9.4.1 Erosion of Soil

Effective soil erosion control is dependent upon decreasing the impact of drops of rain and the velocity of water running over the surface of soil (Andraski et al. 1985; Timm et al. 2002). This involves soil surface protection by utilizing mulch or cover crop to prevent soil striking, directly by the rain. This is done by avoiding the practices that compact the soil, thus causing reduction in the infiltration (Hillel 2007).

Many studies have been done in order to understand the protection of soil provided by the straw blanket spread on the ground especially for corn stover. According to one study, it was assessed that even after the removal of corn stover from the soil, it still provides adequate protection against the erosion of soil, considering implicitly the impact on recycling of nutrients and taking into consideration the local yield and tillage practices (Graham et al. 2007). A study revealed that 20-30 % of the corn stover could be removed to still provide the adequate cover for protection (Wilhelm et al. 2004). In another study which preceded the previous one, an estimation was done which stated that the amount of stover that is required for keeping the soil ero­sion to a level that is acceptable was dependent highly on the crop management and practices of tilling that ranged from 1 to 8 mg ha-1 (Wilhelm et al. 2007). Environmental protection agency (EPA) considers that 100 % of corn stover could be removed when no tillage is used and there is a decrease in the amount to 35 % when conservationist agricultural practices are used. In practices of conventional tillage, no residue is removed from the soil (EPA 2010). Sheehan et al. (2004) built a life cycle model stimulating corn stover collection in the state of Iowa for the production and use of a fuel mixture consisting of 85 % ethanol and 15 % gasoline, by volume. The individual impact on dynamics of soil carbon, erosion of soil, agro­nomic aspects of collection of stover and transport, and conversion of bioethanol was separately modeled. For the conditions in Iowa corn field, the average mini­mum amount of the residues that could be left on the field was 4.9 and 2.5 mg ha-1 for the typical operation of tilling and no-till operation, assuming that the corn is continuously grown.

Erosion of soil in case of sugarcane is generally limited as compared to conven­tional agricultural crops such as soybeans and corn, since the canopy closes rap­idly, thus providing cover to the soil, and disturbance of soil is limited to the replanting period (once every 5 or 6 years). However, losses in the soil for sugar­cane may dramatically vary depending on many factors like the annual rainfall, the management and system of harvesting, etc (Sheehan et al. 2004).

A recent study considered effects of no tillage techniques and also conservation of soil practices like contoured seeding, ripping and furrowing, use of absorption terraces, unburned harvesting, and others (De Maria and Dechen 1998). Other stud­ies revealed that during an experiment of over eleven years, there was no significant effect of production of sugarcane on the soil horizon thickness or physiochemical composition of the soil (Macedo 2005) . The increase in mechanical harvesting (without straw burning) reduces erosion of soil due to mulching effect of straw (Macedo 2005).

In a study conducted by Andrade et al., quantification of economic and technical impacts on nutrient and soil losses through erosion in the sugarcane cultivation in Brazil was carried out. The greatest losses of nutrients of soil as well as the erosion occurred in areas of burned sugarcane. Taking into consideration the average of five cuts, burned sugarcane lost 56.45 % of K, 48.82 % of soil, and 60.78 % of P more than the unburned sugarcane (mechanical harvesting). On the average, the nutrient replacement cost for burned cane is 16.96 $/ha which was higher than the unburned cane. The unburned sugarcane had lower cost of production as compared to the burned sugarcane and higher average economic return with respect to the burned sugarcane (Leal et al. 2013).

Erosion rates are also influenced by management of straw on the ground. Burned or buried straw as well as the straw on the surface results in soil erosion rates of 20.2, 13.8, and 6.5 mg ha-1a-1 and runoff of 8, 5.8, and 2.5 % of rainfall, respectively (Macedo 2005).