​New irrigation model increases rice productivity

The Araguaia river valley, in the state of Tocantins, has recently awakened its potential for rice production. “However, the pioneer farmers brought technical-cultural experience from the traditional rice farming developed in the floodplains of Rio Grande Sul”, explains agronomist André Borja Reis.

According to Reis, despite the merit of enabling a production system in an agricultural frontier area, producers in that region in the north of the country also began to face, concomitantly with the growth of the planted area, the conflict over the use of water. “This was the motivation to develop a doctoral research project in the floodplains of the Araguaia River valley.”

The research was supervised by Professor José Laércio Favarin, from the Department of Plant Production and was recently defended in the postgraduate program in Phytotechnics, at the Escola Superior de Agricultura Luiz de Queiroz (USP/ESALQ). The results promise to strengthen the recent agricultural frontier and substantially increase rice production productivity levels, including in regions with similar soil attributes.

To kick-start the studies, the researcher sought to answer questions that suggested the breaking of a paradigm: is irrigation management using a continuous flood layer really necessary in tropical floodplain conditions? How would an alternative irrigation regime that saves water impact rice productivity? “It was time for agronomic research to make its contribution, and answer a practical question with due scientific foundation”, he states.

According to the agronomist, continuous flood management is widely spread in traditional rice growing regions, such as Rio Grande do Sul, Asia and the USA. “This management provides full compliance with the plants’ water demand, and also favors the absorption of nutrients.

Thus, to propose an alternative management to continuous blades in tropical floodplains, it would be necessary to understand the reasons why the soil-water-plant relationship would be different in this environment, and how the absence of blades would impact nutrient absorption, in particular nitrogen (N). , nutrient most required by the crop.

To answer these questions, André Borja Reis analyzed the results of a field experiment in the municipality of Lagoa da Confusao (TO), around 200 km southwest of Palmas (TO), for three years. Five irrigation regimes were tested: the traditional continuous blade, two combinations of alternating flooding and drainage in different cycles, saturated soil without the formation of a blade on the surface, and unsaturated soil with moisture content close to field capacity. “In all irrigation regimes, doses of N were applied to determine whether irrigation management would be capable of interfering with crop productivity. And urea enriched with the stable isotope was also used ¹⁵N to investigate the fate of N from fertilizer”, details the author of the work.

Results – The study showed that soils have hydraulic characteristics that allow greater water movement and, therefore, plants easily access the water necessary for their physiological functions even in unsaturated conditions. Thus, the flooded floodplain paradigm would be broken. “The soils of the tropical floodplain show a high degree of clay weathering and the structuring of the A horizon into micro aggregates, which gives pronounced macroporosity. These characteristics allow for hydraulic conductivity up to 10 times greater than soils from traditional cultivation regions.” The result showed less need for water application through irrigation and, consequently, reduced loss due to percolation and direct evaporation of the water layer. “In the three harvests, up to 50% per year were saved, compared to traditional continuous blade management”, celebrates the author.

Nitrogen – Another important aspect brought nitrogen balance assessments. “The irrigation regime with unsaturated soil proved to be the most suitable for the region. The recovery of N from fertilizer was 25% greater in this irrigation regime, which indicates a relevant reduction in losses of this nutrient”. This indication makes the results even more promising, as nitrogen losses are harmful to the environment, and increasing the efficiency of use by the crop helps to reduce the environmental impact. “Data on moisture content and forms of exchangeable nitrogen in the soil demonstrated that the irrigation regime impacts transformation processes, and that under these conditions excess water, as in traditional management, favors loss processes such as denitrification and leaching”, he assesses. André Borja Reis.

Saving water without restricting plant growth and greater N recovery allowed rice to perform better. “Over the three years, productivity was 5% to 15% higher in the treatment with unsaturated soil compared to continuous soil, and also with a reduction in the dose of fertilizer”, he adds.

In summary, André Borja Reis reinforces that the results demonstrated that it is possible to rethink the management of rice irrigation in floodplains, and that management technology should be developed based on the attributes of the region's soils, instead of incorporating recommendations for other agroecosystems. “Saving water from the irrigation regime with unsaturated soil will make it possible to expand the planted area and produce more rice with less water resources, fertilizers, and the profitability of agricultural activity. And it also demonstrates that research attentive to the technical challenges that agriculture faces is also the vocation of public research in Brazil”, he concludes.

This work was supported by Fazenda Dois Rios, which provided the area and helped conduct the experiment; National Council for Scientific and Technological Development (CNPq), through a scholarship; from the São Paulo State Research Support Foundation (Fapesp), with structural contributions to the Multi-User Plant Production Laboratory; from the Coordination for the Improvement of Higher Education Personnel (Capes), with assistance in the sandwich doctorate stage, carried out in the USA.