Study indicates challenges in soil fertility after floods

The soil of a citrus orchard in Vale do Caí, affected by the climatic event in May this year, was the subject of a study carried out by the Department of Soils of the Faculty of Agronomy of the Federal University of Rio Grande do Sul (UFRGS), the Secretariat of Agriculture, Livestock, Sustainable Production and Irrigation (Seapi) and Emater-RS. 

The results of the work were published in Technical Note No. 5, which is part of a collection of publications produced by the three institutions. The objective is to provide technical and scientific support for soil recovery guidelines and the foundations of Rio Grande do Sul's production systems. 

Teams from UFRGS, Seapi and Emater-RS collected soil samples from an orchard that had been hit by the flood. The samples were collected from an area where three meters of coarser sediments had been deposited by the flood; from another area where there was a layer of a few centimeters of finer sediments; and from the original subsoil of the orchard, located below these sediments. The soils were analyzed according to their granulometric composition (types and sizes of particles), acidity, organic matter content and nutrient concentration.

The study revealed significant variations in some parameters depending on the type of sediment deposited. 

The granulometric analysis showed that the coarser sediments resembled the original subsoil, while the finer sediments were richer in silt and organic matter. “This exemplifies the importance of case-by-case assessment for decision-making,” highlights Amanda Posselt Martins, professor of Soil Fertility at UFRGS.

The impact of the loss of the topsoil was clear, with very low levels of organic matter found in the soil. “Management measures that promote an increase in these levels should be recommended in recovery actions, as the fertility of Rio Grande do Sul’s soils is highly dependent on organic matter,” adds Amanda. 

On the other hand, management methods such as liming should be used with caution. “Acidity and lack of calcium and magnesium were not a problem observed in any of the situations presented in this study, as well as the micronutrients manganese, copper and zinc”, says Tales Tiecher, professor of Soil Chemistry at UFRGS. 

According to the results of the study, the nutrient that requires the most attention in recovery actions tends to be phosphorus. “Phosphorus is specifically retained in clay particles, which were carried by the waters to more distant locations and did not settle in the productive areas of the Caí Valley. In this sense, crop diversification to favor soil biota and the application of phosphate fertilizers in recommended doses based on soil analysis are strategies that should be adopted to recover the productive potential of these areas,” assesses Tales.

For the director of the Department of Plant Protection at Seapi, Ricardo Felicetti, the results of the study highlight the complexity of the scenario for recovering soils affected by floods. “Each situation is unique, dependent on precise assessments and integration between mechanization, fertility and organic matter recovery measures,” he concludes.