Sub-soiling is a cultivation method of soil horizons under the surface, which does not produce prism inversion. The aim is to break the compact layers of soil which restrict the movement of water, air and the penetration of roots. (Soil Conservation Society of America 1982).

The principal agricultural implement used is the subsoiler, "a heavy, very resistant metal tool which is placed into the soil and pulled by bulldozer, causing the rupture of the compact layers". Subsoiling loosens the soil, increasing both aeration and drainage, improving the distribution of moisture, weeding and ridding the soil of stones in order to favor root growth.

When the soil is compacted, the growth of roots is restricted and water retention is limited which increases run-off and erosion. Rice (1983), stated that crops with restricted superficial root development in the top horizons, never reached their production potential.

In general, subsoiling allows for an increase in the production of various crops such as soya and maize, in lands which have been minimally tillaged (Griffith et al . 1986, Ortíz 1985).

The appropriate period to subsoil is determined by the crop-planting season, the type of soil, the amount of moisture in the soil, the climate and how susceptible the soil is to erosion. In the United States, for example, it is recommended that in heavy clay soils, the practice should be carried out during Autumn, when soils have a suitable moisture content (Green et al . 1981).

Cassel (1979) mentioned that subsoilers have other uses besides breaking the compact layers of soil. It is also possible to pull a metal cylinder, shaped like a bullet, through clayey, moist soils to make small tunnels which allow drainage or the irrigation of subsoil layers.

When the soil is moist, less energy is required for subsoiling, but the soil volume disturbed is reduced, that is why major benefits are obtained when the soil is relatively dry (London 1991). According to Cassel (1979), the optimum moisture range is between 1 and 9.8 bars (pF 3 and 44), which corresponds to a condition close to field capacity (London 1991).

Plantations located in regions with a tropical, humid forest eco-system, possibly never reach the "optimum" moment to subsoil. For example, in Numar Group oil palm plantations in Coto (Costa Rica), the "window" for this type of labor is very short. On the contrary, in the regions of Quepos (Costa Rica) and San Alejo (Honduras), there are longer, drier periods which permit more flexibility for subsoiling.

As advantages of subsoiling, it is mentioned that the reduction of density and hardness, and the increase in the volume of macropores, improve aeration and drainage and increase the rate of water infiltration (Caliman et al. 1990). Studies in Coto found that the moisture content and the porosity (large pores) exercised an effect on the production of oil palm by 61 and 18% respectively (Durán et al . 1993, Durán 1994).

In the same region, Ortíz and Durán (1993) found that subsoiling, plus one harrow pass, improved the growth and initial yield of oil palm, compared to the control without tillage. In this experiment, all treatments which used some type of tillage, showed better growth than the control (no tillage).

Where there are no compaction or drainage problems, subsoiling is unnecessary. In case where there are compact horizons, it is important to consider the economic benefits of subsoiling or doing some other tillage practice.

References

Caliman, J.; Concaret, J; Aubry M., 1990. Labor de subsoleo en una plantación de palma aceitera. Presentación del implemento adecuado y de las condiciones de realización. Oleagineaux 45(8-9):393-394.

Cassel, D.K. 1979. Subsoiling. Crops and Soils. American Society of Agronomy. Madison, WI. p. 7-10.

Durán, N.; Ortíz, R.A.; de Bruin, S. 1993. Utilización de tensiómetros para la medición del status de humedad del suelo y su relación con la producción de palma aceitera. Congreso Agronómico Nacional. San José, Costa Rica p. 217.

Durán, 1994. Evaluación del efecto de algunas propiedades físicas de suelos y precipitación en la producción de palma aceitera. Congreso Técnico. Grupo Numar. Coto 47. Costa Rica.

Green, D.; Woolley, D.G.; Mullen, R.E. 1981. Agronomy Principles and practices. Burgess Publishing Company. Minneapolis, MN. p. 134-149.

Griffith, D.R.; Manneuring, J.V.; Box, J.E. 1986. Soil and moisture management with reduced tillage. In Sprague M., and F. Triplett (Eds.) No-tillage and Surface-Tillage Agriculture. John Willey P. and Sons. New York. p. 19-58.

London, J. R. (Ed.). Booker tropical soil manual. Longman Scientific and Technical. London. p. 88.

Ortiz, R.A.; Durán, N. 1993. Oil Palm growth as affected by different tillage systems. American Society of Agronomy Agron. Abstr. Madison, WI.

Ortiz, R.A. 1985. Chemical and physical properties of long-term multiple cropping systems as affected by tillage. Agronomy Dept. IFAS. University of Florida. Gainesville. 127p.

Rice, R.W. 1983. Fundamentals of no-till farming. American Association for vocational Instructional Materials. Athens, GA. p. 59-60.

Soil conservation Society of America. 1982. Resource conservation glossary. Soil Conservation Society of America. Third Edition. Ankeny, IA. p. 171.