Essay: Soil and nutrients

Questions concerning soil fertility, erosion and the use of fertilizers are currently at the forefront of the global agricultural agenda. By the middle of the 21st century, enough food will need to be produced to sustain approximately 9 billion people (Pretty el al., 2010). However, agricultural fields that are ploughed using conventional methods have increased rates of soil erosion, which threatens agricultural sustainability (Montgomery, 2007). Consequently, practices such as conservation tillage are considered preferable, as they reduce soil erosion, enhance soil organic carbon and reduce nutrient depletion (Cerri et al., 2007). However, practices such no-till are not being implemented worldwide; only 6% of global cropland area was managed using conservation tillage in 2006 (Huggins and Reganold, 2008; Lal, 2007). Therefore, agricultural and soil scientists are searching for more sustainable forms of land use and more efficient agricultural production practices (Pretty et al., 2010).
Agricultural modernization has not occurred equally worldwide. The slash-and-burn agricultural system persists in the tropical world despite of its wide land use dynamic; showing areas where this system persists; other with change in fallow length and areas in transition (i.e., land use intensity) (van Vliet et al., 2012). In the slash-and-burn system, farmers cut down the vegetation, and dry and burn it to clear the terrain and fertilize the soil (called “enriched ash-bed”) (Kang and Sajjapongse, 1980; Nye and Greenland, 1964; Reuler and Janssen, 1993; Tanaka et al., 2005). Fire is an important environmental phenomenon that regulates terrestrial biogeochemical cycles (Hogue and Inglett, 2012; Bod?? et al., 2014). On the other hand, fire also causes significant changes in soil physical, mineralogical, chemical, and biological properties (Certini, 2005; DeBano et al., 1998).
Following a fire, soil erosion tends to increase, accompanied by nutrient loss. This process plays a major role in the depletion of soil nutrients that can occur in slash-and-burn systems; the post-fire gain in nutrients can be offset or reversed by soil erosion. Most studies have focused on the effects of fire on soil properties and nutrient improvement. Few studies have sought to understand soil erosion and nutrient loss in slash-and-burn agricultural systems; such studies are controversial and some authors argue that the losses are sustainable (de Neergaard et al., 2008; Rodenburg et al., 2003; Tanaka et al., 2005). Other research highlights the possible unsustainability of this agricultural practice (Borggaard et al., 2003; Gafur et al., 2003; Juo and Manu, 1996).
In slash-and-burn agriculture the losses of nutrients has been studied extensively; however the study of losses of several nutrients altogether, in particular with attention to micronutrients is scarce. To contribute to this gap, the objectives of this study were to (1) estimate nutrient loss during cropping by comparing the pre- and post-burn nutrient budget, (2) assess soil loss according to land slope (6?? and 11??), and (3) evaluate the effect of ash covering the ground in reducing soil erosion.
Therefore, assessing the dynamics of soil and nutrient losses at this stage i.e., cropping phase is a key factor in determining nutrient balance to ensure adequate crop yield and the restoration of vegetation and soil fertility during the fallow period. In addition, acknowledging that soil losses occur during the early period of cultivation (i.e., first rains), simple and low-cost tillage practices, such as contour-felled log erosion barriers, can be proposed to help farmers retain some of the nutrients in the plot.