The Pulse meter measures the volumetric moisture content of your media. This is the ratio of water to everything else in your media (including soil, roots, organic matter, etc). For example, if you take 5 litres of dry media such as coco coir and add 5 litres of water to this you would have 50% volumetric water content.
The other main way moisture content is measured in soil is by gravimetric water content. This is the same as volumetric though it is the ratio of the weight of water to the weight of everything else. For example, if you take 500g (~18oz) of wet media, put it in an oven at 120 °C (250 °F) for 12-24 hours, measure it again at the end and find the weight has dropped to 250g (~9oc) that would be a gravimetric moisture content of 50%.
The two can be easily changed between if you know the bulk density of your media
The main issue in interpreting moisture content as a measure is that it is affected by things like compaction of the media, root mass and the like. This means if you have a young plant in new media it may be able to hold up to 60% water before becoming saturated as the media is very airy. The same plant a year later will have much more compact media containing many more roots, so it may only hold up to 40% moisture before it is at saturation. This means if you are going to compare between plants of different ages, media types, pot/bag sizes or other things that could affect the maximum water holding capacity of the plants root zone, this needs to be taken into account.
Calibration
To help with comparison and understanding of moisture content, Pulse can be calibrated to the maximum water holding capacity, also known as field or pot capacity, of your root zone. This is the maximum amount of water that the medium around you plant, be it the pot, bag, slab or area of ground can hold without draining due to gravity. After saturation it is essentially the maximum amount of water that will be available for your plant to access over a prolonged period.
This calibration serves to main purposes;
- It simplifies concept moisture content. 100% now means the maximum water holding capacity, so you can easily see how far away from this you are
- It simplifies the comparison between different media types/plant types/pot sizes/etc
To calibrate to this simply select “Calibrated Media” from the menu in the top left corner of the app and tap on the “+” button in the bottom right of the “Calibrated Media” screen, then follow the onscreen instructions.