Foliar fertilization (FF) was reported to increase seed yields of soybean [Glycine max (L.) Merr.] in field experiments in Iowa, but yield increases were not consistent. The researchers hypothesized that FF should minimize nutrient depletion from leaves during seed development, and thereby delay the resulting decrease in leaf photosynthesis. To test this hypothesis, we conducted a field experiment in 1976 to determine the effect of FF on leaf element concentrations, gross leaf photosynthesis (Pg), and soybean seed yield. Five weekly foliar sprays were applied during the seed‐filling period of ‘Bragg’ soybean grown at Gainesville, Florida on Kendrick loamy sand (loamy, siliceous, hyperthermic grossarenic paleudult). At weekly intervals, we sampled upper leaves and total canopy leaves and analyzed them for N, P, and K, and measured Pg of upper leaves in mid‐day sun with a 14CO2 gas flow technique. Foliar applications of N, P, K, and S increased the N, P, and K concentration of total canopy leaves from 3.28, 0.24, and 0.92% to 3.48, 0.29, and 1.32%, respectively. FF significantly increased upper leaf Pg only at two late sample dates when seed growth was nearly complete and most leaves had already senesced and dropped. Even though nutrient concentrations were increased, FF did not significantly affect yields nor did it extend Pg duration or delay maturity. Treated soybeans yielded 3617 kg/ha compared to 3825 kg/ha for control soybeans. Leaf Pg and concentrations of N and P in leaves progressively decreased during seed‐filling until maturity but K did not decline. Leaf Pg was positively correlated with N (r = 0.87) and approached zero at approximately 1.75% N, a concentration similar to that of senesced, recently abscised leaves. Maximum Pg was predicted at 4.6 to 6.0% leaf N. Leaf Pg and percent P were also positively correlated. The relationship of Pg to leaf N during N removal from leaves can potentially be used to model photosynthetic decline during seed‐filling.

Evidence of differential sensitivity to drought between symbiotic N fixation and leaf photosynthesis in legumes has been measured under greenhouse conditions. If the greater sensitivity of N accumulation to drought is substantiated under field conditions, then important questions concerning legume production under water‐limited conditions must be considered. Consequently, the relative sensitivity of N and biomass accumulation to drought was examined in a 2‐yr study on ‘Biloxi’ soybean [Glycine max (L.) Merr.] grown in the field on Arredondo fine sand soil (loamy, siliceous, hyperthermic Grossarenic Paleudults). Various irrigation treatments were imposed during vegetative growth after canopy closure when solar radiation was almost fully intercepted. Shoot and root plus nodule biomass and N accumulation were determined from periodic harvests. Solar radiation use‐efficiencies were computed from regressions of biomass against cumulative solar radiation, and were found to range from 0.53 g MJ−1 under daily irrigation, to 0.23 g MJ−1 under the most severe drought treatment. Nitrogen accumulation rates were computed by regression of N accumulation against time, and were found to decline from 0.31 g N m−2 day−1 for the daily irrigation to 0.003 g N m−2 day−1 for the most severe drought treatment. In both years and all three drought treatments, the N accumulation rate was decreased relatively more than the solar radiation use‐efficiency for biomass accumulation. These results indicate that a high sensitivity of N accumulation to soil dehydration may be an important constraint on soybean productivity.

Mycorrhizae improve plant nutrient uptake and are known to affect the water relations of plants grown in growth chambers and greenhouses. This paper summarizes a 3‐yr field study that tested the effects of mycorrhizae and water management on the growth and grain yield of maize (Zea mays L.). In each year, two inoculation treatments (inoculated or not with Glomus etunicatum Becker and Gerdemann) and three water‐management treatments (fully irrigated, moderate stress, and severe stress) were applied to fumigated and fertilized Millhopper fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult). Inoculum was placed in a furrow 10 cm deep at an average rate of 1500 propagules per meter of row. Six to 7 wk after planting, colonization ranged from 0 to 6% of total root length on noninoculated plants and from 10 to 61% on inoculated plants. Twelve to 13 wk after planting, colonization ranged from 2 to 30% on noninoculated plants and from 21 to 56% on inoculated plants. Water stress had little effect on root colonization. By 52 d after planting, one more leaf had appeared and one additional leaf had formed a collar on inoculated plants. Inoculation increased the concentrations of P and Cu in both shoots and grain on all measurement dates. Overall, grain yields (0.306) and total above‐ground biomass yields (0.458 Mg ha‐1 cm‐1 of water) increased linearly with irrigation. A positive response to mycorrhizal inoculation was constant across irrigation levels (0.802 for grain and 1.170 Mg ha−1 for biomass). Therefore, the proportional response of maize to inoculation with G. etunicatum increased with increasing drought stress.