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PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS:AGRICULTURE

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PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS:AGRICULTURE PHYSICS

PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS:AGRICULTURE PHYSICS

–         Influence of solar radiation on plant growth.
–         Influence of wind, humidity, rainfall and air temperature on plant growth.
–         Soil environmental component which influence plant growth.

Agriculture physics is concerned with physics environment in relation to plant growth.

(a)   Influence of Radiation Environment on Plant Growth

Radiation environments. Refer to radiations present in the atmosphere, commonly coming from the sun.

Components of solar radiation

The main components of solar radiation are:

(i)   Visible light

(ii)    Infrared radiation, and

(iii)   Ultraviolet radiation.

PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS:AGRICULTURE 

HEATING EFFECT OF SOLAR RADIATION ON PLANTS

Positive effect

An optimum amount of heat on plant favours the process of photosynthesis.  This enables a plant to make its own food and hence provide its growth.

Negative effects

(i)  Excessive solar radiation (ultraviolet light) on plants leads to bleaching of green pigment (chlorophyll).  This lowers the amount of food produced by photosynthesis to plant and hence a plant may die.

(ii) Excessive solar radiation on plants leads to excessive water loss in the form of water vapour commonly on plant leaves (transpiration).  Hence wilting (drying) of plants may occur.

(b)  Influence of Aerial Environment on Plant Growth

Aerial environments refer to the atmospheric condition resulting from a series of processes occurring in the atmosphere.  These include air temperature, wind, humidity and rainfall.

WIND EFFECT ON PLANT GROWTH

Positive effects

(a)  Wind acts as pollinating agent for some plants and hence favours plant productivity.

(b)  Wind also favours evaporation of water from plant leaves and thus maintains water balance for proper plant growth.

Negative effects

(a)   Excessive wind on environments leads to plant breaking or cutting of tree branches.  This may lead to the death of plant.

(b)  As the wind speed increases further, cell and Cuticular damage occurs, followed by death of plant tissue, and a gnarled appearance becomes more apparent.

(c)  At low wind speeds, the effect seems to be an increase in transpiration, which results in water stress.  This stress causes the plant to adapt by decreasing leaf area and internodes length, while increasing root growth and stem diameter.

(d)  Strong wind may also cause shade off flowers; this lowers plant productivity.

PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS:AGRICULTURE
Effect of Rainfall on Plant Growth

Positive effect

An optimum amount of rainfall on plants favours its growth.  Water is a raw material for the process of photosynthesis from which plants obtain their food and hence their growth.

Negative effect

Excessive rainfall leads to water logging in soil which in turn leads to root spoil and hence the death of plant.

Effect of Humidity on Plant Growth

Positive effect

Favourable humidity on plants help plants to conserve water for various activities and in seeds helps the development of new leaves.

PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS:AGRICULTURE

Negative effect

Low humidity results into a greater rate of transpiration and hence may result into plant drying.

Effect of Air Temperature of Plant Growth

Positive effect

An Optimum temperature on plants enhances enzymic activities which in turn gives favourable conditions for plant growth.

Negative effect

(a)   High temperature denature enzymes commonly for photosynthesis and hence the death of plant.

(b)  Low temperature inactivates the plant growth enzymes, hence low growth rate.

PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS:AGRICULTURE

Wind Belts

Wind belts are seasonal strong wind moving in a specified direction in a certain region of the earth.

The global wind belts are formed by two main factors:

(i)                  The unequal heating of the earth by sunlight and

(ii)                The earth’s spin.

Here is a simple explanation of the process

The unequal heating makes the tropical regions warmer than the Polar Regions.  As a result, there is generally higher pressure at the poles and lower at the equator.  So the atmosphere tries to send the cold air toward the equator at the surface and send warm air northward toward the pole at higher levels.

Unfortunately, the spin of the earth prevents this from being a direct route, and the flow in the atmosphere breaks into three zones between the equator and each pole.

These form the six global wind belts: 3 in the Northern Hemisphere (NH) and 3 in the Southern (SH).  They are generally known as:

PHYSICS FORM SIX-ENVIRONMENTAL PHYSICS:AGRICULTURE

(1)  The Trade winds, which blow from the northeast (NH) and southeast (SH), are, found in the sub tropic regions from about 30 degrees latitude to the equator.

(2)  The Prevailing Westerlies (SW in NH in SH) which blow in the middle latitudes.

(3)  The Polar Easterlies which blow from the east in the Polar Regions.

Effects of wind belts to plant

1.  Wind belts because the loss of plant leaves and flowers hence lower plant productivity and growth.  Loss of leaves lowers the rate of photosynthesis.

2.    Wind belts sometimes cause plants to lean in direction of moving wing. This changes their direction of growth

3.    Trees are broken by the strong wind.

(c)   Soil Environment Components Which Influence Plant Growth

Soil is composed of both rock particles and organic matter (humus) – the remains of plants and animals in various stages of decomposition.  The humus serves as food for many living organisms.  Within the soil is a large population of animals, plants.  These break down the humus into soluble substances that can be absorbed by the roots of large plants.

Components of a soil

Soil is composed of:

(    (a)   Air, 25% by volume which supports life of soil organisms,

(b)  Water, 25% which dissolves minerals so that are easily absorbed by plants,

(c)  Organic matter (humus), 5% by volume,

(d)  Inorganic matter (minerals), 45% by volume,

(e)  Biotic organisms, micro – organisms like earth worm, centipedes, millipede, bacteria which decompose organic matter.

Types of soil

(i)    Sandy soil

(ii)     Silt soil,

(iii)    Clay soil, and

(iv)    Loamy soil (sand + silt + clay soil mixture)

Water Movement in the soil

Two forces primarily affect water movement through soils, (a) gravity and (b) capillary action.

Capillary action refers to the attraction of water into soil pores – an attraction which makes water move in soil. Capillary action involves two types of attraction – adhesion and cohesion.

Adhesion is the attraction of water to solid surfaces.

Cohesion is the attraction of water to itself.

Speed of water in a particular soil type depends on:

(i)   How much water is in the soil, and

(ii)  Porosity of the soil.

The movement of water in the solid is mainly due to gravity.  The porosity gives a measure of how much water the soil can hold and the rate at which water flows through the soil.  Large pore spaces give a faster rate and vice versa.

An experiment to study water movement in soil

An experiment to demonstrate the rate of flow of water in the soil is done using a glass tube and sand type filled in it.  Water is poured into the tube and the time taken for water to reach the bottom of the tube in notes.

Unsaturated soil Saturated soil
Soil type Water speed Soil type Water speed
Sandy Fastest Sand Slowest
Loamy Moderate Loamy Moderate
Clay Slowest Clay Highest

 

i. Sand soil have large pore spaces thus allows water to travel downwards through it at a fastest rate.

ii. Clay soil can hold water as has very fine pore spaces.

iii. Loamy soil allows water movement at a medium rate.

Heat transfer in the soil

Within the soil heat is transferred by a conduction process. Since soil is poor conductor of heat most of the heat from the atmosphere appears at the surface of the earth.

An optimum soil temperature favours plants growth but a high temperature can lead to the rotting of plant roots.

(d)  Techniques for the Improvement of the Plant Environment

Plant environment can be improved by using wind breaks, shading and mulching.

Shading

Shading is the process of obstructing plants from excessive solar radiation.

Positive Impacts of Shading

1.   Prevents excessive loss of water by plants through transpiration.  This enhances plant productivity.

2.    Preserve moisture in the soil and hence water supply to plant.

Mulching

Mulching is the process of covering the soil by dry leaves, grasses and or papers.

Benefits (Advantages) of Mulching

1.    Improve soil moisture. Bare soil is exposed to heat, wind and compaction loses water through evaporation and is less able to absorb irrigation or rainfall.  Using mulches, the soil has greater water retention, reduced evaporation, and reduced weeds.  Mulch can also protect trees and shrubs from drought stress and cold injury

2.    Reduce soil erosion and compaction. Mulches protect soils from wind water, traffic induced erosion and compaction that directly contribute to root stress and poor plant health.

3.      Maintenance of optimal soil temperatures. Mulches have shown to lower soil temperatures in summer months.  Extreme temperatures can kill fine plant roots which can cause stress and root rot.  Mulches protect soils from extreme temperatures, either cold or hot.

4.      Increase soil nutrition. Mulches with relatively high nitrogen content often result in higher yields, but low nitrogen mulches, such as straw, sawdust and bark, can also increase soil fertility and plant nutrition.

5.   Reduction of salt and pesticide contamination. In arid landscapes, evaporating water leaves behind salt crusts.  Because mulches reduce evaporation, water is left in the soil and salts are diluted.  Organic mulches can actively accelerate soil desalinization and help degrade pesticides and other contaminants.

6.      Improve plant establishment and growth. Mulches are used to enhance the establishment of many woody and herbaceous species.  Mulches improve seed germination and seed survival, enhance root establishment, transplant survival, and increase plant performance.

7.    Reduction of disease. Mulches will reduce the splashing of rain or irrigation water, which can carry spores of disease organisms to stems and leaves of plants.  Populations of beneficial microbes that reduce soil pathogens can be increased with mulches.  Mulches can combat disease organisms directly as well.

8.     Reduction of Weeds. Using mulches for weed control is highly effective.  Mulches can reduce seed germination of many weed species and reduce light, which stresses existing weeds.

9.     Reduce pesticide use. Mulches reduce weeds, plant stress, and susceptibility to pests and pathogens which translates to reduced use of herbicides, insecticides, and fungicides.

Mulch Problems (disadvantages of mulching)

1.     i. Acidification.  Some types of mulches can increase soil acidity.

2.    ii. Disease.  Many mulches made from diseased plant materials can be composted or treated at temperatures that kill pathogens that can be transmitted to healthy plants.

3.     iii.Pests.  Many organic mulches, especially wood – based mulches, have the reputation as being “pest magnets”.

4.   iv. Weed contamination. Improperly treated crop residues and composts as well as bark mulches are often carriers of weed seed.  Mulch must be deep enough to suppress weeds and promote healthy soils and plants.  Weed control and enhanced plant performance are directly linked to mulch depth.

v. Wind Breaks

Wind breaks are long rooted strong plants (trees) that are used to obstruct the path of wind or to slow down the wind.

Windbreaks provide many benefits to soil, water, plants, animals and man. They are an important part of the modern day agricultural landscape.  Windbreaks come in many different sizes and shapes to serve many different conservation purposes.

In agriculture, wind breaks protect small growing plants from strong blowing wind

Advantages of Windbreaks to Plant Environment

1.     i. Control soil erosion. Windbreaks prevent wind erosion from causing loss of soil productivity. This eliminates plant roots stresses and thus favours plant growth condition.

2.     ii. Increase plant yield. Windbreak research substantiates that field windbreaks improve crop yields which offsets the loss of production from the land taken out of cultivation.

3.      Pesticide sprays. Windbreaks control pesticide spray drift and provide buffers to delineate property lines and protect neighbors.

EXAMPLES:  SET A

Example 01

(a)  What is agriculture physics?                                                                             (02 marks)

(b)  What are the components of a soil? How do they support the life of a plant?  (06 marks)

(c)  Explain briefly how soil temperature affects plant growth.                               (02 marks)

Example 02

(a)   What do you understand by the word environmental physics?                (01 marks)

(b)  Explain how the following climatic factors influence plant growth: air temperature, humidity, rainfall and wind.   (06 marks)

(c)  What are wind belts?  Explain the effect of wind belts on plant productivity. (03 marks)

Example 03

(a)   What is mulching?                                                                                            (02 marks)

(b)  Give two advantages and two disadvantages of mulching.                               (04 marks)

(c)  Discuss the heating effect of solar radiation to plant growth.                       (04 marks)

Example 04

(a)   Explain two factors that primarily affect water movement in the soil           (03 marks)

(b)  Explain the soil environment that favours high crop yield                               (04 marks)

(c)  What is shading and what is its purpose?                                                            (03 marks)

Example 05

(a)   (i)  Mention the components of solar radiation.
(ii)  How do those components affect plant growth?                             (04½ marks)

(b)  What are wind breaks?                                                                                (02 marks)

(c)  What are the advantages of wind breaks to plant environment?                      (03½ marks)

 

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