We carried out assays of foliar B and nutritional monitoring of commercial soybean crops in experimental and commercial production sites, respectively, in the municipality of Chapadão do Sul, Mato Grosso do Sul State, Brazil. The dystrophic Red Latosol predominates in the experimental site and in most of the municipality [33]. The climate in the region is humid tropical (Aw) (Köppen classification) with a two-month dry season and an average annual rainfall of 1550 mm [34].
The nutritional status was monitored in the 2015/2016 harvest of commercial crops, in a no-tillage system in straw, but with different phytotechnical managements. We used cultivars P98Y30, W791, M7739, CANCHEIRO, TEC7849, N7737, DESAFIO, M9144, SYN1288, BG4184, M7339 IPRO, AS3797 IPRO, M8210 IPRO, NS7670, and 98Y52. In each crop, a perimeter of one hectare was delimited for leaf sampling. Yield was estimated in kg per hectare of soybean grain, adjusted for water content in the grain at 13%. We were having licenses to collect soy crop.
This research was not conducted with endangered species and all methods in this study were carried out in compliance/accordance with relevant institutional, national, and international guidelines and legislation.
The foliar B fertilization test (calibration test) was designed to determine the B response curve on crop yield as well as leaf B levels in soybean cultivar DESAFIO. The soil of the experimental site presented 0.32 mg B dm-3 with extraction by hot water, a content interpreted as low [35]. The experimental design was randomized blocks, with five B doses (0, 300, 600, 1200, and 1800 g ha-1), applied as boric acid, with five replications per treatment.
In pre-sowing, 100 kg ha-1 of KCl was applied like broadcast application. At sowing, on Nov 24, 2015, we applied 115 kg ha-1 of monoammonium phosphate (N:11% and P2O5:52%). The total experimental site comprised 852.5 m2, divided into 25 experimental plots of 11.0 m long and 3.1 m wide. Each experimental plot consisted of seven planting rows, where the three central lines were the useful area for the evaluations with eight central meters used per row.
Boron was applied via foliar with a CO2 pump sprayer, adjusted to a spray volume of 150 L ha-1. To increase B absorption, 0.15% surfactant (nonionic polyoxymethylene surfactant) and 1% urea were mixed with the syrup. Each dose was divided into three applications, two in the vegetative phase (V2 and V5) and one at the beginning of flowering (R1). The applications were carried out in the morning at a temperature near 25ºC, relative humidity of about 80%, and wind speed near 7 km h-1.
Soybean leaves were sampled in the experimental site at stage R1 (beginning of plant flowering) at 10 days after the last B application. Each sampling site was represented by the random collection of 25 fully expanded leaves, from the third trifoliate, with petiole, counted from the plant apex [36].
The leaves sampled in the calibration test plots and in the commercial crops, containing the petiole, were washed in deionized water, in a detergent solution (0.1% v/v), and then rinsed with hydrochloric acid solution (0. 3% v/v) and deionized water. Subsequently, the leaves were dried in a forced circulation oven at 60-70 °C until reaching constant mass and then ground in a mill.
The nutrient contents were analyzed to different digestion processes [37]: microwave (K), sulfuric (N) and nitroperchloric (P, S, Ca, Mg, Mn, Fe, Zn, and Cu). After digestion, the leaf samples were analyzed to determine the contents of S, Ca, Mg, Mn, Fe, Zn, Cu (ICP-OES plasma spectrometry), K (flame photometry), and P (molecular spectrophotometry). Total N was determined by Kjehdahl distillation. For each nutrient, we identified leaf samples with nutrient contents within ± 95% range of the mean in the 165 samples data set (was used data set of the soybean yield and B contents in experimental plots and commercial crops).
The harvesting of in commercial crops farms was mechanized and carried when the plants was reached full maturation. In the experimental site, it took place on 05 april 2016, obtaining the productivity (Bags ha-1) of the plots. The bag is equivalent to 60 kg of grain. We were having licenses to collect soy crop.
The intermediate functions to generate the nutritional indices for the DRIS were calculated following the formula originally [28] and by the original Beaufils formula with logarithmic transformation of the relationships between nutrients (DRIS Logarithmic ratio - DrisLog) [17].
For both formulas to calculate the nutrient balance index by the DRIS method, the F-Test criterion [15] was used to select the direct and inverse relationships between the nutrients to compose the calculation of the DRIS indices, called the “DRIS F-Test (DrisFtest)”.
For the first case, when there was no data transformation [28], the variance ratio criterion [14] was also used, here called the “DRIS Variance ratio (DrisVar)”
The use of all nutritional ratios was an alternative tested, without any selection [16], called “DRIS All ratios (DrisAll)”.
To apply the DrisFtest and DrisVar criteria, the F value was calculated by the ratio between the variance (S2) of subpopulations from low to high yield for the direct and inverse relationships between nutrients. The F tabulated was obtained based on the degrees of freedom by the number of crops in the low- and high-yield population minus one, at a significance level of 5%. Samples with yield above the mean +0.25 of standard deviation were considered high-yielding populations.
We used the samples that contained the 11 nutrients to obtain nutritional standards by the CND method (norms), determining the means of the multivariate relationships of high yield [18]. After obtaining the CND norms, the nutrient balance index was obtained [10].
The criterion of the nutrient responsiveness method (NR) was used to interpret the nutrient balance index obtained by the DRIS and CND methods through the Average Nutrient Balance Index (NBIa) (IBNm) [19], which consists of grouping the boron balance index (B-index) into two classes: insufficient when the B-index was negative and, in module, lower than the f x IBNm (nutrient responsiveness range – NRr). In all other cases, the B-index was considered balanced. The values of 0.25, 0.50, and 1.00 for f were used.
We carried out the nutritional diagnosis of the experimental plots using the DRIS and CND methods, classifying the plots as B deficient or B sufficient. The accuracy of correctness of these diagnoses was performed by the APD procedure [26] by comparing the diagnosis given by the diagnosis method (DRIS and CND) with soybean true nutritional status (TNS).
The TNS is obtained by analyzing the soybean yield response in each experimental plot as a function of B application. Comparing a plot with B application with another without B application or with B applied at smaller doses. When the Yield Response (YR) was of at least 1%, 5%, or 10% in soybean yield showed that the plot was truly B deficient/insufficient. In all other cases, the plot was considered to be truly adequate/balanced.
The deficiency diagnosis of the methods (DRIS and CND) was considered true (TDef) when there was a yield increase with the nutrient addition and false (FDef) when there was no yield increase. The sufficiency diagnosis of the methods (DRIS and CND) was considered true (TSuf) if B application did not increase yield and false (FSuf) if there was a yield increase (Table 4) [26].
The quality of predictions was then quantified by different indicators, namely accuracy (Acc), net yield response (Net d(Y)) [26], deficiency ratio (DR), accuracy for deficiency (AccDef), and accuracy for sufficiency (AccSuf) (BEVERLY, 1993). A new calculation was introduced for the accuracy test, which is the ratio of the sufficiency radius (SR). The diagnostic quality indicators were created according to the number of counts of indicated categories [22, 26].
The Acc is given by the percentage sum the of true diagnosis cases and obtained by the “equation (1)”, where n is the total number of comparisons performed.
Acc = 100* (TDef/n + TSuf/n) (1)
AccDef and AccSuf correspond to the percentage of correct answers in relation to the total of deficiency or sufficiency diagnoses, respectively, and were obtained by the “equation (2) and (3)”, where VDef and ∑Suf are the sum of cases of deficiency and sufficiency, respectively.
AccDef = 100 x TDef / ∑Def (2)
AccSuf = 100 x TSuf/ ∑Suf (3)
The DR was calculated by the ratio of true deficiency for false deficiency diagnoses and the SR was calculated by the ratio of true sufficiency for false sufficiency diagnoses, they was obtained by the “equation (4) and (5)”, respectively.
RD=%TDef/%FDef (4)
SR=%TSuf/%FSuf (5)
The Net d(Y) was obtained by the “equation (6)”. In this equation, the mathematical operation consists of adding or subtracting the productivity module of each plot, depending on whether the diagnosis is considered true (|P_VDEF| or |P_VSUF|) or false (|P_FDEF| or |P_FSUF|), for each state nutritional deficiency/insufficiency or sufficiency/balanced.
Net d(Y) = |P_VDEF| + |P_VSUF| - |P_FDEF| - |P_FSUF| (6)
The Parity Grade (PG) was also measured between the different diagnoses of the nutritional status of B provided by each of the diagnostic methods used. Between these methods, the true nutritional status was determined experimentally. The PG was calculated by the frequency of cases with the same diagnoses (in agreement between the methods) in relation to the total of diagnoses compared with each other.