Nutrients Required for Plant Growth
The previous lesson examined the key nutrients important for animal growth. In the table below, we will now look at the key nutrients required for plant growth.
| Mineral | Source | Main Functions | Deficiency Disease |
| Macro-nutrients | |||
| Calcium (Ca) | inorganic fertilisers; Ca ions in the soil | part of the plant cell wall; transport and rention of other elements | chlorosis |
| Magnesium (Mg) | inorganic fertilisers; Mg ions in the soil | component of chlorophyll (pigment for photosynthesis); activates many enzymes required for growth | chlorosis (the low production or loss of chlorophyll in plant leaves) |
| Nitrogen (N) | inorganic fertilisers in the form of nitrates; symbiotic nitrogen-fixing bacteria in roots | component of chlorophyll; nucleic acids and proteins; seed and fruit production | stunted growth; smaller leaves |
| Phosphorus (P) | inorganic fertilisers in the form of phosphates; low amounts in the soil | photosynthetic process; part of nucleic acids and cell membranes; root growth | stunted growth, blue/green leaves |
| Potassium | inorganic fertilisers; K ions in the soil | needed for protein synthesis, photosynthesis, enzyme activation, opening and closing of stomata; | chlorosis; curling leaf tips; brown scorching, poor fruit quality |
| Sulfur (S) | inorganic fertilisers | protein synthesis; root growth; chlorophyll formation; promotes activity of enzymes | chlorosis |
| Micro-nutrients | |||
| Iron (Fe) | inorganic fertilisers; Fe ions in the soil | component of the enzyme that makes chlorophyll | chlorosis |
| Zinc (Zn) | inorganic fertilisers; Zn ions in the soil | part of growth-regulating enzyme systems | poor leaf growth |
| Sodium (Na) | inorganic fertilisers; Na ions in the soil | maintains salt and water balance | reduced growth |
| Iodine (I) | inorganic fertilisers; I ions in the soil | needed for energy release during respiration | poor growth |
Use of Fertilisers
When crops are regularly grown and harvested on the same piece of land, the soil becomes depleted of one or more nutrients. Fertilisers are natural or non-natural mixtures of chemical substances that are used to return depleted nutrients to the soil, improve the nutrient content of the soil and promote plant growth. Inorganic nutrients (such as nitrates and phosphates) are added to the soil in the form of inorganic fertilisers.
Effect of Fertilisers on the Environment
Using large amounts of fertilisers can be harmful to the environment. Fertilisers wash off into rivers where they are poisonous to plant and animal life. The accumulation of fertilisers in rivers can lead to a process known as eutrophication. This process occurs when excessive nutrients (nitrates and phosphates) from the land (typically from fertilisers) run off into rivers and lakes. This leads to high growth of water plants. Plants grow and produce food by photosynthesis which requires high quantities of oxygen.
The high oxygen demand of the rapidly growing water plants removes oxygen available to other organisms in the rivers and lakes. These organisms then suffocate and die due to lack of oxygen. The biodegradation of the dead organisms results in a massive increase in bacteria, fungi and algae degrading the dead organic matter, which also require oxygen. This further depletes the available oxygen, and further contributes to the death of fish and other aquatic species.
Image credit: Siyavula
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Natural Fertilisers: An Application of Indigenous Knowledge Systems
The fertilisers discussed above are non-natural inorganic compounds such as nitrates, phosphates etc. However, as a means of reducing the negative impact of the inorganic fertilisers discussed earlier, organic fertilisers that occur naturally can be used. Natural fertilisers consist of organic compounds derived from manure, slurry, worm castings, peat, seaweed etc.
Natural fertilisers supply nutrients to the soil through natural processes such as composting. This means that the nutrients are released back to the soil slowly, and excessive nutrients do not wash off into rivers causing over-fertilisation and eutrophication. However, the use of organic fertilisers is more labour-intensive and the nutrient composition tends to be more variable than the inorganic fertilisers. As a result it is difficult to know for sure whether the particular nutrient required by the plant is actually being supplied by the natural fertiliser.
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