Affordable, open sap testing built with growers
Open Source Ag is studying how commercial pocket sap meters work and designing an original, open instrument that growers can afford to use every week of the season. We are building it in the open, testing it against lab results, and inviting growers to shape it from the start.
Why Sap Analysis
A few drops of sap pressed from a leaf or petiole show what a plant is taking up right now. Dry tissue tests remain the standard for a full nutrient picture, and sap testing adds something they cannot: a quick, frequent check during the season, close enough to the moment to adjust an amendment plan.
The tools for that quick check cost more than small growers can cover. A single-ion pocket nitrate meter lists at about $400, and its replacement sensor at about $170. Add potassium, calcium, pH, and EC, and a grower is buying a drawer of separate meters and a steady supply of sensors that wear out.
What We Are Taking Apart
Every pocket sap meter rests on well-published science. Inside is an ion-selective electrode: a polymer membrane carrying a chemical that attracts one ion, paired with a reference electrode. The meter reads a very small voltage, corrects for temperature, and converts that reading to parts per million through a two-point calibration.
None of those principles are secret. Researchers have printed low-cost nitrate electrodes on plastic sheets using a PVC membrane and a common quaternary ammonium compound, with responses close to the theoretical ideal (Periodica Polytechnica). Fully printed nitrate sensors, reference electrode included, are already described in the literature (Sensors via PMC).
Our reverse engineering work stays on the side of principles. We study how the measurement works, then design our own parts, firmware, and calibration. We are not copying any manufacturer’s sensor, software, or branding. Commercial meters serve as working tools and comparison instruments in our pilot.
What We Are Building
- One reader for many nutrients. A single handheld reader built on a current microcontroller, designed to read nitrate first and then add potassium, calcium, sodium, pH, and EC over time. One reader replaces a set of separate meter bodies.
- Newer, replaceable sensors. The long-term goal is low-cost printed sensor strips that read from a single drop of sap, so the part that wears out is the part that costs the least.
- Data stay on the farm. Models will support calibration, flag worn sensors, and correct for ions that interfere with each other. Processing happens on the device or the grower’s phone, and readings stay in open file formats the grower controls.
- Kits at cost. As a nonprofit, Open Source Agintends to offer kits at cost, with designs, build guides, protocols, and validation results published openly.
A second track, the Open Maturity Meter, shares the same reader and app. It estimates sugar and dry matter in grapes, avocados, and other fruit without cutting the fruit, to support harvest timing.
How the Pilot Works
Our first field pilot pairs sap readings with field conditions and a simple action log. We start with existing handheld meters to see what works on real farms. Cost and test results will guide which parts we incorporate into the Open Sap Meter and which we build ourselves.
The question guiding the pilot is whether repeated sap readings and simple field notes can support a grower in spotting nutrient stress, irrigation imbalance, or crop decline earlier than visual observation alone.
- What we measure. Nitrate, potassium, and sap Brix from leaf or petiole sap, with pH as an option.
- The same method every round. The same plants, the same leaf or petiole type, and the same time of day, ideally morning, every one to two weeks. Sap values shift with timing and sampling method, so consistency matters more than complexity.
- Field conditions on every record. Weather, recent irrigation, fertility inputs, growth stage, and visible stress get logged alongside each reading.
- An action log. Each reading ends with a short note such as “no action,” “watch next week,” “inspect irrigation uniformity,” “compare with a lab sample,” or “trial a small fertility adjustment.” The grower decides whether a reading justifies a change and records why.
- Outside comparison. Matching samples go to a lab at key crop stages, and handheld trends get compared with plant condition, crop quality, and yield notes.
Sap Brix and the Open Maturity Meter measure different things. Sap Brix is the sugar reading in pressed leaf or petiole sap, which we are testing as a general stress signal to check against irrigation, pests, and nutrition. The Open Maturity Meter estimates sugar and dry matter inside the fruit to support harvest timing.
The pilot also works as an apprenticeship. One person leads sampling, calibration, and meter care. A second handles field logging, photos, and data quality. A third manages local data storage, analysis, graphs, and the report draft. The farm lead makes the management decisions. This division turns the work into a training model other farms can repeat.
The first cycle runs on two farms, with one crop block each, over 8 to 10 weeks of weekly readings. At the end, we ask two questions: did the readings change decisions, and were those decisions useful?
Where We Stand
A note from Kelly Cooper, Open Source Ag
I want these instruments to earn a place in a grower’s season. That means each one has to serve a real, recurring decision that costs money to get wrong, and it has to work across many types of grow sites, well beyond a single demo plot.
Our current read on sap testing is that the measurement is the reachable part. Ion-selective sensing is mature, and the electronics are ordinary. The harder question is what a given number means for a specific crop at a specific stage. Sap reference ranges are far less settled than dry tissue benchmarks, and results shift with the plant part sampled, the time of day, and even the method used to press the sap (MDPI Agronomy). One recent field project built its own preliminary sap ranges by pairing sap and tissue samples from the same plants, because local guidelines did not exist (Palouse Conservation District via SARE).
So we are positioning the Open Sap Meter as an affordable, frequent check on nitrate and potassium trends during the season, used alongside the lab tests growers already rely on. We will earn any larger claims through validation. Every reading in our trials gets compared against a commercial meter and an accredited lab, and we plan to publish what we find, including where our tool falls short.
Here is how we think about the difference. If a commercial sap analysis kit produces 20 results, for example, we are planning for the at-home kit to produce a ballpark of around 12. We see that as 12 more results than a grower who finds sap analysis unaffordable would otherwise get, rather than a shortfall against a commercial provider. Success with DIY analysis could lead a grower to make important use of commercial products, and we value what those products offer. We are offering an alternative for growers who are not in a position to use commercial products, or who want an alternative that supports amendment choices within organic and regenerative guidelines.
We are also building whether or not grant funding arrives on any particular timeline. Funding would speed the work. It does not decide whether the work happens.
What Comes Next
Everything on this page describes our current intent. We work with agile, open source methods, so the plan will change as we learn. Each change will come from conversations with growers about their priorities and challenges, along with what the data show.
- This fall and winter. A small core team is testing existing handheld meters and bench testing a first nitrate reader of our own, comparing field readings against lab results on our own and partner farm sites.
- Early prototypes. Working units built from written assembly steps, with a real per-unit cost recorded and shared.
- 2027 season. Field validation across a larger group of California vineyards, orchards, groves, an avocado ranch, and vegetable farms growing crops such as tomatoes, peppers, cucumbers, and brassicas, with sampling timed to crop stages such as bloom and veraison.
- Open release. Published in a public repository as each stage is validated: a sampling guide, a local-first data form and CSV template, a calibration and cleaning checklist, a guide to interpreting readings cautiously, seasonal case studies from partner farms, a grower workshop module, build guides, and datasets.
Shape It With Us
Growers know which readings would change a decision on their operation, and that knowledge shapes what we build. Participation can be as light or as involved as suits you:
- A conversation about your nutrient and harvest decisions
- Feedback on prototypes as they become ready
- Hosting a 2027 test site, with sampling support and lab results shared with you
- An ongoing role deciding what gets built and improved next
Growers of every size are welcome, and organic certification is not required.
