Monitoring soil nutrients using a simple cafetiere-based extraction with paper-based readout.&#160;
Authors: Richardson Samantha , Al Hinai Samira , Gitaka Jesse , Mayes Will , Lorch Mark , Pamme Nicole 
Published: March 2021

&lt;p&gt;Routine monitoring of available soil nutrients is required to better manage agricultural land&lt;sup&gt;1&lt;/sup&gt;, especially in many lower and middle income countries (LMICs). Analysis often still relies on laboratory-based equipment, meaning regular monitoring is challenging.&lt;sup&gt;2&lt;/sup&gt; 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.&lt;sup&gt;3&lt;/sup&gt; 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.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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&lt;sup&gt;-1&lt;/sup&gt; 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 (&amp;#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&lt;sup&gt;-1&lt;/sup&gt; range. Moreover, a pad to monitor pH was developed, employing chlorophenol red indicator, with linear response achieved over the relevant pH 5-7 range. &amp;#160;&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;[1] &lt;em&gt;Europ. J. Agronomy&lt;/em&gt;, 55, 42&amp;#8211;52, &lt;strong&gt;2014.&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[2] &lt;em&gt;Nutr. Cycling Agroecosyst.,&lt;/em&gt; 109, 77-102, &lt;strong&gt;2017.&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[3] Sens Actuators B, 30, 126855, &lt;strong&gt;2019.&lt;/strong&gt;&lt;/p&gt;