While a soil test involves many important nutrients, the pH level is critical in optimizing the microbial activity in the root zone. This is especially important for organic gardeners because a proper pH will allow critical nutrients to be released as the plants mature.

Nutrient availability for soil pH levels
University of Missouri Extension Service

As you can see in the chart, there’s a fall off of phosphorus availability as pH declines (or becomes more acidic – moving to the left on the chart). Root growth is dependent on adequate levels of phosphorus. We can also see how acidity affects other critical metals such as calcium and magnesium when pH drops below neutral (pH 7). These are among the macronutrients frequently cited as deficient in soil tests.

But altering pH can be difficult. Clay soils generally have a high degree of buffer capacity, meaning that the soil is resistant to change. Adding organic matter to the soil also increases both the buffer capacity and the acidity of the soil. While plants do well in soils ranging from a pH of 6 to 6.5, peak microbial activity occurs when the pH is between 6.3 and 6.8.
So why is this important?

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Scientist have found that microbial activity must be considered when analyzing soil fertility. You might think of this phenomenon as a “just in time” delivery of essential nutrients as well as an inoculate that prevents soil borne diseases from harming the plants. Failing to take this into account may result in recommendations that exceed the plant’s requirements.

Years (okay decades) ago I read a study that was instigated by a group of organic farmers who were concerned with nitrate pollution in the groundwater and streams in their locale. Of course their fingers were pointed at the “chemical” farmers in the watershed. But the results of the environmental study indicated that the organic farms were generating more nitrates than the farmers using chemical fertilization.

Not only are manures notoriously inconsistent in nutrient content, but the ability of humus to increase microbial activity and therefore the release of nutrients wasn’t being taken into account. The fact is living organisms in the soil generate nitrogen rich materials as they excrete their waste.

A worm’s digestive system, for example, can produce castings that are 5 times higher in nitrogen than the material it eats. However, worm casting are also neutral (approximately pH=7) so they do not contribute to soil acidity.

Nutrients like phosphorus can also be less available in the spring when soils are cold, compounding the problem of a soil that is too acidic.

The University of Missouri Extension Service provides a comprehensive soil analysis for $10 plus postage. Their recommendations include liming requirements to offset acidity and balance the pH. When submitting a sample you can request a recommendation for a target pH – say 6.5. The laboratory can also analyze compost and manure samples for fees ranging from $20 to $40 for a complete test. More information on these services can be found here: Missouri Extension Service – Soil Analysis Information

You can obtain soil sample boxes from your local Extension Office: Directory of Offices