Autores: Flavio H. Gutierrez Boem,¹ Pablo Prystupa,¹ and Gustavo Ferraris^2 
1Cátedra de Fertilidad y Fertilizantes, Facultad de Agronomía, Universidad de Buenos Aires, Argentina 
²Instituto Nacional de Tecnología Agropecuaria, Estación Experimental Agropecuaria Pergamino, Argentina
 Año 2007
 

Abstract

In soybean, seed number and yield is largely determined by the environmental conditions between initial bloom and the beginning of seed filling. Four field experiments were conducted to determine the effects of two sources of sulfur (S) on crop growth during the critical period of seed number determination and yield. Ammonium sulfate and gypsum were applied at a rate of 15 kg S ha-1. Seed yield was closely related to the number of seeds per m2 (R2 = 0.93), but seed number was not related to crop growth between bloom and the beginning of seed filling. There was no effect of fertilization on aboveground biomass accumulation until the seed filling period began. The results from this study suggest that a moderate S deficiency reduced seed yield by affecting crop growth during the seed filling period. This late effect of a moderate S deficiency could be a consequence of the known high sulfate mobility in soils and low S remobilization in plants.

Keywords: soybean, sulfur deficiency, seed yield, sulfur source

Introduction

In soybean, as in most other grain crops, yield is strongly related to the number of seeds or grains per unit land area (Egli, 1998). Environmentally induced reduction of yield is usually associated with variations in seed number, while it is rarely related to variations in seed size (Egli, 1998). There is a critical period during crop development when stress effects on seed number are the highest. Critical periods for seed number determination have been defined in different crops like wheat (Fischer, 1985), maize (Tollenar, 1977), and sunflower (Cantagallo et al., 1997). In soybean, seed number is largely determined by the environmental conditions between initial bloom (R1; Fehr and Caviness, 1977) and the beginning of seed filling (R5) (Egli, 1998). During this period, flowering, pollination, and fertilization occur, pods are produced, and seed development begins. It was observed that seed number is related to crop growth rate during this period in experiments where incident radiation (Jiang and Egli, 1995), sowing date (Egli and Bruening, 2000), and distance between rows (Board et al., 1999) were manipulated. This relationship was observed in soybean crops where growth was not limited by nutrient availability. The working hypothesis for this study was that the effect of sulfur (S) deficiency on soybean seed yield is primarily mediated by a reduction in crop growth during this critical period and, consequently, seed number.

Sulfur deficiency is a frequent constraint on grain crop production in several agricultural regions of the world (Schnug and Haneklaus, 1998). In the Argentinean Pampas, S deficiency has been reported in the last decade (Martinez and Cordone, 1998) and fertilizer consumption has increased sharply since 1999 (FAO, 2004). The two sources of S most widely used in Argentina are gypsum and ammonium sulfate (AS). They have some differences that could affect their agronomic effectiveness as a source of S for a soybean crop.

Water solubility of AS is higher than that of gypsum; at 20.C, water solubility of AS is 754 g l-1 and is only 2 g l-1 in gypsum (Merck, 2005). Even though the agronomic effectiveness of fertilizers is related to their solubility, all sulfate fertilizers are assumed to have similar agronomic effects (Pedersen et al., 1998). Similar responses to S fertilization were observed when the applications of different sulfate fertilizers were compared in rapeseed (Gupta et al., 1997), wheat (Oates and Kamprath, 1985; Mitchell and Mullins, 1990), cotton (Mullins, 1998), and pastures (Murphy, 1990).

Ammonium sulfate is also a source of nitrogen (N) since it contains ammonium. A larger response to AS compared to gypsum could be a consequence of a direct effect of N fertilization. Also, according to "the law of the optimum," N supply could increase S uptake or utilization efficiency when N and S are both deficient (de Wit, 1992). In soybean crops, high soil nitrate concentrations decrease N fixation (Laysell and Moloney, 1994). As a consequence, N fertilization at seeding usually does not increase yield (Beard and Hoover, 1971; Welch et al., 1973; Bodrero et al., 1984; Bharati et al., 1986; Touchon and Rickerl, 1986). However, in some agricultural regions, low rates of N at seeding have increased soybean yield and, even, nodulation (Ying et al., 1992; Rani and Kodandaramaiah, 1997; Starling et al., 1998; Bhangoo and Albritton, 1976). This could be a consequence of the low N fixation at the beginning of the rop cycle. During the vegetative period the main source of N is the absorption of nitrate from the soil (Zapata et al., 1987). 

The objectives of this work were to determine the effects of two sources of S on crop growth during the critical period of seed number determination, number of seeds, mean seed weight, and yield.

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