Subsurface drip irrigation (SDI) and GYP-FLO in processing tomato production

A replicated trial was set up to study the physico-chemical changes following 4 years of subsurface drip irrigation (SDI) in a commercial processing tomato field near Shepparton, Victoria, Australia. The study also compared the effect of 0 and 150 L ha-1 of the product GYP-FLO injected through the established 4-year-old SDI lines. Soil samples were collected from five different locations around the dripper emitters to assess the cumulative effects of this irrigation method and also the application of GYP-FLO on some soil physico-chemical properties. Water-stable aggregate (WSA), Emerson dispersion, particle size distribution (PSD), electrical conductivity (EC), soil pH, exchangeable cations (Ca²⁺, Mg²⁺, K⁺ and Na⁺) were measured and exchangeable sodium percentage (ESP), Ca:Mg ratio and effective cation exchange capacity (ECEC) were then calculated from these primary data. The results showed that, after 4 years of SDI, there were significant increases in EC (P<0.01), soil pH (P<0.01), exchangeable sodium (P<0.01) and ESP (<0.05) with distance from the subsurface emitters. These results suggest salts (EC) and some exchangeable base cations (Na+) are washed outward and away from the emitters by the irrigation water. Exchangeable sodium increased to more than double on the extremity of the beds. PSD of samples from under the emitters had a higher proportion of medium sand (200-600 microns), suggesting that 'fines', mostly silt and clay, might also be migrating outward from the emitters. The findings indicate that measurable physico-chemical changes do occur in soils under established SDI systems in relatively short timeframes (4 years). An application of the commercial product GYP-FLO at 150 L ha-1 had no significant effect on WSA or the Emerson dispersion values, but there was a significant decrease in exchangeable magnesium levels, leading to a significantly higher Ca:Mg ratio and lower ECEC. No significant differences were observed in crop yield or quality as a result of the applied GYP-FLO product. GYP-FLO was described on the label as a form of 'liquid gypsum', although our laboratory analysis showed that it was a mixture of finely ground calcium carbonate (lime) and elemental sulfur (S). We measured its sulfur content to be 5-12% lower than the label stated. We therefore recommend regular independent testing of such products to ensure that they meet their stated label formulations and elemental concentrations. This is important, as longer-term SDI applications may cause changes in soil physico-chemistry greater than measured here, and irrigation-applied soil conditioners may be required in order to maintain soil health.