Conservation Tillage

Through adapted tillage techniques, moisture loss in the soil can be reduced and soil quality and agricultural production can be improved. Thus, water use for irrigation can be decreased and the local water cycle is optimised. Soil moisture loss (and also soil degradation) can be minimised through different techniques of soil moisture conservation: soil cover and reforestation, mulching, soil amendments and different tillage techniques.

Soil tillage comprises all the physical, mechanical, chemical or biological actions conducted to prepare the seedbed for seed germination, emergence and establishment, root development (the amount of soil water available to plants is governed by the depth of soil that roots can explore (the root zone) and the nature of the soil material) and crop growth (INFONET BIOVISION 2011; BETTER SOILS n.y.). Conservation tillage limits the devastating consequences of drought, conserves soil moisture (thus decreasing water use for irrigation) and some techniques even enhance the local nutrient cycle (e.g. mulch tillage)(UNEP 1998; FRANZEN 1997).

When soil is saturated, all the pores are full of water, but after a day, all gravitational water drains out, leaving the soil at field capacity. Plants then draw water out of the capillary pores, readily at first and then with greater difficulty, until no more can be withdrawn and the only water left is in the micro-pores. The soil is then at wilting point and without water additions, plants will die (BETTER SOILS n.y.).

The amount of soil water available to plants is governed by the depth of soil that roots can explore (the root zone) and the nature of the soil material. Because the total and available moisture storage capacities are linked to porosity, the particle sizes (texture) and the arrangement of particles (structure) are the critical factors. Organic matter, carbonate levels, and stone content also affect moisture storage. Poor structure, low organic matter, low carbonate content and presence of stones all reduce the moisture storage capacity of a given texture class. Clays store large amounts of water, but because they have high wilting points, they need significant rain to be able to supply water to plants. On the other hand, sands have limited water storage capacity, but because most of it is available, plants can make use of light showers regardless of how dry they are before the shower. Plants growing in sand generally have a denser root system to enable them to access water quickly before the sand dries out (BETTER SOILS n.y.) (read also the paragraph “Soil Texture” in the crop selection factsheet).

Water holding capacity (mm/cm depth of soil) of main texture groups. Figures are averages and vary with structure and organic matter differences. BETTER SOILS (n.y.)

(Adapted from UNEP 1998)

Strip Cropping

Strip cropping is the farming of sloping land in alternate, contoured strips of inter-tilled row crops and close growing grasses (or other ground cover crop), aligned at right angles to the direction of natural flow of runoff. The close-growing strips slow down runoff and filter out soil washed from the land in the inter-tilled row. This control of runoff also allows increased opportunity for infiltration of the runoff and, thus, increased moisture in the soil. The strip widths can be varied depending on the soil type and slope.

Contour Farming

Contour farming involves aligning plant rows and tillage lines at right angles to the normal flow of runoff. It creates detention storage within the soil surface horizon and slows down the rate of runoff, thus giving the water the time to infiltrate into the soil. The contour bunds are earth banks 1.5 to 2.0 m wide, forming buffer strips at 10 to 20 m intervals, and are important for the functioning of the technology. The effectiveness of contour farming for water and soil conservation depends on the design of the systems, but also on soil, climate, slope aspect and land use of the individual fields.

Zero or Chemical Tillage

In this approach, the land is not tilled at all. Chemical tillage uses herbicides to control weeds, avoiding the need to till the soil. This tillage technique conserves water in the soil profile since the soil is not tilled and exposed to the drying (evaporative) elements of the atmosphere. The moisture is retained within the soil profile. The new crop is generally planted directly into the stubble of the previous crop.

Mulch Tillage

Mulch tilling involves covering bare soil with mulch or plant litter to prevent or reduce the evaporation of soil moisture and minimise the erosive energies of rain falling directly onto soil particles. Mulching is different from soil amendment.The mulch is usually crop residue such as maize stove, sorghum trash and wheat straw. In cases where these are not available, or are eaten up by animals, gravel can be used as mulch.

Reduced or Minimum Tillage

Reduced tillage is a practise in which the soil is tilled to some extent but not completely inverted. There are several ways of achieving reduced tillage. For example, the plough can be supplemented with discs or a chisel harrow, and the land ploughed in narrow strips, coinciding with the spacing of the row crops, leaving the intervening space untilled. Reduced tillage means a smaller volume of soil is exposed to erosion and moisture loss by evaporation; hence, conserving moisture.

One technique of reduced tillage is called dibble stick planting, where a planting stick or machete can be used to create holes to plant the seed in an un-ploughed field with stubble/crop residue. The cut hardwood stick from the bush is sharpened at one end and used to make planting holes. The holes are made in lines at evenly spaced intervals that make it easier to weed and apply fertiliser or manure.

No-till and Soil Cover

No-tillage is generally defined as planting crops into soil that has remained untilled after the harvest of the previous crop. Conservation tillage includes the WHEN and HOW this tillage is done. The "when" basically refers to the moisture-state of the soil. Conservation tillage takes into account both environmental and tillage factors.This system involves opening a narrow slot only wide and deep enough to obtain proper seed coverage and with at least 30% mulch cover. Permanent, continuous no-tillage should follow, while the soil should remain covered by crop residues or green manure cover crops. Crop residues should remain undisturbed on the soil surface after seeding. Read more about it in INFONET BIOVISION (2011).

The greatest moisture loss is usually experienced during the first tillage trip on the field. However, additional trips result in additional drying in the tillage zone. Moisture loss from tillage is especially high during dry springs, when the seedbed can dry. Spring tillage should be shallow only to limit moisture loss. Conserving spring moisture to allow seed germination is especially difficult with small seeded crops like canola and in dry, windy conditions. Use of a packer or harrow to fill in and seal the ground during the tillage operation may be important.

Fall tillage influences the amount of snow catch, which is important in many years to help recharge soil moisture after a previously dry summer and fall. Snow catch is influenced by surface roughness, but mostly by stubble height. Conservation tillage, and especially no-till, increases the amount of stubble left on the surface.

There are limited operation and maintenance needs associated with these technologies. Maintenance involves keeping the structures in a functional state, e.g., maintaining the contour bunds to prevent erosive runoff of stormwaters, or maintaining the grass strips to retain their functionality. When mechanised ploughing is carried out, equipment maintenance is important in order to ensure the correct depth of penetration of the ploughs, seed injecting devices and other cultivating equipment. Generally, conservation farming requires equipment modifications relative to conventional farming techniques, and may require some initial capital investment to replace unsuitable machinery, although conservation tillage may be done using more traditional, manual farming methods (adapted from UNEP 1998).

(Adapted from FRANZEN 1997)

• Consider adoption of no-till farming. In many cases, adoption of no-till does not require purchase of expensive equipment, but some modification of equipment is likely.
• Use tillage implements that retain crop residues, leave a rough soil surface and limit soil drying, such as undercutters, harrows and sweep style cultivators.
• Avoid tillage with implements that bury more than 50% of crop residues, such as discs with greater than 60 cm diameters, and mouldboard plows.
• Limit tillage trips.
• Perform tillage at right angles to the prevailing winds (usually north and south with a west prevailing wind).
• Follow the contour of the land in hilly landscapes to intercept water in the event of heavy rainfall.
• Control weeds through herbicides rather than tillage.

Soil Compaction

Heavy vehicles can lead to soil compaction and reduce water infiltration by reducing the soil pore volume. Consequently oxygen and water are squeezed out and can hardly infiltrate again (WIKIPEDIA 2011). The five key principles to prevent it are (FAO 2003):

• Reduced or zero tillage
• Controlled traffic
• Permanent soil cover
• Rotations and cover crops
• Costs Considerations

For minimum tillage, labour and operation cost are about $40/ha. For zero tillage, the costs are a function of the cost of chemicals and work out at about $120/ha (Zimbabwe). In general terms, the methods are cheaper than conventional tillage due to the reduced demand for ploughing, but are subject to some opportunity costs at the time of conversion from conventional tillage (UNEP 1998).