Abiotic factors and their control

• Temperature is important in agriculture to maximise the length of the growing season, as it can warm enough for a species to survive, but not grow. For example, grass can survive under 5*C, but not grow.
• Temperature is important in agriculture because if it is warm enough it can provide frost free periods. Frost damages crops and inhibits their growth, so preventing this maximises their growth.
• Temperature is important in agriculture as it impacts evaporation. Higher temperatures increase the rate of evapotranspiration, which increases crop water requirements.
• Temperature is important in agriculture because it affects the rate of biochemical reactions. Photosynthesis is increased with higher temperatures, which increases plant growth.
• Temperature is important in agriculture because it can affect the thermoregulation of organisms. Keeping animals warm can reduce heat losses and increase the amount of energy put into growth.
• Temperature can be controlled in greenhouses by solar heating, burning fuels during cold weather and preventing overheating through ventilation.
• Temperature can be controlled to prevent frost in orchards. This can be done through burning fuels in the orchards or dispersing the cold air with a fan.
• Temperature can be controlled in fields by covering crop seedlings with transparent woven plastic cloth to help retain warm air close to the soil.
• Temperature can be controlled with livestock by providing shelter that can be heated with heating or cooled with ventilation.
• Light is important in agriculture as it affects the rate of photosynthesis.
• Light is important in agriculture as it controls photoperiodism. This affects the growth and development of crops. Some plants require longer periods of light each day for flowering, like oats, and others require shorter days, like maize. Day length can affect the reproductive function of some livestock species as well. For example, long day lengths increases milk production and poultry grows best with shorter days but egg production is greater when days are longer. Some livestock mate during shorter days e.g sheep.
• Light can be controlled with artificial lighting. This can be used to extend the growing season for crops or produce another mating season for sheep in the spring with autumn lighting conditions.
• Water is important in agriculture because it is a physiological solvent in all living cells.
• Water is important in agriculture because it allows nutrients to be absorbed by plants as ions in water. For example, nitrogen as NO3-, phosphorus as PO43- and potassium as K+.
• Water is important in agriculture because in cells it produces cell turgidity which provides support.
• Water is important in agriculture because it is used to transport materials such as glucose, oxygen, and mineral nutrients.
• Water is important in agriculture because it replaces 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 is important in agriculture because of gas exchange. It is lost during the process of evaporation from the cells in the stomata, as the stomata opens to allow carbon dioxide to enter for photosynthesis. If there is a shortage of water, the stomata close to prevent dehydration and death. The plant may survive, but gas exchange and growth will stop.
• The amount of water supply affects crops in agriculture. It 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.
• The reliability of the water supply affects crops in agriculture. This is because it is difficult for farmers to plan activities if they do not now 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.
• The quality of the water supply affects crops in agriculture. Substances dissolved in irrigation water can cause problems. A high salt concentration 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.
• In agriculture, waterlogged soils can cause problems such as higher risk of fungal diseases or soils becoming 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.
• Agricultural methods used to reduce soil water levels include: excavation of drainage ditches or installation of drainage pipes; deep ploughing; avoidance of soil compaction by machinery or livestock; provision of conditions to encourage worms, for example, soil organic matter.
• Water shortages can cause problems for crops. Plants lose water by transpiration during dry weather but this 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.
• Water shortages can cause problems for livestock. In semi-arid areas, livestock 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.
• Agricultural methods to increase water availability include: crop irrigation; soil mulching to reduce evaporation losses from the soil surface; 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 is a measure of the ability of the soil to support plant growth. It is a combination of soil properties such as the availability of nutrients and water, aeration, texture, and structure.
• The macronutrients needed for soil in agriculture are: nitrogen (NO3-), phosphorus (PO43-), potassium (K+), calcium (Ca2+), magnesium (Mg2+) and sulfur (SO32-).
• The micronutrients needed in soils in agriculture are: iron (Fe2+, Fe3+), zinc (Zn2+) and copper (Cu2+).
• Nutrients can be made available in the soil by natural processes - legumes have symbiotic nitrogen-fixing bacteria in root nodules (Rhizobium), some free-living soil bacteria fix nitrogen (Azotobacter) and the rotation of crops 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. These include: faecal material, animal food production wastes and plant food production wastes.
• The advantages of organic fertilisers are: many are waste products and may be locally available; they increase the soil humus content; they increase soil biota populations.
• The disadvantages of organic fertilisers are: nutrient composition can’t be controlled; nutrients are slowly released as the material decomposes, so they must be used as part of a long-term cultivation plan; many are bulky with a high water content so transport is expensive; usually can’t be added to a growing crop.
• The advantages of inorganic fertilisers are: nutrient composition can be controlled to meet specific crop requirements; the nutrients are released rapidly.
• The disadvantages of inorganic fertilisers are: some require large amounts of energy during manufacture by the chemical industry; they don’t add organic matter to the soil, so soil humus levels and soil biota populations may decline; some are toxic to worms; some have high solubility and may be leached after application; raw material supplies for manufacture may be limited.
• Nutrient application methods include: cultural methods - using natural processes such as bacterial fixation and weathering release the nutrients in the soil or a crop rotation cycle that includes livestock to add manure to the soil; mechanical application - normally used to spread organic and inorganic fertilisers.
• Hydroponics involves the growth of crops in a nutrient solution rather than a solid growth medium. It is usually carried out in greenhouses as a part of an intensive system. Productivity is maximised by controlling limiting factors as much as possible.
• The advantages of hydroponic production are: nutrients supply is optimal so this is not a limiting factor for growth; all the roots are in contact with the nutrient medium, so the roots are smaller and more growth is directed into the harvestable crop; there is no soil to hold pathogens; no weeds; harvested crops still has roots so it stays fresh for longer; the harvested crop is attractive for consumers as it has no soil on it.
• The disadvantages of hydroponic production are: intensive production involves high inputs of nutrients and energy; a high level of technical knowledge is needed.
• Aeration is when the soil has large spaces between the soil particles. This is important because many important soil processes are aerobic such as root respiration, nitrogen fixation and decomposition. Gravity naturally causes soils to become compacted. This is counteracted by tunnelling by detritivores and the growth of plant roots, especially larger plants whose larger roots create drainage channels when they die and decompose.