Plant Nutrient Requirements

Plants need about 17 nutrient elements to enable them to carry out their metabolic functions.  Of these 17 fourteen can be supplied through the soil.  The fourteen can be divided into two broad groups, the macro nutrients which are needed in large amounts and the micro nutrients which are needed in minute quantities.

They are:

Macro     

	Nitrogen (N)
	Phosphorus (P)
	Potassium (K)
	Calcium (Ca)
	Magnesium (Mg)
	Sulphur (S)

Micro

	Cobalt (Co)
	Iron (Fe)
	Boron (B)
	Molybdenum (Mo)
	Maganese (Mn)
	Copper (Cu)
	Zinc (Zn)
	Chlorine (Cl)

These nutrients have the following functions in the plants:

N - Control growth and fruiting of most plants

	is important in protein synthesis and is an integral part of all amino acids.
	is important in chlorophyll.
	controls the vegetative and reproductive stage of the plant.

P - essential in cell division

	essential in root growth.
	essential in energy transformation in cells.
	necessary for starch deposits in storage organs.

K - essential to flowering

	believe to control H2O relations of plant cells.
	believe to catalyze many biochemical reactions especially photosynthesis and synthesis of amino acids.

Ca - cement cell walls

	neutralize harmful excessive amounts of organic acid by-products.
	important in strengthening cell walls.
	important in the growth of meristems.

Mg - key element in chlorophyll molecules

important in the transport of phosphates.

S - essential in some amino acids

Fe - important in the chlorophyll molecule

B - seems to be related to Ca (if Ca: B ratio is upset characteristic symptoms develop).

Co - necessary in symbiotic N fixation by rhizobium and soyabean and by alfalfa.

Mn - catalyses biochemical reactions in cells

Zn - "

Cu - "

What is a Fertilizer?

A fertilizer is any substance added to the soil to supply one or more plant nutrient.

Why use Fertilizers?

To maintain adequate yields it now becomes necessary to supply these depleted nutrients to the soil in the form of fertilizers.

Kinds of Fertilizers

There are two major types of fertilizers:

(a) Organic fertilizers and manures:  Organic fertilizers are usually waste products from industrial processing of parts of plants and animals.  They contain more N & P than manures and hence are classed as fertilizers.  They are used intensively in horticulture.  The N & P are not water soluble hence they act as slow release fertilizers also, they contain little or no soluble salts hence they can be applied in large quantities without damage to crop roots.

Organic manures (OM) are composed mainly of wastes and residues from plant and animal life.  They contain much carbon and small percentage of plant food which are usually fixed in the carbon.  Most of the carbon is used as food for soil organisms.  OM have the ability to improve soil structure either directly through their action as bulky diluents in compacted soils or indirectly when the waste products of the animal or micro-organisms cement soil particles.  OM includes slurries and sludges and farm yard manure (FYM).  

(b) Inorganic fertilizers:  These are simple chemical compounds made in a factory (Urea) or obtained by mining (Ca₃ (P₂O₄)₂), which supply plant nutrients.  They are not residues of plants or animals.  Inorganic fertilizers should be taken in its broadcast form to include materials supplying N, P, K, Ca, Mg, etc.

Diagnosing Nutrient Needs of Plants

Determination of the type and amounts of a nutrient to be used could be best done using soil and foliar analyses.  These will give the amounts of nutrients present in the soil and plant tissue.  From this the limiting level of each nutrient would be identified and the amount necessary to bring it to an acceptable level, or, the amounts of each nutrient required to increase yields can be determined.

Factors Affecting Fertilizer Use Efficiency

The efficient use of fertilizers entails the use of the right kind of material, using it at the right time and the right amount and placing it in such a way that plants make maximum use of the material.  Failing to observe these criteria can result in the material being lost to the plant due to:

   Volatilization and Nitrification - N fertilizers

Leaching - N, K, Ca, Mg and some micronutrients

Fixation - N, P, K, Ca and some micronutrients

Erosion - all materials

Antagonism - P, Zn, P, Cu, CaCO₃, K fixation

The type of soil is also important in determining the kind and amounts of fertilizer to be used.  In acid soils applied P will be fixed by Al, Fe or Mn, depending on the pH value of the soil, hence the amounts and type of P fertilizer material used will be very critical.  As the pH value approaches neutral P will be released into the system.  Above pH 7 fixation of P by Ca becomes the problem.  The use of ammonium sulphate (NH₄)₂ SO₄) is not recommended on soils below pH 7 due to the acid forming properties of this fertilizer material.

Sandy soils have a high leaching potential hence the frequency of application of most fertilizers is increased (N, K etc).  On the other hand soils with high clay content has poor leaching potential but high run-off potential.  Hence fertilizer material on these soils could readily be lost due to volatilization and nitrification (N) and run-off (N, P, K etc).

Timing of Fertilizer Application

The timing and frequency of application of fertilizers is dependant on the age of the plant, the type of plant, the type of fertilizer material, and the type of soil.

At the early stage of crop development it is essential that the fertilizer be given to the crop within the first two weeks (7-14 days) after emergence so as to enhance vigorous growth both below  and above ground.  The next critical stage for fertilizer application (annuals) is around the time for flower initiation.  In perennials fertilizers should be applied just after the rainy season to coincide with the development of new "flushes" or at flowering.  Some fertilizer can remain unchanged in the soil for long periods because of their slow release nature (P) and one application will provide nutrients for up to three years (Cooke, 1982) while either due to volatilization and leaching (N) or leaching (N, K etc) needs to be applied more frequently.

Method of Applying Fertilizers

In applying fertilizers care should be taken that the fertilizer is placed near enough for the roots to readily absorb it while at the same time the concentration is not so high as to cause injury to the root.  Two methods of applying fertilizer in Guyana are recognized.  These are:

i) Broadcast: - Here the fertilizer is spread over the entire soil surface.  It is very effective in broadcast-seeded rice cultivation, in seed bed preparation and in pasture, where the plants are placed close to each other.

ii) Placement: - The fertilizer is placed in a row alongside the plant or in a circle around the plant.  In this method the fertilizer should be placed about two inches in the soil and about two inches from the seed/seedling.  In the case of larger plants, with the exception of coconuts, the fertilizer is placed around the dripline of the plant canopy.  In the case of coconuts the fertilizer could be placed one to two metres from the trunk.

Over Use of Fertilizers

Overuse of fertilizers could result in more harm than good.  Excessive use of Nitrogen will predispose the plant to disease (blast in rice) and pest attacks due to the softer cell walls present.  In some species the use of nitrogen often induces/prolong the vegetative phase of the crop and can subject the crop to pest and disease attack due to the prolonged maturation stage.  Excess Nitrogen can also induce Zn and Cu deficiency.  These nutrients act as a catalyst/carriers in many biochemical reactions in the cell hence their presence in insufficient quantities will result in a reduction of plant productivity.  Phosphorus excess will result in a Zn and Cu deficiency similar to N.  Deficiency in P will result in reduced root growth and a stunted plant, sugars and carbohydrates will accumulate in the leaves thereby predisposing the plants to attacks by pests and diseases that find the leaves ideal for feeding.  Potassium is an important nutrient in plant nutrition and disease management for it enhances the ability of the plant to withstand disease attacks.  In fact there is a positive correlation between K deficiency and blast incidence in rice. Calcium is essential for cell wall development, hence a deficiency of calcium will result in plants with cell walls that can be penetrated easily by pests and diseases. Excess use of either K, Mg or Ca can upset the Ca:Mg or Ca:K or K:Mg ratio in the soil this will result in the nutrient in excess being taken up by the plant at the expense of the other.  This will result in the plant showing deficiency symptoms for the nutrient that is less.  These critical ratios also exist for the other nutrients and creating imbalances in the system that will result in deficiency symptoms being exhibited by the crop.

Calculating Nutrient Requirements from Different Fertilizer Materials

In many occasions in Guyana we are given a fertilizer recommendation e.g. a farmer was advised to use 100 lbs 15:15:15 on his 5 ac. plot; but arriving at the store realized there was no 15:15:15; instead there were urea, (N₄₅P₀K₀), Triple Super Phosphate (N₀P₄₅K₀) and Muriate of Potash (N₀P₀K₆₀).  How would you help him to get the nutrients that he needs?  Or suppose he was told to use 100 lbs urea 70 lbs TSP and 50 lbs Muriate of Potash and found that they only have urea, TSP and 15:15:15.  How can he make up the levels of nutrients that he needs.

In the first instance the fertilizer materials available are:

                   Urea         TSP      Muriate of Potash

Nutrient       N   P   K   N  P  K   N   P   K

   %           45   0   0    0  45  0    0   0   60

For his 5 acre plot the farmer needs the following nutrients in the following amounts:

        N      P     K

   75 lbs  75lbs  75 lbs

*P here represents P₂O₅ and K represents K₂O.

100lbs of urea contains 45lbs N

   "       "  TSP      "       45lbs P₂O₅

   "       "  MP        "       60lbs K₂O

...Amount of urea needed = 100/45 X 75 = 165 lbs urea

...Amount of TSP needed = 100/45 X 75 = 165 lbs TSP

...Amount of MP needed = 100/60 X 75 = 125 lbs MP

In the 2nd case the fertilizer materials available are:

               Urea        TSP       15:15:15

Nutrient    N  P  K    N  P  K   N  P  K

    %       45  0   0    0  45  0   15 15 15

100 lbs urea = 45 lbs N

80 lbs TSP = 36 lbs P₂O₅

50 lbs MP = 30 lbs K₂O

200 lbs 15:15:15 will supply all the K₂O (30 lbs), 30 lbs P₂O₅

...we need to supply 15 lbs N and 6 lbs P₂O₅ from the urea and TSP

...amount of TSP needed = 100/45 X 6/1 = 13.3 lbs TSP

...amount of urea needed = 100/45 X 15 = 33.3 lbs urea

Conclusion

Fertilizers could be used as a complement to the other cultural and chemical management strategies in IPD & WM.  However it is necessary for the user to understand:

a)     What is a fertilizer

b)     Why we fertilize

c)     How to use the fertilizer efficiently

	type
	timing & frequency of application
	placement

d)     Effect of under and over utilization of fertilizer

e)     How to make the maximum use of the fertilizer material available.

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