Hernán Sainz Rozas

DIAGNOSING SULFUR DEFICIENCY IN SPRING RED WHEAT: PLANT ANALYSIS

Sulfur (S) availability indicators are necessary for rational fertilizer use. The goals were to assess the predictive capacity of: i) malate:sulfate ratio in leaf; ii) total nitrogen (N):S ratio in aerial biomass; and iii) total N:S ratio in grain. Six experiments were carried out in Argentina for two years. Between 90 and 100% of samples were correctly diagnosed by total N:S ratio during tillering, and critical N:S ratios varied from 14.8:1 to 16:1. At the same time, malate:sulfate ratio diagnosed correctly between the 35 and 65% of the samples. Grains with S deficiency were determined as those with a total S concentration lower than 0.15% and a total N:S ratio higher than 13.3:1. Validation of these new thresholds allowed determining that 77% of the samples were correctly diagnosed. A linear association between grain N:S ratio and N:S in aerial biomass during stem elongation was found (r2 = 0.76–0.78, respectively).

Usefulness of Foliar Nitrogen-Sulfur Ratio in Spring Red Wheat

ABSTRACT Sulfur (S) availability indicators are necessary for a rational use of fertilizers. Experiments were carried out to evaluate the sensitivity and stability of nitrogen (N):S ratio in spring red wheat. Yield responses were determined in three replications over five experimental sites. Sulfur application did not increase N concentration in plant, whereas the effect on total S concentration was varied. Results showed 25, 35, 80, and 85% correctly diagnosed samples at stages Z22, Z24, Z31, and Z39, respectively. In treatments without S limitations, a decrease in N:S ratio was observed as the crop cycle advanced. Nitrogen: S ratio differed according to the site; however, these variations tend to decrease as the crop cycle advance. In summary, for spring red wheat, N:S ratio in plant is a useful method for S deficiencies diagnostic from the end of tillering to flag leaf.

STABILITY OF FOLIAR NITROGEN:SULFUR RATIO IN SPRING RED WHEAT AND SULFUR DILUTION CURVE

A key aspect of any sulfur (S) availability indicator is its stability throughout the crop cycle. Nine experiments were conducted with the objective to evaluate the stability of the nitrogen (N):S ratio in aerial biomass (W) in spring wheat, and to define the critical curve of S dilution. As the crop cycle progressed, a decrease (P < 0.05) in total N:S was determined. This lack of stability was explained by a lower S dilution in relation to N (P < 0.05). A decrease (P < 0.05) in N accumulation rate in relation to S was obtained as the crop cycle progressed, which points out that S accumulation in relation to N is later. A first approach was determined to the definition of the S dilution critical curve (Sc) from tillering beginning to stem elongation end (Sc = 0.37 W−0.169; r2 = 0.71 and n = 24).

Long-Term Sulfur Fertilization: Effects on Crops and Residual Effects in a No-Till System of Argentinean Pampas

A long-term experiment has been conducted between 2001 and 2008 at Balcarce, Argentina, to determine the effect of sulfur (S) fertilization on S concentration in grains, crop yield, and residual S in soil. Two treatments were evaluated: annual S application to crops (15 kg ha−1; S1) and a control with no S fertilization (S0). Sulfur fertilization only increased wheat yield (22% of the crops in the experiment). However, S application increased S concentration in grains in wheat, soybean, and maize (56% of the crops). Although, for all years, the S mass balance was positive for S1 and negative for S0, no differences in soil S extracted as sulfate (S-SO4 −2) content previous to the crop sown were determined. The absence of differences in S accumulation in aboveground vegetative biomass and grain of the maize used as a check also suggest that long-term S fertilization did not affect the soil S availability for crops.

Comparing Nitrate-N Losses through Leaching by Field Measurements and Nitrogen Balance Estimations

Nitrogen (N) balance method is a valuable tool for estimating N losses. However, this technique could lead to incorrect estimates of the amount of nitrate (NO3−N) leaching if processes relevant to N losses are not considered properly. The aim of this study was to compare NO3−-N leaching losses estimated using an N balance (nonrecovered N, Nne) with data of NO3−-N leaching losses (Nl). The experiment was made on a Typic Argiudoll soil planted with corn (five growing seasons) under 0, 100, and 200 kg N ha−1. The ceramic soil-water suction samplers were installed (1 m deep). Drainage was estimated by the LEACH-W model. The greatest overestimation with the N balance method occurred for years with annual rainfall below the historical average and at times of high NO3−-N availability. Future research should focus on investigating mechanisms of N losses relevant under limited water availability.

Determination of Sulfate Concentration in Soil: Depth of Sampling

To diagnose sulfur (S) deficiency, methodologies based on soil sample analysis and simulation models have been used, all of which require determination of S availability. The objective was to evaluate the possibility of estimating sulfate concentration up to 60 cm deep, using values from soils 0–20 or 0–40 cm deep as predictive variables. A set of data from 22 fertilization experiments on winter and summer crops was used. For wheat, a close association (P < 0.0001) was determined between sulfate concentrations 0–20 and 0–60 cm deep (r2 = 0.88). When the sulfate concentration at 0–40 cm deep was used as predictive variable, the model r2 increased to 0.97. Similar results were obtained for summer crops (r2 = 0.68 and 0.94 for 0–20 and 0–40, respectively). The integration of all 22 experiments showed two linear models (P < 0.0001) to estimate sulfate concentration up to 60 cm deep (r2 = 0.79 and 0.95 for 0–20 and 0–40, respectively). These results help to simplify soil sampling and would enable more use of S diagnostic methods by producers.