plant density and crop productivity

7/29/2019 Plant Density and Crop Productivity

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Plant density and crop productivity

THE CONCEPT OF COMPETITION

Two plants, no matter how close, do not compete with

each other so long as the growth resources are in excess of

the needs of both. When the immediate supply of a single

necessary factor falls below the combined demands of the

plants competition begins.

According to Donald (1963) competition occurs wheneach of two or more organisms seeks the measure it wants of

any particular factor or thing and when the immediate supply

of the factor or thing is below the combined demand of the

organisms.Harper (1983) defined competition as the struggle

between individuals within a population for available

resources, when the level of resources is below the combined

need of the members of the population.


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1 THE NATURE OF COMPETITION If all the plants in the community are nearly equal in

competitive ability, they will tend to share equally in the

supply until it is exhausted and then simultaneously, to

suffer the effect of depletion of the pool e.g Water.

In the case of nutrients, the capacity to draw from the pool

is in varying degree, an expression of the differing ability of

plants to make use of the nutrients in different physical andchemical forms.

The concept of a pool or store of material is not valid in the

case of light. There is no store of light energy in the

immediate environment of the plant. Instead, light isavailable as a passing stream which must be intercepted

by the leaves if it is not to be permanently lost to the plant.


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1.2 MECHANISM OF PLANT POPULATION STRESS

Yield per plant is linearly correlated with

available space. Increasing the available space 10 times (100 to

1000 cm2) increased per plant yield 10 times

(3.6 to 35.6). Increasing the available spaceper plant two times (100 to 200 or 500 to

1000 cm2) increased yield 2 times (3.6 to 7.4

or 17.9 to 35.6).


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TABLE 5.1 PLANT POPULATION AND GRAIN YIELD

OF RICE (IRRI 1964)


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The fact that grain yield per plant is proportional to available

space is attributed by some authors to mutual shading

(Papadakis 1970).

Influence of light can be separated from that of soil bygrowing plants in pots of different depths and varying the

distance between them. The influence of depth is notorious

(Papadakis 1954).

The fact that grain yield is proportional to available space

explains why weeds reduce crop yields.


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Nutrients increase the yield when the soil is poor in

fertility. As the dose increases, the incremental yield

gradually decreases and may be negative at higher

doses.

Plant roots fill the surrounding soil with substances,

injurious to root growth. The theory of injurious

substances of the rhizosphere has been confirmedby Papadakis (1968). The theory explains why

plants grown in solution have a stunted growth and

they grow better when the solution is changed

frequently or absorbing substances such asactivated carbon, resins, etc are added.


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2. PLANT POPULATION AND YIELD RELATIONSHIPS

Efficient interception of radiant energy incident to the crop

surface requires adequate leaf area, uniformly distributed, to

give complete ground cover. This can be achieved bymanipulating stand density and its distribution over the land

surface.

Fig 1 Relationship between plant population and yield on four occasions

(Hypothetical).


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The agronomist is concerned with efficient use of

resources by the crop. It is desirable for him to define the

relationships between plant population and crop yield

quantitatively for advising the farmer on optimum plantpopulation for realising maximum yield.

Two aspects of competition are important in determining

the effects on yield: the amount (intensity) and the time

of onset. It is clear, that competition occurs early at high plant

populations. At very low densities, with most crop plants,

competition may not occur at all and resources are not

efficiently used. Selection of plant population must avoid

insufficient use of resources at low levels and excessive

competition at high levels (Harper 1983).


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Density-yield relationships

Holliday (1960) suggested two density-yield relationships:

parabolic and asymptotic.

PARABOLIC RELATIONSHIP

The parabolic response curve is typically a flat-topped one with

decrease in grain yield on both sides of an optimum (Fig 2). The

curve could be fitted by a quadratic equation:

y=a+bx+cx2

where,y = Yield per unit area,

x = Plant population, and

a, b and c = regression constants


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ASYMPTOTIC RELATIONSHIP

When yield is the product of vegetative crop growth, the

density-yield relationship is asymptotic. In an asymptotic

relationship, with increase in density, yield rises to a

maximum and then relatively constant at high densities.

The curve for biological yield can be defined by the

expression for a rectangular hyperbole.1

y = Ax x ---------------

1 + Abx

Where,

Y = dry matter yield per unit area,

A = the apparent maximum yield per plant,

x = number of plants per unit area, and

b = the linear regression coefficient of the

reciprocal of yield per plant and plant population.


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3 CROP RESPONSES TO POPULATION CHANGES

As plant density increases, most components of yield of theindividual plant, in general, are reduced.

The factors for which competition may occur among plants

are nutrients, light, oxygen and carbon dioxide. Other factorsof growth such as temperature and humidity are not

commodities in finite supply and hence are not the subject of

competition.


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3 COMPETITION FOR GROWTH RESOURCES

3.1 COMPETITION FOR NUTRIENTS

There is sufficient evidence to derive the general principle

that as fertility status is improved, so the density requiredto give maximum yield by annual crops increase.

Conversely, as plant density increases up to a certain

limit, the crop will continue to respond to higher levels of

added nutrients.


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3.2 COMPETITION FOR LIGHT

Competition for light may occur whenever one plant casts a

shadow on another or within a plant when one leaf shades

another leaf. ). Maximum light interception by the canopy can

be achieved by improving the foliage pattern and increasing

plant density.

Improving foliage pattern: An efficient plant type is the one

with most leaves, but that in which the inter-plant and intra-

plant competition for light is reduced to a minimum, so thatthe canopy as a whole intercepts maximum light.

Increasing plant density: Light intensity required for

maximum photosynthetic rate by individual leaf is about 1500-

2000 foot-candles, whilest light intensities in the field mayreach 10000 foot-candles (Donald 1963). Hence, there is

scope for increasing the plant population to increase the

efficiency of plant canopy as a whole to utilise the available

sunlight.


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3.4 EFFECT OF PLANT POPULATION ON CROP

Plant densities influence crop growth considerably.

Widely spaced plants, usually, have circular root distribution

as against interpenetratedroot growth at high densities,

With increasing density and competition for light, plant

height may be markedly increased,

There will be marked increase in lodging at high plant

densities,

High plant densities may decrease protein and oil content,

At very high plant densities, seedling mortality is common.

However, this self thinning

will not reduce plant density to that giving highest grain yield,and

In general, high densities are conducive for build up of pests

and diseases.


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4. CROP GEOMETRY

The way in which the crop plants are arranged in the field

(spatial arrangement or plant rectangularity) is usually

referred to as crop geometry. Keeping the total plant

density constant, crop geometry can be varied by

manipulating inter and intra-row spacings.

1 SQUARE ARRANGEMENT2 RECTANGULAR ARRANGEMENT

plant density and crop productivity

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