Protein quality of soybean [Glycine max (L.) Merr.] seed could be enha
nced by increasing the concentration of S-containing amino acids. The
N:S ratio of soybean seed tissue is an indicator of protein quality. T
he objective of this study was to compare N and S accumulation and dis
tribution in the soybean plant under varying levels of S availability.
Soil S levels were varied in two greenhouse trials and tissue S and N
contents were monitored. A field experiment with 0 and 60 kg ha(-1) a
pplied S was also conducted. In the two greenhouse trials, seed yield,
rate of dry matter increase, and rate of N accrual by shoots all incr
eased at least five-fold as S availability increased from less than 15
(zero added S) to 60 mg available S per plant, but showed little resp
onse to higher levels of S. Rate of S accrual was strongly related to
S availability. In the field trial, there were no responses to added S
. Leaf, pod, and seed tissue appeared to accumulate S in proteins, whe
reas, root and stem tissue appeared to accumulate S as sulfate. Harves
t index values for S were consistently less than those for N, indicati
ng S is not remobilized to seed as efficiently as is N. Seed S:N ratio
was linearly related to the rate of S versus N accrual on a whole-pla
nt basis. We estimate that a 50% increase in the amount of S-containin
g amino acids in soybean seed may require a 65 to 80% increase in S ac
crual by shoots to satisfy the increased demand for S.