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Sulfur Fertilization Effects on the Constancy of the Protein N:S Ratio in Low and High Sulfur Accumulating Crops¹

An accurate quantitative separation of protein from non-protein in plant tissue is necessary for answering the question whether the protein N:S ratio, (N/S)p, is constant or is affected by plant part, age, or nutrient supply. Previous studies in our laboratory have shown that plant tissue extracted with hot 0.5% acetic acid to remove non-protein N (NPN) prior to Kjeldahl analysis give results very close to those for true protein N (PN). The objective of this study was to determine the effect increased S rates during growth had on the constancy of the (N/S)p ratios of low and high S-accumulating crops.

Corn (Zea mays L.), soybean (Glycine max L. Merr.), and cowpea (Vigna Unguiculata L. Walp.) were selected as low S-accumulating plants and tomato (Lycopersicon esculentum L. Mill.), cotton (Gossypium hirsutum L.), and okra (Abelmoschus esculentum L. Moench.) as high S-accumulating crops. Plants were grown hydroponically with S levels of 0, 16, and 32 ppm and constant N levels (126 ppm N) and harvested when the 0 ppm S-treated plants showed moderate S deficiency. Samples were analyzed for protein S (PS), non-protein S (NPS), total S (TS), PN, NPN, and total N (TN).

Increased S levels increased the dry weights of corn, soybean, cowpea, and tomato tops and soybean roots. Protein S had a greater demand for limited S than NPS. Non-protein S began to accumulate only after PS demands were satisfied. Low S-accumulating crops had higher and more constant (N/S)p ratios than high S-accumulating crops. High S-accumulating crops showed a slight but consistent decrease in (N/S)p ratios due to proportionately higher PS levels when S supply was increased. When S supply was sufficient, the (N/S)p ratios were 15 to 16 for corn, 20 for soybeans, 15 for cowpeas, 12 for tomatoes, and 8 to 9 for cotton and okra. The accuracy of the (N/S)p ratio is contingent on the method of fractionating protein from the non-protein fraction.

Key Words: N Fractions • S Fractions • Protein precipitants • Non-protein extractants • Zea mays L. • Glycine max L. Merr. • Vigna unguiculata L. Walp. • Lycopersicon esculentum L. Mill. • Gossypium hirsutum L. • Abelmoschus esculentum L. Moench

² Chemist, Agronomy Dep. and associate professor, Horticulture Dep., respectively, Coastal Plain Stn., Tifton, GA 31793.

Received for publication August 31, 1981.