Soil Tests

Soil testing.  For some of my friends it conjures up images of exam day, with an empty math quiz on the desk before them, but no pencil, glasses or calculator, no knowledge of the subject, sitting in a required class they had forgotten to attend all semester.  Perhaps it’s the word “test” that causes them to roll their eyes at the merest mention of the words.  What do you do with that?

Perhaps it would be better if we called them “soil revelations”.  Rather than just guessing what is going on in a room, we part the curtain and take a peek.  We put our ear to the door and listen to the sounds and conversations within.  We get a few facts under our belt before we act as if there were bells or bears inside.  Who wouldn’t want the knowledge a revelation brings?

The ultimate revelation we are looking for is how well our crops do for us.  If growing is part of our business, what exactly brings us success?  If we are growing for our own use, what brings us fulfillment? To be sure, the soil “revelation” results are only a means to an end.  They are the finger pointing in the general direction of the moon, certainly not the moon itself.

The most basic type of soil revelation is this – what minerals are available under the best of circumstances? If all goes well and we have great weather, water, healthy and diverse soil biology, excellent seeds and hearty transplants – what minerals will be available to my plants?  In other words, what are my available mineral reserves? How big is my mineral savings account?

The test for mineral reserves is the Mehlich 3 or AA 8.2 test, depending on soil fizziness.  These tests reveal the minerals in the soil that may be bio-available, as has been determined by field tests. For example, the we know the test for phosphorus works because when we apply phosphorus according to the test results, we get an increase in yield.  The Mehlich 3 test uses a mix of acids at pH 2.5 to remove phosphorus and cations from their exchange sites. However, with calcareous soil the test over estimates calcium and magnesium due to the presence of carbonates.  The AA 8.2 is at present the only reliable test for calcareous soils.

The minerals most readily available to the plant are those that are dissolved in water around the roots.  The Saturated Paste test reveals these minerals.  As the name implies, soil is saturated with water to just above its ability to hold it.  The water is then vacuum extracted from the soil and analyzed for its mineral content.  Only a small percentage of the soil mineral reserve will be present in the paste test. This is a very useful test for determining the presence of excess “salts” or dissolved minerals in the soil that could impede the growth of salt-sensitive plants.  It is also very useful for determining whether minerals in the soil reserve are indeed readily available to the plant. It is another set of information that can help reveal what is going on in the soil.

The saturated paste test provides the most accurate information when it is made using the actual irrigation water.  The irrigation water may contain minerals and/or be of a pH that affects the outcome of the test.

If you suspect your irrigation water is not entirely pure you may want to test it independently.  Minerals in irrigation water can build up in the soil and by identifying this problem up front, you may be able to mitigate the effects by periodic leaching. If you are using a municipal water supply your water quality may change as water sources change throughout the year. You may be able to obtain a water test report from your water bureau, or you may want an independent test.

Soil tests are different from a mineral assay. A mineral assay is used to determine all the minerals in the soil, as if it were to be mined for minerals. Some minerals are bound up with extremely tight chemical bonds and the bonds are too strong for biological forces to break.  They don’t show up on a standard soil test but they may on an assay.

Knowledge is a wonderful thing, allowing us to make informed decisions about our garden farm.  Happy Growing!