The term soil tillage denotes various techniques of working soils mechanically in order to create or maintain favourable conditions for crop cultivation, good soil structure being the most important factor. Hand tools and animal- or motor-powered machinery are used to loosen soils, to crumble soil clods, to break superficial crusts, to give a specific form to the soil, such as an even surface, ridges or furrows for planting and/or irrigation, to mix and incorporate fertilisers, to control weeds and to mulch or incorporate stubble and crop residues after harvest.
Goals and methods in soil tillage have evolved considerably during the last decades. Besides conventional intensive tillage involving the plough, systems of reduced or conservation tillage have emerged, aiming to maintain humus and avoid soil compaction and erosion.
As tillage systems have strong influence on soil organic matter and the water balance in crop/soil systems, they are relevant to agriculture water management as well as in relation to carbon sequestration. Tillage represents a large share in the labour budget of smallholder farms, where soils are still mostly worked with hand tools, thus the labour productivity of tillage is an important factor of agricultural intensification aiming at improving food security.
Types and purposes of soil tillage operations
Soil tillage consists of various regular operations during the cropping season and some operations which are carried out on specific occasions only, like preparing new fields after land clearing or drainage and sub-soiling for long-term improvement of the land. There is an enormous variety of implements used for tillage. In smallholder farming and small-scale gardening these are mainly different types of hand tools like hoes, spades, rakes, etc. Animal and motor-powered tillage equipment adapts to the scale of operations and allows an immense increase of labour productivity (Mechanisation).
Occasional operations:
- Levelling land for irrigated or rain-fed crops, including drainage or irrigation methods like ditches
- Breaking soil crusts at the surface
- Deepening rooting space and improving drainage by loosening sub-soil and breaking plough pans
Regular operations in conventional intensive soil tillage:
In most cases, the ideal strived for is a clean, finely-tilled, well-structured soil over the whole arable layer with no weeds and no crop residues on the soil surface. Four main groups of operations can be distinguished:
- Stubble or post-harvest tillage: Shallow incorporation of crop residues with hoes, rotary tillers, etc. shortly after harvest to clear weeds and incorporate crop residues leaving a rough soil surface to allow for improved water infiltration and erosion control. Preparation of main tillage.
- Primary or main tillage: Creating a porous soil structure in the rooting zone of crops mechanically, through loosening, mixing, crumbling the soil for achievement of optimal physical, chemical and biological soil characteristics. This operation is carried out between two cropping cycles, in mechanised agriculture mostly involving mouldboard ploughs to invert soil layers for improved nutrient availability and weed control.
- Seedbed preparation: Create finely crumbled seed beds for easy germination with the help of harrows, rotary equipment, etc.
- Crop management tillage: Pre- and post-emergence weed control, in mechanised agriculture often with harrows, breaking capillarity for reduced evaporation at the same time, forming ridges for improved crop development.
- Combined tillage operations: In mechanised agriculture implements allow different operations to be combined in one go. For example, seedbed preparation may be combined with sowing or planting and covering seeds and/ or other propagation material with soil, and with creating optimal seed/soil contact with the help of packers or rollers.
- Other operations that can be carried out at different moments of the cropping cycle are the incorporation and mixing of mineral and organic fertilisers, and the sowing of green manure or catch crops.
Conservation tillage:
Recurring problems in intensive soil tillage systems with soil erosion and degradation of soil structure lead to the development of conservation tillage, also reduced or minimum tillage, where intensity of conventional tillage is reduced and crop residues are left on top of the soil in order to stabilise soil structure. Mouldboard ploughing is given up and replaced by the use of loosening and mixing implements. The frequency of tillage operations is minimised. Crop residues are left on the soil or are only partially incorporated and maintain as far as possible a permanent soil cover. In no-tillage farming, or conservation agriculture soil tillage is given up entirely in favour of creating permanent mulch covers into which direct sowing is practised. Weeds are mainly controlled by herbicides.
Effects of tillage
Tillage operations may strongly affect physical, chemical and biological processes in the soil. A standard operation like ploughing after harvest loosens the compacted soil, increasing macro pore space filled with air, which again stimulates soil life and leads to an accelerated breakdown of organic matter into minerals and/or its transformation into humus. The mineralisation of soil organic matter through microbiological activity provides plant nutrients, present in the soil water solution or bound by the soils’ CEC, which in many tropical soils is highly depended on the humus content. Sufficient organic matter and microbiological activity contributes to the formation of soil aggregates, improving soil structure. Good soil structure is beneficial in many terms: it influences the water balance as it increases the water-holding capacity of soils (field capacity and water available to the plants), improves infiltration of rain or irrigation water into the soil and also its drainage (percolation). These beneficial effects of soil organic matter and humus persist as long as there is enough organic matter supply to keep the microbial decomposition and transformation processes in balance A repeated tillage intervention without supply of organic matter will create short-lived mechanical tilth. But aeration will disturb the aforementioned equilibrium, lead to loss of humus which ends up with a degradation of soil structure and compaction of soils. With it the danger of capping soil surface increases, and the amount of run-off water and erosion. Furthermore, soils become more vulnerable to compaction when heavy machinery is used, as stable soil aggregates are lacking. On heavy soils plough pans may appear and lead to waterlogging. A weak soil structure results in lower water holding and cation exchange capacity.
Tillage and carbon sequestration
On global scale soils are the third largest pool of carbon (Lal 2010). Soil carbon appears as inorganic (primary carbonates) and organic form, the latter being relevant for carbon sequestration hence mitigation of greenhouse gases. Soil organic carbon originates from plant residues, soil organisms, humus and manure and comprises up to about 5% of the mass of the arable soil layer. It is estimated that soils under cultivation have depleted their carbon stock 50 – 70% due to erosion, C-decomposition and leaching. But adapted agricultural management practices can restore organic carbon in the soil, conservation or no-tillage practices being an important part of it. At the same time, they can contribute to improving productivity of the land and providing further ecosystem services, like water retention, water purification, increased biodiversity, etc.
Labour demand of hand tillage and mechanisation
In smallholder farming systems of the tropics soil tillage with hand tools like field hoes is one of the most time-consuming operations. It limits the potential of intensification of production systems and the ensuing advances towards food security. This, together with increasing labour costs, has been the driver for the development of labour-saving implements and for mechanisation. Concepts of improving labour productivity through mechanised tillage have to adapt to the particular situation of resource poor farmers.
References
GIZ/Krause, R. et al: (1984) Soil tillage in the Tropics and Subtropics. Eschborn
Lal, R (2010) : Soil carbon sequestration. SOLAW Background Thematic Report - TR04B, FAO
Lal, R (993):Tillage effects on soil degradation, soil resilience, soil quality, and sustainability. In: Soil and Tillage Research, Volume 27, Issues 1–4, October 1993, p. 1–8 Landwirtschaftkammer)
FAO (1993): Soil tillage in Africa: needs and challenges., Rome
Further reading and external links
Lal, R. (1995) Tillage Systems in the Tropics: Management Options and sustainability implications. FAO Soils Bulletin 71. Fome
Sims B. S. and Kienzle J. (2006) Farm power and mechanization for small farms in sub-Saharan Africa. Fao, Rome
Landwirtschaftkammer Rheinland-Westfalen: Standortangepasste Bodenbearbeitung. In: http://www.landwirtschaftskammer.de/landwirtschaft/ackerbau/boden/index.htm (accessed 15 Jan 2013)
