Measurement variability in soybean water status and soil-nutrient extraction in a row spacing study in the U.S. southeastern coastal plain

Full-season determinate soybean [G. max (L.) Merrill] was grown in the field in a humid climate for 3 seasons (1979-1981). Variability in several methods of determining basic relationships between soil and plant water status in a range of canopy configurations and treatment effects on soil-nutrient extraction were examined. In each year, 2 cultivars, Davis (group VI) and Coker 338 (group VIII) were planted in 4 row spacings. In 1980 and 1981 the experiment was expanded and split for irrigation and row orientation (N-S or E-W). Post-harvest soil samples were collected and analyzed to determine if irrigation, row spacing, or cultivar influenced K, Ca and Mg extraction patterns. During the growing seasons, parallel leaf diffusive resistance (Rs) was poorly correlated with xylem pressure potential (.psi.x), canopy temperature (Tc), canopy minus air temperature (.DELTA.T), leaf vapor pressure deficit (LVPD), and atmospheric vapor pressure deficit (VPD) in single factor correlations. Xylem pressure potential was highly correlated with Tc, .DELTA.T, VDP and LVPD, but was poorly correlated with soil water potential. Both .psi.x and Tc were significantly affected by the imposition of shade from a 60% shading cloth within as little as 1 min of shade imposition. Irrigation raised .psi.x by only 2.2 bars over the 2-yr observation period, in spite of large differences in soil water potential when irrigation was imposed. The impact of canopy configuration was not measurable in any water relations parameter except IR-determined Tc. Correlation of Tc and .psi.x was significantly more reliable when limited to a single variety, row spacing and row orientation. Aspect of IR temperature measurement also significantly affected observed Tc. Analysis of post-harvest soil samples indicated that narrow (50 cm) row spacing in 1980 and irrigation in 1981 significantly decreased post-harvest Melich No. I extractable K, but none of the cultural practices influenced extractable Ca or Mg at P (0.05). In 1980, extractable K within soybean rows was significantly greater than between rows. Similar trends were observed for Ca and Mg in 1980 and for all 3 nutrients in 1981, but those differences were not significant at P (0.05). Overall, these measurements quantify the difficulty in relating soil and plant water status and identifying nutrient extraction patterns in sandy soils within the humid southeastern coastal plain.