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What are the basic types of soil tests? What do they tell you? Which test is the best for your soil?

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 Standard Soil Test

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, clean and adequate 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 standard 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, 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 (fizzy) 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.

Standard test results are the basis of soil amendment recommendations.  OrganiCalc uses standard test results to calculate the amendments needed to balance the soil minerals in a way that has been shown to produce excellent quality and high yields on vegetable crops.

The Saturated Paste Test

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, and is left for 24 hours for the minerals to dissolve.  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 same irrigation water as will be used on the crop.  Irrigation water is rarely pure and likely contain minerals and/or be of a pH that affects the amount of minerals that are dissolved.  There is no additional cost to testing with irrigation water; you just need to provide the sample at the time of testing.  A one pint (1/2 liter) bottle of water is enough to do up to three paste tests.

Nitrate and Ammonium Test

Nitrate and ammonium are both sources of nitrogen for soil microbes and ultimately for plants, however they are not the only sources.  Amino acids and other proteins are also present in organically managed soils and can be taken up by plants (as can entire microbes(!), but that is a different story).  In “conventional” chemical agriculture, nitrate and ammonium may be added directly to the soil as salt fertilizers.  In certified organic agriculture, these fertilizers are not allowed.  However, all the other organic sources of nitrogen such as seed meal or composted manure provide nitrogen which is broken down into nitrate and ammonium by the soil biology (see the nitrogen cycle).  The nitrate and ammonium test can provide a good indication of the amount of plant available nitrogen available in the soil at that particular moment.  Soil nitrogen levels fluctuate, especially with tillage and soil disturbance.  They are best viewed as a snapshot rather than as a long term truth.

The Mineral Assay

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 that even the extremely acidic reagent of the Mehlich 3 test cannot break.  They don’t show up on a standard soil test but they may on an assay. These tightly bound minerals may be accessible to microbes under the right circumstances. After all, trees do grow in what appears to be solid rock.

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