements.
Nine of the elements are required in large quantities and are designated as macro- nutrients. The others are only required in small amounts and are the micro- nutrients or trace elements. The macro-nutrient elements are present in soils as ions and may be derived from the parent rock, released from organic matter by the activities of soil micro-organisms, or added in the form of fertilisers. For example, calcium and magnesium occur in limestone and dolomite. Dolomite is a rock which is processed into dolomitic limestone and used as a liming material on acidic soils to reduce the acidity.
In Figure 8.9, three of the major nutrients are called primary elements. These are nitrogen, phosphorus and potassium and can be supplied to crops in the form of inorganic fertilisers (see Topic 8.6). Calcium, magnesium and sulphur are known as secondary elements. Calcium and magnesium help to improve soil aggregation. This affects aeration and tilth in clay soils. Sulphur, needed by all plants for protein synthesis, is obtained from rainwater, farm manure or superphosphate fertiliser added to the soil.
The roles of the major nutrients in crop production are summarised in Table 8.5. Nutrient Role in crop production Signs of deficiency
Nitrogen Needed for protein synthesis; promotes general growth and Stunted plant growth, poor root and shoot juiciness of fruits and grains. development and yellowish leaves.
Phosphorus Speeds up cell division; promotes growth and development Stunted growth, particularly of root systems. of root systems.
Potassium Essential for chlorophyll development; necessary for Leaves become mottled with scorching at the photosynthesis; promotes root systems; influences fruit- edges.
setting.
Calcium Essential for growth and development of root tips; essential Stunted growth; yellowish colour in leaves. for cell wall development.
Magnesium Essential for chlorophyll formation; involved in many Chlorosis (yellowing of the leaves). enzyme reactions.
Sulphur Needed for making proteins. Thin or slender plants; pale green or yellow leaves; late ripening of fruits.
Table 8.5 The roles of the major nutrients.
The micro-nutrients are only required by plants in very small quantities. If there is ^ta deficiency, indicated by poor growth of a crop, they can be supplied to the plants
List the NINE macro-nutrients required by plants. as foliar sprays (applied to the leaves) or combined with other fertilisers and added
Which of these macro-nutrients come from soil? to the soil.