The response of selected legume crops to sources and rates of Sulphur

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Singabapha, Primo Jimmy
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There are concerns about increasing incidence of sulphur (S) deficiency in agricultural soils due to increased cropping intensity, development of high yielding crop varieties and the use of S-free fertilizers. Zambia recorded cases of S deficiency in the 1950' s and subsequently noticed grain yield responses by maize to S applications. Reductions in crop yields as a result of this deficiency are sometimes mistaken for nitrogen deficiency. A field study was conducted in Lusaka to determine whether cowpea, soyabean and green gram would respond to gypsum and elemental-S as sources of sulphur. Gypsum and elemental-S were each applied at 0, 15, 30 and 45 kg S ha"1 rates. The experiment was designed as a split-split plot arranged in a RCBD with four replications. The average legume grain yield with gypsum was 1410 kg ha"1 while it was 1392 kg ha"1 with elemental-S. These were not statistically different. The three legumes showed a significant variation in their grain yield. Cowpea produced 2300 kg grain ha"1, soyabean 1327 kg grain ha"1 and green gram 575 kg ha" Soyabean and cowpea both had significantly higher protein content in their grains than green gram. Grain yields did not vary significantly with the different S rates applied. The overall mean yield of the control was 2209 kg grains ha"1 while the grain yields of 15, 30 and 45 kg S ha"1 treatments were 2393 kg ha"1, 2317 kg ha"1 and 2282 kg ha"1, respectively. Protein concentrations and contents were not significantly different between the two sources of sulphur. Gypsum treatments gave 283 mg protein g"1 and elemental-S gave 274 mg protein g"1. Soyabean grains contained significantly more protein (375 mg g"1) than both cowpea (230 mg g"1) and green gram (231 mg g"1) . However, on a unit area basis, protein contents of soyabean (498 kg ha"1) and cowpea (529 kg ha"1) were not significantly different due to a relatively high yield of cowpea. Leaf analysis results showed that S uptake from gypsum was 7.1 mg S g"1 dry matter (DM) and 7.6 mg S g"1 DM from elemental-S. These were not statistically different. Sulphur uptake and assimilation was not different between the legumes, and cowpea had 7.6 mg S g"1 DM, soyabean had 6.8 mg S g"1 DM and green gram had 7.7 tng S g"1 DM. However, cowpea had significantly higher total-N (52 tng N g"1 DM) than both soyabean (39 mg N g"1 DM) and green gram (37 mg N g"1 DM) . The average organic-S : N ratio for gypsum treatments was 1:34 (0.029) whereas it was 1:36 (0.028) for elemental-S, signifying no significant difference. Cowpea had a significantly wider ratio (1:48 or 0.021) than both soyabean (1:32 or 0.031) and green gram (1:30 or 0.033) . Soyabean and green gram did not show a significant variation in this ratio. The organic-S : N ratio did not significantly vary with increasing S rates. The 0 kg S ha'1 gave a ratio of 1:37 (0.027), while 15 kg S ha"1 gave a ratio of 1:45 (0.022) and each of 30 and 45 kg S ha"1 gave a ratio of 1:30 (0.033) .
Legumes , Sulphur