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

Resposta à aplicação e recuperação de enxofre em cultivos de casa de vegetação em solos com diferentes teores de argila e matéria orgânica Responses to sulfur application and recuperation in greenhouse crops in soils with different clay and organic matter content

A dinâmica e a disponibilidade do enxofre têm sido pouco estudadas em relação a outros nutrientes, apesar da sua importância como nutriente à planta. Este estudo teve como objetivo avaliar a resposta de algumas culturas à aplicação de sulfato (S-SO4-2) em solos com diferentes teores de argila e matéria orgânica, bem como avaliar a recuperação de enxofre, através de um balanço deste elemento no solo e na planta. O experimento consistiu de seis cultivos sucessivos (canola, soja, feijoeiro, gergelim, trevo vesiculoso e trigo), em vasos em casa de vegetação, utilizando-se quatro tipos de solos e quatro doses de S-SO4-2, com quatro repetições distribuídas ao acaso. Avaliou-se a produção de matéria seca, o teor e o acúmulo de enxofre absorvido pelas culturas. Amostras de solo foram coletadas na camada de 0-10cm após cada cultivo, para determinação do teor de S-SO4-2 disponível. Dentre as culturas estudadas, somente a canola respondeu à aplicação de S-SO4-2. Para as demais culturas, não houve relação entre os teores de S-SO4-2 da camada de 0-10cm e a produção de matéria seca. Solos com diferentes teores de argila e matéria orgânica tiveram comportamento similar em relação à resposta das culturas à fertilização com enxofre; entretanto, a disponibilidade deste nutriente foi maior nos solos mais argilosos.<br>The sulfur (S) dynamic and availability is less studied than others nutrients, even though S is an essential nutrient for crops production. This study was carried out to evaluate the crop responses to S-SO4-2 application in different soils and to study S recuperation by balancing it in soil and plants. The study was composed by a greenhouse experiment with six successive crops (canola, soybean, black bean, sesame, clover and wheat) using four soils and four S-SO4-2 levels (0, 5, 10 and 20mg kg-1) to evaluate immediate and residual effect of S-SO4-2 application. Soil samples were taken from 0-10 cm layer before and after each crop and analyzed for S-SO4-2. It was evaluate the dry matter production and S-SO4-2 absorbed by plants. Only canola responded to sulphur application. Soybean, black bean, sesame, clover and wheat did not response to sulfur application. There was not observed correlation between soil S-SO4-2 available in 10cm topsoil and crop yields. Soils with different clay and organic matter content showed similar behavior of crop response to sulfur application. The increase of S-SO4-2 availability with fertilization was greater in soils with more clay content