Fertilizers and their Uses


Fertilizers and Their Uses

According to the World Bank’s population projections, the world’s population will increase from 6 billion people in 1999 to 7 billion in 2020. All these people will have to be housed, dressed, and above all, fed. Up to 90 percent of this necessary increase in food production will have to come from fields already under cultivation.

Fertilizers will continue to play a decisive role in increased food production. It is estimated that, globally, roughly 40% of the world’s dietary protein supply in the mid-1990 originated in synthetic nitrogen.

Therefore, in order to obtain high yields, fertilizers are needed to supply the crops with the nutrients the soil is lacking. With fertilizers, crop yields can often be doubled or even tripled.

We will discuss fertilizers and their types, importance of each fertilizer in crop nutrition, deficiency symptom of each fertilizer, methods of calculating fertilizer rates, methods of fertilizer application and determining fertilizer needs are treated.


Plant Nutrients - their Roles in Plant Growth and their Sources

For proper plant growth, a regular supply of plant nutrients especially the essential ones, is necessary. Plants absorb a large number of elements from the soil, air and water during their growth period, but not all of these are essential. Only 16 elements have been found to be essential for all plants and four others have been found to be essential for some plants.


Essential elements for plant growth

For element to be so classified, it has to fulfill the following criteria:

1. A deficiency of the element makes it impossible for the plant to complete the vegetative or reproductive stage of its life cycle

2. The deficiency symptom of the element in question can be prevented or corrected only by supplying that element 

3. The element must have a direct influence on the plant, and must be directly involved in the nutrition of the plant, quite apart from its possible effect in correcting some microbiological or chemical condition in the soil or culture medium.

Essential elements like carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur are the elements of which proteins and hence protoplasm is composed. The other ten elements which are essential for plants are potassium, calcium, magnesium, iron, manganese, molybdenum, copper, boron, zinc, and chlorine.

The four elements which are essential only for some and not for all plants are sodium, cobalt, vanadium and silicon.


Sources of nutrients

The following elements are derived:

a. From air; carbon (C) as CO2 (carbon dioxide)

b. From the water: hydrogen (H) and oxygen (O) as H2O (water)

c. From the soil, fertilizer and animal manure: nitrogen (N) a considerable amount of nitrogen is also fixed by leguminous plants through root nodule bacteria.


Macro and Micronutrients

Macronutrients are needed by the plants in large amount, and large quantities have to be applied if the soil is deficient in one or more of them. Within the group of macronutrients, which are needed for plant growth in large amounts, the primary nutrient such as nitrogen, phosphorus and potassium. Calcium, magnesium and sulphur are sometimes called secondary nutrients due to their secondary importance in plant nutrition.

In contrast with macronutrients, micronutrients or trace elements are required in only minute amounts for correct plant growth and have to be added in very small quantities when they cannot be provided by the soil.


Functions of nutrients

Nitrogen (N)

1. Nitrogen is the motor of plant growth. It makes up to 1 to 4 percent of dry matter of the plant. It is taken up from the soil in the form of nitrate (NO3 - ) or ammonium (NH4 + ).

2. In the plant it combines with compounds produced by carbohydrate metabolism to form amino acids and proteins.

3. Being the essential constituent of proteins, it is involved in all the major process of plant development and yield formation.

4. A good supply of nitrogen for the plant is important also for the uptake of the other nutrients.


Phosphorus (P)

1. It constitutes 0.1 to 0.4 per cent of dry matter of the plant.

2. It plays a key role in the transfer of energy.

3. It is essential for photosynthesis and other chemical-physiological process in the plant.

4. It is indispensable for cell differentiation and for the development of the tissues, which form the growing points of the plant.


Potassium (K)

1. Potassium makes up 1 to 4 percent of the dry matter of the plant.

2. It activates more than 60 enzymes.

3. It plays a vital part in carbohydrate and protein synthesis.

4. Potassium improves the water regime of the plant and increases its tolerance to drought, frost and salinity.

5. Plants well supplied with K are also less affected by diseases.


Magnesium (Mg)

1. It is a central constituent of chlorophyll, the green pigment of the leaves which functions as acceptor of the energy from the sun, thus, 15 to 20 per cent of the magnesium found in plant is contained in the leaves.

2. Mg is also involved in enzyme reactions related to the energy transfer of the plant.


Sulphur (S)

1. It is an essential constituent of protein and also involved in the formation of chlorophyll.

2. In most plants it makes up to 0.2 to 0.3 per cent of dry matter.


Calcium (Ca)

1. Calcium is essential for root growth

2. It is a constituent of cell wall materials.


Micro nutrients or Trace elements

These are iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), chlorine (Cl) and boron (B). They are part of the key substances in plant growth and are comparable with vitamins in human nutrition. Being taken up in minute amounts, their range of optimal supply is very small.


Addition of Plant Nutrients to the Soil

Plant nutrients can be supplied to the soil by adding the following:

1. Organic manure

2. Green manures and other crop residues

3. Concentrated organic manures

4. Commercial fertilizers

5. Soil amendments


a. Organic fertilizers/manures

Farm yard manure, compost, sludge, green manures and other bulky sources of organic matter are known as bulky organic manures. These manures supply plant nutrients in small quantities and organic matter in large quantities. These manures have a direct effect on plant growth, on the humus content of the soil, so improving its physical properties, and on microbial activities in the soil.


i. Farmyard manures

The term farmyard manure (FYM) refers to the refuse from all animals of the farm although as a general rule the bulk of this is produced by cattle. The richest and most concentrated manure is poultry manure which is particularly good for vegetable production. Farm yard manure consists of two components- solids and liquids in a ratio of approximately 3:1. The solid portion is made up straw that has been used for bedding and dung. Dung is mostly undigested food and urine is a fluid waste product. More than 50% of the organic matter in dung is in form of complex products, often of lignin and protein, which are similar to humus. FYM contain on the average 0.5%N, 0.25% P205, and 0.5% K20. Generally, 30% of N, 30% of the P205 and 50% of the K205 in farm manure are available to plants.


Importance of farmyard manure

1. Farmyard manure is source of nutrient especially nitrogen, potash and some trace elements.

2. It influences the physical properties of the soil.

3. Farmyard manure increases the humus content and consequently the water holding capacity of the soil.

4. It improves the structure of the soil by making it more granular, better aerated and better drained. The manure also tends to reduce soil compaction which is often associated with continuous cultivation.

ii. Compost

Compost is well-rotted vegetable matter which is prepared from farm and town refuse. Compost is prepared in trenches of various sizes and shapes. The accumulated refuse is well mixed and then spread in the trench in a layer of about 0.3 m. this layer is then well moistened by sprinkling over it slurry of cow dung and water, or earth and water. Subsequent layers of the same thickness of mixed refuse are then spread on the heap and moisten. After about three month it is now fully decomposed and should be taken out of the trenches formed into conical heaps above ground and covered with earth. After one or two months, the compost will be ready for use. The N, P and K contents of farm compost are on the average 0.5%, 0.15%, 0.5%, respectively, while those of the town compost are 1.4%, 1.0%, 1.4%, respectively.


Advantages of composting organic matter

1. The carbon: nitrogen ratio is improved because carbon dioxide is released to the air by micro-organisms.

2. Improve the structure of the soil by making the soil friable, crumbly and easier to handle and work upon.

3. The heat generated may kill weed seeds and other pathogenic organism.

4. It is the cheapest source of organic manure.

iii. Green manures

This is the practice of growing and ploughing in green crops to increase the organic matter content of the soil. Green manure crops are usually fast growing annual legumes and grasses. They are usually incorporated in the soil when they are green and succulent. Crops which grow rapidly even on poor soils and produce an abundant mass of green leaves and tops can be used as a green manure crop.

Advantages of green manure

1. It increases the organic matter content of the soil.

2. It improves soil structure.

3. Makes phosphorus and certain trace elements available to plants.

4. Checks erosion and leaching.

5. Helps to control weeds by acting as a smother crop.

b. In-organic/commercial fertilizers

What is a fertilizer? Any natural or manufactured material, which contains at least 5 percent of one or more of the three primary nutrients (N P K), can be called fertilizer. Industrially manufactured fertilizers are called mineral fertilizers.

Fertilizer may contain one or more of the essential nutrients. Those that contain only one of the major elements are described as single, simple or straight fertilizers. Those that contain two or more of the major elements are classified as mixed or compound fertilizers. Nitrogen, phosphorous, and potassium are the main plant nutrients and these three provide the basis for the major groups of fertilizers.

With the rapid increase in population and rise in standard of living there is increasing demand for food and feed grains. To meet up with the food demand it is necessary to intensify field crop production. Achieving and sustaining high crop yield of desired quality is only possible through the use of commercial fertilizers.

Although there have been tremendous increases in fertilizer using in tropical Africa over the years, utilization is still on a very small scale relative to the total needs. There is a wide gap between the national requirements for fertilizers and their actual use by farmers.


Chemical Fertilizers

a. Nitrogenous fertilizers

The nitrogen in many straight and compound fertilizers is in the ammonium (NH4 ions) form, but this is quickly changed by the bacteria in the soil to the nitrate (NO3 ions) form.

Most crop plants such as cereals take up and respond to the NO3 ions faster than to the NH4 ions, but some crops, such as rice, potatoes and grasses, are equally responsive to both forms.

On the basis of the chemical form in which nitrogen is combined with other elements in a fertilizer, nitrogenous fertilizers may be classified into four groups:

i. Nitrate fertilizers

In these fertilizers, nitrogen is combined in nitrate (NO3) form with other elements. Such fertilizers are sodium nitrate (NaNo3), having 16 percent N, calcium nitrate [Ca (NO3)2], having 15.5% N, and potassium nitrate (KNO3), having 13.4% N and 44% K.

Nitrate fertilizers are quickly dissociated in the soil, releasing the nitrate ion for plant absorption. As such they are readily absorbed and utilized by the plants. The great mobility of the nitrate ions in the soil has the advantage that, even when applied to the surface of the soil, the nitrogen quickly reaches the root zone. They are therefore very often used as side and top dressings.

However, there is also the increased danger of leaching of these fertilizers.

All the nitrate fertilizers are basic in their residual effect on the soil and their continued use may reduce soil acidity.


ii. Ammonium fertilizers

In these fertilizers, nitrogen is combined in ammonium (NH+ 4) form with other elements examples of such fertilizers:

1. Ammonium sulphate [(NH4)2SO4], having 20% N,

2. Ammonium phosphate (NH4H2PO4) having 20% N and 20% P or 16% N and 20% P;

3. Ammonium chloride (NH4Cl), having 24-26% N,

4. Anhydrous ammonia, having 82% N and 5. Aqueous ammonia having 28% N.

When added to the soil, the ammonium ion is temporarily retained by the colloidal fraction of the soil until it is nitrified. These fertilizers are much more resistant to loss by leaching because the ammonium ions are readily adsorbed on the colloidal complex of the soils. Most ammonium fertilizers have acidic residual effect on the soil.

iii. Nitrate and ammonium

Fertilizers these are fertilizers that contain nitrogen in both ammonium and nitrate forms. Examples of such fertilizers:

1. Ammonium nitrate (NH4NO3), having 32.5% N,

2. Ammonium sulphate nitrate (ASN)[ (NH4)2SO4 . NH4NO3], having 26% N,

3. Calcium ammonium nitrate (CAN) [Ca (NH4NO3)2 ], having 25% N.

These fertilizers are readily soluble in water and suitable for use under variety of soils and cropping conditions. The nitrate nitrogen of these fertilizers are readily available to plants for rapid growth and the ammonium nitrogen resists leaching losses and can be utilized by the plants at a later stage.

These fertilizers are acidic in their residual effect on the soils.

iv. Amide fertilizers

These fertilizers are carbon compounds, and so are called organic fertilizers.

Important fertilizers in this group are:

1. Urea [Ca(NH2)2], having 46% N and

2. Calcium cyanamide (CaCN2), having 22% N. These fertilizers are readily soluble in water and easily decomposed by micro-organisms in the soil.

In the soil they are quickly changed into ammonical nitrogen and then to nitrate form.


General recommendation on the use of nitrogen fertilizers

a) For rice, it is recommended to use ammonia-forming fertilizers such as ammonium sulphate, ammonium chloride and urea. In case these fertilizers are not available, ammonium-nitrate fertilizers such as ammonium sulphate nitrate, ammonium nitrate and calcium ammonium nitrate should be used. For the rest of field crops, all nitrogenous fertilizers are equally effective.

b) Continuous use of ammoniacal or ammonium-forming fertilizers on acidic soil should be avoided as it tends to make the soil more acidic. c) All nitrate fertilizers are best suited for side and top dressing.

d) Since they are easily leached, they should not be applied in large quantities in light sandy soils or during heavy rains. e) The entire recommended dose of nitrogen should be applied in 2 or 3 splits.

b. Phosphorus fertilizers

Crop plants absorb phosphorus in the form of negatively charged ions such as ܪܱܲ4 2or H2PO4 - Phosphorus fertilizers can be classified into three groups depending on the form in which phosphoric acid is combined with calcium.

Phosphorus fertilizers containing water-soluble phosphoric acid or monocalcium phosphate [Ca (H2PO4)2]: such fertilizers are super phosphate, ordinary or single, having 16-18% P2O5; double super phosphate, having 32% P2O5; triple super phosphate, having 46-48% P2O5 and ammonium phosphate, having 20% N and 20% P2O5 or 16% N and 20% P2O5.

These fertilizers are quickly absorbed by the plants, since plants absorb phosphorus as H2PO4 ions. Water-soluble phosphoric acid is rapidly transformed in the soil into a water-insoluble form. As such there is no danger of loss of nutrients by leaching.

This group of fertilizers should not be used in neutral or alkaline soils and not inacidic soils. Under acidic condition, phosphoric acid is converted into monocalcium phosphate, and there is less chances of the phosphate being fixed as iron or aluminum phosphate.

Fertilizers containing citric-acid, soluble phosphoric acid or dicalcium phosphate [CaHPO4]; such fertilizers are basic slag, containing 14- 18% P2O5: dicalcium phosphate, containing 34-39% P2O5 and rhenania phosphate, containing 25-76%. These fertilizers are particularly suitable for acidic soils.

Fertilizers containing insoluble phosphoric [Ca3(PO4)2]: such phosphatic fertilizers are rock phosphate, having 20-40% P2O5: raw bonemeal, having 20-25% P2O5 and 3-4% N and steamed bone-meal, having 22% P2O5. These fertilizers are well suited for strongly acidic soils or organic soils which require large quantities of phosphorus fertilizers to raise the soil fertility.


Principles of effective utilization of phosphorus fertilizers

a) Granular fertilizers with a high degree of water-solubility are more effective on acid and neutral soils than powdered fertilizers.

b) On acid and neutral soils, band application of powdered fertilizer with a high degree of water solubility will give better results than mixing the fertilizers with the soil.

c) Water soluble fertilizers give the greatest response when applied in band.

d) To get optimum response from addition of phosphorus fertilizers other nutrients must be in adequate quantities.

c. Potassium fertilizers

All potassium fertilizers consist essentially of potassium in combination with chloride, sulphate, or nitrate. Almost all potassium fertilisers are water soluble. The following are examples of potassium fertilisers:

a) Potassium chloride [KCl] or muriate of potash, having 60-63% K2O

b) Potassium sulphate [K2SO4 ], having 50 – 53 % K2O5 and 18% of sulphur

c) Potassium-magnesium sulphate [K2SO4. MgSO4] having 22% K2O5

d) Potassium nitrate [KNO3], having 13% nitrogen and 44% K2O5

e) Potassium metaphosphate [KPO3] having 40% K2O and 60% P2O5.


Principles of effective utilization of potassium fertilizer

1. All potassium fertilizers are equally available to plants because all of them are readily soluble in water.

2. Potassium fertilizers containing sulphur, magnesium or sodium have some additional agronomic importance on some soil because of the presence of other elements.

3. Potassium fertilizers containing chlorine or sulphur should be used with caution as they may be injurious to some crops.

d. Compound fertilizers

Compound fertilizers supply two or three of the major plant nutrient elements (i.e. nitrogen, phosphorus, and potassium). They are produced by mixing the straight fertilizers such as ammonium nitrate, ammonium phosphate and muriate of potassium or by more complex chemical processes. The chemical composition of compound fertilizers is usually given as the ratio of nitrogen, phosphorus and potassium expressed as elemental N, P2O5 and K2O respectively. A 15:20:10 compound fertilizer therefore contains 15% N, 20% phosphorus P2O5 and 10% potassium expressed as K2O.


Advantages of compound fertilizers

a) The mixture is usually dry, fine and well mixed and can be applied by hand as well as through a fertilizer drill.

b) The mixture usually contains all major plant nutrients.

c) It saves the farmer time and labour.

d) It does not form lumps or deteriorate in any way if it is not used immediately.


Grades of compound fertilizer

Low grade mixtures are 6:12:6, 5:10:10, and 9:9:0 etc. and high grade mixtures are15:15:15, 20:20:20.

Using high grade fertilizers have some advantages like:

1. Low cost per unit of plant nutrient

2. Lower cost of transportation, labour, and storage

3. Increased the speed of application in the field.

e. Slow release fertilizers

Slow or controlled release fertilizers contain a plant nutrient (usually nitrogen) in a form, which after application delays its availability for plant uptake significantly longer than a common fertilizer. This effect is obtained either by coating a common (nitrogen or NPK) fertilizer with sulphur or with a semi-permeable polymer material or by special chemical nitrogen compound formulations.


Advantages of slow release fertilizer

1. Labor saving, instead of several split application only one for the whole growing periods.

2. Reduces toxicity to seedlings even with high application rates.


Disadvantages of slow release fertilizer

The cost per unit of nutrient is considerably high than that in common fertilizers.

f. Nitrification and urease inhibitors

Nitrification inhibitors are compounds which, when added to nitrogen fertilizers (containing the nitrogen in form of ammonia (NH4 + ) causes delay in the transformation of the ammonium-ion (NH4 + ) held by the adsorption complex into nitrite (NO2) and further to nitrate (NO3 - ) through the activities of soil bacteria, thus preventing leaching of nitrate not taken up immediately by the crop.

Urease inhibitors are compounds that depress the transformation of the amide-N in urea into ammonium for about 10 to 12 days; thus preventing, or reducing, evaporation losses of ammonia to the air when the weather stays dry or the urea cannot be incorporated into the soil immediately after application.

Both nitrification and urease inhibitors are strongly mixed with the nitrogen fertilizers before spreading and then spread together in the mixture.


Calculating the rate of fertilizer to be applied

The amount of fertilizer to be applied per hectare on a given field is determined by:

1. The amount of nutrients needed by plant for optimum growth and productivity

2. The availability of nutrient in the soil (level of soil fertility)

3. The moisture status of the soil

4. The type of crops to be grown

5. The types and grades of fertilizers available.

Usually mineral fertilizers are delivered in 50-kg bags while the nutrient content (active ingredients) is given in percentages e.g. N 15 P15 K15. Meaning that each 50kg bag contained 15% N 15% P 15% K.


Steps to follow in calculating the rate of fertilizer to be applied

i. Determine the quantity of active ingredients contained in each bag, this could be done by dividing the percentage of each nutrient by two. Example, N15 P15 K15 would contain active ingredients N 15/2= 7.5kg P15/2= 7.5kg K 15/2=7.5kg. If the recommendation is to apply N60-P60-K60 per hecter, the easiest option for the farmer is to buy a multi nutrient (compound) fertiliser grade N15-P15-K15. One 50-kg bag contains N7.5kg -P7.5kg-K7.5kg of active ingredients.

To find the number of bags required to supply the recommended nutrients:

i. 1 bag = 7.5kg x = 60kg find X X = 60 divided by 7.5 = 8 bags of NPK.

ii. Example: how many bags of ammonium sulphate (AS) (with 21% N and 24% S) are needed to supply 60 kg/ha of N? 21 divided by 2 gives 10.5. Thus, approximately six bags of AS would supply 60.5 kg of active ingredients of N. In addition, six bags of AS will supply 72 kg/ha of sulphur. Thus eight bags of 50 kg of N15-P15-K15 are needed to apply the recommended rate of 60 kg/ha N, 60 kg/ha P2O5 and 60 kg/ha K2O.

iii. When the recommendation per hectare isN60-P30-K30, with 50- kg bags of a N15-P15-K15 grade the farmer would divide 30 by 7.5 = 4. Since each 50kg bag contains 7.5 kg of active ingredients of NPK.

In this case he should apply only four 50-kg bags of NPK fertilizer per hectare, giving half of the recommended rate of nitrogen and the full rate of phosphate and potassium as basal dressing. The remaining 30 kg/ha N should be applied in the form of a straight nitrogen fertilizer as one or two top-dressings in line with good agricultural practices.


Time and Method of Fertilizer Application

To achieve maximum benefit from fertilizers, it is most essential to apply them at the right time and in the right place. The amount and timing of nutrient uptake depends on various factors, such as crop variety, planting date, crop rotation, soil and weather conditions. For good agricultural practices, the farmer chooses the timing and the quantity in such a way that as much as possible of the nutrients is used by the plants. For optimum crop use efficiency and minimum potential for environmental pollution, the farmer must apply the nutrients as near to the time the crop needs them.

This is particularly important for mobile nutrients such as nitrogen, which can easily be leached out of the soil profile, if they are not taken up by the plant roots.

i. Time of applying nitrogen

Fertilizers Since nitrogen is required throughout the growth period and nitrogenous fertilizer is lost through leaching, it is better not to apply too much nitrogen at one time. The split application of nitrogen throughout the growing period will ensure greater efficiency and plants would not suffer from nitrogen deficiency.

ii. Phosphorus

This element is required in greater quantities during the early growth period and as all phosphorus fertilizers become available to growing plant slowly, it is always recommended that the entire quantity of phosphorus fertilizers be applied in single doze before sowing or planting.

iii. Potassium

This element is absorbed right up to the harvest stage but it becomes available slowly. It is therefore always advisable to apply the entire quantity of potassium at sowing time.


Methods of Fertilizer Application

The method of application of fertilizers (organic manure or mineral fertilizers) is an essential component of good agricultural practices. A fast start and continued nutrition is essential for sustained maximum profit. It is important to place some of the fertilizer where it will intercept the roots of the young plant and to place the bulk of the nutrients deeper in the soil.

Nitrogenous fertilizers are easily soluble in water and have mobility, so they can be applied on the soil surface. Phosphorus fertilizers moves slowly from the point of placement, it should be placed closer to the plant roots. To reduce phosphate fixation, phosphorus fertilizers should be so placed that they come into minimum contact with the soil particles and are close to the plant roots.

Potassium fertilizer moves slowly in the soil, they should also be placed near the root zone. Based on these principles, the following methods are used to apply fertilizers.

i. Broadcasting

The fertilizer is spread over the entire soil surface to be fertilized with the objective of distributing the whole quantity of fertilizer evenly and uniformly and incorporating it in the plough layer. It is used mostly on dense crops not planted in rows or in dense rows and on grassland. It is also used when fertilizer should be incorporated into the soil after application to be effective (phosphate fertilizers), or to avoid evaporation losses of nitrogen (urea, diammonium phosphate).

Incorporation through tilling or ploughing-in is also recommended to increase the fertility level of the entire plough layer. Whether the fertilizer is broadcast by hand or with fertilizer spreading equipment, the spreading should be as uniform as possible.

ii. Row or band placement

This refers to the application of fertilizers into the soil close to the seed or plant and is employed when relatively small quantities of fertilizers are to be applied. When fertilizers are placed along with, or close to the seed or plant in bands or pockets, the roots of the young plants are assured of an adequate supply of nutrients and this promotes rapid early growth. This method of placement also reduces the fixation of phosphorus and potassium. When seeds or plants are sown close together in a row, the fertilizer is put in continuous band on one or both sides of the row. This method of application is referred to as row placement, and is used for potatoes, maize, tobacco, cotton, sugar cane, etc. Where crops are cultivated by hand and planted in hills, the recommended grams of fertilizer are placed in the row or planting hole, under, or beside the seed, and covered with soil, this is known as hill placement. Great care has to be taken such that no fertilizer is placed either too close to the seed or to the germinating plant to avoid toxicity.

iii. Top dressing

Top-dressing (broadcasting the fertilizer on standing crop) is mainly used for small and large grain crops and for crops such as forage, wheat and barley.

Top dressing of additional nitrogen is done when:

i. a single application of the total nitrogen needed at sowing might lead to losses through leaching and run-off.

ii. Or where crops show a special need for nitrogen at certain stages of growth. Top dressing of potassium, which does not move in the soil to the same extent as nitrogen, might be recommended on light soils, i.e. applying the total amount divided into a basal dressing and top-dressing.

Phosphate hardly moves in the soil at all. Hence, it is usually applied before or at sowing or planting time (basal application), preferably in combination with potassium and part of the nitrogen.

Side dressing: this is also another form of top dressing where fertilizer is spread between the rows or around the plants. Maize, cotton, sugar cane, trees and other perennial crops are normally side-dressed.

iv. Foliar application of fertilizer

Foliar application refers to the spraying of the leaves of growing plants with suitable fertilizer solutions. It is used mainly to correct micronutrient deficiencies.

To minimize the risk of leaf scorch, the recommended concentration has to be respected and spraying should preferably be done on cloudy days and in the early morning or late afternoon.

v. Direct application into the soil

With the help of some special equipment, anhydrous ammonia (liquid fertilizer) and nitrogen solutions can be applied directly into the soil. There is very little plant injury or wastage of ammonium if the material is applied about 10cm below the seed, and the soil is moist.

vi. Application through irrigation water

Straight or mixed fertilizers which are easily soluble in water are allowed to dissolve in the irrigation stream. The nutrients are thus carried into the soil in solution. The fertilizers most commonly applied through irrigation water are nitrogenous fertilizers.


How to Determine Fertilizer Needs

To determine fertilizer needs for crops and soils in your locality you must know two things:

1. The status of nutrients in the soil

2. How much of each nutrient is needed to get the highest or most profitable (optimum) yield?

There are several approaches to finding the answers to these questions.

1. Fertilizer recommendation of crops.

2. Nutrient hunger signs on growing crops (deficiency symptoms).

3. Soil tests or analyses to determine the fertilizer nutrients and amount needed.

4. Plant or plant tissue test in the field.

5. Fertilizer field trials


Hunger signs in plants (nutrient deficiency symptoms)

If plants do not get enough of a particular nutrient they need, the symptoms show in the general appearance as well as in the colour of the plant.

Very typical symptom are: the nutrient deficient plants are stunted , the leaves have a pale green colour or a very dark bluish green colour, yellowish or have reddish spotting or striping. At harvest, yield is reduced, sometimes severely.


Nitrogen deficiency symptoms

1. Stunted growth.

2. Loss of green colour, yellow discolouration of leaves from tip backward, older leaves brown.

3. Lower leaves may die premature while the top of the plant remains green.


Phosphorus deficiency symptoms

1. Stunted growth.

2. Leaves turn dark bluish green, purpling and browning from tip backward.

3. Plants slow to ripen, remaining green.

4. Fruits may be misshapen, grain is poorly filled.


Potassium deficiency symptoms

1. Stunted growth.

2. Leaves show discolouration along outer margin from tip to base.

3. Outer edges of leaves yellow or reddish, becoming brownish or scorched and dead; leaves wilted.

4. Lodging.

5. Tree leaves are yellowish, reddish, pinched, cupped or curved.

6. Fruit is small, may have lesions or injured sport, poor storage and keeping quality.


Magnesium deficiency symptoms

Yellowish discolouration between Greenleaf veins, followed by blotching and necrosis (death of tissue), starting at lower older leaves.


Sulphur deficiency symptoms

1. Whole plant is yellowish (often mistaken as N deficiency).

2. Yellowish of upper leaves, even on newest growth.

3. Delayed crop maturity.



Calcium deficiency symptoms

1. Young leaves turn yellowish to black and curved or cupped (brown spot).

2. Plants appear to wilt.

3. Fruits may appear rotten.

4. Roots are malformed.


Boron deficiency symptoms

1. Leaves frequently misshapen and crinkled, thick and brittle, white, irregular spots between veins.

2. Growing tips of buds die, with bushy growth near tips extension growth inhibited with shortened internodes.

3. Water-soaked, necrotic spots or cavities in beet and other root crops and in the pith of stems.

4. Fruit small and poorly formed, often with corky nodules and lesions.

5. Low seed production due to incomplete fertilization.


Zinc deficiency symptoms

1. Stunted growth of leaves.

2. Fruit trees with typical shortened bushy shoots.

3. Chlorotic stripes (white bleached bands) between the leaf veins in lower part of leaf.

4. In some cases leaves have an olive green or grayish green colour (very similar to P deficiency).


Iron deficiency symptom

Young leaves with typical chlorosis between green veins, along the entire length of leaves (usually on calcareous soils).


Soil Amendments

Soil amendments are the substances used for correcting the acidity or alkalinity of the soil. In high rainfall areas, there is considerable leaching of bases leading to the formation of acidic soil, while in low land areas, saline and alkaline soils occur.



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