The use of palm leaf mats in soil erosion control

Geotextiles have been used for many years in different parts of the world to promote soil conservation and to combat erosion. Such geotextiles may be synthetic (usually some form of plastic, sometimes with wire), or natural (usually some form of fibrous material). Work carried out at the University of Wolverhampton (UK) on the effectiveness of mats made from palm tree leaves sourced from the Gambia, West Africa led to a research project funded by the EU, which ran from October 2005 to February 2009, comprising the participation of four EU countries (UK, Belgium, Hungary and Lithuania) and six developing” countries (Brazil, Gambia, South Africa, Thailand, China and Vietnam). Research carried out in South Africa used mats made from the leaves of the Lala palm (Hyperhene coriacea). These mats are easy to make, flexible, durable and completely biodegradable. They cover approximately 40% of the soil surface, allowing space for vegetation to emerge, and add 1.3 kg of dry organic matter to each m2 of soil. Furthermore, they have a water retention capacity of 1.8 l kg-1 m-2, their N, K, S and P percentages are high, they have low sodium and aluminium values and a favourable C/N ratio. Firstly, trials were done on 20 South African soils and 10 mine tailings materials using a rainfall simulator. The soils varied considerably with respect to their textural, chemical and mineralogical properties as well as annual precipitation and geological origin. Erosion parameters varied greatly within, and to a much lesser extent between, the two different materials. Several significant correlations were obtained. Sediment load (SL) had the best correlation with kaolinite content and with fine sand content, while for runoff, the best correlation was with organic carbon content. When the samples were covered with palm mats values for final infiltration rate (FI) percentage stable aggregates (SA) and inter-rill erodibility (Ki) values were similar to those of bare materials and the amount of runoff was slightly higher. SL, however, was reduced by +65%. The next stage was to carry out a range of field trials, using runoff plots. Plots at four localities (Bergville, Ladybrand, Roodeplaat and Mabula) were used. Results showed that average runoff under the palm mats decreased by between 38% and 70%, compared to bare soil. Sediment concentration under the mats decreased by between 38% and 89%, using three combinations of slope, mat density and mesh size. Splash erosion at Roodeplaat decreased by between 62% and 68%, while re-vegetation at Ladybrand and Mabula increased by between 38% and 58%, with organic carbon content and topsoil accumulation also increasing under the mats. Various trials (using both the rainfall simulator and runoff plots) were carried out to evaluate the effects of reduced mat density and increased mesh size. Results from the other participating countries (25% to 95% reduction in runoff) confirm that there is much potential to use organic, bio-degradable, easy to manufacture geotextiles such as palm leaf mats, especially to combine employment opportunities with enhanced environmental protection in many susceptible areas of South Africa.