H.A. Elwell, Senior Research Engineer, Institute of Agricultural Engineering Harare, Zimbabwe.
Zimbabwe lies within the tropics between 150 30' S to 220 30' S and 250E and 330 E. The country is 389 000 km2.
Three broad relief regions are generally recognized on the basis of elevation: the Lowveld (below 900 m), the Middleveld (900-1200 m), and the Highveld (1200-2000 m). In addition, a narrow belt of mountains (2000-2400 m), called the Eastern Highlands, runs north to south along the eastern border with Mozambique; and the deep cleft of the Zambezi River Valley forms the boundary with Zambia in the Northwest.
Climate is largely influenced by relief, rainfall increasing with altitude. Mean annual rainfall varies from below 400 mm in the extreme south of the Lowveld to above 2000 mm on isolated mountain peaks in the Eastern Districts. Middleveld rainfall ranges from 500 to 700 mm and that of the Highveld from 800 to 1000 mm.
The rainfall pattern is distinctly seasonal with approximately 90% falling in the six months from 1 October to 31 March. Much of the rain falls as intense tropical downpours and is characterized by its extreme variability in both space and time. Three seasons can be distinguished:
Analysis of the mean rainfall figures for the period October to April provided by the Department of Meteorological Services shows that the mean value for the last 30 years (1961-1990, 635 mm) is 41 mm lower than that for the previous 60 years (1901-1960, 676 mm).
The sandy, relatively infertile soils that cover some two-thirds of the country constitute the main soil type in the communal areas. Isolated areas of heavier more-fertile soils occur throughout the country, the largest pockets being on the Highveld. Fertile irrigable balsatic vertisols occur extensively in the southern Lowveld.
Zimbabwe can be divided into five agro-ecological zones.
Zone I: comprises 5835 km2 situated in the Eastern Highlands, where the climate is cool and moist and the high annual rainfall (>1000 mm) is relatively reliable. The land can be used
intensively for dairy farming, forestry, orchards and plantations.
Zone II: embraces 72 745 km2 of intensively cropped farmland in the northeast Highveld receiving 700-1000 mm of rain annually. It is the major dryland cropping area producing the highest yields of maize, tobacco, cotton, soybeans and other crops and is the main source of irrigated wheat.
Zone III: comprises 67 690 km2 of semi-intensively cultivated land on the Middleveld and Highveld receiving 650-800 mm of annual rainfall. The area is used for ranching and cropping, the main crops being drought-tolerant varieties of maize, cotton, sorghum and soybeans. A large proportion of communal land falls in this region.
Zone IV: covers 128 370 km2, mainly in the western and the northern areas of the country experiencing 450-650 mm of rainfall annually. The land is suited to semi-intensive animal husbandry and is marginal for cropping. A large proportion of the communal land falls in this zone.
Zone V: comprises 112 810 km2 of very hot low lying land of the southern Lowveld and Zambezi Valley systems, where the annual rainfall is less than 650 mm. The area is suited to extensive ranching with intensive crop production using irrigation on pockets of fertile soils.
Annual farm output is valued at almost Zim $2 billion, about 20% of the gross domestic product. Three-quarters of the population depends directly on the land. Agriculture contributes 40% of the inputs to the manufacturing sector and supplies 40-50% of exports.
Production is diverse compared with many tropical countries. Tobacco, maize, cotton and sugar dominate crop production with wheat, coffee, sorghum, groundnuts, tea, citrus, coffee and vegetables making significantly smaller monetary contributions.
Maize dominates crop production, covering more land than all other crops (approx 1.5 million ha). One seasons's maize sales can earn more than Zim $100 million.
Tobacco production has steadily increased since 1980. Most Virginia tobacco is grown on large scale commercial farms north of Harare. Burley tobacco is favoured by small-scale communal farmers mainly because of the less rigorous curing required.
Dryland cotton grown in the central and northern parts of the country and under irrigation in the Lowveld, supplies the needs of the local textile industry and provides a 70% excess for export.
Cotton seed provides more than 50% of the vegetable oil in Zimbabwe.
Maize, sorghum and vegetables are the principal subsistence crops of the country. Production for family consumption remains paramount in the majority of peasant farming areas in Zones III and IV, but increasing quantities of maize and cotton are being marketed from communal areas in the more favoured agro-ecological zones. Communal farmers have increased their share of the maize crop from 6% in 1980 to 50% in 1988.
Cotton, sunflowers and groundnuts are major cash crops for communal farmers. Peasant farmers now produce half the total cotton crop, 75% of the sunflower and 80% of the sorghum. Coffee has been promoted as a peasant crop in the Eastern Highlands but production remains small.
In the pre-colonial period, shifting cultivation was practised, predominantly in Zones I to III; while cattle rather than cropping was the major activity in Zones IV and V. Under shifting cultivation, the bush was cut at shoulder height (to allow it to regenerate quickly) and the branches burned to provide nutrients in the form of ash. A form of zero tillage was practised (holing out). The piece of ground was cropped for three or four years at the most and was allowed to revert to bush before the soil showed signs of degeneration. It remained fallow for about 20 years before it was considered fit for another cycle of cropping.
The mixed cropping technology used at that time was simple but maximized soil fertility, minimized erosion and weed labour and reduced the risk of crop loss. Isolation of lands, mixed cropping, long fallows and healthy plants minimized crop losses caused by pests and diseases.
This shifting cultivation was no threat to the environment and was sustainable in every sense but cannot now be practised because of the shortage of land brought about by the colonial Land Apportionment Act (which confined the indigenous population to 41% of the available land area) and the rapid increase in this population.
The period of European settlement (1900-1980) saw the development of large-scale commercial farms using a combination of modern technological methods based on annual ploughing, inorganic fertilizers, insecticides and herbicides. Up to now these methods have served the short-term economic goals of the commercial farmer quite well. On the other hand the only technique communal farmers have been able to afford to any extent has been annual ploughing.
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Recent local experience has shown, however, that annual ploughing leads to rapid degradation of soil fertility and structure through loss of organic matter, giving higher input costs, increased runoff and excessive losses of soil and nutrients (Elwell 1989; Stocking,1986). Furthermore, evidence is accumulating in other countries that existing levels of chemical use lead to the destruction of the on-farm and wider ecology and endanger human health and life. A direct consequence has been the rapid lowering of soil fertility in the communal areas, resulting in poor yields, increased runoff and soil loss and widespread environmental damage. On commercial farms the liberal use of mineral fertilizers has offset to some extent the deterioration in potential soil fertility, nevertheless yields in this advantaged sector are considerably lower than the optimum and production costs significantly higher (Elwell 1991).
Truly traditional land preparation, an integral part of shifting cultivation, consisted of holing out with a hand hoe or sharp pointed stick. Minimal soil disturbance was involved in what, today, would be classified as a form of minimum or zero tillage. Weeds were hand-pulled or hoed.
In this century, communal farming has moved rapidly from shifting cultivation to intensive settled agriculture. But, because of lack of appropriate research and an inadequate resource base, the practices tend to be poor imitations of commercial methods.
Shallow mouldboard ploughing is widely practised but almost no crop residues are returned to the land. Levels of applied fertilizer are inadequate and industrial herbicides and insecticides cannot generally be afforded. Poor soil fertility, large soil losses, loss of nutrients and lack of plant available moisture and draft power are major constraints to production (Norton 1987a; Oliver and Norton 1988).
Today, ploughing with a single furrow mouldboard plough drawn by oxen is the most widely used land preparation practice in Zimbabwe communal areas. It is estimated to be used on 73-90% of the cultivated area (Contill 1990). About 5-25% of the remaining area is
estimated to be ploughed by hired tractor, 1-15% cultivated by hand hoe and less than 1% is under any form of conservation tillage, usually ridging parallel to the contour or ripping into bare ground.
The ox-plough was vigorously introduced by the Extension Services in the 1920s and quickly rose to become the most widespread land preparation method. Generally the ploughing is of poor quality (100-150 mm compared to the recommended depth of 200-230 mm) and is frequently done twice to control weeds, once during winter and a second time soon after the first rains. The recommended system of early ploughing is estimated to be practised by less than 50% of the farmers having access to oxen. Shortage of draught animals is one of the major ploughing constraints, with up to half the farmers in some areas having to hire oxen for land preparation (Norton 1987b). Under these circumstances ploughing is normally done once late in the season (Dec-Jan) after the rains have penetrated deeply enough to facilitate ploughing.
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Annual mechanized ploughing with disc ploughs is the most widely used method of land preparation on large commercial farms. Farmers normally plough in early winter after the previous crop has been harvested when there is usually some residual soil moisture. This reduces the costs of ploughing, enables the required depth to be attained and controls winter weeds. After ploughing the land is frequently left to allow clods to break down by the weathering action of wet-dry cool-warm cycles. However, many farmers disc harrow the cloddy land immediately to conserve subsoil moisture and to control winter weeds (Norton 1987c).
In recent years evidence has emerged to show that annual ploughing reduces soil organic matter to low levels at which aggregates in clay soils are no longer water-stable. Collapse of structure results in increased draught power being required for ploughing and for multiple disc-harrow operations to reduce clod sizes. One of the direct effects has been a marked increase in tractor size and the cost of land preparation (Elwell 1989). Mineral fertilizers, insecticides and herbicides are now an integral part of the annual ploughing system.
Disc harrowing with rolling is probably the most popular method of preparing a satisfactory tilth (Norton 1987c). Multiple passes are required on clay soils in poor structural condition, which is almost invariably the case.
Arising from the tendency of disc harrows to pulverize the soil, a few farmers use spring tines for cultivation and a yet smaller number have purchased cage rollers. Research has shown however that the cage roller also pulverizes the soil, whereas no damage could be detected from use of spring tines (Elwell 1986).
Ploughing was initially most often carried out up and down slope. This direction was preferred by the early farmers because it is generally easier to get a long run than across the slope. Thus, turning time could be reduced by up-and-down slope ploughing. The obvious erosion caused in the steeply-graded opening and closing furrows did not influence ploughing direction at that stage.
In the first soil conservation booklet (Aylen and Roberts 1937), the erosion-control benefits of contour ploughing were recognized and it was thought at that time the introduction of contour ridge systems would encourage farmers to plough across the slope.
In communal areas, the intensive drive in the 1950s and 60s to protect arable land by the construction of large contour ridges did encourage some communal farmers using light ox-drawn equipment to plough parallel to the contour, though most farmers continued to plough up-and-down by using the contour channel and bank as a turning area.
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The introduction of contour ridges on commercial farms, however did not provide even this slight benefit. Practice making continued to be influenced by economics than by the need to conserve the soil. Ploughing direction continued to be up-and-down the slope, with the contour ridges being ploughed-out and then remade at the start of every season. The extra cost of remaking the ridges was not taken into account by farmers though they continued to complain at the expense involved in making these same conservation works.
In spite of efforts by the extension services, up-and-down slope operations continued to be widely used in the 1960s and 70s, and are used to this day on tobacco farms.
In the late 1960s Pereira (1968) strongly advocated contour ploughing as a standard practice throughout the tropics, even on gently sloping land. A few years later a survey in a commercial tobacco growing area (Karoi) found up-and-down slope operations so prevalent that more than half the conservation works in the district were being damaged annually (Erasmus 1971); and, in a general survey to identify the extent and causes of erosion on commercial farms nationwide, up-and-down slope operations were rated as the third (planting) and seventh (ploughing) most prevalent causes of soil erosion. At about this period, instigated by the Institute of Agricultural Engineering, an extension drive was launched by Conex to improve ploughing techniques in general, including the promotion of contour ploughing. This drive, combined with increased emphasis on contour ridging, decreased the number of farmers ploughing up-and-down slope.
Although there are no exact figures available, it is estimated that about 60% of communal farmers and about 70% of commercial farmers contour plough as a routine. Up-and-down ploughing is most prevalent on commercial tobacco and cotton farms and on recently opened-up uncontoured lands in the communal areas.
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TABLE 33
Typical annual losses of soil, rainfall and nutrients for maize lands (adapted from
Elwell and Stocking 1988)
| Soil loss t ha-1 |
Rain loss % |
Organic carbon kg ha-1 |
Value of N and P
fertilizers* $ ha-1 |
|
| Communal sandy soils Contour ploughing Tie-ridging |
50 2 |
30 5 |
535 21 |
94 4 |
| Commercial red clays Contour ploughing Tie-ridging Rip into residues |
15 2 1 |
20 5 5 |
231 21 15 |
50 4 3 |
* at 1988 prices
No measurements of soil loss and runoff from ploughing up-and-down slope have been made locally. Qualitative visual assessments of erosion damage during the 1974 national erosion survey are available as indicated earlier in this section, and crude quantitative estimates can be obtained from the local soil loss estimator SLEMSA (Elwell 1980).
Much accurate (but site-specific) data are available for contour ploughing (Hudson 1962; Elwell 1971; Elwell and Stocking 1988; IAE 1989-90; Vogel 1991a).
Typical estimated values of average annual losses of soil, water and nutrients for two conservation-tillage systems and contour ploughing are given in Table 33.
Ridging and tie-ridging (standard and no-till)
Traditional tie-ridging (with annual ploughing) has never been a popular or widespread in Zimbabwe and was virtually unknown before the early seventies.
Meikle (1972) produced data to show the significant moisture conservation advantages of this technique on a phyllite soil with a marked tendency to cap. Although the technique did not gain wide acceptance as a result of this research, it became popular among farmers in the experimental locality (Tiltman 1984) and is commonly practised there to this day.
In a seven year trial at the IAE, 1971-72 to 1977-78, on red clay soil, tied ridging gave slightly higher yields overall than conventional tillage, largely because of its better performance in dry years (Smith 1988).
During the period 1983-84 to 1987-88, tie-ridging was included as a treatment in a nationwide programme of on-farm observational tillage trials to observe the yields merits of tie-ridging and ripping into bare ground compared to conventional tillage. However, no significant difference in yields could be shown because of the high variation in yield arising from management preferences (Stevens 1989).
A moisture conservation trial was begun in 1983-84 at the Cotton Research Institute, Kadoma, to compare the effects on cotton yields of raised beds, tie-ridging and conventional till. Potholing and wet ripping were imposed as subtreatments on conventional tillage. The soil was a reddish brown clay on a 1% slope. The results up to 1986-87 (the last published information) showed tie-ridging to give the highest and most consistent yields and conventional tillage the lowest. Potholing and ripping was found to improve the performance of conventional cultivation but yields were still poorer than for tied-ridging. Planting on raised beds gave only marginally higher yields than conventional tillage (CRI 1977-88).
In spite of its performance in reducing soil loss and runoff to low levels and in increasing crop yields under most tested circumstances, the tie-ridge system has major disadvantages which render it unsuitable for widespread adoption (particularly in communal areas in Zimbabwe). These are that powerful draught is needed and that substantial effort and time are needed to prepare the lands each year: ploughing, ridging and tieing. A further conceived disadvantage is that reports from outside Zimbabwe indicate that tie-ridging has occasionally given rise to high soil losses due to the level crop ridges being overtopped during heavy rainstorms (Prestt 1986).
In 1981-2 development work began, therefore, at the IAE Borrowdale, on a modification called no-till tied-ridging (Elwell and Norton 1988). The amount of time and draught required in the standard tie ridge system was reduced by incorporating no-till principles; and erosion caused by level ridges was overcome by grading the crop ridges (1:100 max to 1 to 250 min) to carry excess runoff from the land. This was later found to also give major soil-drainage benefits in high rainfall years and on wet soils.
The merits of the no-till tied-ridge system are currently being investigated in Zimbabwe on two sites by a joint AGRITEX/GTZ project. Both are on granitic sandy soils common in communal areas. One site is in Zone IV (450-650 mm mean annual rainfall) and the other in Zone IIa (750-1000 mm). The programme consists of joint on-station fundamental research and on-farm adaptive trials.
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Preliminary results indicate that no-till tied-ridging reduces soil loss and runoff to very low levels (Vogel 1991a) and though, compared to conventional annual ploughing, more draught power and time are required in land preparation in the first year, less power and time needed in subsequent years.
The technique has produced higher yields than conventional ploughing or ripping into residues at the wetter site and lower yields at the drier site, though the rainfall amount and distribution has been extraordinarily unfavourable so far during the period of measurement. The reasons for the variation in yield are being investigated, and data are being collected to evaluate the time involved in land preparation and weeding. New techniques are expected to be developed to remove obstacles to farmers' adoption of the system (Vogel 1992; 1991a; 1991b).
Growing crops on ridges (annual ploughing without ties) has been practised for many years in Zimbabwe, both as a surface drainage (water removal) system and as a soil and moisture conservation technique. Removal of excess surface water is effected by steep graded ridges up-and-down slope or at an angle to the slope; while conservation of soil and moisture is aided by ridges constructed at low gradients (1:250). Locally, all tobacco is grown on ridges, mainly graded up-and-down the slope, though increased yields and significant reductions in soil and runoff losses and considerable reduction in mechanization costs have been claimed for contour ridging (Elwell 1973; Elwell et al. 1974; Hudson 1957).
TABLE 34
Soil losses on well-drained granitic sand on a 6% slope (Elwell 1980)
| Crop and cultivation systems | Soil
loss (t ha-1) |
| Maize on the flat Tobacco ridged up-and-down Tobacco ridged on-contour |
15.8 21.6 2.4 |
Methods of ridging parallel to the contour and of no-till tied-ridging have been described by Elwell et al. (1974) and Elwell and Norton (1988) respectively.
Estimated soil losses for tie-ridging are given in Table 33. Comparisons of soil loss between on-contour ploughing, ridging up-and down slope and ridging on-contour are given in Table 34 for a tobacco/maize rotation.
In Zimbabwe, conventional tillage is the term used for ploughing on-contour, followed by harrowing to prepare a seedbed. Tractor-drawn disc ploughs are used on commercial farms for the basic ploughing operation, followed by disc harrowing and rolling (usually a Cambridge Roller) to break down the tilth. Several discings and/or rolling are usually necessary on clays because of the unavoidable degradation of soil structure and compaction involved in modern farming technology.
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For communal areas, a single furrow mouldboard plough has been developed as the basic tillage tool. Ox-drawn drag spike harrows are most commonly used for secondary tillage operations. Since most of the soils are sands and compaction is not normally severe, few passes are needed to prepare the tilth. However structural damage caused by annual ploughing related to the rapid loss of organic matter (with compaction where tractors are used) is considered to significantly reduce the moisture and nutrient holding capacity of these soils (though there are currently no research data to support this view).
Recommended depth of ploughing (230-250 mm) was determined by tillage trials carried out by a multi-disciplinary team 1965-75 (Smith 1988). However communal farmers can rarely achieve this depth because of poor draught availability (weak oxen), inadequate skills in plough setting and the poor state of the ploughs themselves. A depth of 100-150 mm is more commonly attained resulting in lower yields and greater erosion (see Plate 28).
The effect of structural damage on soil properties arising from conventional till compared to ripping into residues or tine planting into residues is shown in Table 35 for continuous maize on clays and sands. Organic carbon contents and the proportion of water stable aggregates were found to be lowest under conventional annual ploughing irrespective of whether residues were returned or not. An important finding was that, contrary to existing extension recommendations, ploughing-in maize residue did not significantly improve soil structure compared to plots where no residues were returned. However the highest values attained were still considerably lower than on virgin land.
Rainfall simulator experiments on red clay soils have shown losses of soil and rainfall to increase dramatically as organic carbon and water-stable aggregate levels decline (Elwell 1986).
Much research has been carried out in Zimbabwe from the late sixties to the mid eighties into the effectiveness of reduced or minimum tillage systems compared to conventional tillage. Most of the work was appropriate to the mechanized commercial sector with the communal sector benefiting indirectly (based on the assumption that communal farmers would eventually generate or have access to the resources required to practise the techniques).
TABLE 35
Mean organic carbon and water-stable aggregate levels as influenced by tillage
treatment
| Organic carbon % |
Water-stable aggregates % |
|
| Red clay soil* Ploughing - no residues Ploughing - residues Ripping into residues Virgin ground** |
1.1 1.2 1.5 2.5 |
5.0 5.2 16.6 47.0 |
| Sandy soil*** Ploughing - residues Tine plant Virgin ground |
0.52 0.58 0.74 |
5.0 10.4 14.0 |
* IAE Annual Report 1988-89.
** Elwell, 1986.
*** IAE Annual Report
1989-90.
The effects on crop yield of a wide range of reduced tillage practices were compared with yields from conventional tillage (plough, disc and roll). The most common crops studied were maize and cotton, though soybean and wheat featured in rotations on some trials.
Tillage practices in "minimum" categories included wheeltrack planting, rip and disc, rough ploughing, ripping, discing and combinations of these with various residue management options such as returning all residues, grazing or burning.
Developed in the late 1960s to early 70s, wheeltrack planting is rarely seen today in Zimbabwe. The method consisted of the land being rough-ploughed. The crop was then planted into the soil crushed by the wheels of the tractor, pulling the planter. The method was said to considerably reduce land preparation costs and increase infiltration (Meikle 1972) but lost popularity because of weed and germination problems.
Smith (1988) concluded from commercial- and communal-orientated trials that, while reduced tillage systems with residues returned gave higher infiltration than conventional till, they were likely to give lower infiltration (higher runoff) when residues were burned. He found yields to be generally lower from reduced tillage systems than from conventional till, which he attributed to increased weed and pest levels and the poorer rooting (see Tables 36 and 37 for typical examples).
When substantial amounts of residues were left on the soil surface, Smith (1988) found reduced tillage systems gave higher yields in dry years, particularly on sloping ground and for low infiltration rate soils; but lower yields of maize in wet years, a fact he attributed to loss of fertilizer through leaching and adverse changes in soil physical properties.
In addition, Smith (1988) concluded that reduced tillage systems require lower levels of most inputs than conventional tillage, but also required higher levels of management to minimize adverse effects of pests, weeds and difficulties in handling surface residues.
Norton (1983), after carrying out a theoretical accounting exercise including assumptions of increased weed and pest control expenses for reduced tillage systems, provided figures indicating lower profits per hectare compared to conventional till except when residues are burnt off. However, recent farm-scale research (ART 1990; Winkfield 1992) and on-farm practice (Oldreive 1989) show that net returns per hectare from reduced tillage systems, particularly those approaching zero till, can be impressively better than for conventional tillage.
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From small field plots on a crusting silty soil at Henderson Research Station from 1972/73 to 1975/76, Rankin (1977) found soil loss and runoff from ripping into crop residues to be approximately half the losses from conventional tillage, while crop yields (cotton) were not significantly affected.
In a three year experiment on red clay soils at the IAE (Elwell, 1983) found soil loss from wheeltrack planting to be 64% of the mean loss from conventional till, while the mean loss from zero till without residues was 162% that of conventional till. Similarly runoff from wheeltrack planting was 77% that of conventional till and zero till runoff from bare soil increased to 135%.
Two years' results on sandy soil more typical of communal land conditions in the higher rainfall areas were similar to those of Rankin (1977) with soil loss from ripping into mulch being 41% of the loss from conventional till.
TABLE 36
Mean organic carbon, water-stable aggregate and yield levels as influenced by minimum
tillage treatment
| Organic carbon % |
Water-stable aggregates % |
Mean maize yield*** t ha-1 |
|
| Red clay soil* Ploughing - no residues Rip & disc - no residues Rip & disc - residues Ripping residues Ripping residues |
1.2 1.3 1.4 1.3 1.5 |
5.2 7.5 11.1 6.4 16.6 |
6.4 6.4 5.8 5.8 5.2 |
| Sandy soil** Ploughing - no residues Rip & disc - residues Strip till - residues Rip & disc - residues (with interrow rip) |
0.5 0.6 0.6 0.7 |
3.0 9.8 8.5 8.0 |
7.2 6.3 6.4 6.4 |
*IAE Annual Report 1988-89.
** IAE Annual Report 1989-90.
*** Average of
9-11 years.
The effect of mini-mum tillage treatments on soil properties and yield as recorded in IAE research programmes are illustrated in Table 36.
The IAE investiga-tions showed that mini-mum tillage treatments improve soil structure compared to convention-al till with the greatest improvement being for ripping into residues, a practice which involves less soil disturbance than the other treatments.
Results such as these have led to the general conclusion that, for Zimbabwe condi-tions, the less soil disturbance and the greater the organic biomass entering the soil the better. Solutions remain to be found however to redress the problem of lower yields experienced from reduced till systems, although the drop in yield had not been great.
Included in this category are systems requiring no modification of the soil, the only disturbance being that necessary to weed, plant the seed and place the fertilizer. Local systems meeting this qualification are: spike harrowing, tine planting into residues (ripping to 150 mm along or between old crop rows only), no-till strip-cropping (ripping 50 mm along crop rows only), Badza holding out and methods of direct drilling and broadcasting seed into residues.
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Spike or drag harrow
From 1965-66 to 1968-69 a unique treatment was included in a frequency tillage trial in Zone IV. It consisted of normal ploughing in the first year followed in subsequent years by simply dragging a spike harrow over the untilled surface to remove weeds. The results in terms of maize yield were compared to a minimum-till rip and disc treatment and to conventional ploughing. The spike harrow was the design commonly used by communal farmers. Rip and disc was found to give similar yields to conventional till while spike harrowing tended to yield only slightly less. The ground is prepared finely, to be bare and loose on the surface, with firmer layers below. Soil losses and runoff are likely to be high from this treatment therefore it is not considered a conservation treatment but is used only to save time and draught during land preparation. The value of promoting this treatment in times of draft shortage is currently being considered to offset effects of the severe drought on ox draft.
Tine planting into residue
The most common and widely-researched approach to residue farming in Zimbabwe is to fertilize and plant by shallow ripping (150 mm) either along the old crop row (by tractor) or between crop rows (by oxen).
To qualify as a conservation tillage system the local requirement is for residue to cover at least 30% of the total ground area at the start of the season. Since residue in communal areas is fed to the cattle, peasant land can rarely meet this requirement and the method is usually more applicable to commercial farms.
TABLE 37
Mean organic carbon, water-stable aggregate and yield levels of zero and conventional
tillage on paired trials (all residues returned)
| Organic carbon % |
Water-stable aggregates % |
Mean maize yield*** t ha-1 |
|
| Red clay soil Plough (dryland)* Tine plant (dryland) Plough (irrigation)** Tine plant (irrigation) Plough (irrigation)*** Direct drill (irrigation) |
1.1 1.4 1.4 1.8 1.6 1.7 |
5.7 19.3 45.9 49.7 24.8 31.9 |
5.3 4.2 8.0 7.7 |
| Sands Plough (dryland)** Tine plant (dryland) Plough (dryland)** Tine plant (dryland) |
0.5 0.9 0.5 0.6 |
3.0 158 5.2 10.4 |
7.2 6.0 6.0 4.1 |
*Annual Report 1988-89.
** IAE Annual Report 1989-90.
*** 9-11 year
means.
As can be seen from Table 37, provided residue is returned, zero tillage systems have consistently improved soil structure compared to conventional tillage. Irrigation (in this case wheat-maize-soybean rotations) has resulted in greatly improved soil structure (residues returned) compared to dryland farming. Large improvements in soil structure occurred even for irrigated convention-al till (compared to dry-land), though zero till still scored higher.
Although the effect on soil structure of zero till treatment with resi-dues removed has not been researched in Zimbabwe, the data for ripping into and without residues (reduced till), Table 36, indicate that structural degradation can be anticipated if zero tillage systems were applied locally without residues.
Generally, contrary to findings elsewhere, the local research experience has been that tine planting is likely to give lower yields than tine conventional till (Table 37). However, it is thought that this may be due to inadequate monitoring of lime and nitrogen requirements in the early trials.
However, there is no doubt that ripping or tine planting into residues lowers soil and rainfall losses from arable land. Average annual soil losses of 1-2 t ha-1 have been measured from this treatment on sands (Vogel 1991a) and long term average annual losses are estimated to be about 1 t ha-1 on red fersiallitic clays (Elwell and Stocking 1988).
No-till strip-cropping
This recently (1988) devised system is still in the development stage but already shows great promise for controlling soil, rainwater and nutrient losses; and for improving soil structure. To date, on red clay soils, annual soil losses have been less than 0.5 t ha-1 and runoff less than 2%. In addition, the proportion of large water-stable aggregates is 30-35%, almost the same as on virgin ground.
The system consists of alternating on-contour strips of dense cover crops and row crops, making up an entire rotation planted together on the land between two adjacent contour ridges. The following year the crop strip adjacent to the contour channel moves up to just below the contour bank above it and all crops move down one strip. Crop strips vary in width to accommodate the proportion of any particular crop in the rotation (within reason).
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A rotation of 70% maize, 20% soybean and 10% rapoko, with soybean and rapoko planted as cover crops at high density, has given low losses and high yields.
The only land preparation involves shallow ripping to 50 mm with a spiked tool to place the seed. Maize is underplanted with pumpkin, dolichos lab lab and cowpea to spread risk and to suppress weeds. Weeds are removed by hand tools.
In the trials so far no chemicals (as fertilizer insecticides or herbicides) have been applied, so that the potential of the system for reducing external inputs when organic amendments are readily available, can be assessed. In good rainfall years, yields of maize and soybean have equalled those of conventional farming and greatly exceeded conventional farming yields in the recent serious drought. Preliminary results indicate that this system is the closest approximation to an intensively-cropped sustainable farming system yet devised in Zimbabwe.
Badza holing out
Badza holing out has been extensively researched in Zimbabwe as it is a common method employed in communal lands where ox power is in short supply. Invariably, planting on research trials has been carried out on bare ground on the assumption that field residues from the previous crop will have been removed.
In this system, the land is either entirely or, more commonly, partly cleared of weeds and a hand hoe used to make a planting hole. Fertilizer is either placed in the bottom of this hole and covered (with soil) before the seed is placed or an extra fertilizer hole prepared alongside.
Generally, yields for this treatment have been lower than for conventional till and weeds more difficult to control. Soil losses from 2 years of measurement on granitic sands on a 4.5% slope indicate losses of soil to be similar to or slightly less than under conventional till (Vogel 1991a). However, losses from holing out are expected to increase with time as the soil becomes more compact and as the soil organic matter declines due to non-return of residues.
Although this system is often referred to locally as a conservation system because draught power is conserved, it can only truly be considered as such when the minimum level of mulch (30%) is available as soil cover.
Direct drilling and broadcasting
Strictly speaking the term zero till applies to methods involving no soil disturbance whatsoever, a condition not easy to achieve. Broadcasting is, however, one of the few treatments meeting this requirement. In this system, the seed is broadcast over the top of the previous crop residues and, if necessary, the residues shaken to ensure the seed reaches the soil.
In the direct drilling method, small seeds such as rapoko, sorghum, soybean, wheat, barley etc. are planted directly into very shallow furrows cut into the previous crop residues. Weeds are normally controlled by herbicides. The effect of direct drilling on soil properties in a wheat-soybean rotation are shown in Table 37.
No soil loss or runoff figures are available locally for either direct drilling or broadcasting; but the losses can be inferred from similar systems such as tine planting into residues and no-till strip-cropping, the magnitude depending largely on the percentage and nature of the residue and the type and density of the crop planted.
No precise figures are available giving the number of farmers in Zimbabwe who have adopted conservation tillage practices. However, it is thought to be 5-10% of commercial farmers, dropping to below 1% of communal farmers (discounting Badza holing out in bare ground).
Several commercial farmers have been practising conservation tillage systems for a long time, however, and have adequately demonstrated the value of these systems in terms of reduced inputs and increased profits (Tiltman 1984; Oldrieve 1989). These few innovative and far-sighted farmers are well ahead of their community and in some ways of research.
Whereas much effort has been directed towards the development and extension of conservation tillage systems suitable for large-scale commercial enterprises, the peasant sector has barely benefitted in the past, a factor accounting in large measure for the poor adoption rates among communal farmers.
Research and extension workers are now attempting to redress this situation by combined on-station research and on-farm demonstration trials. These are being undertaken throughout Zimbabwe by various aid agencies usually working through departments of the Ministry of Lands, Agriculture and Rural Resettlement or through NGOs (CARD 1992; Gotora 1992; SRI 1992).
Within the government sector, research and extension/training for conservation tillage are the responsibility of the Ministry of Lands, Agriculture and Rural Resettlement. Research functions are the responsibility of the Department of Research and Specialist Services (agronomy, soils, plant protection and farm systems) and the Institute of Agricultural Engineering, a Branch of Agritex (tillage, machinery testing and soil and water engineering). The communal farmer is the priority target for government organizations.
Outside government, The Agricultural Research Trust, conducts research into conservation tillage systems with large-scale commercial farmers as its primary target. Extension work is carried out principally as field days held at the ART Farm near Harare and on suitable local farms, usually ones where trials are being carried out. The work is funded by the Commercial Farmers Union and from interested crop-commodity associations (ART 1990).
Within the Ministry of Lands, subject specialists within the Department of Agricultural Technical and Extension Services (Agritex) are responsible for disseminating research results to farmers. The results are gleaned from published work or collected during field days at research stations or training courses, and through seminars held by researchers. These subject matter specialists are responsible for training Agritex field extension officers (degree and diploma holders) and these officers, in turn, are responsible for training lower echelons of staff, the principal contact with the farmer being the Agricultural Extension Worker (AEW).
Specialists organize and run courses for officer grade staff using the expertise of the Training Branch of Agritex. The Training Branch has no direct teaching functions, that being the responsibility of the specialists themselves. The Branch's responsibility is confined to teaching these specialists training techniques and in general extension methods; and for ensuring that all officers in the Department are put through the obligatory in-service training courses. Many optional courses are also provided by specialist branches.
The training of AEWs is normally decentralized, taking place at provincial level. Some specialist training may also be decentralized, depending on the demand and nature of the training. Extension messages targeted for farmers are formulated by extension specialists and appropriate techniques demonstrated to farmers through field days and demonstration trials.
The Institute of Agricultural Engineering itself has a training wing situated at the Agricultural Engineering Training Centre (AETC). Communal farmers, AEWS and Agritex officer grade staff are taught to handle and train oxen for land preparation and other farm operations; and in the use of a variety of ox-drawn implements. In addition, farm workers (usually commercial), AEWs and Officer grade staff are taught proficiency in the operation and maintenance of tractors and associated equipment. Agritex field officers (only) are provided with courses designed to give them a general appreciation of local soil and water conservation problems and techniques, while conservation specialists are given in-depth training on this subject.
Largely arising from the government policy of prioritizing communal farmers, commercial farmers now depend to a large extent on non-governmental organizations for advice and training. Foremost among these organizations are crop-commodity associations, fertilizer and mechanization companies, private consultants and farming unions.
All the above bodies, government and non-government, promote conservation tillage techniques to some extent but particularly active in this field are the Institute of Agricultural Engineering, Department of Research and Specialist Services and the Agricultural Research Trust. Preliminary recommendations on the application of conservation tillage techniques recently issued by the AGRITEX/GTZ Contill Project are given in Appendix I.
The major technical constraints in communal areas to the adoption of conservation tillage techniques based on minimum and zero tillage are:
Historically, prior to independence, communal farmers were assumed to be aspiring commercial farmers. On this basis, almost all research was directed at the commercial sector and the developed technologies "handed down" to communal farmers. Even today, agriculturalists find it difficult to appreciate that most of Zimbabwe's smallholders can never aspire to being commercial farmers because of land size, low rainfall, infertile soils, lack of finance and other resources both material and human.
The only real constraint to the widespread adoption of conservation tillage by Zimbabwe's commercial farmers is the shortage of suitable equipment on the market. This has not been a great obstacle to innovative farmers, who have met the challenge by modifying their existing equipment. However, among the fears commonly expressed during field days are: possible increases in pests and diseases, poorer germination, more expensive weed control and reduced yields, none of which have materialized in practice (Oldrieve 1989; Winkfield 1991). One is forced to conclude therefore that unwillingness to change is the major constraint to farmer adoption of conservation tillage techniques. Nevertheless unfavourable economic conditions and deterioration in availability of imported inputs are now forcing many farmers not only to reconsider their attitudes to run-of-the-mill conservation tillage techniques but to take an active interest in low-external-input farming systems in general.
The major constraint of the adoption of conservation tillage in the socio-economic sphere is reluctance to change. Very few farmers have shown the foresight, flexibility and innovation necessary to meet the challenges posed by soil degradation, deterioration of the on-farm ecology and declining profitability caused by existing standard farming technologies.
Most of the commercial farmers who have changed (5 - 10%) have done so simply as a response to market forces, primarily the rising cost of fuel and not for ecological reasons or sustainability. The pressure from fuel costs does not apply to the communal sector, however, which may account (in part) for the lower adoption rates (less than 1%).
In the communal sector shortage of labour is a major constraint to the adoption of conservation tillage techniques. Weeds often are troublesome in the first few years while management adjustments are being made. The extra attention weeding requires is not onerous on commercial farms with mechanized weeding or herbicide application facilities, but can require high labour inputs when handhoeing (Vogel 1992). Labour shortages are often acute in communal areas due to the migration of active males to the towns in search of employment. In many communal areas the predominance of children and old people is very noticeable.
A further disadvantage of male migration is that the decision-maker, the family head, is not present to see to the day to day running of the farm; nor to make the major decisions necessary for an effective change in farming methods to be undertaken. Since the family head is rarely there to take part in agricultural training programmes or field days, the probability of change is small. It must be recognized too that care of the communal plot has a low priority in family needs since the main source of income is external (wages of migrant workers) and the land is looked upon as merely a retirement location. The resources as such have therefore no immediate intrinsic value and health, education, water and transport invariably score higher than soil conservation on the communities' scale of priority needs.
Since a large proportion of the money available in rural areas is from external sources, pressures from agricultural market forces which might otherwise have induced change, have little effect.
Generally, in their current decision-making, farmers in the commercial sector are motivated by short term profit whereas communal farmers are attracted by systems offering less effort. Unfortunately, truly sustainable agricultural systems are unlikely to satisfy the requirements of either farming group since they inevitably involve both greater effort and long term thinking. A major shift in attitudes is necessary therefore before the new sustainable techniques are readily adopted.
Among other economic and social constraints associated with the adoptions of conservation tillage techniques are:
Many commonly used implements have a role in conservation tillage or can readily be adapted.
Initial deep ploughing is often necessary to remove compacted zones and chemical pans. Ripper tines are useful for this purpose too as well as for providing opening furrows for planting into residues.
The single furrow ox-plough (mouldboard) is the most favoured implement for building ridges in the no-till tied-ridging system in communal lands; though the light cultivator can also be used for this purpose. On commercial farms satisfactory ridges can be made with the tobacco disc-ridger.
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Disc harrows, however, are not now recommended locally for conservation tillage systems as insufficient residues are left on the soil surface and much damage is done to the soil structure by the creation of a powdery fraction in the soil. Spike harrows do not pulverise the soil but they are not recommended as they require a bare surface for efficient operation.
Special machinery is, more often than not, developed by simply modifying existing equipment. For example, planting and fertilizing into residues has been facilitated by placing a disc coulter ahead of the planter shoe or fertilizer tine to cut through the trash. An alternative strategy is to place sweeps or trash wheels ahead of the planter/fertilizer assembly to clear a planting path. The residue can be chopped before planting.
The following description of machinery developments in the commercial sector in Zimbabwe has been condensed from Oliver (1989), who only describes equipment that leaves adequate crop residues on the land.
At least four different make of chisel tine are available locally; the Smith & Bennet "C" tine, the Bain "Tempest" chisel tiller which is built of spring tines with over-centre breakback mechanisms, the Power "Commander" consisting of forged spring tines with additional leaf spring loading, and the Farm Mechanization "Vibro Flex" imported spring tine.
The "Tempest" and "Commander" have a cage roller fitted as a standard feature.
Four makes of conventional ripper from Duly, Bain, Imco and Power are locally available. They consist of rigid tines with shear bolt breakaways and most have a cage roller attached. These rippers were originally designed for a complete field ripping operation with tines set 45 cm apart. More recently they are being used for rip-on-row planting system (into residues), set at the desired row spacing and fitted with fertilizer boxes directing the fertilizer down the back of the tine. Planting is done into the pre-fertilized rip line either by machine planter or by hand.
The locally designed "Bateleur" is a slant ripper similar to the "Paraplow". It is being used fairly extensively as a primary tillage tool in place to the disc plough or conventional ripper. It can be fitted with disc coulters for use in heavy residues, which results in no incorporation of the trash. Two other slant tines are the "Contine" and "Mini Contine". The Mini Contine is being developed into a vertical tine mini ripper to be drawn by small tractors through residue.
Deep subsoiling is often performed with an ordinary ripper tine but a deep subsoiler, the "100 hp Tine", is available. Subsoiling is required to remove pans and compaction in the profile.
Those farmers who broadcast their small grains into the residues of the previous crop, use a roller or light disc harrow to shake the seed through the residues and then irrigate to germinate the crop.
A small number of imported planters and drills such as the John Deere Maxemerge with disc openers, a Bettinson Drill and a Moore Drill, have been brought into Zimbabwe.
Locally designed and manufactured commercially available direct planting machinery is limited, however, to the "Trash Freeway" unit. It consists of double discs to open a path through the residue with minimum soil disturbance, followed by mini slant tine rippers to form planting furrows. Fertilizer is directed down tubes at back of the tines. The unit is provided with two types of disc openers: double concave discs suitable for a wide ranges of conditions and single wavy discs for sparse trash in soft or friable soil.
A locally manufactured "Matador" seed drill also been used successfully for planting soybeans into cereal or maize residue. However, the equipment does not adequately insert all the seed into moist soil and germination rates depend upon the post planting rainfall pattern or on irrigation.
A few machinery developments of note have taken place appropriate to the communal sector.
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A prototype automatic ridger and tie-maker developed and tested at the IAE could not meet the exacting requirements for ridge and tie size and has not been approved. Further development work is needed.
An ox-drawn disc ridger is being developed as part of the AGRITEX/GTZ Contill project. It consists of two adjustable discs angled to form a wide "V" shape. The discs can be adjusted to give the required size and shape of ridge and can be set parallel to form running wheels. The unit looks rather heavy but draft is much less than for the normal mouldboard-type ridgers. The finished unit is likely to be too expensive for individual communal families and may better suit co-operatives.
The ties are made by scraping the metal piece along the furrows between the ridges until sufficient soil has been collected to make a dam about one half to two-thirds the height of the ridges. Ideally the ties should be spaced every 1 to 1.5 m along the furrows.
A tractor-mounted automatic tie-maker consists of a line of discs mounted behind the lines of disc ridgers with eccentric cams to lift the discs every 2 m in order to deposit the scraped up soil.
An interesting innovation is to mount an old plough share or disc behind a light cultivator and, since the cultivator can be used to make ridges, this modification enables the ridges and ties to be made in one operation.
Servicing of equipment and minor repairs on both large and small scale farms are normally carried out by the owner and his staff. Most commercial farms have well equipped workshops and semi-skilled mechanics on the payroll.
In communal areas, some minor repairs can be done at growth points where parts can be taken for modification or repair.
On large farms, major repairs are referred to the nearest local dealer, usually a private garage, the mechanic being able to visit the farm on request.
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Small farmers are often remote from dealers and/or cannot afford to pay the costs levied by the visiting mechanics. Normally, therefore, farmers attempt to do major repairs themselves and usually abandon the tractor (or other sophisticated equipment) if they fail. Few small farmers own sophisticated equipment (basic equipment being ox-drawn tools such as the single furrow mouldboard plough and maybe a cultivator). Most tractors in this sector are 15-20 years old and parts are not readily available.
A GTZ/IAE programme is in operation to strengthen the skills at rural growth points. The main objective is to train rural blacksmiths to undertake minor repairs and to construct a variety of simple agricultural and other hand tools (knives, axes, hoes etc) from scrap metal.
Mechanics are trained through national apprenticeship schemes. The minimum qualifications required are five "O" levels including mathematics and English. The prospective apprentice is required to apply to the Ministry of Labour, Manpower and Social Welfare to be registered. He/she is then authorized to search for a suitable employer and to work under a suitably qualified artisan for four years. The apprentice is required to attend part time courses at an approved technical college and to pass the required examinations of the college. Officials of the Ministry carry out yearly on-the-job inspections and the supervising journeyman is required to submit regular annual reports detailing the apprentices progress. Those qualifying attain the level of D1 artisan direct. Prospective artisans who do not have the required "O" levels can attend courses at Vocational Training Centres and progress from D3 to D1 artisans grades through a system of trade testing operated by Ministry staff.
Comprehensive training in the maintenance and operation of agricultural equipment is offered at the Institute of Agricultural Engineering Training Centre (AETC) Borrowdale. The Institute is a Branch of the Department of Agricultural Technical & Extension Services (AGRITEX) under the Ministry of Lands, Agricultural and Rural Resettlement. The AETC has two sections for this purpose: Animal Power and Tractor Power.
The Animal Power section trains communal farmers, Agricultural Extension Workers (AEWs) and officer grade staff in the maintenance and operation of tractors and tractor drawn equipment.
The Tractor Power section trains farmers (mainly from the large scale sector) and cooperative drivers (small scale) in the maintenance and operation of tractors and tractor drawn equipment. Major dealers of agricultural equipment also offer training to farmers who purchase expensive items of specialised equipment from them.
Some Commodity Associations also provide training mainly on tractors and associated equipment. Such training is provided by the Tobacco Training Institute, Harare, and the Cotton Training Institute, Kadoma.
In addition to government-provided training, ox-training is carried out by about six small organizations throughout Zimbabwe (NGOs).
Although a comparatively large amount of research and development work has gone into various conservation tillage systems in Zimbabwe, farmers in both large and small scale sectors have been slow to adopt them. This reluctance can be attributed primarily to conservatism, rather than to technical or socio-economic factors, though the latter obviously play a part.
Nevertheless, adverse climatic changes, which seem to now be the rule in Africa rather than the exception, combined with spiralling input costs, are likely to be factors stimulating change in the years ahead.
It must be recognized that none of the currently available conservation tillage techniques are truly sustainable in terms of preserving soil, rainwater, nutrients, soil structure and the ecosystem.
Zimbabwe's farmers, teaching institutes, research and training organizations are still orientated towards chemical farming at levels likely to be harmful to the environment and to human health. A dramatic change in attitude is necessary before progress can be made towards the development and practice of truly sustainable technologies.
Nevertheless, some hesitant steps have been taken locally to reduce the environmental damage resulting from annual ploughing combined with monocropping and over-reliance on chemicals.
Tine planting into residues has the potential to reduce losses of soil, rainwater and nutrients to levels close to sustainable ones, and significant improvements in soil structure have been recorded under this treatment; but the technology still depends on large inputs of chemicals and has been tested for only a limited number of crop rotations, As it stands, this system is generally unsuited to the small scale sector due to the shortage of residues for this purpose and because of increased pests and diseases (from the residues) requiring chemical inputs which the resource-poor cannot afford.
The locally developed system called no-till tied-ridging is particular suited to the communal areas as hardy residues are recommended to be fed to the cattle and would be a hinderance in land preparation if left on the surface. Losses of soil, rain and nutrients are reduced to very low levels under this system but no significant improvement in soil structure has been recorded. Limited rotational practices have been tested and the technology remains dependent upon a high level of chemical inputs.
The experimental no-till strip-cropping system is the closest approximation to a sustainable low-external-input system yet devised locally. Negligible soil, rainwater and nutrient losses have been recorded with soil structure being maintained at levels similar to virgin ground. No chemicals have been employed so far on trial plots and yet yields equal to or better than conventional plough-chemical farming have been achieved. Further research is required to enable the system to be established on degraded soils, to assess yield levels under different combinations of cropping technologies and fertilizer/compost applications; and to select combinations which optimise returns while minimizing ecosystem damage.
At the last meeting of the Soil and Water Resources Committee of the NRB, the Chairman asked if members of the Contill Project could recommend to the Board circumstances where each conservation tillage is most likely to apply. Below are the recommendations from the team.
The recommendations should be considered as preliminary as the research is not yet complete.
Present extension emphasis is to promote method in small-scale farming areas.
Present extension emphasis is to promote method on commercial farms.
