Copper, manganese, and zinc fertilization effects on growth of soybean on a calcareous soil

High pH may limit micronutrient availability for soybean (Glycine max L. Merr.) on Blackland soils found in northeast Texas. However, it is not clear whether preplant additions of micronutrients will enhance soybean growth or yield on this soil. In three separate greenhouse studies, a pH 8.3 Houston Black clay (fine, smectitic, thermic Udic Haplusterts), with. DTPA-extractable concentrations of 0.74 mg copper (Cu) kg(-1), 3.74 mg manganese (Mn) kg(-1), and 0.47 mg zinc (Zn) kg(-1), was treated with CuSO4 (0, 25, 50, 75, and 100 ppm Cu), MnSO4 (0, 10, 20, 30, and 40 ppm Mn), or ZnSO4 (0, 2, 4, 6, and 8 ppm Zn). Pot size was 19 L and soil dry mass was 10 kg. Soybean (cv. Hutcheson) seed were planted in either February or April 2000 and seedlings were thinned to three per pot at the first true leaf stage. One leaf from each plant was harvested at growth stage R3 for nutrient analysis. Between 20 and 110 days after planting, five to six nondestructive leaf chlorophyll readings were obtained. Plants were destructively harvested at R6 (mid podfill) for nutrient and yield determination. No visible symptoms appeared through early reproductive growth, but by growth stage R5 (beginning seed), leaves from the 0 ppm Cu treatment were chlorotic. Chlorophyll (SPAD 502) values of the 3rd uppermost leaf during the R3 to R5 growth stages were greater when Cu, Mn, and Zn were added to the soil than in the 0 ppm treatment. Fruit yield (seed plus pod walls) at growth stage R6 was greater in pots receiving 25 to 100 ppm Cu, 20 to 40 ppm Mn, and 4 to 8 ppm Zn than in the check treatment. Total biomass was increased by 25 to 100 ppm Cu, but there was only a trend for Mn or Zn to increase biomass. Because fruit yield increases were observed as a result of an addition of the micronutrients applied individually, a combination of micronutrients may increase yield further. There was no evidence that the higher rates of Cu, Mn, or Zn caused reduced growth. In summary, these greenhouse results indicated that field studies testing the effects of Cu, Mn, and Zn on soybean yield on this calcareous soil are warranted.