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Boron doses applied to soil during coffee development

The range between boron deficiency and toxicity in the coffee crop is narrow and the crop toxicity tolerance depends on the soil type, initial soil content and plant age. The objective of this study was to evaluate different levels of boron in the coffee crop during development and grown in a red clayey Oxisol. Treatments consisted of boron applications (zero; 2.48; 4.96; 9.92; 19.8 and 39.68 kg ha-1) in the boric acid form and applied to soil surface in the area of the tree crown projection. The six treatments were outlined in a randomized block design with four replications and ten plants per plot. The cultivar used was Catuaí IAC 144 with sixteen months old, grown in an Oxisol in Campinas, SP, Brazil. Foliar boron content was analyzed as a function of time by regression analysis. The levels in fruit and yield were analyzed by regression according to boron doses. It can be concluded that higher boron doses in the plant can be observed when fruits are at the first development stage. High doses can reduce yield productivity, being each kilogram of applied boron responsible for a reduction of 0.33 coffee bags. ha-1.


Introduction
The low crop yield across the world is mostly due to nutritional disorders, including it's the mineral toxicity or deficiency (Amaral et al., 2011). During years of potential higher yields for the coffee crop, micronutrient deficiency can be observed, such as boron (B) (Martinez et al., 2003), due to its lower availability in Brazilian soils under cultivation, especially in the Cerrado region (Fernandes et al., 2012a). Its main source is the organic matter (Carmo et al., 2012), and due to the small availability in Brazilian soils, the nutrient must be supplied to the crops (Fernandes et al., 2013). The proper correction of B-deficient soils with the use of fertilizers with boron depends on various factors, such as application form source and time (Rosolem et al., 2007), beyond the soil type (Fernandes et al. 2012 b). Nutritional balance is extremely important because some nutrient in excess can inhibited others (Prado, 2008).
The borated fertilization in coffee is widely used, but the use of higher doses can cause plant toxicity, because the range between appropriate and toxic levels is relatively narrow for most crops. The B mobility varies widely among plant species, which has implications in the expression of its eficiency or toxicity in different species and in the management of fertilization with this micronutrient (Viégas et al., 2004;Bastos & Carvalho, 2004). Symptoms of B toxicity in coffee are characterized by the appearance of yellow-green spots on older leaves, where the micronutrient would stay (Brown & Hu, 1998), evolving to the appearance of small dark spots, complete burning on the edges of the leaves and its fall (Malavolta, 2006).
Knowledge of appropriate B dose to be provided for young coffee plants requires more attention, because the toxicity may be more severe in this period (Mattiello et al., 2009), since its sufficiency range is narrow (Gontijo et al., 2007). However, little is known about this micronutrient effects during the coffee formation stage. Therefore, this study aimed to evaluate the coffee crop tolerance in the formation phase to the application of high boron doses in a red clayey oxisol.

Material and Methods
The At harvest, which began when fruit was 15% of green (Matiello et al., 2010), the fruits were divided according to the maturation stage the measurements were carried out with the use of graduated containers. After determining the volume, the values have been converted to the equivalent of processed coffee (kg ha -1 ).  Bingham (1982).
These data were submitted to analysis of variance by F test (P <0.05) and regression analysis for significant effects using the SISVAR ® statistic program (Ferreira, 2011). The coefficients of each model components were tested and the significant models with higher coefficient of determination were choosed.

Results and Discussion
The application of B doses in the soil resulted in a linear increase in the B foliar contents of coffee for all evaluated months, corresponding to winter, spring, summer and fall seasons ( Figure   2). It was noted that the highest B content achieved the leaf sample that was taken 90 days after the nutrient application (Figure 2b (Prado, 2008;Tomaz et al., 2011). In September, the foliar B content increased (Figure 2b) during the period before coffee flowering, when the plant requires large amounts of this element.
Other authors also founded similar results, and reported that the increased demand for B by the plant occurred during the reproductive phase (Blevins & Lukaszewski, 1998 Currently, the practice of using B before and after flowering in coffee is commonly used to obtain greater "fixation" during flowering, increasing its productivity (Fernandes et al., 2012b). Between January and March (240-300 days after application), it was observed that the boron content in leaves decreased (Figure 3). This element was transported to fruit, that participates in cell division, differentiation and carbohydrate transport from leaves to fruit (Prado, 2008). For the higher dose, this decrease was not observed, probably due to the large nutrient amount added to the soil. shows that the doses that caused B toxicity to the coffee crop were different for cultivated soils, and the clayey is more tolerant to higher doses of this nutrient. In another study, Correa et al. (1985) evaluated the coffee seedlings response to boron application in two Oxisols (75 and 11% clay), reaching the conclusion that the adsorption capacity depends on soil texture, which means, higher the content of clay, higher the adsorption.

Conclusions
Higher levels of boron are achieved when fruits are in the first stage of maturity (December).
The reduction in yield due to boron toxicity occurs even when the visual symptoms can not be observed.
In clay soils, for each kilogram of boron added, the reduction of 0.33 coffee bags ha -1 was observed.