Plant nutrition management in farmi

Plant nutrition management in farming systems: Interaction with efficient water use at watershed level
LAND ORGANIZATION, PLANT NUTRITION MANAGEMENT AND CROPPING SYSTEM MANAGEMENT AND THEIR IMPACT ON WATER CIRCULATION IN THE WATERSHED
http://www.fao.org/docrep/v5400e/v5400e0d.htm
In most farming systems, the land is organized for the purpose of agricultural production in three main areas: the cropped area; the grazed area; and non-arable areas which include forests, rangelands and agriculturally-unproductive land. The ecological conditions prevailing in these three areas, the socio-economic conditions and the available know-how and genetic resources largely determine, through complex interactions, the geographical extent of these areas in a region. The historical patrimony is always very important and, in general, it is difficult strictly to connect the present development of the areas with the current conditions of farming. Their management by farmers, herders and foresters/hunters has an important impact on the availability of plant nutrients and water for the cropping systems implemented in the cropped area. Many traditional farming systems have developed differentiated use of these categories of land in which the transfer of water, biomasses and plant nutrients is organized.
Under conditions of low population densities, slash and burn practices are used in order to make available, through the burning of natural biomasses, a large part of the plant nutrients stored in the natural vegetation during long-term fallows. The productivity of these plant nutrients is low: run-off disperses a large quantity of them and leaching evacuates many of the nutrients in excess of what the crops can take up under the generally-extensive management of cropping systems. However, the infiltration of the soil usually remains quite high and the transfer of water within the landscape is limited where the proportion of cropped area is also limited.
Replacement of forests by perennial crops, industrial crops or traditional food crops (bananas, oil palm trees and other fruit trees) is less detrimental to the water balance and creates less plant nutrient losses in the watershed than slash and burn practices, when the population density increases. However, the capacity for replenishment of the plant nutrient reserves of the soil disappears through perennial cropping, because the period of storage through long-term fallows no longer exists. Cover crops may assist in providing some nitrogen if sufficient light is provided to the crop. Intercropping of annual crops between perennial crops at an early stage of the plantation is a valuable method for reducing the leaching of plant nutrients derived from the destruction of the natural vegetation. It also increases the efficiency of labour, sharing in particular the cost of weeding between the perennial and the annual crop, especially when pruning of the former is done at the same time (e.g. coffee, nuts). However, more water and more nutrients are consumed, which is detrimental to the perennial crop if there is a shortage of one of these two inputs (as with coffee plantations in Western Cameroon).
A. Angé, Chief, Plant Nutrition Management Service, FAO, Rome
With a further increase in population density, perennial use of large areas for crop production takes place, in rotation with fallow periods of varying duration. Farmers then have to face the problems of both water management and plant nutrition management for sustainable use of the land. In very rainy areas, the evacuation of surplus of water is the main problem, while in semi-arid tropical and temperate areas the shortage of water is a risk, as sometimes is the excess of water. In humid areas, the leaching and run-off of nutrients and the low efficiency of nutrients created by waterlogging are factors currently limiting crop yields. In other areas, insufficient water supply creates stress for the crop and lowers the efficiency of available plant nutrients, while leaching and run-off may also occur.
The competition for resources in the same area, by various social groups, is often a factor of destabilizing the proper management of land, water and nutrients. Thus, the design of improved general management often depends on negotiations between these groups resulting in trade-offs for access to the resources. In most cases, successful integration of various land use patterns, optimizing the management of water and nutrients, is the result of a long historical process, governed by powerful local hierarchies and well-established rules. These successful examples are adapted to a set of market conditions, production techniques and social attitudes, and there are many examples of disruption of ancient equilibriums, superseded by the evolution of population densities, social rules and political conditions, and by the modification of economic conditions. The development of rice paddies in Asia and of irrigated terraces in the Middle East and the Andes are examples.
The export of crop harvests mines the plant nutrient reserves in the soil if they are not replaced by natural supply from rain and irrigation water, sedimentation, dust, weathering of soil minerals and big-nitrogen fixation. Farmers have therefore thought up various methods for sustaining the availability of plant nutrients for their crops. There are two traditional ways: fallows and the transfer of nutrients. Fallows provide the accumulation of the natural plant nutrient supply over a certain period of time in order to make such a stock available for the following period of cropping. It must be noted that fallow periods also recreate better water management conditions through the alleviation of the crusting of the soil surface, the improvement of soil porosity by insects and deep rooting by pluriannual species, and replenishment of the soil organic matter content. Fallows imply the management of an area with the rotation of the cropped area in space and time. The extent and spatial organization of fallows have an important impact on the circulation of water in the watershed, by their influence on infiltration and run-off. Plant nutrients are also transferred from non-cropped areas to cropped areas: manures resulting from the grazing of forests, pastures and fallows; harvesting of natural straws, litter and leaves from forests and woodlands; and the transportation of topsoils.
Such forms of transfer have, in the long term, a great impact on water circulation in the watershed, if they exceed the natural renewal of these resources. Thus, the resulting denudation of hills in China and Vietnam and the overgrazing of large areas in the Andes, the Sahel and soudanian areas of Africa have dramatically increased run-off and finally degraded the cropped land, also threatening fertile alluvial soils and irrigated areas through flooding and abundant coarse sedimentation. The transfer of plant nutrients extends the management of those nutrients from the local cropped area to the whole territory concerned by the transfer. Plant nutrient management is then no longer a matter of the nutrient availability in the fields but is a problem of plant nutrient harvesting in farming systems and in a collective area.
In this respect, the connections between livestock systems and cropping systems are interesting. The organization of cropped and grazed areas is largely dependent on the social organization for the management of the livestock system. With very limited livestock, the organization in open fields will be prominent. When livestock systems are more important, hedges control the circulation of animals and also divide the rural space into management units, if the availability of the labour force is not sufficient to keep the herds away from the crops, or if the social organization is not adapted to the control of large herds by few herders, as in many semi-arid countries. When the control of animals is properly organized, open fields may co-exist with various levels of development of the hedges. However, the landscape is organized to preserve the access of animals to the essential resources: fodder in the rainy season in the upper parts of the landscape, fodder during the dry season in the lowest part of the landscape, access to drinkable water, access to crop residues after harvest. In most cases, access of farmers to the manures from these herds is well-regulated. The corresponding management of the landscape has important implications for the water circulation in the catchment. Maintaining the upper and lower parts of the watershed under natural vegetation protects it against erosion, but herders frequently set the vegetation on fi re, expecting new growth, and destroying the natural habitat of pest flies. This reduces the protecting effect of the vegetation and causes extensive losses of plant nutrients.
Intensification of livestock systems, through the collection of crop residues and the production of fodder, further modifies the management of the watershed and the balance of plant nutrients in the landscape. The denudation of the soils from intensive harvesting of crop residues facilitates runoff from rains coming before new planting. In contrast, the development of artificial pastures is an efficient method for controlling run-off. In most cases, the two practices concentrate the plant nutrients in the limited area where manures are spread. However, the development of leguminous fodder crops has an important role in restoring the nitrogen balance in the watershed.
Many traditional farming areas, especially in Africa and Latin America, are organized in concentric territories. The first of these, close to the houses, receives most of the domestic wastes and manures, and most of the available labour. This is the high intensification ring where, in general, food security is the primary issue for the farmers. The productivity of water for crops is very high, influenced by the ready availability of