Agricultural production is influenced by external factors:

  • Human factors
  • Biotic factors
  • Climatic factors
  • Edaphic factors.

Human Factors

These are human characteristics which affect the way decisions are made and operations carried out.

Level of education and technology:

  • Skills
  • Technological ad van cements .

Human health/HIV-AIDS:

  • These affect the strength, the vigour, vision and the determination to work.
  • HIV/AIDS is the biggest threat to human health today and has long lasting effects on  agriculture, such as;
  • Shortage of farm labour.
  • Loss of family support.
  • Low living standards leading to despondency and hopelessness.
  • Increased criminal activities.
  • More time spent by the Government and NGO’s in Carring for the sick.


  • Stability in the countries’ economy affect agricultural production.

Government Policy:

  • These are governmental laws which have been enacted to protect farmers, land and livestock.

              They include:

  • Food policy
  • Policies on control of livestock parasites and diseases.
  • Policies on marketing of both local and export products and others.

Transport and communication:

  • For agricultural goods to move from the farm to the consumers.

Cultural practices and religious beliefs:

  • These activities hinder important changes in a society that may bring agricultural development.

Market forces:

  • Demand and supply forces which affect prices of commodities in a free market.

Biotic Factors

These are living organisms which affect agricultural production.

  • Pests – Destructive organisms which destroy crops.
  • Parasites – These are invertebrates which live in or on other living organisms.
  • Decomposers – Organisms which act on plants and animal tissues to form
  • Pathogens – Micro-organisms which cause diseases.
  • Predators – Animals that kill and feed on other animals.
  • Pollinators – They transfer pollen grains from the stamens to the pistil of a flower.
  • Nitrogen fixing bacteria -They are micro-organisms which convert atmospheric nitrogen to nitrates ready for use by the plants.

Climatic Factors(weather elements).

  • Rainfall,
  • Temperature,
  • Wind,
  • Relative humidity
  •  Light.

Weather – Atmospheric conditions of a place at a given time period.

Climate – weather conditions of a place observed and recorded for a period of 30-40 years.


Supplies Water:

  • Which is necessary for the life process in plants and animals.
  • Which makes the plant turgid hence provides support.
  • Acts as a solvent for plant nutrients.
  • Cools the plant during transpiration.
  • Which is used as a raw material in photosynthesis.

   When plants lack enough water they respond in different ways as follows:

  • By closing the stomata to restrict water loss.
  • Hastens maturity.
  • Some will roll their leaves.

  Other plants have developed permanent adaptation to water stress such as:

  • Growing needle like leaves.
  • Develop fleshy leaves for water storage.
  • Develop long roots.
  • Wilting and death in extreme conditions.

Important Aspects of Rainfall:

Rainfall reliability;

  • This is the dependency on the timing of the onset of the rains.

Amount of rainfall;

  • Quantity of rain that falls in a given area within a given year.

Rainfall distribution ;

  • The number of wet months in a year.

Rainfall intensity;

  • Amount of rainfall that falls in an area within a period of 1


  • This is the degree of hotness or coldness of a place measured in degrees Celsius.
  • Cardinal range of temperature ­ is the temperature required by plant to grow and thrive well.
  • Optimum range of temperatures – the best temperature for the best performance of plants.

Effects of Temperatures on Crop Production:

Low temperatures:

  • Slow the growth rate of crops due to slowed photosynthesis and respiration.
  • High incidences of disease infection.
  • Improves quality of crops such as tea and pyrethrum.

High Temperatures

  • Increase evaporation rate leading to
  • Wilting.
  • Hastens the maturity of crops.
  • Increase disease and pest infection.
  • Improves quality of crops such as pineapples, oranges and pawpaws.


Wind is moving air.

Good effects of wind include:

  • Seed dispersal
  • Cooling of land
  • Pollination in crops
  • Brings rain bearing clouds

Negative effects of wind:

  • Increases the rate of evaporation of water.
  • Causes lodging of cereals and distorts perennial crops.
  • Increases evapo-transpiration.
  • Spreads diseases and pests.
  • Destroys farm structures.

Relative humidity

  • The amount of water vapour in the air
  • Affects the rate of evapo-transpiration.
  • Forms dew which supplies soil with moisture under dry conditions.
  • High humidity induce rooting in cuttings.
  • Increases disease multiplication and spread. 


  • Provide radiant energy harnessed by green plant for photosynthesis.

Important aspects of light:

Light intensity ;

  • The strength with which light is harnessed by chlorophyll for photosynthesis.

Light duration;

  • The period during which light is available to plants per day.
  • Plant response to light duration is known as

Short-day plants require less than 12 hours of daylight to flower and

Long-day plants – require more than 12 hours of daylight to flower and seed.

Day-neutral plants require 12 hours of daylight to flower and seed.

Light wavelength;

  • This is the distance between two – successive crests of a wavelength.
  • It dictates the difference between natural and artificial light.
  • Chlorophyll absorbs certain wavelengths of light.

Edaphic Factors Influencing Agriculture

  • These are soil factors.
  • Soil is the natural material that covers the surface of the earth,
  • Made of weathered rock particles and decomposed animal and plant tissues, and on which plants grow.

Importance of Soil

  • Provides anchorage to the plants by holding their roots firmly.
  • Provides plants with mineral salts/ nutrients which are necessary for their growth.
  • Provide the plants with water.
  • Contains oxygen necessary for respiration of the plants and soil micro-organisms.

Soil Formation:

  • Soil is formed through weathering process.
  • Weathering is the breakdown and alteration of the parent rock near the surface of the earth to a stable substance.
  • Weathering process is a combination of disintegration (breakdown) and synthesis (build up) process.
  • Weathering process is continuous.

Types of Weathering

  • Physical weathering
  • Chemical weathering
  • Biological weathering


Agents of Weathering

 Physical Agents of Weathering

  • Include wind, water, moving ice and temperature.
  • Wind – carry materials which hit against each other to break into fragments.
  • Water – intensity of rainfall causes breakdown of rock.
  • Moving ice – has grinding effects which tear off rock particles.
  • Extreme temperature cause rocks to expand and contract suddenly peeling off their surface.

Chemical Weathering

  • Affects the chemical composition and structure of the rock.
  • Involves processes such as ;
  • Hydrolysis,
  • Hydration,
  • Carbonation


  • The process by which soluble minerals in the rocks absorb water and expand weakening the rock thus leading to disintegration.


  • The process whereby water dissolves soluble minerals in the rock weakening it.


  • The reaction of rock minerals with oxygen to form oxides which break easily.


  • The process whereby carbonic acids formed when rain water dissolves carbon dioxide,
  • It reacts with calcium carbonates in limestone causing it to disintegrate.

Biological Weathering

This involves the action of living organisms, plants and animals on the rocks.

  • Burrowing animals, for example, termites and moles bring soil particles to the surface exposing them to other agents of weathering.
  • Big animals like, elephants, buffaloes, camels and cattle exert a lot of pressure on the rocks as they step on them due to their heavy weights causing the rocks to disintegrate.
  • Earthworms take part in the decomposition of plant matter with the soil particles.
  • Man’s activities like, mining and quarrying expose rocks to the surface during excavation. These activities breakdown large rocks into smaller rock particles.
  • Plant roots force their way through the cracks in the rocks thus widening and splitting them.
  • Humic acids formed when plant tissues decompose react with the rocks weakening them further.
  • Plant remains-decompose adding humus into the soil.


Factors influencing soil formation

  • Climate- (rainfall, temperature and wind)
  • Biotic factors – living organisms.
  • Parent material– Nature and properties of the original rock from which the soil is formed.
  • Time – length of time during which the soil forming processes have taken
  • Topography – influences the movement of disintegrated materials.

  • It is the vertical arrangement of different layers of soil from the ground surface to the bedrock.


  • These layers are also referred to as horizons.


  • The layers show differences in their contents and physical properties such as colour, texture and structure.


  • The layers include: organic matter region, top soil, sub-soil, weathered rocks and parent material.


Organic Matter Region

  • First layer of the soil found on the surface.
  • Made up of leaves and other plant remains at various stages of decomposition.
  • Some soil organisms may also be found here.

Top Soil

  • Has a dark colour due to the presence of humus.
  • Is rich in plant nutrients and well aerated.
  • It is a zone of maximum leaching (zone of eluviations)


  • It is compact and less aerated.
  • It is a zone of accumulation of leached material (zone of aluviation) from the top layers.
  • Deep rooted crops have their roots growing up to this region.
  • Hard pans normally form in this layer

Weathered Rocks

  • It is also called substratum.
  • Rocks at various stages of disintegration are found in this zone.
  • Most of the materials found in this zone originate from the parent rock.

Parent Rock

  • It exists as a solid mass which is un-weathered.
  • It is the source of the inorganic composition of the soil.
  • The water table is on the surface of this rock.

Soils Formed in Situ and Soils Deposited

  • Soil formed in the same place and remains there is said to be in situ.
  • However, soil can be formed due to deposition of soil particles carried from its original site of formation to another area which is usually in the lower areas of slopes.
  • Such soils are said to have been formed through deposition.

Soil Depth

  • This is the distance between top soil layer and the bottom soil layer in a profile.
  • It dictates root penetration and growth
  • Deep soils are more suitable for crop growth since they contain more nutrients.
  • Have a larger surface are for root expansion.
  • Deep soils facilitate good drainage and aeration.

Soil Constituents

Organic Matter – Dead and decaying plants and animal remains

Living Organisms – Soil organisms and plant roots.

  • Micro-organisms (bacteria, protozoa and fungi)
  • Invertebrates -termites,
  • Earthworms and molluscs.
  • Higher animals – rodents and others.

Inorganic or Mineral Matter

  • Formed from the parent materials.
  • Supply plant nutrients
  • Form the skeleton and framework of the soil.


  • Found in the pore spaces of the soil.
  • Used for root and organism respiration
  • Used for germination of seeds.
  • Helps in decomposition of organic matter.
  • Regulates soil temperature.
  • Regulates the movement of water through capillary action.


  • Dissolves mineral salts
  • Maintain turgidity in plants.
  • Used for germination of seeds
  • Used by soil organisms.
  • Regulate soil temperature
  • Dictates the amount of air in the soil.

        Water in the soil exists in three forms namely:

  • Superfluous/Gravitational Water
  • Found in the large spaces (macro-pores) in the soil particles.
  • Held by gravitation forces.
  • When the pores are saturated, the soil is said to be waterlogged.
  • It moves and may cause leaching.


  • Hygroscopic Water
  • Water found in thin films on the soil particles.
  • Held by strong adhesive forces between water and soil particles.
  • Does not move and hence not available for plant use.


  • Capillary Water
  • Occupy micro-pores in the soil particles.
  • Held by cohesive forces between water molecules.
  • Moves through capillary action
  • Available to plants for use.


Soil Structure

  • This is the arrangement of soil particles in a soil horizon.

Types of Soil Structure

  • Single-grained
  • Crumby
  • Granular
  • Prismatic
  • Columnar
  • Platy
  • Blocky

Importance of Soil Structure on Crop Production

Soil Structure Influences

  • Soil aeration
  • Soil drainage and water holding capacity.
  • Plants root penetrability and anchorage.
  • Microbial activities in the soil.
  • Circulation of gases in the soil.

 Farming practices which improve the soil structure are:

  • Application of inorganic manure into the soil.
  • Tilling the land at the right moisture content.
  • Crop rotation.
  • Minimum tillage.
  • Cover cropping.

Soil Texture

  • It refers to the relative proportion of the various sizes of the mineral particles of soil.

    Importance of Soil Texture on Crop Production;

  • Influences soil fertility
  • Affects the organic matter content
  • Influences the drainage of the soil.
  • Influences soil aeration.
  • Influences water holding capacity.
  • Influences the capillarity or movement of water in the soil.


Soil Textural Classes

 Sandy Soils

  • Made up largely of sand particles.
  • Have large pore spaces hence poor in water retention.
  • Easy to till (light soils).
  • Freely draining.
  • Low fertility due to leaching of minerals.
  • Easily erodible.

 Clayey Soils

  • Made up largely of clayey particles.
  • Have small pore spaces hence good in moisture retention.
  • Difficult to till (heavy soils).
  • Poorly ‘drained.
  • Expand when wet, crack when dry.
  • High capillary.
  • Rich in plant nutrients.

Loam Soils

  • About equal amounts of sand and clay.
  • Moderately good in both moisture and air retention.
  • Fertile soils.

Soil Colour

  • This depends on the, mineral composition of the parent rock and the organic matter content.
  • Soils containing a lot of iron are brownish, yellowing and reddish in colour.
  • Soils with a lot of silica are white.
  • Soils with a lot of humus are dark or grey.

Soil pH

  • This refers to the acidity or alkalinity of the soil solution/the concentration of hydrogen ions in the soil solution.
  • Soil pH is determined by the concentration of hydrogen ions (H+) or the hydroxyl ions (OH) in the soil solution.
  • A pH of less than 7 means that the soil is acidic.
  • A pH of more than 7 means that the soil is alkaline.
  • As the hydroxyl ions (OH) in the soil increase the soil becomes more alkaline.


Influence of Soil pH Crop Growth

  • It determines the type of crop to be grown in a particular area.
  • Most crops are affected by either very acidic or very basic soil pH.
  • Soil pH affects the choice of fertilizers and the availability of nutrients to crops.
  • At low pH the concentration of available iron and aluminium in the soil solution may increase to toxic levels, which is harmful to plants.
  • Very acidic or low pH inhibit the activity of soil micro-organisms.
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