A soil test seems like a bit of a mystery. Sending soil to a lab sounds complicated and expensive. In reality, it’s the opposite, and the recommendations you get back are well worth the few minutes and small fee.

Every state has a Cooperative Extension service, and most of those perform soil tests for the public. In a few cases, your state’s lab has contracted with a neighboring state to provide this service, but the process is the same. Search online for your state plus soil test, and you’ll find the right page. Each service has a slightly different sampling procedure, but the basics are to remove the mulch and grab a few trowels of soil from the specified depth at different spots in the garden. Mix it up, put it in a bag, and send it in. It’s easy, fast, and pretty straightforward.

Professional lab tests are easy, but are at-home tests just as good? What information will you get with your soil test results? And what should you do with the results once they arrive in your inbox or by snail mail? 

What about at-home or DIY tests?

Look online, and you’ll see an endless number of kits and gadgets designed to help you test your soil at home. Some are worth the money, some aren’t, and others cost as much as a professional lab test. The most accurate and highest-rated soil tests available to purchase on the internet are simply a sample kit you mail to a private lab, no different than you’d find with your state’s Extension service, and they cost about the same. 

Electronic testers with probes to stick into the soil can have accuracy problems. While they seem like a quick and easy method, check what information they are actually providing. Most will measure pH, which is helpful, and the rest of the readouts are often not useful for gardeners. For example, one of the more popular testers reads “fertility” as a percent, with no indication of which nutrients are present or in what amounts.

Many DIY kits provide small mixing tubes and capsules or bottles of a reagent chemical. The soil sample is mixed with water, the reagent is added, and then the color is compared to a chart. While these may give a general idea of what nutrient levels are present, they often only provide a general descriptor such as high or low. For pH, these home tests can be a useful quick check to see whether your soil adjustments are taking effect, but you shouldn’t rely on them for an accurate diagnosis. In most cases, you’re better off sending a sample to the lab.

Results typically included in a professional soil test

A certified lab provides accurate, trustworthy results. Most laboratory soil tests provide results for pH, percent organic matter, phosphorus, and potassium. Depending on the lab, you may get results for nutrients like calcium, iron, magnesium, and others as part of the basic package, or you can add them for a small extra fee. Your test results may display actual numbers, like 155 ppm of phosphorus, or a range, such as greater than 50 ppm. Most labs will also test your soil for lead contamination for an additional fee. If your garden is near buildings erected before 1978, when lead paint was phased out, or in an area with likely old industrial contamination, you may want a lead test as well as a standard soil test.

Perhaps most useful, professional soil tests will recommend what to do about your results. In a recent example from my garden, the test explained how much nitrogen I should add based on my intended use (cut flowers), and noted that while my phosphorus and potassium levels were high and no inputs were needed, those high levels would not be an issue for my plants. 

Percent organic matter is also often included and can be useful either as a baseline (this is where I started) or as an indicator you are on the right track. Organic matter in the soil improves soil structure, provides fertility, improves drainage, increases nutrient storage and exchange sites, and supports soil microbial life. Many of our soils are low in organic matter because of their parent material or past mistreatment, and having only 1-2% organic matter isn’t uncommon. Many university publications provide a range of 5-10% organic matter as the sweet spot for gardening. 

How to make use of soil test recommendations 

Your soil test will either reveal that everything is going swell in your garden and no action is needed, which is its own recommendation, or more likely, that a few small adjustments will improve your results. Start with the lab’s interpretation and recommendations rather than judging the numbers alone. If you want to track changes over time, use the same lab, since testing methods can vary from lab to lab.

As mentioned, a soil test can help us avoid wasting money on nutrients you don’t need. If your soil test reveals adequate levels of phosphorus and potassium, then further fertilizer applications of those nutrients may not only be unnecessary, but also wasteful or even harmful. Some new gardeners go a bit overboard on the compost, and a soil test might reveal that your organic matter percentage is already high, and you could skip a year of compost. Below are a few common adjustments your soil test might reveal. 

Adjusting pH

We often assume changing soil pH is as simple as tossing out a bit of lime or elemental sulfur, but in reality it’s more complicated. Soil has a natural capacity to buffer, or resist, pH changes. When we simply toss out some amendment without a true measurement of pH and our soil’s buffering capacity, we are throwing darts in the dark. 

Your soil test will recommend pH adjustments based on the intended use you provided with the sample, and it will take your soil’s buffering capacity into account. Recommendations are often given for finely ground lime at a rate of pounds per 1000 sq. ft. If your garden is 20’x50’, that’s a thousand square feet, and you get away with no math for this exercise. If your garden is larger or smaller, you’ll have to adjust.

To adjust for your garden, find the square footage in production, divide it by 1000, then multiply by the recommended application rate. 

Hypothetical Example: Your soil test recommendation is to add 26 pounds of lime per thousand square feet. For a garden with a production space 12 feet wide and 20 feet long

• 12’ x 20’ = 240 sq. ft

• 240 sq. ft / 1000 = 0.24

• 26 pounds per thousand sq. ft x 0.24 = 6.24 pounds of finely ground lime

Caution: The above example is to explain math only. Every soil has a different buffering capacity, which your soil lab will test when making recommendations.

Lime applications can take a year or more to be fully effective. Applying in fall and working the lime into the top 6-8 inches of soil will help provide some adjustment by spring. If your test recommends lowering the pH with elemental sulfur or other products, those dosages will be provided as well.

Adding nitrogen

Most soil tests won’t provide a specific nitrogen level; instead, they'll recommend how much to add based on the use you provided when submitting the sample. Remember nitrogen is highly mobile in the soil and is also easily depleted. It is the most likely nutrient to be deficient, since it can be leached out of the root zone by rains and irrigation, or simply taken up and used by plants faster than it is replenished by fixation and atmospheric deposition. 

Like the lime example above, your nitrogen recommendation will likely be given per unit of area. Some labs will provide a recommendation in pounds of a specific product (like a 46-0-0 fertilizer), and some as pounds of the actual nutrient itself. 

When the test recommends an amount of a specific fertilizer:

Example: Apply 1.2 lbs of the specified high-nitrogen fertilizer per 100 sq. ft. Simply adjust this for the size of your garden. An inexpensive kitchen scale can make measuring easy. 

When the test recommends how much of the actual nutrient to add:

Example:  Add 0.15 lbs of nitrogen per 100 sq. ft.

In this case, you’ll need to do the math to convert your fertilizer product to the recommended application rate. Remember that the NPK numbers on the bag are the percent by weight of nutrient (nitrogen, or phosphate and potash in the case of P and K) in the bag. Using a 12-0-0 organic blood meal product as an example, we know that for every pound of this product, 12% of it, or 0.12 pounds, is actual nitrogen. 

Given we need 0.15 pounds of nitrogen and there are 0.12 pounds per one pound of fertilizer, dividing 0.15/0.12 = 1.25 pounds of this fertilizer needed for the hypothetical 100 sq. ft bed.

Apply nitrogen, if needed, in 2-3 increments spaced throughout the early and mid growing season. Avoid applying nitrogen at the end of the growing season, as it won’t be used before plant growth stops, and may leach out of the soil by spring.

Warning! Do not use weed-and-feed type fertilizer products in your garden. The “weed” part of the product name usually implies it includes a broad-leaf herbicide, which will kill your flowers and vegetables. Choose products meant for gardens, not lawns.

Adding phosphorus and potassium

Just like the nitrogen recommendations, your soil test will reveal any needed additions of phosphorus or potassium. If your soil test showed no issues with potassium, but a deficit in phosphorus, you could choose a high-phosphorus fertilizer, such as a 0-51-0 product, and follow the soil test recommendations.

While nitrogen should be applied immediately prior to and during the growing season, in most soils, phosphorus and potassium can be applied at any time as they are not readily leached or lost from the soil profile.

Improving organic matter percentage

Soil organic matter is important, and in our gardens we often need to provide annual amendments to keep it high. The way we garden often works against naturally maintaining soil organic matter. We frequently disturb the soil, remove old dead plants, and harvest crops. All these things reduce soil organic matter or remove soil carbon without restoring it. Adding compost when needed, using mulch (shredded leaves, clean grass clippings, pine straw, etc.), and letting non-diseased plants overwinter in our gardens to begin decomposition are all excellent ways to maintain and improve soil organic matter percentage.