Wheat is a crop that plays a fundamental role in human nutrition worldwide, with an estimated increase of 0.8% in global consumption in the years 2023/2024 (Conab, 2023). Brazil currently occupies the 14th position in the world ranking, with production estimated at 10 million tons in the 2023/2024 harvest (USDA, 2023), despite this, this amount does not meet the internal demand for wheat in the country, making it necessary to import this cereal .
In this way, producers have sought efficiency strategies in the use of inputs that aim to increase productivity and, consequently, the profitability of rural producers. Nitrogen (N) is one of the nutrients most required by wheat crops, having a close relationship with its productive potential, playing an important role in the development of the plant's photosynthetic activity and the formation of protein in the grains.
Wheat is highly responsive to nitrogen fertilizer. Currently, the main form of nitrogen used by producers is urea, however, we must take some care when applying, as this nutrient can undergo several reactions in the soil that can increase its losses, reducing its efficiency.
How to measure the amount of nitrogen?
To measure the amount of nitrogen necessary for the good development of the crop, it is essential to carry out soil sampling and analysis. To achieve this, there are procedures that must be followed. As mentioned by the Soil Chemistry and Fertility Commission - RS/SC (CQFS), soil sampling can be done at any time of the year, however, it is recommended to carry it out two to three months before sowing, with the Ideally, on average, 15 subsamples per homogeneous plot at a depth of 0 cm to 10 cm in the case of consolidated direct planting.
In the case of nitrogen, the parameters that must be considered for the recommendation are the organic matter (OM) content included in the analysis, the expected yield of the crop of interest, the losses of applied N and the predecessor crop, which will determine whether greater mineralization or immobilization of nitrogen will occur.
The recommendation brought by CQFS is to apply 15 kg to 20 kg of N per hectare at sowing and the remainder as cover between the tillering and stalk elongation stages.
To determine the required quantity, the manual provides data based on an expected yield of 3 t/ha, that is, 50 bags of wheat, where:
Contents lower than or equal to 2.5% of M.O. 60 kg of N/ha is recommended when a legume such as soybean, vetch or clover precedes wheat, or 80 kg of N/ha when a grass such as oats, ryegrass or corn precedes wheat;
Between 2.6% and 5% M.O, 40 kg of N/ha (legumes) and 60 kg of N/ha (grasses) are recommended;
Contents above 5% M.O, a dose equal to or less than 20 kg of N/ha is recommended, regardless of the predecessor crop.
Furthermore, for yields greater than 3 t/ha, 20 kg of N/ha must be added to the cited values in the case of legumes as a predecessor crop, and 30 kg of N/ha for grasses for each additional ton of grains to be produced.
How to improve nitrogen use efficiency?
Urea is the main source of nitrogen used in wheat cultivation, but losses due to volatilization and/or leaching of this nutrient are common when incorrectly managed. Therefore, it is essential to pay attention to numerous environmental factors, management and phenological status of the plants to optimize the use of this fertilizer.
Nitrogen leaching occurs when the nutrient is lost in the soil profile carried by rainwater. Volatilization refers to the loss of the nutrient to the atmosphere, when amidst the transformations that urea undergoes, NH3 (ammonia) is formed, which is a highly volatile gas.
Splitting the nitrogen dose is a strategy that minimizes losses of this nutrient and also optimizes the efficiency of its use by the plant, as it will be available in the critical phases of the crop's development. In regions such as the Cerrado, where wheat cultivation is under central pivot in a period in which there is no rain during the entire wheat cultivation, it is common to incorporate all the urea before sowing, aiming to increase nitrogen efficiency. This practice is used to eliminate losses due to NH3 volatilization, which is the main path of nitrogen loss in production systems.
Therefore, the application of nitrogen is recommended based on observation of the plant's apical meristem, in the phases comprising double ring (tillering) and terminal spikelet (Elongation) (figure 1 and 2, respectively). To observe the apical meristem or growth tip, it is necessary to collect plants in the field and dissect them with the aid of a histological needle.
