PDF Just by visually examining soil, it can be hard to tell if there is enough organic matter in it to assure adequate levels of humic acids. These acids bind soils together, free up minerals for plant use and just generally help the soil along. Happy soil = happy plants … it’s just that simple.
However, even without expensive lab tests, you can get a ‘close enough’ estimate as to whether or not you have sufficient levels of humic acids in your soil simply by measuring the amount of organic material present.
The Fist Test
There are two means of getting this “guesstimate” of soil fertility that can be performed at home with next to no equipment or lab skills.
The first is sometimes called a ‘fist test’ and will take only a few seconds to run. It’s answers, though, are so broad that you really have to be familiar with your soil to make much use of it. Don’t despair, though: running this fist test is a big part of how you get to know your soil.
The fist test will tell you the approximate ratios of sand, clay and organic material. It gives you no clue what the soil chemistry might be on a finer level. When you get all done, you will be totally clueless about the NPK ratios and levels in your soil except that you will know if they are within reason.
Here’s How to do the Test
Take a handful of moist soil (water lightly if it is too dry at present) and squeeze it, making a fist with your hand. Then open your hand and poke the soil lump apart with your fingers. Take note how well it holds together.
When you first open your hand, the lump of soil should rest right where it is. Then, as you prod it lightly with your fingers, it should yield to the pressure, making jagged fracture lines. If it does, you’ve probably got good soil. Whatever you’ve been doing, keep doing it.
If it simply crumbles easily and essentially completely, there is not enough organic material and clay in it to hold the sand together. You’ll want to amend this soil because it won’t hold water and it won’t yield its nutrients to the plants, either. That’s the simple version, the technical reasons for these results would fill your average DVD.
We’re not going to get into that today, okay?
If the lump won’t hold together long enough for you to poke it at all, there is too much sand and you need to add both clay and organic material to this soil. Bags of crushed clay are commercially available as kitty litter (get the cheap, unscented, kind) and as driveway absorbent for gas stations. Either way, add a little, work it in, water well, wait a few days and test again until you get a nice lump that sticks together until you give it a good poke. From there, it’s all good.
On the other hand, if the lump takes a finger imprint without fracturing, there is not enough organic material and sand to hold the clay apart. Take a piece of such a lump and roll it between your fingers. If you can roll it between your fingers, or if you can smear it against your palm, there is definitely too much clay in the mix … add some sand to go with that compost!
Note that, whether you are adding sand or clay to the soil, you also want to add compost. Compost is a universal constituent of healthy soil and having too much of it is what is known as a ‘self-correcting problem’. If you have a lot of organic material in the soil you will get a lot of biota to eat it. There; problem solved.
The Shake Test
The second method is sometimes called a “shake test”. Yes, I am making these names up but only because I can’t remember what their proper technical name is.
Who cares?
A direct numerical measurement of soil condition is worth a thousand “maybe’s”. This test takes about 15 minutes once or twice a year and costs less than a penny per test.
This is a simple, albeit crude, method of testing the organic matter ratio in your soil. Since there is no need for more precision, and since it will take only a few minutes of your time, it is worth giving a try.
Soil in jar with water after 1 week.
Here’s how to ‘do the science’
You will need a trowel, some jars with straight sides, water and a ruler as long as the jars are tall. Pint canning jars or quart mayonnaise jars work just fine.
Using a trowel, select soil samples from a half-dozen locations in your garden. Since we aren’t paying for these tests, collect it from two strata: the first 6? / 150mm and the second 6? / 150mm just to examine the difference. Keep the strata separate. That is, all the top soil goes in one batch, all the deeper soil goes in another. Mix each batch separately and place a trowel full (the exact amount can vary without changing the results) from each batch in a quart jar, somewhat less in a smaller jar. Add water to just below the shoulder of the jar, cap tightly and shake vigorously. Unless you own a centrifuge, set the jars aside to wait for the water to clear. Clay is extremely fine and sinks slowly. Depending on the amount of clay in the soil, this could take anywhere from a couple of days to nearly a month. That’s okay, we are in no hurry.
Just for fun, you might also make up a third jar with a 50/50 mix of the leftover soil from the two strata.
When the water has cleared (or you have run out of patience!) measure the amount of soil on the bottom and the amount of organic material floating at the top. This is the ratio you were looking to quantify. Divide the inches / mm of the organic material by the inches / mm of the soil at the bottom to get the percentage of organic material.
How to Interpret the Results
If the organic level is below 3%, you need to take serious action soon. Add as much organic material as you can as deeply as you can and measure again in a few (3-6) months. Manure, food waste, compost, grass clippings, tree leaves … if it’s organic, in it goes!
If it is in the range of 3-10%, your soil is at a healthy maintenance level. Could it be better? Yes. But it will take considerable and constant effort to get it above this range and keep it there. Better to simply keep doing what you have been.
Somewhere around the 10 – 25% mark, either your sample got contaminated or you are testing too soon after applying fresh material. The more material you have in your soil, the more micro-organisms there will be to consume it. The more micro-organisms, the faster the material gets depleted. It takes a near-constant application of organic material to keep levels much above the 10% mark and this near-constant effort is enough to suck much of the fun out of casual gardening. More than that, going beyond the needs of the plants is never an improvement.
If you work really, really hard and get the organic material really, really high, your plants will look at you and say:
(cue Clare Peller)
“Where’s the soil?”
Practical and Educational
This simple measurement might also make a good earth sciences experiment in junior high school … simply compare the ratio found in fertile soils in open lawn and beneath trees with that found in barren soils such as parking lots or worn-down paths in the lawn. In senior high school, a chemistry class might want to take this further with a consideration of the various salts and acids present, the permeability and the pH levels found. A biology class might want to catalog the varieties of life present in the soil and consider how they interact. Ideally, the biology and chemistry classes could work from the same bulk soil samples and then compile an aggregate set of data to draw conclusions from.
(This post is very much a ‘work in progress’. It has enough information in it to be useful, but not enough to be definitive. For instance, the interpretation of the shake test needs more work. It’s useful now, but I need to double check some numbers and so on. Check back here from time to time as I continue to develop this idea.)
