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Machinery, tools and equipment

7. Direct seeding

Planting stick or hand hoe


Direct seeding is practised in a lot of places in the tropical world, although the terminology is not used as such.
Plate 37.
A farmer in the steep lands of southern Honduras using a planting stick to sow his maize. (A.J. Bot).

Plate 38.
"Frijol tapado" or broadcast beans, sown over the residues of the former vegetation in Costa Rica. (A.J. Bot).

Seeding or planting in large parts of Africa is done by using a hand hoe. The hand hoe used for planting purposes usually differs from the one used for tillage and weeding in that the blade is thin and narrow. Basically we are talking about direct seeding/planting if planting is done without any prior land preparation. In Latin America a planting stick (huizute in El Salvador; coa in Mexico, espeque in Honduras, etc.) is commonly used to plant maize and sorghum and beans may be broadcast over the covered surface, without any land preparation. Making sure that the seed falls through the mulch cover, it germinates on the moist soil under the mulch without problems.
Manual direct seeder or hand jab planter


In order to speed up the process of planting, hand jab planters (or matracas in Portuguese) have been developed. It is a hand-held tool that allows the farmer to plant from a standing position and faster than with other hand tools (average 2 days per hectare). The tool comprises two long levers joined with a hinge to form a V with a pointed tip. The metal pointed tip is pushed into the soil. By closing the levers the tip is opened to release the seed and fertiliser into the soil. At the same time new seed and fertilizer is charged into the metering mechanism. The planter is pushed into the soil at every step, allowing a regular spacing. Disadvantages include:
  • The wooden levers are sometimes too weak and easily damaged by powerful farmers.
  • The planting beak becomes clogged with soil, especially when the implement is used by novices in very moist clayey soils.
There are two types of jab planters: with broad tip and with narrow pointed tip. The former type is used on prepared land, for example when the planting line is ripped. The second one with the narrow pointed tips is designed for manual no-till planting and thus more recommended for conservation agriculture.
Plate 39.
A group of farmers learning how to use jab planters to plant through crop residue. (J . Ashburner).

One of the modifications to the planter is a second hopper, opposite the seed hopper, for fertilizer. This allows the farmer to fertilise and plant at the same time. If the jab planter is provided with a second hopper to apply fertilizer and seeds in one operation, it has two separate delivery tubes and points to make sure the seed and fertilizer are not deposited too close to each other.
Plate 40.
The distance between seeds and fertilizer deposited in the soil by a hand jab planter is about 2 cm wide and 1-2 cm deep. (A.J. Bot).


Animal traction and single-axle-tractor drawn planters


Direct seeding implements for animal traction and single-axle-tractors have been designed to manage residues on the soil surface and at the same time to place the seeds and possibly fertilizer in the soil. Therefore direct seeders have following working elements:
  • A disc to cut through the surface mulch and open a slot in the soil.
  • A furrow opener to place the fertilizer - often a chisel point.
  • A furrow opener to place the seed - either a chisel or a double disc.
  • Wheels to control the planting depth and eventually press the seed row.
  • Press wheels to close the slot in order to secure a good contact between soil and seed.
Figure 2. Basic design of a direct planter - IAPAR prototype Gralha Azul (adapted from Ribeiro et al. 1999)


The efficiency of the disc that cuts the (cover) crop residues depends on various factors:
  • Soil conditions: texture, resistance to penetration, humidity and porosity.
  • Straw and residue conditions: resistance to cutting, humidity, quantity and management.
  • Seeder: weight and dynamics.
  • Disc: size, shape and profile.
Plate 41.
Cutting disc. (T. Friedrich).

For good results it is recommended:
  • To work during the warmest hours of the day (after 10 in the morning).
  • Work when the straw is either green or completely dried, never when it is only wilted.
  • Operate when soil moisture is at the point of soils being friable.
  • When using animal traction, never try to seed when more than 5 tons of dry matter per hectare is left on the surface.
Inefficient cutting leads to an accumulation of residues between the different parts of the seeder and results in seed and fertilizer placement problems, i.e. irregular spacing or complete absence of seeds (Ribeira et al., 1999).
Plate 42.
Accumulation of residues occurs when they are too humid or the implement is adjusted incorrectly. (S. Vaneph).

The soil needs to be firm enough to facilitate cutting through the residues, otherwise these will be pressed into the soil, resulting in hairpinning and bad soil contact for the seed. Blockage of the equipment could also result from incomplete residue cutting on soft soils (Casão and Yamaoka, 1990). The cutting discs have either smooth edges that facilitate penetration into the soil or wavy or fluted edges if more soil movement is desired, for instance when Fusarium infestation is expected and a drier soil is needed, or to improve disc rotation and avoid blockage.

Furrow openers for seed and fertilizer on animal traction or micro-tractor planters are usually a chisel tine or hoe, double discs or by rolling punch type injection. Usually the furrow opener is placed just before or on the tip of the tubes that drop the fertilizer and the seeds.

The performance of the furrow opener depends on its geometric characteristics; the forward speed; the texture and density of the soil; the quantity of residues; and the vertical loading placed on it by the seeder. It can take the form of:
  • A chisel tine: commonly used in soils that have a higher resistance to penetration, but results in more problems with clogging up the implement with residues, and can not be used in areas with stones, trunks or a lot of roots. Chisels are preferred for animal drawn implements as they require less weight due to the better penetration characteristics.
  • Double discs, either the same diameter or not and offset at an angle forming a "V" to each other. The additional effect is that residues that may not have been very well cut by the main cutting disc, are cut by these discs resulting in less obstruction of the implement. The implement has less capacity to penetrate the soil, especially in clay soils (Ribeira et al., 1999). Discs of different diameter and offset axles have better self cleaning and penetration characteristics than those of similar diameter. If double disc openers are used at all, then offset double discs of different diameter are therefore the preferred option for animal traction planters.
The following are the most common furrow-types created by animal traction direct planters (Baker et al., 1996):
  1. V-shaped slots.
  2. U-shaped slots.
In conservation agriculture, V-shaped slots are almost always created by twin discs which touch at the leading edge and are angled outwards at the rear. The angle of the V is usually about 10 degrees. Each of the angled discs pushes roughly an equal amount of soil sideways when both discs are at the same angle to the vertical. The biggest advantage of vertical double discs is their ability to handle surface residues without blockage. The construction is relatively simple and maintenance free.
Plate 43.
Planter with double disc coulters and cast iron press wheels. (T. Friedrich).

When the front edges of the two discs leave a gap open at ground level, this can cause problems with the entry of residues and soil. This can be avoided through:
  • Placing a third disc ahead of, or in between the two angled discs, which cuts the residues; or;
  • Positioning one of the two discs forward of the other as to present a single cutting edge; or;
  • Replacing one of the two discs by a smaller one; the larger disc becomes the leading edge to cut the residues.
Disadvantages of V-shaped slots:
  • Needs high penetration forces.
  • Intolerance to sub-optimal soil conditions.
  • Tendency to 'tuck' residues into the slot (hairpinning).
  • Tend to concentrate seed and fertilizer at the base of the slot if applied together.
Slots that can be distinguished from V-shaped ones by a broader base are called U-shaped slots. U-shaped slots are in animal traction and micro-tractor planters usually formed by the following furrow opener types:
  • Hoe or chisel openers
  • Power tiller openers
All of these designs produce some loose soil on the surface near the slot that can be used to cover the slot again. Hoe and chisel type openers burst the soil upwards; power tiller openers chop the soil with a set of rotating blades; and furrow openers scoop the soil out from the slot zone.
Plate 44.
Working elements of a power tiller furrow opener. (T. Friedrich).

Hoe type openers refer to any shaped tine or chisel, which is designed to vertically penetrate the soil. Seed is delivered either down the inside of the hollow tine itself or down a tube attached to it, which is usually open at the back. The biggest disadvantage of hoe openers is the fact that they do not handle even modest levels of residues without blockage, unless a leading disc is placed ahead of the hoe opener to cut the residues.
Plate 45.
Planter with hoe openers for fertilizer and seed following the cutting disc. (T. Friedrich).

Advantages of hoe openers:
  • Low cost.
  • They penetrate the soil better than discs, requiring less weight of the implement. This makes them the best option for animal traction.
  • They do not tuck residues into the slot, but 'brush' them sideways.
  • They do not create smear surfaces at the sides of moist planting slots and so create a better seedbed.
Disadvantages:
  • Problems with stones and other obstacles.
  • Requires a good cutting disc for long residues.
  • Considerable soil movement depending on shape and width.
The rolling punch injection is another form of furrow opener. It handles residues quite well, but tends to clog when used in sticky soils.
Plate 46.
Two row rolling punch injection planter. (T. Friedrich).

Plate 47.
Single row rolling punch injection planter. (T. Friedrich).

For annual crops, it is recommended that fertilizer should be placed about 5 cm to the side and beneath the seeds. In a direct seeder this would mean that the opening device for fertilizer is placed outside the line of work for the seeding slot. However, in animal drawn direct seeders the fertilizer is placed under the seed, but in the same line.
Plate 48.
Detail of the inside of the seed hopper of a direct planter, showing the seed plate and expeller to distribute the seeds. (V.H. de Freitas.)

Seed plates inside the hoppers control the plant density in the field. They are rotated by a transmission from one of the wheels either with a chain or gear. The distance between the soil and the delivery tube inlet defines the precision of planting: if the distance is bigger, the chance is greater of the seeds deviating from the desired plant spacing.
Plate 49.
Both the seed and fertilizer metering mechanisms are activated by the movement of the ground wheels. In this case there is a chain and sprocket transmission. (A.J. Bot).

Seed plates in animal traction planters can be custom made by the planter manufacturer. To reduce seed damage, the diameter of the plate should be large enough, so that the revolution speed of the plate is not too high. Animal traction planters with small metering discs should only be used with oxen and not with horses, as horses have higher forward speeds. Most modern animal traction planters now use standard discs designed for tractor planters, which can cope with the speed of any draught animal.
Plate 50.
Standard tractor planter seed plate in a simple animal traction planter. (T. Friedrich) .

For many years, scientists have thought that the best cover for seeds is loose soil. This thinking has evolved from situations with tilled seedbeds. However, especially under dry conditions, it can be observed that seeds under mulch cover germinate better than those covered by loose soil. Since under tilled conditions (loose soil) the macropore system in the vicinity of the seeds is completely destroyed the soil moisture equilibrium and the capillarity is disturbed. In undisturbed soil, the soil humidity equilibrium is intact providing optimal exchange of moisture between soil particles and pores. This allows the capillary supply of soil water to the soil surface while reducing the evaporation loss with the mulch cover. In conservation agriculture, soil moisture loss takes place in the slot, and depending on the type of slot more or less moisture is lost (Figure 3). Further details of the relative moisture-conserving benefits of the different types of slot can be found in Baker et al. 1999.

Figure 3 Moisture loss from different slot shapes and the position of the seed in the different slots (after Carter, 1994).

The inverted T-slot traps water vapour within the slot, which germinates the seed. Pressing on the seeds in V- and U-shaped slots before covering the seeds improves their performance, especially in dry soils.

The wheels at the back of the implement serve to press humid soil or crop residues on the slot in order to place the seed into close contact with the soil. Some seeders lack this wheel and in that case it is the operators of the planter who put the seeds into contact with the soil, as they walk on the slot.

Surface residues are an important resource for promoting seedling emergence from dry soils and it is possible to obtain more effective seedling emergence from a dry soil by direct seeding than by tillage, provided the correct technique and equipment are used.

The animal traction prototype seeders were originally made to plant one row at a time, but planters are now manufactured that can seed more rows. Multi-row versions might even provide an operator's seat.

Plate 51.
A two row ride-on seeder for animal traction. (T. Friedrich).


Direct seeding equipment for tractors


Figure 4. Basic design of a direct planter - IAPAR prototype Gralha Azul (adapted from Ribeiro et al. 1999)

No till planters or seeders have either all or some of the following components:
  • Hoppers for seed and, if applicable, for fertilizer with the respective metering mechanisms and delivery tubes.
  • Row cleaner, if necessary, to remove excess mulch from the plant row.
  • Cutting disc to cut through residue cover.
  • Furrow opener for fertilizer.
  • Furrow opener for seeds.
  • Seed press wheel.
  • Furrow closing wheel (often in combination with depth control).
  • Furrow press wheel.
Row cleaner
Under some circumstances, direct seeders have a row cleaner just before or combined with the cutting disc. Row cleaners make the seeding more efficient under the following conditions:
  • Heavy or difficult mulch.
  • Delicate seeds.
  • In cold climates to warm up the soil.
Plate 52.
Row cleaner combined with cutting disc. (T. Friedrich).

Cutting disc
A cutting disc is usually necessary to make a clean cut through residue cover and avoid residue collection around the planter elements or the pressing of residues into the seed row. They are particularly important with heavy residue covers and with chisel type furrow openers. The efficiency of the disc that cuts the (cover) crop residues depends on various factors:

The comments on cutting disc performance detailed in the section on animal traction direct planters and seeders, apply equally to this section and should be referred to.

Furrow opener
A furrow can either be opened by a chisel tine or hoe, single disc at an angle to the furrow, double discs or by punch type injection. Usually the furrow opener is placed just before or on the tip of the tubes that drop the fertilizer and the seeds.

The performance of the furrow opener depends on its geometric characteristics, the speed, the texture and density of the soil, the quantity of residues and the pressure placed on it by the seeder. It can take the form of:
  • A chisel tine or hoe: commonly used in soils that have a higher resistance to penetration, but these will result in more problems with clogging the implement with residues, and can not be used in areas with stones, trunks or a lot of roots. Chisel openers, either exclusively or in combination with discs are usually preferred for lightweight no-till planters used with smaller tractors as they require less weight due to their better penetration characteristics.
  • Single discs at an angle to the furrow.
  • Double discs, either the same diameter or not and placed in an angle forming a "V" to each other. The additional effect is that residues not completely cut by the cutting disc, are cut by these discs resulting in less obstruction of the implement. The implement has less capacity to penetrate the soil, especially in clay soils (Ribeira et al., 1999). Discs of different diameter and offset axles have better self cleaning and penetration characteristics than those of similar diameter.
  • Other types of openers, like inverted T and cross slot.
There are different opinions about the function of furrow openers in direct planters and seeders. Some believe that the furrow opener should, while placing the seed into the soil, at the same time provide some seedbed preparation in the furrow. The planters for this purpose often use hoe-type furrow openers and create considerable soil movement. This leads to higher draught power requirements, soil moisture losses and the germination of weed seeds might be stimulated. Under CA it is therefore preferred to use furrow openers which are designed to create a minimum of soil movement, regardless whether they are chisel or disc type.
Plate 53.
No till planting with hoe type furrow openers creating considerable soil movement. (T. Friedrich).

Generally, there are four basic slot shapes (Baker et al., 1996):
  1. V-shaped slots;
  2. U-shaped slots;
  3. Inverted T-shaped slots;
  4. Cross slot.
Plate 54.
No till planting with double disc furrow openers creating hardly any soil movement (on the right side, field planted with hoe-type planter). (R. Dambros).

V-shaped slots are almost always created by two discs which touch in the front and open at an angle at the back. The angle of the V is usually about 10 degrees. Each of the angled discs pushes roughly an equal amount of soil sideways when both discs are at the same angle to the vertical. The biggest advantage of double discs is their ability to handle surface residues, stones and minor obstacles without blockage. The construction is relatively simple and maintenance free.
Plate 55.
Double disc furrow opener, here with offset discs for improved penetration. (T. Friedrich).

When the front edges of the two discs leave a gap open at ground level, this can cause problems with residues and soil entering. This can be avoided through:
  • Placing a third disc ahead of, or in between the two angled discs, which cuts the residues; or
  • Positioning one of the two discs forward of the other to present a single cutting edge (offset discs); or
  • Replacing one of the two discs by a smaller one; the larger disc becomes the leading edge to cut the residues.
Plate 56.
Double disc furrow opener with offset discs of different diameter. (T. Friedrich).

Disadvantages of V-shaped slots:
  • Need high penetration forces
  • Intolerance to sub-optimal soil conditions
  • Tendency to 'tuck' residues into the slot (hairpinning)
  • Tend to concentrate seed and fertilizer at the base of the slot if applied in the same furrow
U-shaped slots have a broader base than V-shaped slots. They are created by a variety of opener designs:
  • Single angled disc type openers.
  • Hoe openers.
  • Power tiller openers.
All of these designs produce some loose soil on the surface near the slot that can be used to cover the slot again. Angled disc type openers scrape soil away from the centre of the slot; hoe and furrow type openers burst the soil upwards; power tiller openers chop the soil with a set of rotating blades; and furrow openers scoop the soil out from the slot zone.
Plate 57.
Detail of single disc furrow opener. (T. Friedrich).

Box 5 Single disc coulters
Characteristics:
  • Produce U-shaped slot
  • Simple and robust
  • Compact (often used for small grain seed drills)
  • Good residue handling
Disadvantages:
  • High penetration force required
  • Considerable soil movement (depending on the angle)

Hoe type openers refer to any shaped tine or chisel, which is designed to vertically penetrate the soil. Seed is delivered either down the inside of the hollow tine itself or down a tube attached to it, which is usually open at the back. The biggest disadvantage of hoe openers is the fact that they do not handle even modest levels of residues without blockage, unless a leading disc is placed ahead of the hoe opener to cut the residues.
Plate 58.
Wide wing hoe type furrow opener. (T. Friedrich).

Advantages of hoe openers:
  • Low cost.
  • They penetrate the soil better requiring less weight of the implement.
  • They do not tuck residues into the slot, but 'brush' them side ways.
  • they do not create smear surfaces at the sides of moist planting furrows creating a better seedbed.
Disadvantages:
  • Problems with stones and obstacles.
  • Require a good cutting disc for long residues.
  • Considerable soil movement depending on shape and width.
Plate 59.
Narrow chisel type furrow opener for reduced soil movement. (T. Friedrich).

The inverted T-shaped slot was developed as a result of inverting the wide-top, narrow-base of the V-shape. The main objective for the development was the ability of the implement to fold the residue-covered soil back over the slot for moisture conservation. The second objective was to find a design that would be able to handle proper fertilizer placement at the same time as seeding. The implement consists of a vertical shank that curves out at its base to form two wings that are inclined towards their fronts at 5-10°. A disc in front is used to cut the residues. The main advantages include:
  • No compaction of the soil.
  • Self-closure of the slot.
  • Maintenance of a constant seeding depth by closely following the soil surface.
  • Capable of seeding very superficially through reduced inclination of the wings.
Plate 60.
Inverted T furrow opener. (T. Friedrich).

Disadvantages:
  • Considerable wear on sandy soils.
  • No proper furrow opening on loose soils.
  • Difficult residue handling (requires a cutting disc).
The Cross-Slot furrow opener is a further development resulting from the inverted T-opener. It provides close to ideal germination conditions for the seed (see box 6). It consists of a vertical cutting disc, with two small wings attached to it on both sides. The wings open a horizontal cut, in which seed is placed on one side and fertilizer on the other, separated by the vertical cut of the cutting disc.
Plate 61.
Detail of Cross-Slot furrow opener. (T. Friedrich).

Box 6. Cross-slot characteristics
Characteristics:
  • Ideal germination conditions for seed
  • Efficient separation of seed and fertilizer
  • Low draught requirement
  • Good residue handling; no hairpinning problem
  • Considerable wear on sandy soils
  • Expensive

The rolling punch injection is another form of furrow opener. The seed is placed between the fingers of two star-wheels which are at an angle comparable to a double disc, punched into the soil and released. It handles residues quite well, but tends to clog when used in sticky soils.
Plate 62.
Rolling punch injection planter (star-wheel planter). (T. Friedrich).

It is recommended for annual crops that fertilizer should be placed about 5 cm besides and beneath the seeds. In a direct seeder this would mean that the furrow opener for fertilizer is offset to one side of the line of work of the seeder. However, in many planters the fertilizer is placed under the seed, but in the same line.
Plate 63.
Detail of Rolling punch injection planter. (T. Friedrich).


The relative merits of different shapes of slots and their effect on seed germination have been discussed in the animal traction section, and should be referred to. The relevant reference is Baker et al., 1996.

Seeders and planters
Different crops and seeds require different ways of seeding or planting. The two major types are:

  1. Seed drill: the seed is planted in a continuous band into the crop row. This method is usually used for small grain crops like cereals. The seed is metered with feed rollers of different designs; either positioned one for each seed line and gravity fed or centrally with pneumatic distribution of the seed to the lines. The machines for this type of seeding are usually called seed drill or seeder.
  2. Precision planting: single seeds, or a predetermined number of seeds, are placed at an equal predetermined distance within the row. This method is usually used for row crops like maize, beans, cotton, sunflower etc. The number of seeds per planting hole and the distance between each planting location is determined by seed plates which have cells or chambers to meter the seed. The metering can either be mechanical or pneumatic, where the air is either as vacuum or as pressurized air used to select the number of seeds per position.
Plate 64.
A mechanical no-till seed drill. (T. Friedrich).

Plate 65.
Pneumatic no-till seed drill with two central metering and distribution units. (T. Friedrich.)

Plate 66.
Small row crop precision planter. (T. Friedrich).

There are some modern no-till planters which incorporate both options in one machine, the seed drill and the row-crop precision planter. The farmer only has to equip the seed-drill-cum-planter with the desired number of furrow openers and connects them to the respective metering mechanism.
Plate 67.
Seed drill-cum-precision planter: on top the feed rollers of the metering system for drill seed, below the boxes for the precision planting seed plates. (T. Friedrich).

Upgrading of old seeders/planters into no-till seeders/planters

No-till seeders and planters often are much more expensive than conventional ones. Therefore the investment in a no-till seeder or planter might create a serious obstacle for the transition process towards CA. In many cases the old conventional seed-drills or planters can be converted at low cost into no-till seeders or planters, either by the farmers themselves or by mechanical workshops. For the conversion the metering systems remains the same, the planter frame might have to be reinforced and the soil engaging parts, namely the furrow openers, are replaced by new no-till units.
Plate 68.
Modifying a conventional precision planter for no-till use: adding a cutting disc in front and replacing the furrow openers for fertilizer and seed. (T. Friedrich)

Plate 69.
Modifying a conventional seed drill for no-till use: adding new cutting discs and furrow openers and modifying the frame. (T. Friedrich) .

Plate 70.
The same seed drill after the finished modification. The upgrading was done by a farmer using modification kits. (T. Friedrich).


Other information on direct seeders


Already a lot of manufacturers of zero tillage equipment have posted their products together with information on the Internet. FAO Agricultural Service tries to bring as much information together as possible in their on-line database on Conservation Agriculture Technology, which is accessible through:
http://www.fao.org/ag/catd/index.jsp

The database provides information on different models of conservation agriculture equipment for manual use, animal and mechanized traction. Technical, agronomic and commercial information for direct planters and seed drills, rippers, equipment for residue handling and specially developed sprayers can be viewed from this site. Complete addresses are provided, including links directly to webpages of the manufacturers.

References


Almeida, F.S. 1988. A alelopatia e as plantas. IAPAR Circular 53. Londrina.

Araújo, A.G., R. Casão Jr., and P.R. A. Araújo. 1993. Recomendações para dimensionamento e construção do rolo-faca. In: Encontro Latinoamericano sobre Plantio Direto na Pequena Propriedade. Anais. IAPAR. Ponta Grossa. p. 271-280.

Araújo, A.G., R.S. Yamaoka and D.A. Benassi. 1999. Máquinas para pulverição em solos de baixa aptidão agrícola. In: Uso e manejo do solos de baixa aptidão agrícola. O. Muzilli and C. Castro Filho (Eds.) IAPAR Circular Técnica 108. p. 154-167.

Baker, CJ., K.E. Saxton and W.R. Ritchie.1996.No-tillage seeding. Science and practice. CAB International, University Press Cambridge. 258pp.

Bertol, O. and O. Wagner. 1987. A knife roller or chopping roller. In: ILEIA Newsletter. Vol. 3:1. p.10-11.

Carter, M.R. 1994. Conservation tillage in temperate agroecosystems. Lewis. Boca Raton. 390pp.

Casão Jr., R. and R.S.Yamaoka. 1990. Desenvolvimento de semeadora-adubadora direta a tração animal. In: XIX Congresso Brasileiro de Engenharia Agrícola, Piracicaba. Anais. p. 766-777.

Derpsch, R. and A. Calegari. 1992. Plantas para adubação verde de inverno. IAPAR Circular 73. 80 pp.

Derpsch, R. 2003. No-tillage, Sustainable Agriculture in the New Millennium; internet homepage http://www.rolf-derpsch.com/

Friedrich, T. 2000. Conceptos y objetivos de la labranza en una agricultura conservacionista. In: Manual de prácticas integradas de manejo y conservación de suelos. FAO. Soil and Water Bulletin 8. Rome. p. 29-37.

Godwin, R.J. 1990. Agricultural engineering in development: tillage for crop production in areas of low rainfall. FAO. Agricultural Services Bulletin 83. Rome. 124 pp.

Krause R., F. Lorenz and W.B. Hoogmoed. 1984. Soil tillage in the tropics and subtropics. GTZ. Eschborn. 320pp.

Moeller, O. 1997. Farmers' Tools. Farmesa, FAO. Zimbabwe. 115 pp.

Monegat, C. 1991. Plantas de cobertura do solo. Características e manejo em pequenas propriedades. Chapecó. 337pp.

Ribeira, M.F.S., A.G. Araújo, R. Casão Jr. and D.A. Benassi.1999. Máquinas para semeadura direta em solos de baixa aptidão agrícola. In: Uso e manejo do solos de baixa aptidão agrícola. Muzilli and Castro Filho (Eds.) IAPAR Circular Técnica 108. p. 139-152.

Vieira, M.J. 1996. Uso del arado de cincel para la producción agrícola y la conservación de suelos y agua. MAG-FAO, San José, Costa Rica. 41 pp.

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