The aim of this paper is to provide an overview of the available technologies for conservation tillage without going deeply into the merits and limitations of each. We trust that the advantages and disadvantages of each system will be discussed in the group work to follow.

We are very much aware that conservation tillage systems are not just different methods of tillage or no-tillage, but rather complete systems of crop production. Therefore at least the following aspects have to be taken into consideration:

• Crop rotations and practices
• Tillage (soil and profile conditioning)
• Planting
• Weed management
• Residue management
• Soil fertility building and cover crops

There are different definitions of what can be truly regarded as conservation tillage system. In this paper we look at four different options of mulch tillage, of which one probably does not qualify as true conservation tillage, plus two systems of planting on ridges or into furrows which will be briefly described and probably do not qualify either as there is not sufficient soil cover throughout the year. But rather than spending much time on definitions we would like to go straight into our presentation. Some (but not all) of the examples we want to show are from our project area in northern Tanzania. Not all of the presented methods are put in practice and tested by our project, therefore we partly have to rely on information provided by others.

2 Present Situation

What we describe here is the typical situation in northern Tanzania, which with some variations we believe to be true for most of the neighbouring countries as well.

In the small-scale farming sector we note a domination of food crops, principally maize and beans. There is little know-how about other crops and about crop rotations.

Tractor mechanisation is based almost entirely on a single tillage implement, the disc plough.

For cultivation, up to three passes by disc ploughs are considered necessary to keep the weeds down. In some areas, ox ploughs (simple mouldboard ploughs) are used for some of the operations, but the basic idea remains the same. After harvest, grazing of cattle leaves the soil barren and compacted until the next cultivation cycle begins.

Over the years, soils have become extremely poor. The soil structure is weak and the organic matter content very low. With fine dust on the surface and compaction below, the first heavy rains carry a lot of topsoil away, but hardly infiltrate the soil below the first few centimetres. Therefore little water becomes available to the crops. All these factors contribute to the generally low yields and the high susceptibility of crops to dry spells.

Most of the farmers in the small-scale sector are aware of decreasing yields, but there is little know-how or initiative to look for ways to change the situation. Quite the contrary is true for the large commercial holdings: most of them have started subsoiling their land and replace the disc plough by chisel ploughs. Some of the farms have stopped ploughing altogether and turned to direct seeding, but the size and sophistication of the machinery they use is completely out of reach for smaller farmers.

3 Available Technologies for Conservation Tillage Systems

Looking now into available options for conservation tillage, it is important to keep the technological and financial restrictions faced by smaller farmers in mind, but without excluding systems geared more towards large-scale commercial farming.

A) Mulch Tillage

Mulch Tillage systems were developed for situations where crop residues could be utilised for soil protection and improvement.

Principles

The system can be classed as a conservation tillage method only if:

• The residue cover to the soil does not fall below 30% at any time of the year;
• It is ensured that all crop roots remain intact;
• It is ensured that no soil limitations exist structurally or fertility-wise to crop establishment prior to adoption of the mulch tillage treatment;
• Soil disturbance is minimised and soil humus optimised;
• All operations carried out on true contour.

Application

• Appropriate to areas with adequate rainfall (>800 mm per year, better >1000 mm per year);
• Avoid poorly drained lands/soils including soils with poor internal drainage e.g. tight clays or soils with inherently poor structure;
• Do not use in situations where crop residues are burnt, grazed or otherwise removed from the land;
• Useful on land slopes from flat to 10%.

Option 1: Reduced tillage by chisel plough

Replacement of the disc plough by the chisel plough (ripper) was the starting point at our project. It involves the ripping of the entire area of the plot in parallel lines at a depth of up to 25 cm. Depending on the implement used, most of the stubble, crop residues and weeds gets mixed into the soil in the process, but a part remains on the surface. Usually at least two passes (plus one pass by harrow) are required at an interval of several weeks to achieve a satisfactory seedbed preparation. Seeding can be done manually, by animal traction (usually mouldboard plough) or with a seed drill or planter.

This system does not truly qualify as conservation tillage, as there is not enough ground cover during the critical period of the first heavy rains. The chisel plough works well in wheat stubble, but not if there are a lot of crop residues such as maize stover, as clogging of the tines occurs. Weed control is not satisfactory, as not all weeds are uprooted and furthernore the tines transport fresh weed seeds to the surface, resulting in further weed growth.

All this leads to the question: Why chisel the entire area? The intensive mixing action clearly is in contradiction to the guideline of minimum soil disturbance. Chiseling might be justified in a situation where the soil structure is very poor, but the aim should be to make tillage unnecessary. Therefore in our project we have arrived at a point where we want to recommend chiseling only during a transitory stage from conventional tillage to one of the further options described below.

Option 2: Ripping between rows

This system is suitable for row crops only. The chisel is only applied where the new crops will be planted, leaving the soil between rows undisturbed. If the previous crop was also planted in rows at a similar row-to-row distance, ripping is done between the old rows to leave the remaining roots intact. The distance between tines is sufficient to greatly reduce the problem of obstruction by crop residues.

For mechanical weed control before cultivation the use of sweeps or winged tines to cut the weeds just below the surface is recommended. Winged tines of a working width of not less than 50 cm are preferable, as they are less likely to be clogged by the mulch on the surface. This technique is suitable only when no stones are present. The alternative is, of course, chemical weed control. The ripping of the soil at the new row-to-row distance can be done by tractor-mounted chisel plough or by animal traction (Magoye Ripper or similar implement).

The seeding follows by hand into the furrows directly behind the ripper tines, with the seed covered and compacted by foot, which is common practice in Tanzania. Alternatively a seed drill or planter could be mounted directly on the chisel plough, with seed tubes to drop the seeds behind the tines. In principle, such a combination could be built at local workshops. Various models have been designed and tested with more or less satisfactory results for example in the USA. In Tanzania, we encountered a simpler solution which, however, we do not generally recommend: A board was placed on the chisel plough, several persons seated on top of it whose task it was to drop the seed into the furrows behind the tines.

Even if Option 2 meets the criteria for true conservation tillage, the question remains: Why till at all? With the soil structure and organic matter content sufficiently improved, the ripping should become unnecessary and bring us one step further to a no-till solution as described in Option 3.

Option 3: Opening of seed furrows

This system is the first real no-till option in our sequence. Again, it is mainly meant for row crops. In most points it is identical to Option 2, except that ripping is reduced to the opening of a seed furrow. The tines must be spaced sufficiently wide apart to avoid clogging by mulch. Alternatively, mulch removers can be used.

The opening of the seed furrows can also be done with animal-drawn implements such as the Magoye ripper. The shallow working depth does not require much draught power, therefore donkeys can be used as draft animals.

In both cases this method invites combination with a seed drill or planter. Again, home-made solutions are possible, but of course sturdy seed drills or planters that serve the purpose are also commercially available. For animal-drawn implements, there are simple seeders that can be mounted on top or behind the ripper.

For large-scale farms there is an interesting variation of this approach: the air seeder. Most of the available models are not suitable for seeding into mulch, because the spacing between tines is not sufficient. However, some truly huge air seeders supposedly operate well in mulch if is not too coarse.

Option 4: Direct seeding

Direct seeding is the classical no-till solution for mechanised agriculture. It requires heavy and relatively expensive machinery, usually equipped with weighted discs that cut through the trash to open narrow seed furrows. The seed is placed behind the discs directly into the furrows, usually together with fertiliser in a way that it does not come into direct contact with the seed. The soil is then recompacted by rubber wheels that follow the seeding elements and also control the working depth.

Weed control before seeding (or together with seeding) is usually done with herbicides, even if in principle the method of cutting weeds with winged tines just below the surface also could also be applied.

There has been much discussion about the advantages/disadvantages of direct seeding. The results in Arusha Region are a bit mixed, but then it has been introduced only recently and farmers are still gaining experience. One principal problems is, of course, that this technology can hardly be made accessible to smaller farmers or village-based hire operators, as the technology is both too sophisticated and too expensive. The situation might change if commercial hire operators start offering services with direct seeding machines, but this will only occur when sufficient demand develops to justify the investment.

B) Cultivation on Ridges or into Furrows

In situations where mulch tillage is not advisable (poorly drained soils) or not feasible (lack of crop residues due to marginal rainfall, grazing or other reasons), conservation tillage systems have been developed to meet these special requirements.

Our project has little direct experience with either of the two systems presented below, but intends to test them together with interested farmers.

No-till tied ridging

This system was devised for medium-to-high rainfall areas where crop residues were not available for mulch tillage to be practised successfully. It consists of a blend of conventional tied ridging and no-till principles.

Principles

• Large crop ridges at sufficient gradient for their furrows to drain into protected grass waterways (maximum of 1%);
• Ridges are permanent and merely maintained;
• Smaller ties every 1-2 metres to keep water spread evenly;
• Crop planted on ridge tops, sides, or in the furrows;
• The crop ridge should be moist throughout at planting i.e. no dry patches inside the ridges;
• Coarse residues are not required and interfere with ridging because of the lack of residues, crop rotations and organic fertilisers are essential for soil conditioning;
• Residues should be returned as manure/compost.

Application

• Can be adopted on any soils;
• Particularly useful on crusting or poorly drained soils;
• Use where there are insufficient crop residues left to practise Mulch Tillage;
• Appropriate for land slopes >2%; upper limits have not been fixed.

Tied Furrows

This method was developed for arid areas where water harvesting was an essential pre-requisite for cropping, and where planting in the furrow was considered necessary because high temperatures in the ridge tops resulted in poor germination.

Principles

• The land is planed into a series of shallow Vee shapes 1.5 metres wide laid out across the slope;
• The Vee slopes are kept bare to maximise runoff into the furrows;
• Residues are not wanted;
• The crop is planted into the furrows;
• The furrows are tied at intervals to keep the rainwater spread out;
• The soil is not tilled and the Vees are merely maintained.

Application

• Has out-yielded other treatments in dry areas;
• Do not use on shallow soils;
• Do not use in medium to high rainfall areas;
• The crop residues can be fed to the cattle.

4 Transition from Conventional Tillage to Conservation Tillage

In a situation where soil has been ruined by many years of conventional tillage and furthermore might be severely eroded (as it is the rule rather than the exception in Arusha Region), it should not be attempted to introduce a no-till system directly, that means without passing through some transitory stage first. The aims to be achieved are:

• To remove plough pans and to alleviate soil compaction in general, and
• To build up organic matter in the soil.

Subsoiling usually is the first step and, in situations where severe hardpans exists, brings spectacular results which also are very useful to convince farmers that positive changes are possible. Depending on soil properties, subsoiling might have to be repeated after some years even under a no-till system.

Deep chiseling of the entire plot might be required during the first and maybe second year before changing to a true minimum tillage or no-till system

Planting all crops at very high densities, if possible in the form of strip cropping or intercultivation of various crops together, helps to build up organic matter in the soil and to control weeds.

Good weed management is critical for a successful transition. Weed growth must be controlled also between cropping seasons to prevent multiplication of weed seeds. This can be achieved either by using cover crops or by mechanical weed control (cutting weeds below surface or slashing).

Comments to Table (next page)

• The table provides an overview of available technologies presented as a matrix
• The rows represent types of operations (such as subsoiling, weeding), sometimes subdivided into several rows where several options exist (for example mechanical or chemical weed control). In the first column these operations are listed. In the second column the implement(s) required for each operation can be found.
• Column 2-5 jcontain the four options for Mulch systems to be presented.
• 
  Some operations, such as subsoiling, may by used with several or all of the options and accordingly spread over several columns.