Die invloed van rywydte en saadbehandelings op sojabone (Glycine max L. Merr.) in verskillende omgewings

Abstract

The increase of soybean (Glycine max L. Merril) production in South Africa is limited by the absence in natural occurrence of the nitrogen fixing symbiont and the lack of standardized production practices. The use of widening row spacing typical for maize production from east to west because of a systematic decrease in precipitation, has not been researched on soybeans. The objective of this study therefore was to determine the effect of row spacing (RS), nitrogen fixing bacteria and molybdenum on the growth and yield of soybeans across a range of agro climatic conditions in South Africa. Ten field experiments were planted during November and December 2000 in five provinces. The trial sites were grouped into three climate zones according to photo thermal units i.e. relative warm, moderate and cool. Determinate and indeterminate soybean cultivars of varied maturity length were used and planted in either 45cm narrow row spacing (NR) or 90cm wide row spacing (WR). Experimental plots were split for seed treatment, which comprised a contro~ Bradyrhizobium japonicum nitrogen-fixing bacteria, Azospirillum brasilense nitrogen-fixing bacteria and sodium molybdate (Na2Mo04.2H20) micro nutrient. Average seed yield for the dry land trials correlated positively with rainfall for November and February (r = 0.89 and 0.55; p = 0.05), while seed mass correlated positively with rainfall for February (r = 0.2; p = 0.05). The high correlation between seed yield and rainfall for November was probably due to the role that precipitation played in sustaining the crop during the time of drought in January. WR tested significantly (p = 0.05) higher for seed yield at the moisture stressed Bethlehem and NR did for the irrigation trial at Bloemfontein. Seed mass tested significantly (p = 0.05) higher for the WR at Bethlehem, Lichtenburg and also at Naboomspruit where supplementary irrigation was applied, but with less moisture per tonne of yield available than Bloemfontein. A significant cultivar x RS interaction for seed yield and seed mass was found at Bethal. At Bethlehem where soybeans were not cultivated previously, the control plots did not nodulate and the lack of nitrogen caused a significant (p = 0.05) decrease in plant height, seed yield and harvest index. At all the other sites where soybeans have been planted before, the control plots did nodulate. With nodule occupancy it was found that WB 74 as well as the previous used WB 1 B. japonicum strain survived in soils where soybeans were cultivated previously. Seed treatments with sodium molybdate caused a significant increase in seed mass at Bergville and a significant decrease in harvest index and seed yield at Lichtenburg. The application of sodium molybdate at Bethlehem caused a significant decrease in seed yield for the 45cm row width, while a cultivar x molybdenum interaction for seed yield indicated differences in sensitivity amongst cultivars. The differences in response to molybdate can be attributed to soil pH, which ranged from a low of3.99 KCI at Bergville to 5.2 KCI at Lichtenburg and Bethlehem. The application of A. brasilense did not affect yield significantly. The results of this study suggest that yield can be increased with wider RS at early planting dates in the lower rainfall areas, that yield can be increased with narrow RS under irrigation or high rainfall conditions, that the presence of B. japonicum is vital for successful cultivation, that A. brasilense did not affect yield and that soybean seed treated with molybdenum significantly increased yield in acid soils but caused a significant decrease in yield in higher pH soil conditions.