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Sulfur and Nitrogen Composition of Fertilized and Unfertilized Alfalfa Grown on a Sulfur-Deficient Soil¹

ABSTRACT

A study was made of the effects of three levels of S fertilization (0, 200, 400 lbs. gypsum per acre) on several forms of S and N present in alfalfa grown on Delhi sand in the San Joaquin Valley of California. Previous studies with alfalfa on this soil have shown that the annual yield could be approximately doubled by an application of 200 lbs. gypsum per acre and that phosphate fertilizer had no significant effects on yield or P content.

Yield data were obtained at each of five successive cuttings of alfalfa. Forage samples were analyzed for total N, amide-N, free-amino-N, total S, SO₄-S, and methionine.

At all harvests, yields were increased by S; slightly higher yields were produced by 400 than by 200 lbs. gypsum per acre, the difference being significant for only the first two cuttings. For the first three harvests, the percentage of total S in the plant was approximately 3 times as great in alfalfa grown with 400 lbs. gypsum as in that grown without fertilization; the differences were progressively smaller at the fourth and fifth harvests. Most of the differences in S content could be accounted for as SO₄-S; however, a consistent difference in reduced-S was also noted, with maximum variation occurring at the third harvest, when the reduced-S contents were 0.11 and 0.17%, respectively, for alfalfa from plots receiving 0 and 400 lbs. gypsum per acre. This difference in reduced-S could not be accounted for as methionine; actually the percentage of the reduced-S present in this form was higher in the unfertilized alfalfa.

Marked differences in N content were found only at the first harvest, when crude protein values were 17 and 24%, respectively, for alfalfa grown with 0 and 400 lbs. gypsum per acre. Contents of amide-N, free-amino-N, and NO₃-N were interpreted as indication that the lower crude protein content of the unfertilized alfalfa was not caused by lack of S in the plants.

The conclusions were drawn that (1) the alfalfa was not enriched in methionine by fertilization with S, and (2) the synthesis of some form of reduced-S was impeded in the unfertilized alfalfa by lack of sufficient available S.

¹ Contribution from the College of Agriculture, University of California, Davis, Calif. A progress report of part of this material was presented to the Western Soil Science Society at the 1954 meetings in Pullman, Wash.

² Assistant Soil Chemist, California Agr. Exp. Sta.

Received for publication March 7, 1955.