11. Agriculture

• The choice of what crop or livestock are cultivated depends on :

  • market demand and access
  • environmental conditions being within the range of tolerance
  • if the environmental conditions or adaptations of a species can be controlled to improve productivity
• Temperature - Length of growing season

  The temperature needs to be warm enough for survival and growth. e.g. grass survives at low temps but wont grow below 5 degrees C
• Temperature - Frost free period

  Some crops are damaged by frost e.g. maize so cannot be grown where frost is likely. Blossoms of fruit crops are damaged by crops.
• Temperature - Impact on evaporation

  Higher temperatures increase the rate of evapotranspiration which increases crop water requirements.
• Temperature - Biochemical reactions

  The rate of biochemical reactions, including those involved in photosynthesis and plant growth, are generally increased by higher temperatures
• Temperature - Thermoregulation

  Mammals and birds use food energy to maintain a constant internal body temperature. Keeping the animals warm reduces heat losses and increases the amount of food energy put into growth.
• Controlling temperature

  • Low lying areas such as valley bottoms are more likely to have frosts as cold dense air collects there
  • South facing slopes in the northern hemisphere receive more solar insulation and are warmer
  • Greenhouses achieve higher temperatures from solar heating
  • Greenhouses can be heated in cold weather by burning fuel
  • Frosts in orchards can be prevented by burning gas, oil, or candles
  • Transparent woven plastic cloth over fields to retain warm air close to the ground + protect seedlings
  • Proving ventilated shelter, buildings + heating to livestock
• Light

  The intensity of light affects the rate of photosynthesis, brighter light producing more rapid photosynthesis
• Photoperiodism

  Affects the growth and development of crops. Some plants require longer periods of light each day for flowering. Day length can also affect the reproductive function of livestock: Long days increasing milk production, sheep mate when days are getting shorter in the autumn.
• Control of light

  Artificial lighting can be used to extend the growing season for crops in greenhouses. The artificial production of autumn lighting conditions in spring will produce another mating season for sheep and a second lambing season in the autumn.
• Water - Physiological functions

  water is the general physiological solvent in all living cells
• Water - Nutrient absorption

  Nutrients are absorbed from soil as ions dissolved in water e.g.

  • Nitrogen absorbed as nitrate ions
  • Phosphorus is absorbed as phosphate ions
  • Potassium is absorbed as potassium ions
• Water - cell water

  Cell water produces cell turgidity which improves support, especially in seedlings
• Water - Transport

  Water is used to transport materials such as glucose, oxygen and mineral nutrients
• Water - Transpiration

  Water is needed to replace the water that is lost during transpiration. Transpiration causes water to be drawn upwards to the leaves, carrying nutrients from the roots with it
• Water - gaseous exchange

  Stomata are the pores in leaves where carbon dioxide dissolves and is absorbed, so it can be used during photosynthesis. Water is lost during this process by evaporation from the cells in the stomata. If there is a shortage of water, the stomata close to prevent dehydration and death. The plant may survive, but gaseous exchange and growth will stop.
• Water supply affecting crops

  Some crops have high water requirement e.g. rice. Some have low water requirement e.g. wheat. Irregular water supplies can cause some crops to expand and split. Humid conditions can increase the risk of fungal diseases
• Water supply affecting crops - amount

  amount of water available to plants is affected by precipitation rates and soil properties such as permeability and water retention. The 'hydrological growing season' is the time during the year that there is sufficient water to sustain growth.
• Water supply affecting crops - reliability

  It is difficult for farmers to plan activities if they do not know how much water will be available. This is a particular problem in areas with unreliable seasonal rainfall or where there are no alternative water supplies for irrigation.
• Water supply affecting crops - Quality

  substances dissolved in irrigation water can cause problems, especially if groundwater or polluted river water is used. A high salt content can cause salinisation, leading to osmotic dehydration of the crop. Heavy metals may bioaccumulate in crops and be a threat to the health of people who eat the harvested crop
• Problems caused by water logged soils

  • higher risk of fungal diseases
  • Soils become anaerobic and create ideal conditions for denitrifying bacteria, but not nitrifying bacteria. This reduces soil fertility as nitrates are lost from the soil more rapidly and replaced more slowly
• Methods to reduce soil water levels

  • Excavation of drainage ditches or instillation of drainage pipes
  • Deep ploughing
  • Avoidance of soil compaction by machinery or livestock
  • Provision of conditions to encourage worms, for example, soil organic matter
• Problems caused by water shortages - crops

  Plants lose water by transpiration during dry weather but it is reduced by closure of the stomata in their leaves. This also stops the absorption of carbon dioxide so photosynthesis and growth stop. Even a moderate water shortage reduces crop productivity. A severe water shortage will kill plants as cell dehydration inhibits cellular biochemical reactions.
• Problems caused by water shortages - Livestock

  Livestock in semi-arid areas may die if there is a water shortage. Shortages can increase trampling damage if animals have to regularly walk longer distances to reach water. This can increase the risk of erosion and desertification
• Methods to increase water availability

  • crop irrigation
  • soil mulching to reduce evaporation losses from soil surfaces
  • Provision of suitable conditions for worms to increase infiltration and reduce runoff losses
  • Reducing soil compaction by machinery and livestock to increase infiltration and reduce runoff losses
  • Adding soil organic matter to increase water retention
• Soil fertility

  a measure of the ability of the soil to support plant growth. It is the combination of soil properties such as the availability of nutrients and water, aeration, texture, and structure
• Soil nutrients

  Plants need a range of nutrients for growth and good health. Nutrients that are needed in large amounts are called macronutrients. Those needed in smaller accounts are called micronutrients.
• Nitrogen

  • Used for protein manufacture
  • Commonly applied to fields as fertiliser
• Phosphorus

  • Used in root growth and ATP manufacture
  • Commonly applied to fields as fertiliser
• Potassium

  • Used in ion transport and seed development
  • Commonly used in fields as fertiliser
• Calcium

  • Used in cell walls
  • Only used as fertiliser if soil is nutrient-deficient or in intensive systems such as hydroponics
• Magnesium

  • Used in chlorophyll synthesis
  • Only used as fertiliser if soil is nutrient-deficient or in intensive systems such as hydroponics
• Sulphur

  • Used in protein synthesis
  • Only used as fertiliser if soil is nutrient-deficient or in intensive systems such as hydroponics
• Iron

  • Used in chlorophyll manufacture
  • Only used as fertiliser if soil is nutrient-deficient or in intensive systems such as hydroponics
• Zinc

  • Used in Enzyme activation
  • Only used as fertiliser if soil is nutrient-deficient or in intensive systems such as hydroponics
• Copper

  • Used in Enzyme activation
  • Only used as fertiliser if soil is nutrient-deficient or in intensive systems such as hydroponics
• Legumes have symbiotic nitrogen-fixing bacteria in root nodules e.g. Rhizobium
• Some free-living soil bacteria also fix nitrogen e.g. Azotobacter
• Crop rotation gives time for weathering to release more nutrients and to even out the demands for particular nutrients by different crops
• Organic fertilisers are animal and plant materials that release nutrients as they decompose. They include:

  • Faecal material: manure/sewage
  • Animal food production wastes: bone meal/fish meal/dried blood
  • Plant food production waste: crop harvest wastes/composed plant waste