What can different soil tests tell you?

Standard Soil Test  – this is a snapshot in time and is one option for setting a chemical baseline awareness for the soil plant’s available nutrients and minerals. The standard soil test views the soil from a more conventional lens, approximating which plant essential minerals and nutrients can be made available using a “weak acid.” Plants and microbes liberate nutrients from the soil using acids, enzymes, and biologically derived compounds. A typical extraction solution used for a standard soil test is the Mehelich 3 extraction solution, which has a pH of 2.5. This test is still the most used to make fertility amendment recommendations for soil or growing media. Most agricultural labs will provide a standard soil test. 

When to use a standard soil test?

It depends on what you want to know.

A few good times to run a standard soil test are: 

• Before establishing a growing system or garden, help create a baseline understanding of the soil.
• If an established growing system or garden is not performing well 
• After harvest, but before winter, assess what nutrients may be limiting for the next growing season. 

“Total” Digest or Acid Extractable Totals Soil Test – This test uses a strong acid like an Aqua- Regia or Nitric-Perchloric acid, which can have a relative pH of close to 1. Because a much stronger extraction solution is used, a more significant percentage of the bound minerals and nutrients can be liberated from the soil’s parent materials. This gives you a view of what is in your soil’s deep reserve.

When to run a “total” soil digest test? This will only need to be done once for each distinct soil type in a growing system, and this test is not appropriate for non-soil-based media. 

Saturated Paste Soil Test – another snapshot. This water solubility test provides insight into what minerals and nutrients are currently contained in the soil solution. Some labs allow customers to send in their irrigation or rainwater to run the analysis. 

When to run a Saturated Paste test – 

• Early season to see what plants will have access to before the soil has warmed and the microbial activity has reached full function. 
• Just before fruiting crops transition into the reproductive phase is another exciting time to run a saturated paste to see where the nutrient drags might appear. This test does not consider the plant/microbe or rhizophagy nutrient acquisition cycle. 

Haney/ Soil Health Analysis – This test is a “new kid on the block.” The Haney Analysis aims to mimic nature with its dual extraction process, designed to simulate natural nutrient cycling more closely. It uses three weak acids, “H3A,” known to be exuded by living plant roots to acquire nutrients from the soil, and a second extraction is made with water, “nature’s solvent.” Biological indicators are also a component of this test. The Haney Analysis will return a soil health score and can be used to make soil amendment recommendations.  

When to run a Haney test? – Yep, another snapshot. Because this test includes soil microbial activity in its assessment, think about what you are trying to learn about your soil before deciding when to run the test. If you are interested in a soil health score, consider running a Haney Analysis during the most biologically active phase of your season, like early summer. If you want to know what nutrients will be available to your plants at planting, test them before planting. When you choose to do the analysis, if you compare this data to the following data set, run the analysis simultaneously each season. Spring nutrient cycles and biological activity will differ from high Summer nutrient cycles and biological activity. For comparable data, set testing parameters. 

Phospholipid Fatty Acids (PLFA) – This estimates the living microbial biomass of different organisms in the soil. PLFAs are an essential structural component of all microbial cellular membranes. The fatty acids contained in the phospholipids are used as biomarkers for:

Bacteria (Gram (+) and Gram (-)

Fungi (Arbuscular Mycorrhizae and Saprophytes)

Protozoa

Undifferentiated Microbial Groups

While this will not tell you which exact organisms are inhabiting the soil, it will give you a glimpse into which functional groups are represented. And yes, it’s a snapshot.  

When to run a PLFA test? As with many of the above tests, think about what you want to know. Are you experiencing nutrient tie-ups or exceptional plant health, and what clues as to why? Have you recently applied bio-fertility inputs or inocula and are curious about activation? Are you just wanting a baseline awareness of your soil’s microbial life? These might be good times. 

These are just a few of the possibilities for soil chemical and biological assessments, and I hope you picked up that there is no perfect soil test and no prescribed time to run a soil test. When investing in soil testing, consider what you want to learn, what management decision you are trying to make, and what you intend to do with the resulting data.