Effect of surface residue and potassium sources on potassium availability in two contrasting soils in South Africa

摘要

Condensed molasses stillage (CMS) is an inexpensive source of potassium (K) that is recycled back to sugarcane fields. Little information on CMS use as a fertiliser is available in the literature. The objective of this paper was to compare CMS as a source of K with conventional inorganic fertiliser. Consideration is also given to the efficacy of CMS when applied on residues left in the field after harvest. Trials were established on two contrasting soils, a pedorhodic (pH CaCl2 4.26, clay 46.6% and organic matter 5.4%) and glossic (pH CaCl2 4.29, clay 15.6% and organic matter 1.5%). Treatments (five replicates) were a control (no fertiliser) (T1), low inorganic K (123 kg/ha K as KCl) and high inorganic N (160 kg/ha N as LAN) (T2), low organic K (123 kg/ha K as CMS) and high organic N (160 kg/ha N as CMS fortified with urea) (T3), high inorganic K (180 kg/ha K as KCl) and low inorganic N (123 kg/ha N as LAN) (T4), high organic K (180 kg/ha K as CMS) and low organic N (123 kg/ha N as CMS) (T5). Additional treatments were burning with tops spread or green-cane harvesting (mulched). At both trial sites, K in the 0-2.5 cm soil layer was significantly lower in treatments with full residue compared to burnt at harvest with tops spread. Comparing CMS versus KCI sources of K, no significant differences were found. Leaf K was also not affected by the source of K; however, leaf K was consistently higher with full residue cover. No yield differences were observed amongst treatments at both trial sites. Only at the glossic site was the yield of the control treatment significantly lower than that of other treatments. This indicates that CMS is as effective as a K source as other inorganic K fertilisers. The data show that application of CMS on top of a mulched layer does not impede K uptake by the crop. Furthermore, the soil and leaf data provide clear evidence of larger K uptake under green-cane harvesting (mulched) relative to burning. It would thus appear that root activity was greater near the surface for the GCH treatment, probably due to more favourable moisture conditions.