Protect and produce
Dimensions
of Need
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| The soil |
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Fertile soil ready for planting chick peas, Jordan. |
Soil fertility is a key factor in determining agricultural potential. All plants take up nutrients from the soil as they grow; these nutrients are removed with any plant that is harvested. Crop rotation or fertilizers are required to prevent even the best soils being depleted by farming. |
Maize farming experiment, Nicaragua. Scientists will use the results to teach local farmers how best to cultivate and fertilize their land. In the absence of any constraints such as availability of water, judicious use of fertilizers can raise yields by 30 percent, but over-use can do more harm than good. |
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A profile of the soil reveals a sequence of horizons, varying in colour and texture according to their composition. Plant roots work their way between the soil particles, binding and aerating the soil. |
Soil covers most of the land surface of the earth in a thin layer, ranging in thickness from a few centimetres to several metres. It is composed of inorganic matter (rock and mineral particles), organic matter (decaying plants and animals), living plants and animals (many of them microscopic), water and air.
Basically, soil forms as rocks slowly crumble away. Air and water collect between the particles, and chemical changes occur. Plants take root, binding the particles together, shielding the surface from the elements, drawing up minerals from lower layers and attracting animal life. Bacteria and fungi break down plant and animal remains into fertile humus.
The speed of this process varies. In prairie regions with ample rain and organic inputs, it may take 50 years to build up a few centimetres of soil; in mountainous areas it can take thousands of years. The process of destruction as a result of misuse or erosion is much quicker. Once completely destroyed, soil is for all practical purposes lost for ever.
Fertile soils teem with life. Porous loamy soils are the richest of all, laced with organic matter which retains water and provides the nutrients needed by crops. Sand and clay soils tend to have less organic matter and have drainage problems: sand is very porous and clay is impermeable. Only 11 percent of the earth's soils have no inherent limitations for agriculture. The rest are either too wet, too dry, too shallow, chemically unsuitable or permanently frozen.
To grow, plants need nitrogen, phosphorus, potassium and a range of other elements. However fertile the soil, growing crops will use up its nutrients. Farmers once compensated for this by spreading animal manure and plant waste on their fields. Increasingly, these have been replaced by manmade fertilizers.
Organic matter maintains the soil structure. It also acts as a buffer for chemical fertilizers, adding to their beneficial effects and reducing possible harm. In fact, the organic content and structure of the soil has to be managed as carefully as the nutrient content.
As agriculture has become more intensive and extensive, mineral fertilizer use has increased. Between 1981 and 1991, the world's annual use of fertilizers rose from 81 to 96 kilograms per hectare of cropland. This average, however, conceals huge differences in usage Zimbabwe, one of Africa's higher users, used only 56 kilograms per hectare a year in 1989-91.
When fertilizer levels correspond to the needs of specific soils and crops and the structure of soil is conserved, yields can be sustained indefinitely. Overuse or under-use of fertilizer can lead to crop failures. Over-application can also cause pollution: excess nutrients leach out of the soil into groundwater, streams, rivers and lakes, making their water unfit for consumption or boosting the growth of algae, which can suffocate entire aquatic ecosystems.
The production of food depends on healthy agricultural systems. These in turn depend on healthy soils.
Percentages of total world land area
Only 11 percent of the world's soils can be farmed
without being irrigated, drained or otherwise
Source: Geography in Diagrams, CUP
Soil texture varies with particle size from clay (fine) through silt (medium) to sand (coarse). The larger the particles the larger the spaces between them so water drains fast through sand but clay gets waterlogged quickly. Texture depends largely on the bedrock - shales yield finer soils than sandstones -but most soils contain a mixture of particle sizes in different proportions. Loam is best for plant growth.
Average annual fertilizer use
Click here to see the statistics
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Luvisols present few problems for agriculture. With moderate management they can be extremely productive. |
Gleysols are poorly drained. With appropriate drainage and water control, however, they can be developed to at least medium agricultural potential. |
Vertisols are dark clays and difficult to work. Good management can bring them to medium or high potential. |
Ferralsols are acid and found in tropical or subtropical lowlands. With appropriate management they offer medium to high potential for selected crops. |
| Main soil association | Soils too dry | Soils too cold | Shallow soils | Sandy soils | Dark clays | Saline soils |
| Soil components | Hot deserts Calcisols, Gypsisols and shifting sand dunes | Very cold areas Permafrost, gelic soil units, glaciers | Leptosols and lithic phases of other soils. Rocky terrain | Arenosols, Regosols, Podzols, other soils with coarse texture | Vertisols, vertic soil units | Solonchaks, Solonetz, saline and sodic phases of other soil units. Salt flats |
| Agricultural potential | Nil for rain-fed agriculture; medium to high locally when irrigation is possible | Nil or very low | Generally low Some potential for grazing | Low to medium depending on nutrients ant moisture management level | Workability problems, but medium to high when well managed | Generally low. Reclaimed land has low to medium production potential |
| Main soil association | Acid soils of sub/ tropical lowlands | Soils of sub/ tropical highlands | Ferruginous (iron- rich) soils | Poorly drained soils | Soils with few problems |
| Soil components | Ferralsols, Acrisols, Alisols, dystric and humic Nitisols, Petroferric phases of other soils | Andosols, euric Nitisols | Ferric Luvisols, Lixisols, ferralic Cambisols | Gleysols, Fluvisols, Histosols, Planosols, gleyic soil units | Luvisols, Cambisols, Chernozems, Kastanozems, Phaeozems; Podzoluvisols, Greyzems |
| Agricultural potential | Medium to high(Nitisols) with adapted crop selection and management | Medium to high if phosphorus fixation problems are overcome | Only medium potential, even with good management | Medium to high potential with adapted water and drainage control. Low in Histosols, Planosols and acid sulphate Fluvisols | High to very high with moderate to good management; low (but with forestry potential) for Podzoluvisols, Greyzems |
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