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Topic of Book
David Grigg explores why certain crops grow better in some regions than others, and how this affects farmers decisions on what to plant.
David Grigg has written some of the best books about agricultural systems. While the topic may seem remote to the modern reader, I believe that agriculture is one of the key foundations of modern progress. And geography has a more powerful impact on agriculture than perhaps any other type of technology. Therefore, understanding geography helps one understand progress and technological innovation.
I would recommend reading Grigg’s other book (listed below) before reading this book.
- Agriculture is far more limited by geography than other technologies.
- The ability of region to grow wheat or rice was a critical determining role in its ability to support large populations and complex societies.
- Farmers make critical choices about which crop will yield the most production and profits on their specific plot of land.
- Temperature and moisture are the critical factors in the geography of crops.
- Plants require some sixteen chemical elements for successful growth, of which nitrogen, phosphorus and potassium are the most important.
- The essence of successful farming is to find a way to harvest crops, which removes nutrients, while maintaining the soil-plant nutrient status.
- In most farming systems, the limiting factor to crop growth is nitrogen.
Other Books by the Same Author:
Important Quotes from Book
Plant requirements in terms of temperature, moisture and plant nutrients are rarely linear. For any plant there are (i) minimum requirements of temperature or moisture without which no growth will take place; (ii) maximum limits, beyond which growth ceases. Between these limits are (iii) environmental characteristics which give optimum growth and development; here are found the highest yields, the weight of the edible part of the crop per hectare.
Farmers are not likely to cultivate the crop near the absolute limit, for economic as well as ecological factors determine where a crop is grown… Farmers will clearly cultivate the crop only within this economic limit. The economic limit, unlike the absolute limit, is not fixed.
A more common situation occurs where two or more crops have a similar optimum area and compete for the zone where both crops give their highest yields.
Net photosynthesis is highest in the mid-latitudes, including southern Europe, the Near East, much of Australia, the USA, southern Africa and southern South America. It is lower towards the equator, including the humid tropics, and poleward.
Temperature is a principal determinant of the geography of crops. Most temperate crops will not grow until temperatures are above 6°C, and for tropical crops this threshold is higher.
Some crops have a remarkable latitudinal range; of the cereals rice and maize can be grown in lowland areas on the equator, but also north of 50° in the northern hemisphere. Rye, oats, sugar-beet and potatoes are grown north of 50° latitude- in the northern hemisphere, but rarely south of 30°. However, these and other temperate crops can be grown in equatorial regions at high altitudes, where temperatures are lower than at sea level.
Soil moisture is essential to plant growth for without water plants wilt and the, and plant nutrients cannot be taken up by the crop. Moisture affects crop growth in two ways. First, crops differ greatly in the amount of water needed to give optimum yields. Wheat and rye for example, can be grown in areas where annual rainfall is between 25 cm and 100 cm, but rubber needs over 178 cm and tea over 254 cm. As over half the earth’s surface receives between 25 and 100 cm of rain in a year, wheat can be widely grown. In contrast only 10 per cent of the earth’s surface has over 178 cm and 5 per cent has over 254 cm of annual rainfall, so that tea and rubber have a much more limited distribution. Second,… there is a relationship between water supply and crop yield, but it is not linear. The yield of wheat, oats, sugar-beet, maize and potatoes increases as water supply increases, but declines after an optimum yield has been reached.
The distribution of mean annual rainfall is not a reliable guide to the possible location of crops, because not all the water is available to the plant; some is evaporated, some transpired. Evaporation rates in the tropics are some four times those in north-west Europe because of temperature differences, and so mean annual rainfalls in West Africa have to be above those in northern Europe to give equivalent soil moisture availability.
A further problem concerns the seasonal distribution of rainfall; an apparently adequate rainfall may be concentrated in a very short growing season, therefore limiting the range of crops that can be grown, as in the Sahel or rainfall may be in the winter when temperature conditions determine that the effective growing season will be in the spring and summer.
In the tropics temperature is a limiting factor for the growth of only a few crops, and it is more commonly the amount and seasonal distribution of rainfall that determines the limits and optimum areas for crop growth.
Establishing the optimum climatic conditions for the growth of a crop is important for understanding the present distribution of crops, but it is by no means the only factor involved. First, not all the suitable areas are necessarily occupied by a crop.
Second, many parts of the earth’s surface have environmental conditions which are optimum for several crops that are in competition for the land. Factors other than environment then determine its use. Third, some crops are grown in suboptimal conditions, when the price of a crop is high enough to justify significant modifications of the environment (such as irrigation). But for all these qualifying remarks, climatic differences are the main determinant of differences in crop distribution on a world scale.
In the optimum soil a wide range of crops can be grown, and high yields are obtained without the need for expensive modifications of the soil. Away from the optimum area, adverse soil characteristics increase, and so fewer crops can be grown and yields are lower. Thus the need to modify the soil by using fertilizers to increase the plant nutrient supply or liming to reduce acidity, increase the cost per unit of output. As with climate an economic margin is reached and later an absolute margin where, for example, excessive acidity, acute waterlogging or a low level of plant nutrients precludes crop growth.
Most crops thrive on deep, well-drained loams… Rice, unlike virtually all other soils, requires an impermeable subsoil so that the water in the padi does not drain away.
Good soils—those which are deep, well drained, neutral and retain sufficient soil moisture for crop growth—are not common. In England and Wales, only 17 per cent of the improved agricultural land is without moderate or severe limitations, and only in eastern England is a high proportion of land in this category. On a world scale, a mere 11 per cent of the total land area is without marked limitations to crop growth, while only 24 per cent of the total area is thought to be capable of growing crops at all.
An important characteristic of a soil is its depth. Many parts of the world suffer from too shallow soils, both in arid regions and in areas such as the Canadian Shield where glaciation has removed the soil.
Plants require some sixteen chemical elements for successful growth and development, of which nitrogen, phosphorus and potassium are the most important. These nutrients are derived from the decomposition of rocks, the decay of organic matter and, in the case of nitrogen, by the fixation of atmospheric nitrogen by bacteria in the soil. Under natural vegetation, nutrients are taken up by the roots of plants and returned to the soil when the plants the and decompose, or through the urine and excreta of herbivores. There is thus a closed cycle of nutrients between soil and vegetation, and little loss from the system. However, once the natural vegetation is cleared and crops planted, there is loss.
The essence of successful farming is to find a way to harvest crops, which removes nutrients, while maintaining the soil-plant nutrient status.
In most farming systems the limiting factor to crop growth is nitrogen. Nitrogen is fixed in the soil by bacteria, which live under two sets of circumstances. First are free-living bacteria, which are not associated with any plant and occur in nearly all soils. They fix very little nitrogen.
Second, and more important, are the nitrogen-fixing bacteria living symbiotically with the nodules that occur on the roots of all legumes.
Areas where soils are constantly being replenished are comparatively rare, but two outstanding cases are the deltas and lower reaches of rivers and regions of recent volcanic activity. In Asia—and particularly in South-East Asia—there is a marked contrast between the population density of the rivers and deltas, and the non-riverine areas. The latter are subject to the problems of leaching and soil erosion touched upon above. The lower rivers, however, gain plant nutrients brought from the catchment basins. These are flat, comparatively easily irrigated, and provide ideal conditions for wet-rice cultivation. The deltas of the Ganges-Brahmaputra, the Mekong, the Irrawady and the Chao Phraya, therefore, all have very high rural population densities. But it does not follow that all deltas have such high densities. Few of the deltas of Africa have been developed agriculturally, and in the Americas farmland is rare in the Amazon basin.
In most temperate regions, an increase in altitude has adverse effects upon farming. As mean annual temperatures decrease with increasing altitude, the growing season—the period with mean daily temperatures above 5.6°C —shortens, commencing later in the year and ending earlier, and so the amount of energy received from the sun is less. With increasing height and falling mean annual temperatures, summer temperatures become more variable and the risk of harvest failure greater.
In the tropics temperature is rarely a limiting factor for crop growth, and it is more commonly lack of moisture that prevents cultivation.
At the micro-scale, the angle and direction—the aspect—of slope may influence the type of farming.
Like wheat, but for quite different reasons, rice requires flat land. Two principal varieties of rice can be distinguished: dry-rice is sown and cultivated like any other cereal; wet-rice, however, is grown in slowly moving water which reaches an average of 100–150 mm on the stalk for three-quarters of the growing season, the water being withdrawn for the ripening stage, harvesting, and to allow weeding. This requires an elaborate system of water control to direct water which comes as rainfall, river floods or from reservoirs. To ensure that the water is the same height upon the stalk demands that fields be level and surrounded by small earthen bunds to contain the water. Such fields, generally called padi fields, but in Indonesia sawah, must be made upon flat land. The production of wet-rice in Asia is thus found overwhelmingly in the deltas and lower reaches of rivers.
In some areas, flat land for padi fields has been created on steep slopes by terracing, but this requires enormous labour; terracing is by no means common in Asia. It is most noteworthy in southern China, where a quarter of the cultivated land is terraced, Japan, and in some of the islands of South-East Asia, especially Java. It is rare in the Indian subcontinent and mainland South-East Asia.
The unusual circumstances of wet-rice cultivation provide other favourable conditions for growth. The rivers bring plant nutrients, in solution and suspension, removed by erosion from the upper reaches of the rivers, and so each annual deposition of silt helps maintain soil fertility. In the lower reaches of rivers the heavier silt carried by water is finally deposited; and this forms clay soils, often impermeable, which are necessary for the padi fields.
First, the poorer the household, the larger the proportion of total income spent on food; as income increases so the proportion of total income spent on food declines. Second, the absolute amount spent on food increases.
(See tables and graphs on page 62-3) 65.7% of all calories in Developing countries is derived from cereals and roots.
In most countries the cheapest source of calories are cereals and root crops, such as potatoes and yams, because they give a high yield of calories —and protein—per hectare. Thus the major food crop in much of Asia is rice, in parts of Latin America and tropical Africa it is maize and in the drier regions of Africa it is sorghum or millet. In parts of West and Central Africa manioc or yams are the chief source of energy. As a consequence most of the countries with low national incomes per capita get a high proportion of their calorie intake from the starchy staples, cereals and roots. In the developed countries, with higher incomes, more of the expensive livestock products can be eaten, and the starchy staples are consequently a much smaller percentage of all calories consumed.
In the developed countries there is a high demand for livestock products, and they make up two-thirds or more of the value of farm production. These animals are fed partly on grass, but increasingly upon grain. In contrast, in the poor countries little land is devoted to growing crops to feed animals, and livestock production is a small part of total output, which is dominated by staple food crops.
Economic Distinctiveness of Agriculture:
In contrast land is a fundamental part of agriculture. The total amount of land that is available is fixed in quantity, as is that of any one nation, save for exceptional cases such as the reclamation of the Zuider Zee. It is also fixed in position. Labour, entrepreneurial skills and inputs such as fertilizer or machines can all be moved from one place to another; land cannot. Land also differs greatly in quality.
But in agricultural geography the location and environment of the farm is given, and the farmer has to decide what combination of crops and livestock will give the most profit or, in the case of the subsistence farmer, what crops will satisfy most of his or her consumption needs. It is rare that a farmer will decide to raise a particular crop and then seek the optimum location.
First, as has already been noted, climate limits the crops the farmer can grow. Second, the time between the decision to grow a crop and the time when it is available for sale is rarely short; it may be as little as six months, but in the case of some perennial crops several years. Third, farmers have a limited control over the quantity they produce, for the size of the harvest varies with fluctuations in rainfall or temperature, and is liable to loss from disease and pests. Fourth, farmers do not have complete control over the quality of their product; thus the timing of a grape harvest influences the quality of wine, while frosts may damage fruit. Lastly, all foodstuffs are perishable.
Agriculture, unlike manufacturing industry, has a very large number of small producers.
In manufacturing industry most plants produce only one good with perhaps an incidental by-product. In agriculture most farms produce several different crops.
For most people in the developed world there is a physical difference between home and place of work; manager and employee both commute to work. This is not so for farmers… On a majority of farms in the world the family provides all or most of the labour on the farm, and on many the family consumes most of the produce. Thus, more than in most forms of economic activity, the cultural, economic and social characteristics of the family determine how the farm is run.
There are two other motives which are as important as maximum profit. First, farmers aim at security and stability in their incomes; maximization is less important. There are good reasons for this. Although England has a temperate climate with few extremes, the income of farmers varies from year to year more than that of firms in other sectors of the economy. Under these circumstances farmers are loathe to take risks with new crops or methods, and reluctant to switch into new enterprises. Second, farmers value the independence that farming as a livelihood gives them, in contrast to most other occupations. This is particularly relevant to the farmer with a small amount of land.
In the 1820s about three-quarters of food production in the USA was consumed on the farm but today virtually all food output in developed countries is sold off the farm; commercial farmers, like non-farmers, buy their food from shops.
The provision of water for crops other than from rainfall has a long history in Asia, but most of the present irrigated area has been established comparatively recently. In 1800 the world’s irrigated area was only 8 million hectares, but had increased to 40 million by 1900. Between 1900 and 1950 a further 80 million hectares were added, and between 1950 and 1988 the figure rose from 120 million to 228 million hectares.
But whereas the inputs so far considered are most intensively used in the developed countries, nearly three-quarters of the world’s irrigated land is to be found in the developing countries, half of it in China and South Asia. There is little in Latin America; in tropical Africa, where half the continent has insufficient soil moisture for crop cultivation only 3 per cent of the arable land is irrigated.
Three features, however, characterize modern agriculture in contrast to traditional agriculture. First has been the introduction of methods of preserving perishable
foods, such as canning and freezing. Second, much of the processing of products has moved from the farm to factories.
See Table on page 140 for Characteristic feature of agriculture in areas of high population density.
- “The Agricultural Systems of the World: An Evolutionary Approach” by David Grigg
- “Alchemy of Air: … the Scientific Discovery that Fed the World” by Thomas Hager
- “A History of World Agriculture” by Mazoyer and Roudart
- “Food, Energy and Society” by David and Marcia Pimentel
- “First Farmers” by David Bellwood
- “People, Plants and Genes: the Story of Crops and Humanity” by Denis Murphy