Soil – Testing Your Soil’s Organic Matter

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Most people know that it is beneficial to have high levels of organic material in their garden soil, whether it is in an orchard, or a vegetable or ornamental garden. Digging in different materials such as compost, animal manures or green manures helps increase the levels of organic matter, and thus, the soil’s biological activity and fertility. The improved soil allows water to penetrate efficiently, supplies nutrients slowly to root zones of plants, and helps root development. In addition, the higher number of organisms such as earthworms helps cultivate and improve the soil.

There is a simple ‘home garden’ way to test the amount of organic matter in your soil. You will actually be testing the ‘biological activity’, which is a direct result of the level of organic material. The test could be used to help to tell whether the organic materials you are adding are having much effect, how long their effect takes to decline, or whether you might have some areas in your garden that are much lower than others in organic matter. 

The test is based on the length of time it takes for soil organisms in the soil to rot a piece of fabric. Where there is a higher level of organic matter in the soil, there will be a corresponding higher level of micro-organisms or ‘activity’, and the fabric will rot faster. The level to which the fabric has rotted over a given period of time is measured by a simple ‘load-bearing’ test.

The test is undertaken as follows: Choose several areas that you want to measure. For example, you might want to compare the results of one garden bed where you have added mulch recently, to another that you have added nothing to for a year, and see what the difference is in biological activity. Or you might have different garden beds where you have added different types of material, and you want to compare which is the better soil treatment.

Take some pure cotton fabric such as an old sheet, and cut it into strips about 30cm long and 3cm wide. Try to make them all the same width, for accuracy of results. Using a waterproof fabric pen, label one end of each strip with the information you want, such as the date, and area of the garden, and what type of organic material has been added. Push a sharp spade into the selected area of soil, to about 15cm deep, to make a slot in which to insert the cotton strip. Fold the strip in half over the blade of the spade and use the blade to push it back into the slot you have just cut. When you draw the spade back out, a couple of centimetres of fabric should be left protruding out of the soil. It is a good idea to take several samples at each selected site.

Leave the fabric in the soil for a period of several weeks, and then remove the strips on the same day. If they have almost completely rotted, they were left in too long, so the test will have to be done again. (If you had several extra strips in the same area, you could remove one each week to see how they are going). The higher the level of biological activity in your soil, the more the fabric will have rotted, and therefore, the weaker it will be. To test the strength of the partially rotted fabric strips, you simply fold one strip over the handle of a bucket and use it to raise the bucket just a little off the ground. Slowly pour water into the bucket until the strip breaks. Measure the amount of water in the bucket with a measuring jug, and record the results, along with the other information that you had written on the fabric. If you have taken several samples in the one area of garden, you can average the results for a better level of accuracy.

This technique can give you useful information about the amount of organic matter in your garden soil, especially if you repeat the test over a period of time, and collect and compare the resulting data.

This article was contributed by Jenny Awbery.