BIOLOGICAL SOIL CONSERVATION
This umbrella term can include reduced tillage, minimum tillage, no-till, direct drill, mulch tillage, stubble-mulch farming, trash farming, strip tillage, plough-plant (for details see Mannering and Fenster 1983). In countries with advanced soil conservation programmes, particularly the USA and Australia, the concept of conservation tillage is the main theme of the recommendations for cropland, and it is also being taken up quickly in other areas, for example southern Brazil. The application is mainly in mechanized high production farming with good rainfall, or for the control of wind erosion where there is large-scale mechanized cereal production. It is less applicable to low input level crop production, or subsistence agriculture.
The principles are equally effective in any conditions - to maximize cover by returning crop residues and not inverting the top soil, and by using a high crop density of vigorous crops. Conservation tillage also has the advantage of reducing the need for terraces or other permanent struc- tures. However there are several disadvantages which hinder the application of conservation tillage in semi-arid conditions:
- dense plant covers may be incompatible with the well-tested strategy of using low plant populations to suit low moisture availability;
- crop residues may be of value as feed for livestock;
- planting through surface mulches is not easy for ox-drawn planters although there may be no problem with hand jab planters.
Surface manipulation such as ridging is discussed in Chapter 5.
One of the reasons for low yields in semi-arid areas is the limited amount of moisture available to crop roots. The available moisture will be increased if the rooting depth is increased and it has been shown that in some cases deep tillage can help, for example on the dense sandy soils (luvisols) in Botswana (Willcocks 1984). Reviewing many studies of experi- ments of depth of tillage on alfisols, El-Swaify finds varied results; deep tillage is beneficial for some crops but not all, and on some soils but not all. Also deep tillage requires greater draught power which is usually in short supply in semi-arid areas.
Ripping or subsoiling can be beneficial, either to increase the porosity of the soil, or to break a pan which is reducing permeability. The deep placement of fertilizer can also be used to encourage more rooting at depth, but again the application of this technique to subsistence farming will be difficult.
Like conservation tillage, this title covers many different farming techniques. It includes any farming practice which improves yield, or reliability, or decreases the inputs of labour or fertilizer, or anything else leading towards improved land husbandry, which we have defined as the foundation of good soil conservation.
Sometimes there is a long history of traditional farming and soil conservation practices which have been tested and developed over periods of time which are long enough to include all the likely variations of climate. These traditional practices should give the best long-term result, bearing in mind that the farmer's interpretation of 'best' may be based on reliability rather than maximum yield. But the semi-arid areas are changing rapidly, and the traditional patterns may be no longer relevant. As Jones (1985) says "while tradition may incorporate the wisdom of centuries of practical experience, it may also be inappropriate where recent demographic pressures have already compelled changes - for instance, the abandonment of bush fallowing or migration onto different types of soil or into more arid areas. There is also the point that the agricultural scientist very often still lacks the recipe for certain success; and you cannot require farmers to adopt new practices that are only 50 percent successful." Possible new techniques should have the same basic characteristics as traditional practices, they should be easy to understand, simple to apply, have low inputs of labour or cash, and must show a high success rate i.e. a high rate of return.
Some of the techniques are:
Farming on a rade is well established in India (Swaminathan 1982).Cultivations and planting are done on a gentle gradient, sometimes together with graded channel terraces. This encourages infiltration but permits surplus run-off at low velocities. Sometimes this may be combined with simple practices to encourage infiltration such as returning crop residues. This seldom provides a complete solution because of the problem of disposal of the surface run-off when it does occur.
Strip cropping is most useful on gentle slopes, where it may reduce erosion to acceptable levels without any banks or drains.
Rotations are another well established and simple practice. The object may be to improve fertility by the use of legumes or to help control pest or disease. In the semi-arid parts of Australia a successful practice is to alternate a cereal crop with a free seeding self-regenerating annual forage legume such as subterranean clover or medicago. Trials of adapting this system in Tunisia are reported by Doolette (1977).
Fallowing is well established and successful in some circumstances but not others. In the drier wheat lands of Australia, a bare fallow in summer is used to build up soil moisture before sowing the winter wheat which receives only barely adequate rainfall. The practice is particularly useful on cracking clay soils. There is a risk of erosion taking place during the summer when high-intensity summer thunderstorms fall on the bare soil (Walker 1982). In East Africa, using this method on sloping land has a high risk of erosion (Pereira et al. 1958), but on gentler slopes in Botswana good results were reported by Whiteman (1975). The practice is not universally successful, partly because subsistence farmers may fail to keep the fallow completely free of weeds, and it is unlikely to appeal in uni-modal rainfall if the result will be twice as much grain half as often. In Syria, ICARDA studied the effect of fallows on moisture conservation in a barley/fallow rotation at six sites with annual rainfalls varying from 260 to 350 mm. At less than 260 mm there was no increase in stored moisture, and farmer-managed fallows had little effect up to 300 mm, but there was potential for increased moisture conservation when fallow land was well managed. This included thorough and deep cultivation of the fallow, good weed control and pest control, a nitrogen status able to make use of the increased moisture, and good seed-bed preparation (ICARDA 1982). Jones (1985) suggests that the best application of fallows might be a system of land management in which sequences of short and long-cycle crops and intervening bare fallows would be planned to optimize water use, since a full profile of stored moisture at planting time permits a crop to produce some yield even in the driest of years. Boersma and Jackson (1977) report the long-practised successful use of summer fallows in semi-arid North America, and point out that a soil depth of one metre is necessary, and preferably 1.5 metres. On the other hand, trials in Israel by Rawitz et al. (1983) showed that the traditional tillage system of deep ploughing and further cultivations in autumn resulted in accelerated erosion and loss of up to 60 percent of the winter rainfall. Basin tillage was more effective, as discussed in Section 5.2.2. Reviewing the result of trials of fallowing in Francophone North Africa, Manichon (1983) concludes that the required conditions for it to work seldom apply in practice. Clearly this is a potentially useful technique but it must be tested in local conditions.
Mixed cropping and interplanting are widely applied traditional techniques. A combination of crops with different planting times and different length of growth periods spreads the labour requirement of planting and of harvesting, and also allows mid-season change of plan according to the rain in the early part of the season (Swaminathan 1982). Another possible advantage may arise from the use of legumes to improve the nitrogen status for the cereal crop. Variations on the theme of mixed cropping, intercropping, and relay cropping are being investigated in the Farming Systems Programme at ICRISAT (1986).
Surface mulching has the advantage of providing protective cover at a time when crop cover is not practical. It improves infiltration, and may also beneficially reduce soil temperature. Possible dis- advantages are:
- the amount of crop residue required may be more than is available from low-level production;
- problems of pest, disease, or nitrogen lock-up;
- the lack of implements which can plant or drill through the mulch;
- organic mulches are liable to be rapidly oxidized in high temperatures.
Successful use of mulching in the semi-arid south west of the USA is reported by Stuart et al. (1985). Trials of different materials and amounts are reported from India (Yadav 1974), and from the dry savanna of northern Ghana (Bonsu 1985).
Timeliness of farming operations is always important, particularly where the rainfall is erratic, and yields can be dramatically affected by planting or cultivating at the right time. Common problems are having to wait for rain to soften the ground because it is too hard to plough when dry, and perhaps then not being able to plant because the ground is too wet. Or a family with only one ox having to wait to borrow another one - hence the interest in the one-ox plough shown in Plate 4.4. Or having to wait for a month after the rains start to get the oxen back into condition for ploughing after a hard dry season. The essence of Farming Systems Research is to look at the whole farming operation to identify the constraints or bottle-necks before starting component research on parts of the system.
Some other techniques should be mentioned, but are beyond the scope of this book, so references are given for the interested reader.
- Deep planting of varieties which can germinate from 15 cm deep, and so delay germination until good rains have fallen (New Mexico, Billy 1981), alternatively, soaking seed before planting when it is desirable to accelerate germination.
- Dry seeding where the onset of rains can be predicted (India, Virmani 1979).
- Improved ox-drawn implements (Ethiopia, ILCA 1985. Kenya, Muchiri and Gichuki 1983).
- Recent developments in tractor-drawn machinery (Australia, Charman 1985).
- Tillage systems (USA, Wittmuss and Yazar 1981; world review, Unger 1984).
The selection and testing of alternative crop varieties and, the selection and breeding of cultivars for semi-arid conditions is relatively new but shows promise (Oertli 1983). However, Jones (1985) warns that this solution will be neither easy nor simple because the main requirement is the ability to survive drought periods and start growing again when the drought is broken. This is controlled by a complex of little-understood attributes.
Other desirable characteristics are a short growing season, drought resistance, and drought avoidance. The latter means the ability of the plant to adjust its growth habit according to the available moisture, for example, by tillering when moisture is available or going dormant when moisture is short, or only carrying through to ripening a proportion of the seed heads available.
The uncertainty of crop production reduces the opportunity for the effective use of manures and mineral fertilizers. There are possibilities for economic returns for a small investment, for example "many semi-arid soils have a low sorption capacity for phosphate, which means that small additions are sufficient to give a substantial crop response and will usually have some residual effect for several years after. This is just as well, for little sustained increase in productivity will be possible in these areas without an improvement in phosphate ability." (Jones 1985). There is also evidence that the availability of potassium can improve water utilization through its effect on turgor pressure or the mechanism of stomatal regulation (Lindhauer 1983).
Supplementary irrigation can be important because the provision of small quantities of water at critical times can have good results, for example to allow earlier planting, life-saving irrigation to carry crops through dry periods, or to increase the availability of soluble plant nutrients.
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