Chapter 20 : Soil Limitations for Crop Productivity in South America

The agricultural practices introduced by European colonisers have been practiced in South America for many years, resulting in depletion of the soil’s natural fertility. The introduction of fertilisers and acidity amendments occurred in the mid-1960s and boosted production of many areas, but still with high soil erosion and low levels of organic matter. The widespread use of conservation systems, such as no-tillage, occurred latter and changed the relationship between soil indices and crop responses. Nowadays, South America represents 47% of the total global area under no-tillage, which covers an area around 56 million hectares. No-tillage reduces the annual rate of decomposition and increases the mean residence time of the soil organic matter. One of the great challenges in different countries nowadays, in the most diverse cropping systems, is to increase biodiversity through the proper use of different species of cover crops, mainly mixed cover crops, in order to improve the microbiota, achieve better soil-plant equilibrium and contribute strongly to enhance the soil organic carbon (C) sequestration. In some countries, the challenge of no-tillage adoption is getting closer to being overcome. The current challenge is working on a cropping system with diversified species and high residue input. The adoption of legume cover crops as a source of nitrogen (N) seems to be very important, resulting in higher accumulation of soil organic matter compared to N fertilisation. The management of soil acidity was, and continues to be, one of the main factors limiting crop yields. The problems of diagnosis are mainly related to the sampled soil layer and the indicators used for decision making. Recent studies have demonstrated that neither the 0-10 cm nor the 0-20 cm soil layer is suitable for diagnosing soil acidity and the potential crop yield in no-tillage systems with chemical restrictions in the subsurface. In these areas, a stratified soil analysis is essential, covering at least one subsurface layer (10-20 cm). The incorporation of limestone may be the best and fastest way to eliminate problems related to soil acidity in the subsurface. Significant increases in crop yields have been observed when using agricultural gypsum based on the diagnostic soil layer of 20-40 cm. Doses between 2 and 3 Mg ha-1 are sufficient to obtain 95% of the maximum crop yield. For phosphorus (P), there is no doubt that the biggest problem is restricted access: by farmers at the micro scale and by countries at the macro scale. When accessible, the inappropriate use of P fertilisers is often noticed. The correct approach would be to raise the available P content above the critical level in the 0-20 cm soil layer, and then reapply the amount exported by crops in the row at sowing time. Regarding potassium (K), although there is an assumption that the tropical soils found in South America have only minerals such as kaolinite and oxides, there are several studies that show that the mineralogy of these soils is not so uniform. It is common to observe situations where 2:1 clay minerals are present and crops do not respond to the addition of K fertiliser, or the available K content does not increase over time. The research on sulfur (S) has advanced and shown that in tropical soils there is a higher positive crop response to S addition than in subtropical soils, regardless of available soil S contents. The evaluation of the 20-40 cm soil layer can support decision making regarding S management. To enhance production of plants and to increase the soil organic matter content, it is necessary to encourage and promote the horizontal and vertical monitoring of soil fertility. Associated with this, it is necessary to establish research networks aimed at improving the establishment of critical levels of soil acidity and available nutrients in the soil to guide decision-making more assertively, thus maximising productivity and promoting more sustainable production

Phosphorus and root distribution and corn growth as related to long-term tillage systems and fertilizer placement Distribuição de fósforo e raízes e crescimento do milho em sistemas de manejo do solo e modos de aplicação de fertilizante no longo prazo

Soil and fertilizer management during cultivation can affect crop productivity and profitability. Long-term experiments are therefore necessary to determine the dynamics of nutrient and root distribution as related to soil profile, as well as the effects on nutrient uptake and crop growth. An 18-year experiment was conducted at the Federal University of Rio Grande do Sul State (UFRGS), in Eldorado do Sul, Brazil, on Rhodic Paleudult soil. Black oat and vetch were planted in the winter and corn in the summer. The soil management methods were conventional, involving no-tillage and strip tillage techniques and broadcast, row-and strip-applied fertilizer placement (triple superphosphate). Available P (Mehlich-1) and root distribution were determined in soil monoliths during the corn grain filling period. Corn shoot dry matter production and P accumulation during the 2006/2007 growing season were determined and the efficiency of P utilization calculated. Regardless of the degree of soil mobilization, P and roots were accumulated in the fertilized zone with time, mainly in the surface layer (0-10 cm). Root distribution followed P distribution for all tillage systems and fertilizer treatments. Under no-tillage, independent of the fertilizer placement, the corn plants developed more roots than in the other tillage systems. Although soil tillage systems and fertilizer treatments affected P and root distribution throughout the soil profile, as well as P absorption and corn growth, the efficiency of P utilization was not affected.<br>O manejo do solo e de fertilizantes ao longo do tempo de cultivo pode contribuir para o rendimento e lucratividade das lavouras. Há, então, a necessidade de se ter e avaliar experimentos de longo prazo para que se consiga entender a dinâmica da distribuição de nutrientes e raízes no perfil do solo e seu efeito na absorção de nutrientes e no crescimento e desenvolvimento da cultura. Foi utilizado um experimento de 18 anos de duração em um Argissolo localizado na Estação Experimental da Universidade Federal do Rio Grande do Sul (UFRGS), no município de Eldorado do Sul, com a sucessão de milho no verão e aveia-preta consorciada com azevém no inverno, sob manejos de solos e modos de aplicação de fertilizantes. Os manejos de solo foram o convencional, plantio direto e cultivo em faixas, e a fertilização fosfatada (superfosfato triplo) foi a lanço, em linha e em faixas. A distribuição do P disponível (Mehlich-1) e das raízes de milho no perfil do solo foi avaliada pela coleta de monólitos de solo no período de enchimento de grãos do milho; a produção da parte aérea seca do milho e o acúmulo de P da safra de 2006/2007 foram determinados, sendo calculada a eficiência de utilização de P. Independentemente do grau de mobilização do solo, o P e as raízes acumularam-se ao longo do tempo na zona fertilizada, principalmente na camada de 0-10 cm. A distribuição de raízes seguiu a distribuição de P, independentemente do manejo de solo e do modo de aplicação do fertilizante fosfatado. No sistema plantio direto, qualquer modo de aplicação de P, o milho apresentou quantidade maior de raízes em relação aos demais manejos. Embora o manejo do solo e os modos de aplicação de P tenham alterado a distribuição do nutriente e das raízes de milho no perfil do solo, a absorção de P e o crescimento do milho, bem como a eficiência de utilização do nutriente pelo milho, não foram influenciados