<p>Routine monitoring of available soil nutrients is required to better manage agricultural land<sup>1</sup>, especially in many lower and middle income countries (LMICs). Analysis often still relies on laboratory-based equipment, meaning regular monitoring is challenging.<sup>2</sup> The limited number of in situ sensors that exist are expensive or have complex workflows, thus are not suitable in LMICs, where the need is greatest.<sup>3</sup> We aim to develop a simple-to-use, low-cost analysis system that enable farmers to directly monitor available nutrients and pH on-site, thus making informed decisions about when and where to apply fertilisers.</p><p>We combine nutrient extraction via a cafetiere-based filtration system with nutrient readout on a paper microfluidic analysis device (PAD) employing colour producing reactions that can be captured via a smartphone camera through an app. Image analysis of colour intensity permits quantitation of analytes. We initially focus on key nutrients (phosphate, nitrate) and pH analysis.</p><p>For extraction of phosphate, we mixed soil and water in the cafetiere and quantified the extracted phosphate via phosphomolybdenum blue chemistry. For example, for 5 g of soil, a water volume of about 160 mL led to optimum extraction. Active mixing, by pushing coffee filter plunger up and down, aided extraction. A mixing period of 3 min yielded maximum extraction; this time period was deemed suitable for an on-site workflow.</p><p>Following nutrient extraction, a simple-to-use readout system is required. For this, we developed colourimetric paper-based microfluidic devices; these are simply dipped into the decanted soil supernatant from the cafetiere and wick fluids based on capillary forces. Chemical reagents are pre-stored in reaction zones, created by patterning cellulose with wax barriers. Our devices contain multiple paper layers with different reagents; these are folded, laminated and holes cut for sample entry. Following the required incubation time, the developed colour is captured using a smartphone. This constitutes a portable detector, already available to envisaged end users, even in LMICs. We have previously developed an on-paper reaction for monitoring phosphates in fresh water in the mg L<sup>-1</sup> working range, with readout after an incubation period of 3 min. This method was adapted here to enable storage at ambient temperatures up to 1 week by incorporating additional acidic reagents. Further pad devices were developed in our group for colour-based readout of nitrate, involving a two-step reaction chemistry. Within a relatively short incubation period (&#8804;8 min) a pink coloured was formed following reduction of nitrate to nitrite with zinc and subsequent reaction to form an azo-dye. This system achieved detection in the low mg L<sup>-1</sup> range. Moreover, a pad to monitor pH was developed, employing chlorophenol red indicator, with linear response achieved over the relevant pH 5-7 range. &#160;</p><p>Our analysis workflow combines a simple-to-use cafetiere-based extraction method with paper microfluidic colour readout and smart-phone detector. This has the potential to enable farmers to monitor nutrients in soils on-site. Future work will aim at integrating multiple analytes into a single analysis card and to automate image analysis.</p><p>[1] <em>Europ. J. Agronomy</em>, 55, 42&#8211;52, <strong>2014.</strong></p><p>[2] <em>Nutr. Cycling Agroecosyst.,</em> 109, 77-102, <strong>2017.</strong></p><p>[3] Sens Actuators B, 30, 126855, <strong>2019.</strong></p>
Estimation of plant productivity and nutrient extraction capacity along the length of horizontal subsurface flow constructed wetland treating swine wastewater
