Use of Ultraviolet-Visible Spectroscopy to Provide Ion-specific Nutrient Control of N, P, K, Ca in Hydroponics Solution

Abstract

Hydroponics is the heart of most systems for controlled environment agriculture (CEA) providing several simple implementations to offer direct control over the moisture and nutrients provided to the crops. The more accurately growers can control the solution being provided, the longer they can reuse the same solution and the more consistent the crop quality. The goal would be direct control over all fourteen to eighteen nutrients that plants need to grow, but the primary nutrients – nitrogen, phosphorous, and potassium (N, P, K) – are often considered to be the most critical. While ultraviolet-visible (UV-Vis) spectroscopy has been proposed for this purpose for years, and been demonstrated to be usable to get within ten percent of the true value for N and K, this paper lays out a succinct and practical approach to calibration and measurement that can be used to implement a near real-time control system that can be used to control N, P, K and calcium (Ca). The intent being to speed adoption of such a technology by making it cheap and easy to implement. Special attention is paid to laying out what information can in theory be reused and what a grower would need to maintain calibration on site. This is presented as a six-step process for deployment based on lessons learned from these experiments. A notional implementation of the control system and a discussion of practical use in lunar greenhouse growing is also provided. Overall the final algorithm only uses four wavelengths of light, a small number of compounds that are already components of Hoagland and similar solutions, and is computationally light enough to be able to run comfortably on a minute-by-minute basis.