Sulfur-supplying capacity of soils and critical sulfur values of forage crops

Abstract

Greenhouse experiments and field surveys were conducted to evaluate the sulfur (S)-supplying capacity of soils in the Chugoku area as well as to identify appropriate diagnostic criteria by which the S status of plants can simply be assessed, and to determine the critical S value in several forage crops. The results obtained were as follows.

1.	The S-supplying capacity of the regosols and andosols was evaluated based on the S uptake of african millet, and on the differential amount of total-S of soils before and after cropping. These values were estimated at 5 mg S/kg of air dried soil for the regosols and 58 mg S/kg for the andosols. The S-supplying capacity of soils could not be estimated only by the content of available-S extracted with 0.01 M Ca(H2PO4)2 and it required the evaluation of the mineralization of soil organic matter.
2.	Several grassland soils contained low amount of available-S and organic matter. In these grasslands, S deficiency of forage crops may easily occur if the soils fail to receive S or if low amount of S are applied.
3.	The concentration of SO4-S in leaves was found to be a more suitable index of S status of forage crops compared with the SO4-S/total-S value or N/S ratio.
4.	The critical values of SO4-S in leaves to achieve 60% of maximum yield of forage were 60 to 160 ppm S, being the lowest for siratro, sorghum, african millet, and perennial ryegrass, followed by 230 to 260 ppm S, for white clover and desmodium. The values of S amounting to 450 to 880 ppm were higher for orchard grass, red clover, italian ryegrass, and tall fescue, while, the highest value of 1,400 ppm was recorded in alfalfa

Key Words:

• sulfur deficiency
• sulfur (S)-supplying capacity
• diagnostic criteria
• critical sulfur (S)-value.