Limiting factors for pasture and cereal production in marginal soils of the southwestern Pampas in the province of Buenos Aires, Argentina

Soils from semi-arid zones, such as the southern boundary of the Argentine Pampas, have low resilience values and are therefore fragile because they are subject to natural erosion processes, such as wind, and anthropogenic degradation, i.e., monoculture (Iacobucci, 2000). The province of Buenos Aires is also affected by varying degrees of erosion. The processes of wind erosion are observed in western and southern regions of Buenos Aires with an intensity that varies locally. In the south of the province, there is also the development of desertification processes that are mainly attributable to the overuse of soil resources in a transitional area located between the Pampas region and the Patagonia region. 

Soil degradation results in a series of negative effects. Structural deterioration hampers the rooting of plants and their ability to absorb water and nutrients; at the same time, this deterioration greatly exacerbates the risk of erosion. A deficient structure results in decreased soil permeability, increased difficulty in water infiltration, increased runoff and less efficient use of the soil structure. The decrease in organic matter and the degradation of the exchange complex causes a loss of nutrients that accelerates the degradation of the vegetation. In recent decades, Argentine agriculture underwent a transformation from mixed farming-based production systems to intensive agriculture. This change resulted in nutrients being extracted at high rates but not replaced in equal magnitudes, generating degradation and depletion processes that threaten the sustainability of production systems (Casas, 2000; Martínez, 2002). Soil quality can be defined as the ability of the soil to function within the surrounding ecosystems such that it maintains biological productivity and environmental quality and promotes the health of plants and animals (Doran and Parkin, 1994). The soil has an essential role in the carbon cycle, acting as a source and/or sink, depending on the conditions. More than 50% of the carbon that "circulates" is contained in soil organic matter. The carbon in soil organic matter (OM) is almost double the content of the gas phase (atmosphere), where it is found primarily as CO , and surpasses the carbon contained in plant matter to an even greater extent. Nitrogen fertilisation favours the increase 2 in soil organic carbon and improves its quality and productivity, thereby increasing the efficiency of atmospheric carbon sequestration (Halvorson et al., 1999). Continuous agriculture promotes the least amount of OM. Fertilisation has been suggested to increase the level of OM by increasing the crop yield and thereby increasing volume of waste (Diaz et al., 1980).

The fertilisation of soils is a common practice to compensate for the absence or deficiency of essential nutrients in the agroecosystem (Deacon et al., 2010). Nitrogen is the main element required for the production of winter grains, such as wheat, a typical crop in the southwest of the province of Buenos Aires in Argentina. Deficiencies in this nutrient affect the yield and protein content. Certain environmental factors reduce the efficiency of nitrogen use in such regions as southwestern Buenos Aires, which is in the marginal zone of the Argentine Pampas. In this region, the cultivation of winter grains, such as wheat and barley, is limited in productivity, due to such factors as the presence of limestone at shallow depths, deficiencies in organic matter, and water stress. Therefore, low doses of nitrogen fertiliser are required (Ferraris, 2008). Organic fertilisers that have the ability to regulate the release of inorganic forms of nitrogen, which are available to plants, are preferred. The lack of water can limit the absorption of nitrogen by the crop; therefore, the availability of nitrogen as related to the water situation of the soil is important (Cáseres et al., 2005). For cereals, such as wheat, significant amounts of P, K, S, Ca, and Mg are also required as nutrients in addition to the micronutrients B, Cu, Mn, Fe, Cu, and Zn (Ciampitti and Garcia, 2007). For the particular case of the production of winter forage cereals (e.g., rye, oats, triticale), the demands of N, P, and S have been specifically confirmed. These nutrients have increased the dry-matter yields in sub-humid and semi-arid zones with fertilisation (Diaz-Zorita et al., 1997; Quiroga et al., 2007). The incorporation of organic matter into the soil increases the movement of P in calcareous soils, which consequently increases the organic P in the soil solution. The soil solution is mobilised by the microbial population, which, in turn, can physically move and aid in the redistribution of P, thereby increasing the efficiency of the P use compared to organic fertilisation with minerals (Hannapel et al., 1964; Aguirre et al., 2007). However, meat production, especially beef cattle production, is a production alternative that is suitable for marginal regions of the Pampas, such as in the southwestern zone of the province of Buenos Aires. It has also been confirmed that winter-cycle species of forage plants have limited growth as a result of the severe nitrogen deficiency (Echeverría and Bergonzi, 1995). Nitrogen fertilisation of perennial pasture and forage is a rarely adopted practice that would affect plant production, ease the forage shortage, and make it possible to sustain a higher animal load (Fernández Grecco et al., 1995). 

Nutrient depletion is a major form of soil degradation. The sludge from effluent treatment plants in agro-industrial processes is a potential source of organic fertilisers (Roy et al., 2003) that can be used to restore the fertility of agricultural soils with better prospects than even inorganic or conventional fertilisation. When biosolids are incorporated as an amendment, many micronutrients that are not incorporated with conventional (synthetic) fertilisation are provided. This incorporation is an advantage, given that the design of fertiliser dosages at the micro-level would be notably costly. The bioavailability of trace elements, such as micronutrients or toxic elements, is not determined by the total concentrations of the elements in question; rather, it depends directly on the chemical properties of the soil, particularly the pH and cation exchange capacity. The oxidation states of the element and the type of complex formed also affect the bioavailability (Myers et al., 1996; Andrade et al., 2005; Lair et al., 2007). 

The application of sludge and organic (plant/animal) waste to the soil should be reconsidered as an economic practice, both from the standpoint of operating costs (the current analysis) and from the standpoint of the environment, given the facts that matter is recycled and it can effectively compete with chemical fertilisers at lower environmental costs. All of these factors support the pursuit of an effectively sustainable agricultural-livestock production method. The use of organic waste would also be an advantage for countries with relatively low industrialisation that could more easily "close" the cycle of nutrient recycling in contrast to highly industrialised countries, where in certain cases, the surface availability is small. Ideally, sustainable agricultural-livestock production would be stable when organic waste arising from the study area is reused within the same area (Schulz et al., 1997). This stability is possible when the surface areas of agricultural land are large, and the generation of agribusiness and domestic waste is not excessive, as is the case in Argentina. One way to improve or restore long-term soil quality is to intervene in the complex processes of agro-ecosystem biocycles. 

Taking into account the limiting soil factors for plant production in marginal zones of the Argentine Pampas, such as the southwestern regions of the province of Buenos Aires (sub-humid – semi-arid zones), the chemical and physical characterisation of two typical soils is shown. The objective is to propose practices, such as amendment with organic waste, that would improve the soil quality and increase the sustainable productivity of cereals and fodder to prevent deterioration of the ecosystem.