Agriculture

Abiotic Factors

Temperature

Importance of suitable temperature

-          Length of the thermal growing season: Warm temps are needed for growth as well as survival. E.g. grass can survive very low temperatures but does not grow at temperatures below 50C.

-          Biochemical reactions: the rate of biochemical reactions, including those involved in photosynthesis and plant growth, are generally increased by higher temperatures.

-          Frost-free period: some crops are damaged by frost, for example, maize, so it cannot be grown in areas where late frosts are likely. The blossom of many fruit crops is damaged by frosts.

-          Impact on evaporation: higher temperatures increase the rate of evapotranspiration which increases crop water requirements.

-          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.

Control of temperature by field locations:

-          Frosts in orchards during the flowering period can destroy the flowers and prevent any crop from being produced. This can be prevented by burning gas, oil, or candles in the orchards, or by dispersing cold air with large fans.

-          Transparent woven plastic cloth over fields helps to retain warm air close to the ground and protect crop seedlings.

-          Livestock can be kept warm by providing shelter or buildings that may be heated. In hot weather, buildings may need to be ventilated to prevent overheating.

-          Low lying areas e.g. valley bottoms are more likely to have frosts as cold, dense air collects there.

-          South-facing slopes in the northern hemisphere (and north-facing slopes in the southern hemisphere) receive more solar insolation and tend to be warmer. Where the crop is sufficiently valuable, temperatures may be raised with more intensive methods e.g.:

o   greenhouses to further enhance temperatures from solar heating

o   greenhouses may be heated in cold weather by burning fuel e.g. gas or paraffin

o   greenhouse overheating can be prevented by ventilation

Light

Importance of light

-          Rate of photosynthesis: Brighter light= Faster photosynthesis

-          Effect of photoperiodism on livestock

o   Poultry grow best with short days while egg production is greatest on longer days.

o   Long day length increases milk production.

o   Some livestock, such as sheep, mate when days are getting shorter in the autumn, with the lambs being born in spring

-          Photoperiodism (daylength): This affects the growth and development of crops. Some plants require longer periods of light each day for flowering, e.g. oats, while others require shorter days e.g. maize.

Controlling Light levels

-          Artificial lighting can be used to extend the growing season, e.g. for salad crops in greenhouses.

-          The artificial production of autumn lighting conditions in spring will produce another mating season for sheep, with a second lambing season in the autumn.

Water

Importance of water

-          Gas 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

-          Replacing transpiration water: Transpiration causes water to be drawn upwards to the leaves, carrying nutrients from the roots with it.

-          Physiological functions: water is the general physiological solvent in all living cells

-          Nutrient absorption. Nutrients are absorbed from soil as ions dissolved in water:

o   Nitrogen is absorbed as nitrate ions: NO₃^1-

o   Potassium is absorbed as potassium ions: K¹⁺

o   Phosphorus is absorbed as phosphate ions: PO₄^3-

-          Transport medium: Water is used to transport materials such as glucose, oxygen, and mineral nutrients.

-          Turgidity: Cell water produces cell turgidity which provides support, especially in seedlings

How water supply affects crops

Some crops have a high water requirement e.g. most rice varieties must be flooded during early growth. Some cereal crops have a low water requirement e.g. wheat. Irregular water supplies can cause some crops to expand and split e.g. tomatoes. Humid conditions can increase the risk of fungal diseases.

-          Amount: the amount of water available is affected by precipitation rates and soil properties e.g. permeability and water retention. The 'hydrological growing season’ is the time during the year that there is sufficient water to sustain growth.

-          Reliability: It is difficult for farmers to plan activities if they do not know how much water will be available. An issue in areas with unreliable seasonal rainfall or where there are no alternative water supplies for irrigation.

-          Quality: Substances dissolved in irrigation water can cause problems, especially if groundwater or polluted river water is used.

-          High salt content: causes 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 waterlogged soils

-          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.

-          Higher risk of fungal disease

Methods used to reduce soil water levels

-          Deep ploughing

-          Avoidance of soil compaction by machinery or livestock

-          Excavation of drainage ditches or installation of drainage pipes

-          Provide conditions to encourage worms e.g. soil organic matter

Issues 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. Increasing the risk of erosion and desertification.

Crops:

-          In hot areas, plants close stomata to reduce water loss due to transpiration.

-          This also stops the absorption of carbon dioxide so growth stops.

Methods used to increase water availability

-          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

Buffer strips

-          Creating strips of vegetation within a field can provide a physical barrier that slows the flow of overland runoff, increases infiltration and prevents soil, sediment and nutrient loss from fields.

-          Riparian buffer strips are next to watercourses.

-          In-field buffer strips are found adjacent to field boundaries and across fields.

-          Buffer strips can contain long grasses, trees and shrubs.

-          Alongside a river they usually require fencing to prevent livestock from accessing both the buffer strip and the watercourse itself.

-          Alternative drinking sources, such as gravity fed drinking troughs, solar pumps or pasture pumps can be used to provide livestock with access to water.

Soil Fertility

-          Soil fertility is 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.

-          Plants need a range of nutrients for growth and good health.

-          Nutrients that are needed in large amounts are called macronutrients.

-          Nutrients needed in smaller amounts are called micronutrients.

-          Nutrients must be added to soil to replace the nutrients removed when the crop is harvested, to replace nutrients lost due to natural processes or if the soil is naturally deficient in that nutrient.

Nutrients could be available by natural processes

-          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

Fertilisers

Organic fertilisers are animal and plant materials that release nutrients as they decompose. E.g.

-          Faecal material: manure/sewage sludge

-          Animal food production wastes: bone meal/fish meal/dried blood

-          Plant food production wastes: crop harvest wastes/composted plant waste

Nutrient application methods include

-          Cultural methods using natural processes such as bacterial fixation and weathering release the nutrients in the soil.

-          A crop rotation cycle that includes livestock will add manure to the soil.

-          Mechanical application is normally used to spread organic and inorganic fertilisers.

-          Hydroponics

o   Hydroponics involves the growth of crops in a nutrient solution rather than a solid growth medium.

o   It is usually carried out in greenhouses as part of an intensive system.

o   Productivity is maximised by controlling limiting factors as much as possible

Advantages of hydroponic production

-          Nutrient 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.

-          There are no weeds.

-          The harvested crop still has roots so it stays fresh longer.

-          The harvested crop is attractive for consumers as it has no soil on it.

Disadvantages of hydroponic production

-          Intensive production involves high inputs of nutrients and energy.

-          A high level of technical knowledge is needed

Aeration

Uncompacted soil has larger spaces between the soil particles which increases aeration. This is important because many important soil processes are aerobic e.g. root respiration, nitrogen fixation, decomposition.  The force of gravity naturally causes soil particles to become compacted. This is counteracted by the tunneling action of detritivores e.g. worms and the growth of plant roots, especially of larger plants whose larger roots create drainage channels when they die and decompose

Farming practices that increase soil compaction e.g.

-          Weight of farm machinery

-          Livestock trampling

-          Reduction in soil OM content (due to some farming methods) reduces worm populations

-          Natural communities have deep-rooted plants, e.g. forests, but most crops have shallow roots, so less root penetration deeper into soil, leading to increased compaction.

-          Ploughing can kill soil organisms by moving them to depths at which they cannot survive

Control of aeration

-          Ploughing turns over and aerates surface soil layers but not deeper layers

-          Adding organic matter provides food for the soil biota that increase aeration

-          Low tillage methods prevent the disturbance to soil and the killing of soil organisms during ploughing

-          Removing livestock from fields when the soil is very wet reduces the compaction caused by trampling

Soil Salinity

-          Dissolved salts in soil water are essential for plant growth as they include soil nutrients e.g. Mg2+ , but excessive salinity can kill plants by osmotic dehydration of their roots.

-          Farmers aim to use methods to avoid increasing soil salinity (especially avoiding irrigating with groundwater that has a high salt content).

-          The salt concentration of the soil increases as the salts are left in the soil when water evaporates from the soil.

-          Extra water can be used to wash the salts out of the soil but this greatly increases water consumption and can cause ecological problems by increasing the salinity of the local river

Soil pH

-          All plant species have their own range of tolerance for pH, usually between pH5 and pH7.

-          A high pH can inhibit nutrient solubility.

-          A low pH can increase the leaching of nutrients and inhibit nutrient uptake.

-          It can also mobilise toxic ions in the soil such as aluminium and lead.

-          Increasing pH can be done by:

o   Adding crushed lime (CaO, Ca(OH)2 or CaCO3 depending on the form).

o   Spreading powdered sulphur

Carbon Dioxide

-          Carbon dioxide concentration can be the limiting factor on the rate of photosynthesis.

-          It isn’t practical to increase the carbon dioxide concentrations for crops grown in fields.

-          However, crop growth rates in greenhouses may be increased by burning carbon based fuel e.g. gas or paraffin.

-          The carbon dioxide is kept in the greenhouse rather than being lost and the increased costs may be justified by the high market value of greenhouse crops

Topography

The undulations of the land surface can affect its suitability for particular crops and the methods that are used.

-          Aspect: Undulations e.g. valleys, produce some areas which are more exposed to sunlight therefore usually warmer).

-          Frost pockets: Cold dense air may collect in low-lying areas, making crop frost damage more likely.

-          Runoff rate: Steeper gradients make soil erosion by surface runoff more likely. Gentle or flat gradients make flooding more likely.

-          Use of machinery: It may be difficult to operate large machinery on land that undulates a lot or where gradients are very steep.

Controlling topography

-          It is not practical to alter the topography of large areas of land but it may be possible in some areas.

-          An area of steep gradient may be changed to a series of flat fields by terracing.

-          This is usually done to retain irrigation water and reduce soil erosion.

-          Areas that are nearly flat may be levelled by machinery so that water drains slowly.

-          This can reduce irrigation needs and can help produce flooded fields for rice cultivation

Relief

-          Altitude of an area can impact the choice of crop species grown as relief can control other factors that crops may need.

-          Livestock:

-          Some species are adapted to high altitudes e.g. goats, sheep, and llamas. They can survive lower temperatures and low atmospheric pressures.

-          Cattle don’t thrive at high altitude because the low atmospheric pressure causes 'high altitude disease' where pulmonary arteries thicken.

-          Crops:

-          Temperatures are often colder at higher altitudes. The low atmospheric pressure increases the evaporation rate

-          Care needs to be taken when setting up new farming areas as It is not possible to control the relief of an area.

Wind Velocity

-          Increased evaporation rates and the drying of soils

-          Crop damage, for example the 'lodging' of cereal crops when they are flattened by strong winds.

-          Increased soil erosion, especially in dry areas