Soybean (Glycine max (L) Memll) growers in Brazil commonly band corrective and maintenance fertilizer near the seed at planting time. Little information existed to substantiate this as the most efficient P placement method. Because P is an immobile element in the soil and a costly import for Brazilian farmers, placement may be critical
This research was undertaken to evaluate the influence of P broadcast and incorporated vs. banded near the seed upon yield and leaf composition of field-grown ‘Bragg’ soybean. The study was conducted on Sao Jeronomo silt loam (Paleudult), low in available P, at 30" S lat about 70 km west of Porto Alegre, Brazil. Experimental design the initial year was a 4 ✕ 4 factorial, three replications, randomized block with P-rates of 0, 35, 70, and 140 Kg P/ha broadcast and/or banded as triple super hosphate. This established with the P broadcast and P incorporation, an available soil-P gadient to determine at what levels banded-P might most influence yield. Second year residual P responses was measured after thoroughly mixing the soil and measuring available P with Bray P2 (0.03N NH1F + 0.1N HCI) and Melhich (0.025N H2O4 + 0.05N HCl) extractors. Plant measurements include yield, lodging index and leaf N, P, and K composition. Statistical relationship between relative yield and Bray P2 available soil-P were evaluated by linear, quatratic, second degree polynomial, logarithmic, and segmented lines methods. Correlation coefficients between P treatments, soil available P, leaf N, P, and K and lodging index provided additional interpretations. DRIS interpretation norms of leaf N, P, and K composition was used to evaluate the nutrient balance.
The 1st year (1974) broadcast-P was significantly correlated at the 1% level to eld (r = 0.41) and leaf-P (r = 0.41) and at the 5% level to lodging index (r = 0.29). Banded-P was significantly correlated at the 1% level to leaf-P (r = 0.61), leaf-K (r = 0.53), and lodging (r = 0.63), but yield, though increased, was not significant at the 5% level (seed yield ranged from zero P of 2,775 to 4,835 kg/ha for 140 kg P/ha broadcast compared to 3,936 k e a for this quantity banded). The maximum lodging index for broadcast P only was 2.7 (about 17% lodged) while banded-P only increased lodging index to 4.3 (about 46% lodging). This suggested an effect of high P levels upon increased lodging that was more pronounced by banding the P resulting in lower yields. Mixing the soil before the 2nd year incorporated the previously banded-P so that the residual P responses were quite similar to the broadcasting of P. Leaf-P was correlated significantly at the 1% level from both broadcast- P (r = 0.49) and previously banded-P (r = 0.57) and yields correlated at the 5% level (r = 0.31 for broadcast-P and r = 0.36 for previously banded-P). The seed yield ranged from zero P of 1,945 to 3,150 kg/ha for 140 kg P/ha compared to 3,338 k o a to this quantity previously banded.
The Bray P2 extractor correlation was higher with broadcast-P and leaf-P than with Melhich extractor and, therefore, was used to interpret relationship between relative yield and available soil-P. The segmented line model attributed 32% of relative yield to Bray P2 available soil-P, which was superior to the linear, quatratic, second degree polynomial, and logarithmic. This model indicated the maximum (93.2%, relative yield) was obtained at 8.83 ppm-P, with no increase in relative yield with increasing levels of Bray P2 available-P.
DRIS evaluation of leaf N, P, and K composition suggest that for zero P treatments the order of nutrient needs were P > N > K for both years of the study. In the 1st year, the addition of P changed the order of need from N > P > K. The 2nd year, the order of nutrient need was P > N > K, regardless of soilavailable P level, and even when leaf-P levels would not be classified as low or deficient. Leaf-N and -P varied more during the 2 years of the study than did leaf-K. High leaf-K levels suggest this nutrient was high with respect to N and P.
Results from this study suggest that banding P near the seed increased P uptake vs. broadcast and incorporation of P, but did not provide highest yields. The plantsoil growth factors obtained from broadcasting P resulted in highest return from P inputs.