Nitrogen (N) is the nutrient, along with potassium (K), required in greater quantities by corn and wheat crops during the cycle. NO is a very dynamic element in the soil and in the plant, and numerous factors, such as the organic matter content and its mineralization rate, meteorological conditions, relief, predecessor crop, soil moisture and fertilizer source, among others, can affect this dynamic.
Nitrogen fertilization in tropical and subtropical regions represents a challenge, due to the dynamics of N in the environment and its mobility in the soil and plants, which can reduce the efficiency of the practice. Currently, the efficiency in N absorption varies from 50 to 70% of the total applied in fertilization. When talking about N in the plant, it is observed that, for every 1 ton of corn grains, the plants absorb around 28 kg of N per hectare (ha), while, in wheat crops, this value is approximately 22 kg N/ha.
Due to the complexity of the interactions that involve the use of N, nitrogen fertilization needs to be managed in the most efficient and sustainable way possible. The correct choice of nitrogen sources, the stage of best response to fertilization and doses consistent with the plant's needs at the time of application are some points to be observed. Therefore, it is increasingly necessary to use tools that enable the maximization of N use efficiency, with the aim of increasing grain yield and crop profitability and reducing N losses and the environmental impact of agricultural activity.
Nitrogen fertilization for cereals in Rio Grande do Sul (RS) and Santa Catarina (SC) is recommended based on three criteria: grain yield expectation, soil organic matter content and predecessor crop (Liming and Fertilization Manual for the states of RS and SC, from the Brazilian Soil Science Society). This way of defining the total dose to be recommended, despite being a consolidated practice and widely used by rural producers, disregards the spatial variability existing in the crop at the time of fertilization, that is, it performs fertilization at a fixed rate. It was designed prior to the growth in the use of Precision Agriculture (AP) practices and tools and, therefore, can be improved, reconciling robust principles on the behavior of N in the growing conditions of Southern Brazil, with principles that consider variability spatial and temporal proposed by AP.
When one considers the spatial variability of a crop, one begins to understand that nutritional demand varies within a field. Also taking into account the temporal variability of N dynamics and the complexity of interactions with soil, plant and environmental factors, it is possible to improve the efficiency of nitrogen fertilization by crops, depending on adjustments in the dose or nitrogen application strategy (crop stage, soil condition, weather forecast, among others) according to the actual demand of the plants (Figure 1). With this understanding, nitrogen fertilization at a variable rate starts to be carried out taking into account the nutritional status of the plants and their variability in the area.
The philosophy of variable rate fertilization used by AP must always be complementary to that carried out based on the fertilization manuals of each region, valuing the principles and accumulation of knowledge on the subject over time. The objective of the AP, in this sense, is to add tools and dynamic indicators that adjust the N dose according to the variability characteristics of the area, the need for the crop and the progress of conditions throughout the cycle, especially in the critical stages for utilization. of N. Therefore, it is not always necessary to increase the “target dose” in relation to the traditional recommendation system, but to adjust it, with variations upwards or downwards, based, for example, on indicators captured by non-destructive analysis of the plant canopy.
An example of the possibilities of using AP tools for N management is the use of information provided by vegetation indices obtained through sensors embedded in different platforms (proximal sensor, drones and satellites). The Normalized Difference Vegetation Index (NDVI), which can be used to estimate the biomass of the aerial part and the amount of N absorbed by the canopy, makes it possible to estimate the “nutritional status” of the crop and its variability. With this information, it is possible to estimate the N levels in plant tissue, infer the amount of N accumulated in the biomass and indicate the necessary dose to be applied to the crop at varying rates (Figure 2).
Thinking about the feasibility of nitrogen fertilization at varying rates, the Federal University of Rio Grande do Sul (UFRGS), Embrapa Trigo and Sociedade Educacional Três de Maio (SETREM) joined forces in the last wheat and corn harvests and developed mathematical models to adjust the fertilization recommendation for wheat and corn, considering spatial variability and vegetation indices. The models, also called “Algorithms”, were developed thinking about the soil and climate conditions of the southern region of Brazil.
The proposed algorithms make it possible to make a recommendation that achieves the most appropriate redistribution of nitrogen in the crop, in relation to the traditional way of applying a uniform dose or fixed rate. The use of these nitrogen fertilization algorithms in top dressing for wheat and corn resulted in grain yields equal to or greater than the traditional form of recommendation and with N savings.
For wheat, data has been collected since 2015 in the Homogeneous Regions of Adaptation of Wheat Cultivars (RHACT) 1 (Coxilha and Vacaria – cold and humid region) and 2 (Eldorado do Sul and Três de Maio – moderately hot and humid region) . The algorithm was built with the expectation of a grain yield of 5.000 kg/ha and the application of nitrogen fertilization in top dressing at the stage of six fully expanded leaves (Stage V6 – HAUN, 1973), this being the period prior to the greatest demand for nitrogen by the crop.
For corn, the use of the fertilization algorithm allowed better redistribution of the N dose applied in coverage (Stage V8). When evaluating the grain yield of the control (without N), which did not receive nitrogen during the cycle, this was 46% and 40% lower than the algorithm's fertilization methods and the standard traditional method, as recommended by the Fertilization and Liming Manual , respectively. When evaluating fertilization methods, a 10% increase in grain yield was observed when top-dressing nitrogen fertilization was carried out based on the algorithm method, in relation to the standard (Figure 3).
One of the main benefits of using algorithmic fertilization methods is to enable the minimization of spatial variability of crops, if N is responsible for most of the variability and, thus, enhance grain yield and optimize the use of inputs and return economic.
The algorithms are easy to implement in the field, due to the direct relationship between the amount of N with the NDVI and the nutritional condition of the plants, making it possible to apply N at a varied rate, using vegetation sensors in corn and wheat crops.
These studies served to generate technical-scientific knowledge, with the potential to provide technical support for rural producers who use vegetation sensors and to support future improvements in nitrogen fertilization algorithms in coverage at varying rates in Southern Brazil in corn and wheat crops. .
By André Luis Vian, Christian Bredemeier (UFRGS); João Leonardo Fernandes Pires (Embrapa Wheat); Alexandre Alan Cassinelli (UFRGS); Marcos Caraffa (SETREM)
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