The physical influences on agricultural activity can be divided into three main groups; climate, soils and relief. Of these, the climate is the chief determinant of agricultural patterns. Soil properties are largely the product of past and present climates, while relief effects are largely expressed through climate changes.
Influence of Climate on Agriculture
All plants need water in order to survive, but their requirements vary widely, as well as their ability to extract water from the soil. Since plants derive the bulk of their water requirements through their root systems, water must be available in the soil in the appropriate quantities. A continued deficiency or excess of soil-water will ultimately cause the destruction of crops. Excessive precipitation may also cause problems of rill, gully and sheet erosion of the soil cover, as well as flooding of fields and serious crop losses in lowland areas. Heavy and prolonged falls of snow in winter may also have serious consequences upon agricultural production. In many parts of Scandinavia and Canada, for example, work on the land virtually comes to a standstill during the winter months. The spring snow-melt may also cause problems of waterlogging of fields and crucial delays in the start of the annual round of work on the farm which, in any event, is concentrated into a critically short period of intense activity. However, on the credit side, a prolonged snow cover does have an insulating effect and reduces frost penetration into the ground so that certain cereal crops may be sown in the autumn without risk of frost damage.
Suitable temperature conditions are also essential for the successful germination of seeds and plant growth. Again requirements vary, but most plants need a minimum temperature of 5-7°C before growth commences. The average number of days per year with temperatures above this threshold level provides a rough guide to the length of the available growing season. A more precise indication may be provided by the calculation of accumulated temperatures: the amount by which each day’s temperature exceeds a threshold figure is added throughout the growing season to give a cumulative temperature figure. For example, wheat, which has a threshold figure of 5°C, requires approximately 1,300 day-degrees of accumulated temperature in order to give optimum yield.
In marginal areas of cultivation where the length of the growing season is scarcely long enough for the successful cultivation of particular crops, serious damage may be caused by spring and autumn frost. For example, in Finland, which lies at the northern limit of grain farming, there is always a considerable risk of failure because of killing frosts or an unusually short summer. Severe killing frosts causing complete crop failure throughout the country occur on average every fortieth year, or once in the lifetime of every generation of Finnish farmers. Less severe crop failures occur once every ten years, and regional crop failures due to frost once in every four years. It appears that many Finnish farmers adopt an over-optimistic view of the frost hazard and underestimate the risks of failure involved. On a local scale, the incidence of frost-hollows may be an important influence on patterns of cultivation, especially fruit crops, which are particularly susceptible to frost damage.
Light is also essential for plant growth, and sufficient amounts of sunshine are necessary for the ripening of crops, In temperate regions, the ripening and harvesting of crops may be delayed during unusually cloudy summers, while in parts of the equatorial region, where temperature conditions encourage rapid plant growth, persistent cloud cover and reduced amounts of direct sunshine may prevent the double-cropping which would otherwise be possible.
The wind is another element of climate which affects farming activities in many ways. One obvious effect of this particular hazard is the damage caused to mature cereal crops by storm winds. Cold local winds such as the Mistral of southern France may cause serious crop losses, while hot, dry winds such as the Sirocco of southern Italy and Malta may have a desiccating effect upon crops. Less obvious is the fact that constant strong winds increase evapotranspiration from crops and lead to increased water requirements to compensate for this fact. The loss of valuable topsoil by wind erosion, especially in areas of dry farming where crops are grown under semi-arid conditions without irrigation, is also an extremely serious problem in many parts of the world. Reference was made in Chapter Two to the way in which problems of crop failure and soil erosion during the 1930s in the Dust Bowl of the USA resulted from a misinterpretation of the agricultural potential of the Great Plains.
Influence of Soil on Agriculture
Any study of farming activity must make reference to the soil since it is the essential material upon which all agriculture is based. It contains minerals such as nitrogen, phosphorus, sulphur, potassium, magnesium, calcium and iron, as well as minute quantities of trace elements such as boron, iodine and cobalt which are necessary for plant growth. All forms of agriculture, whether arable or pastoral, remove certain of these minerals and trace elements from the soil so that fertility and crop returns will ultimately diminish unless these essential constituents are replaced. Soil fertility may be maintained by fallowing, scientifically based crop rotations, and the application of manure or chemical fertilisers.
A companion volume, Physical Geography Made Simple, includes reference to the processes of soil formation (pedogenesis), describes variations in soil texture and composition, and comments on the methods of soil classification. The suitability of any soil for agriculture depends upon its composition, texture and depth, and is determined by a variety of factors, including the nature of the parent material, past and present climatic influences, relief and vegetation, soil organisms, as well as man’s use of the soil which may have the effect of either converting naturally infertile soils into good farmland by manuring, deep-digging, draining and liming, or initiating soil erosion by bad husbandry. It takes nature from 300 to 1,000 years to build up 25 mm of fertile soil. A man by wanton misuse can destroy 200 mm in one or two generations. Justifiably, therefore, soil quality has been described as a response to management.
Loam soils are often regarded as ideal soils because of their richness in plant food, good drainage without waterlogging and general ease of working, although heavier clay soils may be more suitable for certain crops, provided that drainage is adequate. Sandy soils are generally infertile, although they may respond to heavy applications of fertiliser.
Influence of Relief on Agriculture
Three elements of relief — altitude, aspect and gradient-influence patterns of agricultural activity. As mentioned earlier, the effects of altitude are chiefly expressed through climatic modifications, notably the decrease in temperature with increased height. Thus, in southern Scotland, the length of the growing season decreases from about 240 days at sea level to about 180 days at 330 m and 135 days at 600 m. In middle latitudes, other adverse effects of increased altitude generally include higher precipitation, strong winds and deterioration of soil quality. High altitude, therefore, restricts the number and types of crops that may be grown. Where arable farming is practised, reliance tends to be placed on hardy cereals and fodder crops, but in many upland areas, pastoral farming is the dominant type of agricultural activity.
On the other hand, in the tropics, increased altitude provides some relief from the excessively high temperature and humidity of the lowland plains, and provides an improved environment for many crops. In Java, for example, the best crops of tea are grown at heights of 1,200-1,800 m, while in Kenya the main coffee growing belt is located at elevations of 1,400-1,800 m.
On a local scale, another important element of relief is the aspect or orientation of slopes. In the northern hemisphere, south-facing slopes receive longer periods of more Intensive sunshine than their north-facing counterparts. Numerous studies have been made of this effect in deeply cut, cast-west orientated valleys in Norway, Austria, Switzerland and elsewhere. Although de temperature differences between the sunny (adret) and shady (ubac) slopes of such valleys are quite small, they are nevertheless sufficient to cause significant differences in the land use and settlement patterns of the two opposing valley sides. Cultivation extends to higher levels on the south-facing slopes; most villages and farms are found on the sunny side of the valley, whereas the shady, north-facing slopes are often heavily forested and devoid of settlement and cultivation.
Finally, the gradient of slopes imposes an important control on the type of agriculture and methods of cultivation that may be practised in any area. Not only is the risk of soil erosion greater on steep slopes than on gentle ones, but steep gradients also greatly restrict the use of heavy machinery. For example, the use of combine harvesters is normally restricted to slopes of less than 10° with a soil whose structure will not deteriorate under the weight of such heavy machines. In many parts of the world, especially in South-East Asia, complex systems of terracing have been developed to allow steep hillsides to be brought under cultivation.
The physical factors outlined above should not be thought of as absolute controls which impose rigid, unchanging limits on agricultural production. Soils can be modified and improved by the application of fertilisers designed to compensate for specific mineral deficiencies, inadequate soil drainage can be improved by tile drains, ditches and pumping, farming can be extended Into areas of low and unreliable rainfall by irrigation schemes, and the geographical limits of particular crops extended by plant breeding. In the latter Context, great success has been achieved in developing short maturing varieties wat and hybrid maize whereby the cultivation of these cereals has been extended into previously marginal and unsuitable areas. Almost complete control over the physical environment is achieved in farming practices such as the rearing of battery hens, the raising of dairy cattle in large buildings in which the animals are constantly housed and fed, and the cultivation of market-garden crops in soil-less cultures under glass. However, these developments can only be achieved at enormous cost. This, in turn, implies a strong demand and high returns for the products in order to justify the capital expenditure involved.
The flexibility of environmental constraints may be illustrated by a simple model in which a single physical factor exercises a dominant influence on crop distribution, in which the farmers’ information and decision-making are assumed to be perfect, and in which transport costs have no effect.
Under certain conditions of market demand, the area under cultivation of a particular crop might extend to the margin, A-A. However, if demand should increase, or if the government increased the level of its price support for the crop, it is possible to imagine an extension of the cropped area to a new suboptimal margin at B-B. With falling prices or the withdrawal of price support, the margin of cultivation might recede towards, or even beyond, A-A. In each instance, the limit of cultivation correlates with a different isopleth value. In other words, physical factors determine the shape of the crop area, while economic factors determine its extent.