Nitrogen fertilizer in coffee

Nitrogen is required in large quantities and is one of the most important nutrients in coffee crops. And avoiding losses when applying fertilizers, such as urea, has been a challenge for technicians and producers. To minimize the problem, an alternative lies in the use of controlled release products (urea + polymer).

Brazilian coffee farming is largely implemented in soils with low natural fertility, which requires adequate correction and fertilization to achieve high productivity. Among the nutrients, nitrogen deserves special attention, as it is the one required in greatest quantities by the coffee plant.

Nitrogen (N) is the nutrient that has the most significant effects on increasing productivity in coffee cultivation. It is important for performing several functions in the plant's biochemical processes, as a constituent of enzymes, coenzymes, nucleic acids and chlorophyll.

One of the difficulties in recommending nitrogen fertilization is the very complex dynamics of the element in the soil, undergoing several transformations such as: leaching, volatilization, nitrification, denitrication, immobilization and mineralization, thus altering its supply during plant development. In this way, N can become the most expensive element in the coffee production system.

The nitrogen fertilizer most used in Brazil is urea, which, due to its high concentration of N, allows for a lower cost per unit of the nutrient. However, the application of urea to the soil causes an increase in pH in the fertilizer granule region at the time of hydrolysis. This increase in pH causes high losses of N through volatilization in the form of ammonia. These volatilization losses contribute to a decrease in the efficiency of nitrogen fertilizers, which can reach values ​​of up to 35% of the total N that is applied.

One of the alternatives to minimize nitrogen losses is the partial application of nitrogen fertilizers, resulting in greater use by plants. However, splitting is not always the most profitable way for the producer, as it can generate a significant increase in labor costs and also cause soil compaction, due to greater machinery traffic in the area.

In this sense, there are many strategies to improve efficiency in the use of nitrogen fertilizers. These include the use of urease (NBPT) and nitrification inhibitors, the addition of acidifying compounds and the use of urea coated with polymers or gel, generating fertilizers known as slow or controlled release.

Slow or controlled release fertilizers are those that somehow delay or extend their supply to the plant for a period beyond that of a reference fertilizer. A fertilizer whose nutrient availability is immediate, such as urea, is considered a reference.

The use of controlled-release nitrogen fertilizers can provide several benefits, such as increased productivity, labor savings, as it can abolish or reduce the number of installments, and reduced environmental impact due to less nitrate leaching, as long as it is combined with demand. of culture and development conditions. Among the release control mechanisms, one of the most used, basically consists of a polymer layer around a soluble fertilizer (normally urea) that allows synchronization in the supply of the nutrient by the fertilizer and the demand for the element by the plant. However, it is worth mentioning that it depends on the release period guaranteed by the fertilizer manufacturer and also on the soil's humidity and temperature conditions.

Considering the importance of studying this technology in the field of soil fertility, this work sought to evaluate ammonia losses due to volatilization of the widely used conventional urea and controlled-release urea, coated with polymers.

The experiment was conducted in the field, between November 2014 and March 2015 in a dystrophic red Oxisol, in a coffee farm located 7 km from the municipality of Lavras, Minas Gerais.

A randomized block design with three replications was used, with the N sources being: conventional urea (45% N) and urea coated with polymers (39% N). The experimental plots were composed of 12 plants, equally spaced 3,0 m between rows and 1,0 m between plants. The dose of N used was 450 kg/ha, divided into three applications in the treatment with conventional urea, with the interval between each application being 60 days and applied in a single dose for the treatment with urea coated with polymers.

For crop maintenance fertilization, 300 kg/ha of K was applied₂O and 100 kg/ ha of P₂O₅, using potassium chloride (60% K₂O) and triple superphosphate (46% P₂O₅). The micronutrients, boron, zinc and copper were applied foliarly.

To quantify ammonia volatilization, the semi-open collector method was used. To construct the collector, PVC tubes measuring 20 cm in diameter and 50 cm in height were used. 

The PVC pipes were inserted into the soil to a depth of 5cm. Two laminated foam discs with a density of 0,02 g cm were used.^-3 and 2,5 cm thick cut to the same diameter as the tube. The foams were soaked with 85 mL/L of phosphoric acid solution (H₃PO₄) and glycerin in volumes of 40 and 58,8 mL/L (0,87 mol /L of H3PO4) respectively and fixed at heights 25 cm and 45 cm from the ground. The sponge fixed at 45cm has the function of avoiding contamination of the lower sponge. This lower sponge, placed 25 cm above the ground, is used to determine volatilized ammonia. For determination, these sponges were changed on the 1st, 2nd, 3rd, 4th, 5th, 7th, 9th. 11th, 13th, 16th, 19th, 23rd, 28th, 35th and 42nd days after each nitrogen top dressing. To extract the solution from the collected foams, a Buchner funnel was used with the aid of a vacuum pump, with each foam being washed 4 times in sequential extractions with 80 mL of deionized water. Aliquots were taken from this extract to determine the N content by distillation using the Kjeldahl method.

After obtaining the N loss results, as a percentage of the total applied (450 kg/ha of N), significant differences were noted for nitrogen losses (in the form of ammonia), between conventional urea and urea coated with polymers. Comparison between means was performed using the Scott-Knott test (p < 0,05) (Figure 1). 

Conventional Urea achieved a loss of 18,5% of the total N applied, which allows us to infer that of the 450 kg/ha of N applied in the form of conventional urea, 83,5 kg were lost through volatilization. Urea coated with polymers resulted in a loss of 10,46% of the total N applied, that is, only 47,1 kg of N.

Therefore, it can be concluded that urea coated with polymers had lower N loss (8% less) compared to conventional urea, even when applied in a single dose. Thus, the use of controlled-release fertilizers (Urea + Polymer) can be an efficient alternative for fertilizing coffee plants, providing lower losses of N through volatilization and thus reducing machinery traffic in the field, avoiding the need for installments of nitrogen fertilization.

Wantuir Filipe Teixeira Chagas, Dougras Ramos Guelfi, Anderson William Dominguetti, André Luiz Carvalho Caputo, Ufla

Article published in issue 204 of Cultivar Grandes Culturas.