EVALUATION OF PHOSPHATE ION-SELECTIVE MEMBRANES FOR REAL-TIME SOIL NUTRIENT SENSING

Authors: KIM, HAKJIN - NAT INST AG ENG, S KOREA; Hummel, John; BIRRELL, STUART - IOWA STATE UNIVERSITY; Sudduth, Kenneth - Ken

Submitted to: ASAE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 5/16/2005
Publication Date: 7/18/2005
Citation: Kim, H., Hummel, J.W., Birrell, S.J., Sudduth, K.A. 2005. Evaluation of phosphate ion-selective membranes for real-time soil nutrient sensing [CDROM]. American Society of Agricultural Engineers Annual International Meeting. Paper No. 051033.

Interpretive Summary: Conventional soil sample collection and analysis is costly and time consuming, and intensive sampling using these techniques is impractical for variable-rate fertilizer management systems. A rapid, on-the-go soil macronutrient sensing system is needed to improve the efficiency and practicality of site-specific fertilization. Our research is developing such a system, based on ion-selective membrane technology, to rapidly measure soil nutrient levels in soil extracts. In this paper, which reports on part of the overall project, we investigated the sensitivity and speed of response of a PVC-based phosphate membrane. Tests were conducted in two different solutions – a buffer solution used by previous researchers, and a solution used to extract phosphate from soil. For the buffer solution, the phosphate membrane response was comparable to that reported in the literature. In addition, the membrane responded quickly, as would be required for on-the-go sensor operation. However, when using the soil extracting solution, the membrane was almost insensitive to varying levels of phosphate, due to interfering effects of other chemicals in the solution. Modification of the extracting solution does not appear promising, because these interfering chemicals perform key functions in the extraction process. Additional tests are planned to evaluate other types of phosphate membranes, with the goal of identifying a membrane that is not affected by the chemicals present in soil extracting solutions. If successful, this research will provide technology to support variable-rate fertilizer application, so that optimum soil fertility can be provided in each subfield area, resulting in an economic benefit to the producer, and improved environmental conditions for all consumers.

Technical Abstract: A real-time soil nutrient sensor would allow the efficient collection of data with a fine spatial resolution, to accurately characterize within-field variability for site-specific nutrient application. Our goal was to evaluate the applicability of a phosphate membrane to the measurement of phosphate levels in soil extractants and to determine how previously developed nitrate and potassium membranes would be affected by the presence of phosphate. A type of PVC-based phosphate membrane containing an organotin compound, bis(p-chlorobenzyl)tin dichloride, was evaluated, along with the nitrate and potassium membranes, in pH 7 Tris buffer solution and Kelowna soil extractant for sensitivity and long-term stability. The phosphate membranes in the Tris buffer solution of pH 7 exhibited a response over a range of 10**-5 to 10**-1 mol/L phosphate concentrations with an average slope of -28.2 +1.5 mV per activity decade of dibasic phosphate. The response speed of tested electrodes containing phosphate, nitrate and potassium membranes was rapid, reaching an equilibrium response in less than 15 s. However, the phosphate membrane in the Kelowna solution of pH 8.5 was almost insensitive to different phosphate levels from 10**-6 to 10**-2 mol/L due to the presence of a high concentration of fluoride in the solution. In addition, the tin compound-based phosphate membranes had limited lifetimes of less than 14 days. It is not expected that the tested phosphate membranes could be used for phosphate detection in other soil extractants, such as Bray P1 and Mehlich III solutions, because they also contain high concentrations of fluoride.