Agriculture Reference
In-Depth Information
Organic Matter, ORP, pH and ERGS
You'll probably get a number for organic matter on your soil test. The environmentally
friendly way of determining this is to bake the soil and measure the weight loss, because
the carbon burns off. Labs are moving to that over the old method, which used toxic chro-
mium. Still, some labs measure humus, while some measure total carbon, and some labs
just guess based on the color, so it's difficult to know what the number means. Mostly, I use
this number to see the trend — whether or not it's going up with my management practices.
For most people, your soil organic matter will be low, perhaps even down in the 1-2%
range. If you've been working on your soil for a while, it can be around 5% or even higher.
I prefer just looking at my soil color, and looking and seeing how well it's aggregating and
holding moisture and so on. It just takes a bit of experience looking at the soil to be able to
judge the amount of organic matter present.
Oxidation reduction potential (ORP) is a number that has been traditionally used more
for determining compost stability, but it's also being used for soil. It basically measures the
state of soil oxidation. The ideal range is 25-30. Too low and the soil is lacking oxygen, it's
anaerobic. Too high and the soil has so much oxygen that organic matter may be burning
up too fast. Like pH, I don't use ORP for soil management decisions, but you may see it on
a soil test.
While pH is important, we've already seen how knowing the number isn't particularly
useful to us other than to monitor changes over time. ERGS, energy released per gram of
soil, is another number often shown in tests that is covered later in the energy chapter.
