Plants, like animals, require food in the form of nutrients for their growth and development. These nutrients are generally called essential nutrients. Most of these elements occur in the soil in combination with other elements like oxygen and hydrogen. The role of these nutrients, their deficiency symptoms, the ways the nutrients deplete and replenish in the soil will be discussed here. It will also consider the various ways in which certain nutrients are removed and added into the soil in nature.
Essential nutrients are divided into two groups based on the quantity of nutrients that are required by plants. These are the macro nutrients and the micro nutrients.
(i) Macro nutrients: These are nutrients that are required by plants in relatively large quantities, examples include: Nitrogen, Phosphorus, Potassium, Calcium, magnesium, sulphur. They are obtained from the soil.
(ii) Micro nutrients: These are also called trace elements. They are required by plants in minute or small quantities. Plants cannot survive without the trace elements; examples of micro nutrients include, iron, manganese, molybdenum, boron, zinc, cobalt, copper and chlorine. They are also obtained from the soil.
Functions and Deficiency Symptoms of the Nutrient Elements
Nitrogen is a constituent of protein: it is absorbed by plants in the form of ammonium (NH4=)ions. The Functions of Nitrogen are:
(i) It impacts the dark green colour in the leaves.
(ii) It encourages vegetative growth and makes It look succulent.
(iii) It increases the size of grains in cereals.
(iv) It facilitates the absorption of phosphorus and potassium.
(v) Increases carbohydrate synthesis. Excess nitrogen delays fruiting and ripening as well as maturation.
(i) Yellowing of the leaves especially the old ones. This is regarded as chlorosis.
(ii) Stunted growth in plant.
(iii) Chatty stems and leaves.
(iv) Premature dropping of leaves may occur.
Nitrogen deficiency can be corrected by the use of nitrogen containing fertilisers such as ammonium sulphate and calcium ammonium nitrate (CAN) as well as through the use of organic compounds like compost, farmyard manure and green manure.
(i) It is essential for cell division.
(ii) It is necessary for seed germination, root development, flowering, fruiting and seed development.
(iii) It quickens the ripening of fruits.
(iv) It also increases the resistance of plants to some diseases.
(v) It improves the quality of plant produce.
(vi) Improves the palatability of forages and vegetables. Excess of phosphorus leads to iron deficiency.
(i) Premature death of leaves.
(ii) Lodging results due to poor development of roots.
(iii) Stunted growth of green parts of the leaves.
(iv) Reduction in metabolism in certain organs of the plants.
(v) Poor root development.
(vi) Flowering, fruiting and seed development are retarded.
(vii) The colour of the leaves may turn purple, especially in maize.
Fertilisers that contain phosphorus are single super phosphate and triple super phosphate; wood ash and organic manures can also serve as a source of phosphorus.
Potassium is usually absorbed as potassium cation (K-). It has a balancing effect on nitrogen and phosphorus.
(i) It increases the strength of the stem thus preventing lodging.
(ii) It also increases resistance of plant to fungal and bacterial diseases.
(iii) It is essential for chlorophyll formation and the synthesis of carbohydrate.
(iv) It influences the rate of transpiration and respiration.
(v) Helps in the uptake of nitrates from the soil.
(vi) It neutralises organic acids in plants.
(vii) It is important in the formation of grains in cereal and tuber crops like yam.
(viii) It activates various plant enzymes.
(i) The edges of leaves look dry and scorched: irregular chlorosis appear on the surface of the leaves.
(ii) Poor roots and tuber development in cassava, yam, and sweet potato.
(iii) Premature death of leaves.
Potassium containing fertiliser is muriate of potash. Other sources are wood ash, organic manure, oil palm branch and rock minerals like mica, feldspar, etc.
(i) It helps to reduce acidity of the soil.
(ii) It is necessary for growth of plants particularly the growth and proper functioning of the meristems (the growing tips of stems and roots) of plants.
(iii) It tends to thicken the cell walls of plants.
(iv) It aids the translocation and deposition of carbohydrates and protein, in seeds and tubers.
(v) Excess calcium increases alkalinity and reduces Boron utilisation by plants.
(i) It leads to stunted growth.
(ii) Poor root development.
(iii) It also results in the breakdown of the cell wall leading to the malfunctioning of the plants parts. Calcium deficiency can be corrected by the addition of simple calcium salts, such as limestone , quick lime (CaO), slaked lime , and gypsum . These materials are referred to as limes.
Magnesium is supplied to the soil along with calcium in lime materials. It could also come from the weathering of primary minerals, as well as from the decay of organic matter.
(i) It is an essential constituent of chlorophyll molecules necessary for photosynthesis.
(ii) It is a soil conditioner i.e. it controls acidity.
(iii) It helps in the transportation of phosphate ions.
(i) Causes chlorosis or yellowing of leaves along the leaf veins.
(ii) Reduces the rate of photosynthesis.
(iii) It causes stunted growth.
(i) It is a member of a plant protein.
(ii) It is required for nitrogen fixation by legumes.
(i) Stunted growth.
(ii) Disturbed photosynthesis.
(iii) Causes yellowing of leaves.
Generally, in view of the fact that abundant sulphur can be supplied by organic matter, their deficiency is not very common.
The micro nutrients
(i) Iron: It plays an important role in chlorophyll. It facilitates a number of enzyme reactions in the plant. Deficiency may result in chlorosis.
(ii) Molybdenum: It is taken in form of anions . It is necessary for nitrogen fixation in leguminous plants and for protein synthesis. It can be corrected by using sodium molybdate or ammonium molybdate.
(iii) Boron: This is taken in form of borate anions. It is important in protein synthesis. It aids in nitrogen and carbohydrate utilisation. It facilitates the development of roots and formation of fruits and seeds. Deficiency results in poor growth, or at times, death of the terminal buds.
(iv) Manganese, Copper, Zinc: Zinc are activators of enzymes in the body of plants. Their deficiency results in reduced growth of plant. Zinc is also necessary in the formation of some plant growth hormones and in the production and maturation of seeds.
Factors Affecting Nutrient Availability
Many factors are responsible for the short supply of nutrients to plants, these include:
(i) Soil pH or soil reaction.
(ii) Excess of other nutrients.
(iv) Crop removal.
1. Effect of soil reaction or soil pH
pH is the index of hydrogen ion concentration in the soil situation. Some nutrients become available to plant when pH is high (increase in alkalinity). This increase in alkalinity will in turn increase the amount of nutrients, like calcium, potassium, magnesium and molybdenum that is available in the soil.
Similarly, nutrients like iron, manganese, phosphrous and boron will be more available with a decrease in the soil pH. Other nutrients like copper and zinc are available at neutral pH level, for the availability of most plant nutrients is 6.0 – 7.5.
2. Effect of excess amount of other nutrients
Nutrients interact, with one another in the soil. The availability of some nutrients depends on the proportion of the elements in the soil. Balancing of the nutrients thus, is important for their proper utilisation, e.g. an excess amount of soluble iron and aluminum, as well as calcium will reduce the availability of phosphorus in soils. Similarly, when copper, zinc and manganese are in excess, iron tends to be unavailable to plants. Excess calcium reduces the utilisation of boron by plants. Also molybdenum becomes deficient when there is excess of copper. All these elements are said to be antagonistic to each other.
3. Effect of leaching
Nutrients like nitrogen, calcium, magnesium and potassium when in soluble forms, are easily washed from the top soil to the subsoil, beyond root zones. This will make the nutrients unavailable to plants. Leaching also results in soil acidity which in itself can affect adverse nutrient availability.
4 Crop removal
When crops grow they derive nutrients from the soil which they use for the building up of their tissues. When such plants are cut and carried away without allowing them to be returned to the soil, availability of such nutrients is reduced. Removal of nutrients from soil also leads to the concentration of hydrogen ions (acidity).
5. Effect of oxidation
Oxidation is the breakdown of soil organic matter in the presence of oxygen. Ammonium compound in the soil is broken-down (oxidized) to ammonia. Similarly, nitrate may be reduced to nitrogen gas, or nitrogen oxide by denitrifying bacteria. The gases so produced from these reactions are lost to the atmosphere.
6. Effect of burning
When vegetation is burnt, nutrients like nitrogen, carbon, sulphur, phosphorus, etc., are lost to the atmosphere in form of gases. The ash that is left after burning is either blown away, or lost through run off in areas with high rainfall. Burning destroys organic matter as well as kills the micro organisms that help to generate soil nutrients.
Methods of Replenishing Soil Nutrients
A soil is said to be fertile when crops can be successfully grown on it, but the fertility of the soil loses its nutrients each year it is cropped. There are some farm practices that can be adapted to reduce or even prevent these loses. They include:
(i) Crop rotation: This is the growth of different crops continuously on the same piece of land. The crops planted are changed so as to bring about reasonable yield and still maintain its fertility. Roots of crops derive their nutrients from different levels in the soil. Similarly, plants differ in the amount of nutrients removed from the soil. If these crops which differ in nutrient requirement and different feeding levels are alternated on the same piece of land, the fertility of the soil will be maintained for a reasonable length of time.
(ii) Organic manuring: Organic manure are decaying plants and animal substances added to the soil to improve its fertility. It could be divided into three: – namely compost manure, farmyard manure and green manure. Compost manure are made up of plants and animal remains which are stored in either heap or pits and allowed to decay. Farmyard manure is made up of a mixture of faeces, urine and other metabolic wastes with animal beddings. Green manure involves burying of growing crops, which have reached flowering stage into the soil to decompose and add nutrients to the soil. Organic manure acts as a store house from which plant nutrients are released into the soil. It enhances water holding capacity of soil as well as increase circulation of air in the soil. Soil organisms are also encouraged by organic manuring.
(iii) Fertiliser application: Fertilisers are any organic salt chemically manufactured and added to the soil to improve its fertility. Nitrogen, Phosphorus and Potassium are important in fertiliser production, thus, they are called primary or key element, Fertilisers are classified into two on the basis of the nutrients they supply. When fertilisers supply only one key element, it is said to be a single or straight e.g. fertiliser e.g. potassium sulphate which supplies only potassium. On the other hand when a fertiliser supplies more than one primary element, it is called compound or mixed fertiliser e.g. ammonium phosphate contains or supplies nitrogen and phosphorus. Fertilisers are applied to the soil in different ways such as:
– Row placement.
– Top dressing.
– Ring method.
– Band placement.
This is the method of leaving the soil uncultivated for a period of time after being used. Vegetation growing on land shed leaves, branches and flowers fall on the soil and these will turn into humus containing plant nutrients. The legumes growing there can add nitrogen into the soil, as well as prevent erosion. The activities of rodents and micro organism will increase the aeration of the soil, by improving their structure.
(ii) Cover cropping
This is the growing of creeping or spreading plants to cover the surface of the soil with their leaves. This will prevent erosion and leaching and thus prevent nutrient loss from the soil. Nitrogen is also fixed into the soil through the activities of the nitrogen fixing bacteria. Cover cropping helps in nutrient recycling and maintenance of soil especially when the crops are turned into the soil. They also help to conserve soil water and soil micro organisms.
This is the addition of any compound of calcium and magnesium to the soil to reduce its acidity e.g. calcium oxide, calcium carbonate and calcium magnesium carbonate. Liming also adds calcium and magnesium into the soil. Acid soils affect nutrient availability. Also acidity reduces the effectiveness of soil micro organisms that either help in decomposition or in fixing nitrogen into the soil. This is the process of covering the surface of the soil with organic materials such as straw, dry grasses, etc.
Helps in cooling the soil temperature thus reducing evaporation. It also prevents weeds from growing in farms. Mulch also prevents nutrient loss through erosion. Similarly, the organic material (mulch material) can decompose and add nutrients into the soil.
(i) Nitrogen cycle
Nitrogen cycle is a sequence of reactions indicating the various means by which nitrogen is naturally added to and removed from the soil, and the atmosphere. Through this reaction, plants and animals get the nitrogen they utilise in making proteins. The process begins with the fixation of gaseous nitrogen in the soil. There are various ways in which nitrogen is fixed into the soil. One source of nitrogen fixation is through the agency of some bacteria in the soil called nitrogen fixing bacteria. Another form is through electrical discharge in the atmosphere, nitrogen can also be added into the soil by man through the application of nitrogenous fertilisers. In the nitrogen fixation process that involves soil organism, the atmospheric nitrogen is trapped by the nitrogen fixing bacteria and fixed into the soil where they can be used by plants. Note that two types of nitrogen fixing bacteria are responsible for this fixation. There are those that live symbiotically with root nodules of legumes e.g. rhizobium, and those that live freely in the soil e.g. azobacter. Plants take up this fixed nitrogen in the form of ammonium ion (NH4+) or nitrate ions (NO3-). Often, plant parts are eaten by animals, where they obtain the nitrogen they use in building up their tissue. When plants and animals die, they get decomposed. This decomposition or mineralisation is encouraged by putrefying bacteria with the release of ammonia, carbondioxide and water. This process is regarded as ammonification, some ammonia escapes into the atmosphere while some are changed into nitrite and nitrate by special types of bacteria. Nitrosomonas will first change ammonium/ammonia to nitrite while the nitrobacter will then transform the nitrite to nitrate. The process involved in changing ammonia to nitrate is known as nitrification.
Some of the nitrates are absorbed by plant roots, while others may be lost through leaching, erosion and drainage. Others still, may be transformed into their gaseous forms and escape into the atmosphere. The transformation of nitrate to their gaseous forms is called denitrification and is carried out by denitrifying bacteria. Denitrification occurs mainly in poorly aerated soils. During a thunderstorm when it rains, the atmospheric nitrogen is broken up by electrical discharge from lightening. The nitrogen is oxidised and washed into the soil by rain water. Further oxidation changes the oxides of nitrogen into nitrate which the plants can absorb, and the cycle continues. Man often adds nitrogen to the soil through the application of nitrogenous fertilisers, organic manure etc. Through all the above gains, losses and transformation, nitrogen continues to circulate both in the atmosphere and the soil.
(ii) Carbon cycle
Carbon dioxide makes up about 0.03% of the air. This carbon dioxide is taken up by green plants from the atmosphere and used to make carbohydrate in the presence of soil water, sunlight and chlorophyll (photosynthesis). When plants are eaten by animals and digested, absorbed and assimilated, the carbohydrate will be broken down during tissue respiration to produce energy, carbon dioxide and water. The carbon dioxide so produced during respiration is again released to the atmosphere to continue with the cycle. Apart from respiration, carbon dioxide can also be released to the atmosphere through the decomposition of organic matter as well as through burning (combustion).
(iii) Water cycle
Water cycle involves the unending circulation of water from the land-sea surface to the land-sea surfaces. This cycle begins with the evaporation of water from oceans, rivers, lakes, streams, puddles, soils, leaves of plants through transpiration to the atmosphere as vapour, where it cools and condenses into dews and falls back to the land in form of precipitation.
Nutrients are required by plants for their growth and development. These nutrients may be required by plants in large quantities and are called macro nutrients or they could be required in minute quantities (micro nutrients). Nutrient availability in the soil and their utilisation by plants are affected by the following factors-soil reaction, excess of other nutrients, leaching, crop removal, oxidation and burning. The following farming practices can be adopted to reduce or even prevent these losses; crop rotation, organic manuring, fertiliser application, fallowing, cover cropping as well as mulching. Apart from the above mentioned; nature also has a way of adding and removing nutrients such as nitrogen, carbon and water from the soil in form of cycles.