Abstract
Soil redox status is known to influence nutrient availability and grain yield in waterlogged wetland rice; nevertheless, the instruments required to measure it are beyond the financial means of most smallholder farmers in the study area. To test whether the rusting patterns on a zero valent iron rod could be used to discriminate soil redox micro-environments in smallholder fields, the authors tested the method in four Padasekharams in the Thrissur Kole Wetland Rice Ecosystem: Chettupuzha West (170 ac; 68.83 ha), Variyam Kole Padavu (150 ac; 60.70 ha), Madhukkara Thekku Kole (230 ac; 93.08 ha), and South Manalur Kole Karshaka Samithi (214 ac; 86.60 ha). Sixty-four plots, divided into four location categories - near the canal, plots in fields adjacent to a Cabomba-infested canal, interior plots, and Azolla-infested zones- were sampled, with 16 plots in each category. Kruskal-Wallis test showed that location category had a highly significant effect on rust coverage across 64 plots (H(3) = 58.94, p < 0.0001; η²H = 0.916). Near-canal zones (69.38 ± 1.59%) and Azolla-infested field zones (68.29 ± 1.82%) were statistically equivalent (Dunn’s test, ns) but were significantly different from Cabomba-adjacent zones (24.64 ± 0.095%) and interior zones (2.01 ± 0.061%) at p < 0.001. Yield data showed that near-canal zones yielded 77–234 kg ac⁻¹ more paddy than interior zones in the same fields, in accordance with the redox gradient rather than fertilizer dose. Any farmer can perform the field level diagnostic test at the time of transplanting.