Analysis of Organically Farmed Soils Using Ion Exchange Membranes and Chemical Extractions.

Predictions of soil nutrient availability and fertilizer need in both conventional and organic cropping systems are mostly based on interpretation of soil analyses by chemical extraction (CE).  However, soil nutrient pools and fertilizers in organic systems rely more on organic matter and biological processes, such as mineralization, than in conventional systems. Increasingly ion exchange membrane (IEM) technology is being used to measure nutrient availability. But estimates of nutrient availability and fertilizer need based on IEM and CE have rarely been compared, particularly for organically managed soils.

Using soil samples from five organic farms, we examined the variability and correlation between nutrient availability estimates and fertilizer recommendations for NH₄-N, NO₃-N, P, K, Mg, Ca, S, Fe, Mn, Cu, Zn, and B using CE and IEM. Four subsamples were created from homogenized composited sample from representative area in each field. The subsamples were thereafter spited into two equal parts, one for analyses using CE and the other for IEM.  We also compared 24 hr and 7 d laboratory incubation periods with IEM.

Nutrient analyses using IEM were more variable than CE for most elements. However, the greater variability in nutrient availability measured by IEM did not result in greater variability in fertilizer recommendations for N, P, and S based on IEM model used. We also found significant differences between 24h and 7 d laboratory incubation periods in the use of IEM for those nutrients that move to the root mostly by mass flow, but no significant differences were found for those nutrients whose major flow-path is diffusion.

The study also established that incubation period could be a significant factor in the usage of IEM for soil testing. The use of IEM could be a very good tool in the assessment of soil nutrient under organic systems, however further study will be required in correlating the CE and IEM results to nutrient uptake and yield response, and calibrating those responses to fertilizer recommendations for organically managed soils. This study demonstrates the need for further research in using the two methods for assessing both soil N and P in organic farming systems.