The importance of soil testing cannot be stressed enough. Any grower, from a professional farmer to the backyard hobbyist, should know the basics about the soil that they grow in. Hundreds to thousands of dollars per year can be wasted on soil amendments if they are applied without testing to learn just what the soil already has and what it needs to support healthy growth and abundant yields. Testing allows gardeners to make informed decisions on what to add to the soil.
Fortunately, soil testing is not as mysterious or complicated as some folks think it is. Growers don’t need an advanced degree in chemistry or need to be independently wealthy to perform a soil test and analysis. It is both a relatively easy and inexpensive process. Gardeners also have the choice between doing the test themselves or taking advantage of their local cooperative extension service.
Cooperative extensions are usually in every county in each state of the United States (they also exist in various forms throughout the developed world). They are a function of a state’s land-grant college or university. These are institutions of higher learning that are often a state’s university system, or are a private institution that were given state-owned land at some point in its existence for the purposes of promoting research and awarding degrees in the fields of agriculture and engineering (in contrast with those schools that had a liberal arts focus). While the range of services that extension offices vary from state to state, it is near universal that they serve to help folks with their farming and gardening problems and assist with basic services such as soil testing and analysis. They are a great resource for many people and often offer free or low-cost services to their constituents.
Why Test Soil?
Without knowing basic properties like the nutrient levels, organic matter content and acidity or alkalinity levels of a soil or growing medium, it is difficult or even impossible to know what needs to be added to the soil for optimal health. Growers that routinely add various fertilizersor amendments like lime to their soil may be spending money needlessly and, worse, building nutrients in the soil to near-toxic levels.
Every crop has unique nutrient needs and thrives in unique soil conditions. By performing a soil test, growers will learn what is already in their soil in sufficient amounts and what precisely has to be added. Soil testing will reveal the soil’s pH level, organic matter, levels of several different nutrients, cation exchange capacity (CEC), and presence of heavy metals. Some may even help to identify how much sand or clay is present.
Collecting the Sample(s)
Whether growers intend to test the soil themselves or send it to a lab for professional analysis, a sample still needs to be collected. It should not be assumed that the properties of the soil are consistent throughout. Even in small garden areas, it is advised to take several samples and mix them to achieve a composite. There is no minimum number of samples, but there should be no less than one sample per 1,000 square feet of growing area to get an accurate picture. If there are different growing areas or different fields in question, separate samples should be prepared.
When taking a sample, it is important not to contaminate it. Growers should use a clean bucket, a clean trowel, and wear gloves. Also, the sample should be taken from soil that is approximately six to eight inches below the surface. It is important not to include any of grass or mulch that had to be cleared away to obtain the sample. Once all the samples are collected, stir them together using the same tool used to collect the samples. A portion of this blend will be used for the test. If using the services of a cooperative extension, they will generally provide a collection bag that indicates how much soil to fill it with. In general, only about one cup of soil is needed per sampled area.
Testing the Sample
If growers opt to send their samples to an extension, or a service lab they recommend, do not delay. If possible, deliver the sample the same day it is collected or at least get it mailed the same day. If there needs to be a lag between sampling time and delivering or mailing, keep the sample closed and stored in a refrigerator to try and reduce the chance of contamination.
If testing their own soil, growers should do so immediately after collecting the sample. When practical, it is advisable to have the testing materials ready to go prior to collecting the sample.
Most DIY kits do not test the soil itself, but rather the water that runs through the soil. As such, distilled or deionized water needs to be used for irrigating the sample so that the pH is not affected. Make sure to collect this leachate into a clean container. At this point, some kits test the leached water directly, while some require various provided chemicals added to it (meaning growers must mix those in and let them stand for the prescribed amount of time).
Analyzing the Results
Most DIY soil testing kits come with instructions on how to interpret the test results. Some have a color chart where growers match the color of their samples to determine the pH or other various characteristics being tested.
DIY vs. Cooperative Extensions
Growers should take a look at the following considerations to determine which course of action best suits their soil’s or garden’s needs.
DIY Soil Testing
- Options: There are different kits on the market. Some tell just pH, others test for individual nutrient levels, and comprehensive tests do all of the above. Depending on what information a grower seeks, there is usually a kit out there that can match their needs.
- Cost: DIY soil testing can be very inexpensive. There are dozens of types, ranging from a few dollars for an easy, disposable kit to more expensive, more precise kits.
- Results: The results of a DIY soil test can become evident in minutes. Growers don’t need to wait long at all to know what they are dealing with.
- Precision: Most DIY soil kits do not offer the precision of a lab test. Many only give pH range instead of the actual pH.
- Contamination Risk: Even when following instructions carefully, there is a greater risk of contamination when testing at home versus in a controlled lab condition.
Cooperative Extension Soil Testing
- Reliable Results: Tests like these are routinely performed by employees of the extension service or lab, so growers can be fairly confident that their samples were properly handled, processed, and that the results are accurate.
- Thoroughness: Depending on the type of test selected, a lab analysis of a soil sample will reveal the presence or absence of many more nutrients than most DIY kits can offer. They usually show a comparison of a grower’s soil to other area soils sampled or to recommended levels based upon what they are intending to grow.
- Analysis: A lab analysis not only tells growers what they have in their soil, it prescribes a course of action for what is needed. Most will tell growers exactly how many pounds of each nutrient or amendment should be added to bring their soils’ nutrient or organic matter levels to optimum levels. It may also recommend other cultural practices to help growers achieve their growing goals.
- Cost: While not likely to break the bank, growers will generally pay two to three times more for a lab analysis of their soil through a cooperative extension service than if they were to perform the test themselves.
- Waiting for Results: If growers need answers right away because they have high-value crops starting to fail, a lab analysis may not yield the results they need in the time frame required to take action and reverse the problem. Samples generally get analyzed in the order they were received and can be held up by weekends, holidays, employee shortages, etc.
Soil is a complex system of biotic and abiotic forces. A soil test is just a snapshot of what is present at the time of the collection. It should not be done once and considered the final word. It is advisable growers perform a soil analysis at least yearly and even seasonally if they really want to know what’s going on in their garden.