# HKDSE Geography/M5/Physical Factors in Agriculture
## Weather and climate
### Insolation and temperature

Insolation and temperature are important factors in agriculture. Firstly, insolation provides light and heat, which plants need for photosynthesis. Photosynthesis is essential for plant growth. Plants can only grow at temperatures above 6°C.
The second reason is frost, a type of precipitation in which ice is formed on the earth's surface. Frost occurs at the freezing point prevents plant growth. The frost-free period is the range of days between the last frost of spring and the first frost of autumn. Its length determines the length of the growing season.
The amount of insolation is determined by the length of daylight and the intensity of the solar radiation (in turn determined by the angle of the overhead sun). Without going into details we will cover in M6, this means that lower latitudes generally have longer growing seasons than higher latitudes.
* Growing season of certain plantsSugar cane - 250 days (Source) Rice - 240 days (Source) Cotton - 200 days (Source) Wheat - 135 days (Source)

### Precipitation

Rainfall is another important factor. Raindrops will infiltrate into the soil for plants to absorb, and thus is an important water supply for plants. Without water, crops will wither and die, leading to crop failure. This is why prolonged drought leads to a decrease in yield. Rainfall beneficial to crops is known as effective precipitation. In general, crops need 250 mm of effective precipitation per year in temperate regions and 500 mm in tropical regions. Snow is also beneficial to crops as they shield the plants from extreme cold and provide moisture for plant growth when they thaw.
However, heavy, persistent rainfall and rainstorms may cause flooding and waterlogged conditions in the soils, as well as serious rainsplash erosion. Hailstorms will damage crops. This may lead to crop failure as well.
In areas with abundant rainfall and insolation throughout the year, such as South China, 'double cropping and triple cropping may be carried out to increase yields. These areas generally grow wet crops, or water-demanding crops like wet rice. Cotton, which has a long growing season, is also grown in tropical areas. In temperate regions with less rainfall and insolation, oats, wheat and barley are grown. They have shorter growing seasons and demand less water (and are thus known as dry crops).
Seasonal patterns in rainfall determine the type of crop grown. For example, maize needs large amounts of rainfall during their ripening period, whereas coffee requires a dry period.

### Wind

Winds affect plant growth both positively and adversely:
* Light winds facilitate transpiration, which encourages plant growth.
* Onshore winds bring moisture to plants, increasing the water supply for plants. They may also lead to frontal rain.
* Strong winds increase the evaporation rate, reducing soil moisture content. They may also cause soil erosion as the topsoil is blown away.
* Huge winds like tornadoes and typhoons damage crops and lead to crop failure.
* Good winds, bad winds
* In Egypt, a dry, hot wind known as khamsin is harmful to agriculture. It carries sands to agriculture areas.
* In the south of France, cold winds called the mistral hinder cultivation.
* Foehn winds on leeward slopes increase the local temperature. (This will be discussed in E2.)

## Land
### Relief

Altitude is very important in agriculture. The temperature decreases 6°C per 1000 metres; this is known as the environmental lapse rate. Winds are also stronger at higher latitudes. Leaching is more likely at higher latitudes, and organic matter takes longer to be decomposed into humus. (These will be explaiend in M6.) Soils are less mature than in low latitudes. Therefore, it is difficult for crops to be cultivated at high latitudes. In Britain, for example, grass cannot be grown above 300 m.
Aspect is another important factor. In the Northern Hemisphere, south-facing slopes are likely to receive more solar radiation each day. These are known as adret slopes. North-facing slopes in the Northern hemisphere receive less solar radiation and known as ubac slopes. In the Southern Hemisphere, the same holds for north-facing slopes. Also, near the coasts windward slopes, which face the coasts and receive onshore winds, are likely to receive relief rain, which is favourable for agriculture. Leeward slopes face away from the coasts and are said to be in the rain shadow, which hinders agriculture.
Slope gradient is another factor. On level ground, infiltration is easier and there is less surface run-off. Therefore, much of the moisture is retained, which increases soil moisture content, favouring crop cultivation. Soils are also thicker because of the higher infiltration rate, which facilitates chemical weathering. Thus the provide better support for plants. Lam Tsuen Valley in Hong Kong is an example. On sloping ground, surface run-off occurs and little moisture is retained on the slope surface. Moreover, rainsplash erosion is serious, and the topsoil is often eroded away. Thus thin and dry soils are often found on sloping grounds. Terracing is a method to reduce these problems and will be discussed later.

### Soil

Various attributes of soils are key to arable farming:
* Soil moisture: High soil moisture content allows absorption by plants, facilitating growth. However, if the moisture content is too high, waterlogging will occur, drowning crops. This is also related to the water retention capability of soils. Sandy soils retain less water because of their high porosity and permeability. Infiltration and percolation rates are very high. Clayey soils retain much water and are thus generally more favourable for plant growth.
* Soil nutrient content (soil fertility): High soil nutrient content is essential for plant growth. Soils high in nitrate and mineral content will give better yields.
* Soil depth: Deep soils favour agriculture as they provide support for plant roots, especially in agroforestry.
* Soil aeration: Aeration is closely related to soil porosity. Sandy soils have better aeration and air can be easily circulated in and out of the soil. In other words, the soil can 'breathe' better.
* Soil acidity: In general, slightly acidic soils are more favourable for farming because they can maintain better soil fertility.
Soil often determines the type of crop grown.
* Soil type and crop type
* Clayey soils are more suited for plants like broccoli. Crops with dense roots cannot grow because of the densely-packed soil particles. They require grass cover to increase yield.
* Sandy soils are good for root vegetables like carrots and turnips.
* Silty soils are the middle ground between clayey and sandy soils, and most plants can thrive in it.
* (Source)

### Drainage

The drainage of an area is another important factor. Rivers can transport produce from the farm, and farm equipment, seeds and workers into the farm. It also serves as an important water supply in areas where rainfall is not abundant. Thus drainage is an important factor in both pastoral and arable farming.
In floodplains and deltas, sediments washed downstream provide important nutrients for farming. The soil on floodplains is fertile and known as alluvial soils as they contain alluvium. This is discussed in M2. The Nile is an important example, and served as the cradle of the Ancient Egyptian civilisation. The Ganga Delta in Bangladesh is another.