Soil Test Comparison

Soil is what’s under your feet; dirt is what’s under your nails – as many of my professors (especially the soil scientists) would say to any student that mistakenly called the ground we stood upon ‘dirt’.

As mentioned in an earlier post, our first year of ‘gardening’ in this area was a miserable and memorable failure. Clay is a challenge to work if you don’t have the right tools, and even more challenging if you’ve not grown up with it.

Building the raised beds was a challenge to build and fill, as each one is 8’ long and 4’ wide. Once in place, topsoil that had been delivered and dumped onto the driveway was then carried wheelbarrow by wheelbarrow until the beds were filled. Remaining topsoil was piled in the garden until the beds had settled and needed to be further filled.

Over the years, I’ve incorporated well-rotted manure and compost whenever it seemed that the previous years yields were down, or if the plants growth lacked vigor. This was done to build up the soil, improve the soil structure, add humus, increase the available nutrients in the soil, and hopefully, create a biosphere that promotes microbial growth and a hospitable environment for worms, bugs and other organisms that are part of the agrarian cycle.

It’s been about 4 or so years since I’ve added anything to the beds, and last years’ growth indicated that it was time. But as I was tooling about Fort Collins Nursery earlier this spring I notice that there were soil test kits from the CSU Extension agent, and wondered – How is my soil doing, and how do ‘high tech’ soil analysis like those performed at CSU’s Extension agency compare to those that you can get at most home improvement stores and gardening centers?

Testing Soil Fertility Tests

Two commercials available quick tests were compared to the soil testing service available through Colorado State University’s Extension office.

Using the soil sampling instructions provided with the CSU soil sample test kit I collected three samples from 5 of the beds that I’m currently growing in. The samples were collected in a bucket and then spread out upon some cardboard so that the soil could be screened for worms (there were many!), remove obvious plant material, break up clumps, and to allow it to dry out per the instructions. The added benefit was that the individual samples could be fully mixed so that it was a true representation of all of the beds, as opposed to one being more heavily represented than another.

Once the CSU Soil test was packaged, the paperwork filled out and it was ready to be dropped off, I performed each of the quick tests.

soil test prep

Each rapid test kit contains tests for soil PH, and one for Nitrogen, Phosphorous, and Potash (potassium). With the pH chart, soil is added to the tube and then the reactive agent and distilled water. After shaking the solution up, it’s left to site for two or so minutes and then the results are compared to the color chart on the back of the package insert.

The sampled soil was then mixed with distilled water per the package instructions. After the requisite time needed to allow the soil particles to settle, water samples and the active reagents were placed into each of the individual nutrient tubes. As with the pH test, once the solution was shaken and allowed for the indicator colors to ‘develop’.

With the rapid tests, I found that at each step of the process the likelihood of human error affecting the results increased. The tubes are small and hard to fill. The guidelines are nearly imperceptible and the reactive agents are packaged in gel capsules that are difficult to open and the contents poured into the tubes. Tasks that are extremely challenging to complete if you have any sort of hand tremors or waning eyesight.

Soil Test a

When the tubes were compared to the color ‘results’ chart – both tests indicated that the soil pH was high or basic. Both of the rapid tests indicated that the potash (potassium) levels were high, and one of the two indicated that phosphorus was high – I found that the other test was inconclusive. Both rapid tests also indicated that Nitrogen levels were low, though one indicated that there was more nitrogen than did the other.

FFC Soil Tests 03
Depending on the time of year and the demand for soil test analysis, it typically takes two weeks from the time you drop a sample off until you receive the results via email, and a hard copy is sent out to the provided contact information.

While the rapid tests usually cost under ten dollars, the definitive soil analysis was $35, but it came with a 25% off coupon for the next routine soil analysis.

Reading the results from CSU was extremely interesting for this science nerd.

As indicated with the rapid test kits, the soil pH was high, but the difference is that the test from CSU provided a quantitative value as well as an ideal range for plant growth. The rapid tests didn’t have a quantitative value – so it’s hard to tell if ‘high’ is still within ranges that plants can grow in.

Soil Analysis a
The test also indicated if the soil has too much salinity – a problem associated with using reclaimed water or in this case can occur when using manure. The percentage of organic matter is also tested as well as the other primary nutrients and minerals.
Soil Analysis b

I was surprised to find that the levels of nitrogen were as low as they turned out to be and that the phosphorous and potassium were so high.

Knowing this, applying well-rotted manure would have been a mistake. Potassium and phosphorus are somewhat mobile in soil – meaning once it’s been added, it takes a long time to move out or be used. Nitrogen – whether it’s applied organically or inorganically – is extremely mobile as a nitrate – it dissolves easily in water and moves down through the soil (called leaching) out of reach of the plant’s rootzone.

Manure would have provided a short-term solution to plant growth by adding some immediate nitrogen, but it would also apply too much phosphorous, potassium and other micronutrients. Too much of any one nutrient can inhibit a plant’s growth, or fruit/vegetable production, or if levels are high enough the plant can die.

Per the recommendations provided by the soil analysis I’ve opted to apply a healthy but appropriate amount of nitrogen to each bed using blood meal. I’ve never used this before, but given the options that I’ve found in town, it seems to be the best one for the situation.

The bottom line –

It’s a great idea to have a full soil analysis performed every few years to determine the overall health of your soil. While the cost seemed initially high to me, when compared to the cost of the two rapid tests that were used, the seeds and plants that I’ve purchased and the investment in tools and equipment over the years, $35 isn’t a lot, especially if it can help you identify why your garden isn’t growing as well as it should.

Rapid tests are good for broad general results, but they are subject to a lot of human error and shouldn’t be relied on with the same expectation as what can be provided though the extension agency.

Considering that this the first time that I’ve had the garden soil tested after nearly 20 seasons, I’m really happy with the results and I look forward to how the next one turns out.

Until next time, may your knees be green and your spirits light.




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