Are garden soil test kits a good alternative to lab testing?

Many gardeners know that soil testing is one of the most important tools when it comes to growing healthy, vigorous plants. As a result, the UNH soil testing service usually processes hundreds of soil tests every year. Unfortunately, due to public health concern around the COVID-19 virus, UNH is not accepting soil samples until further notice. So, what options do home gardeners have in the meantime?

 If you had your soil tested in a previous year, it is appropriate to continue using the fertilizer recommendations in an old test until you can run a new one. However, if you’ve never had your soil tested or are looking to start a new garden, you may be wondering if home garden soil test kits are a good alternative.

One option is to use a home test kit or handheld soil fertility meter. Many of these kits are available online or at garden centers and range in price from $10 to $60. Complete kits typically promise to test nitrogen, phosphorus, potassium and pH, which are the most important basics.

Though these test kits and meters may seem like the perfect alternative to lab testing, they typically aren’t as accurate. Several studies have evaluated some of the various test kits available to home gardeners and have found that some are more accurate than others, and that accuracy can vary depending on which nutrient is being tested for.

It’s also important to note that test kits may not be using the most accurate tests for NH soils. So while test kits may sometimes test accurately, they don’t always. They can be a useful tool if you’ve never tested your soil before, but it is still worth planning to have your soil lab tested in the future.

Perhaps the most important measurement of a soil test is that of the pH. Soil pH affects nutrient availability, soil microbe activity and heavy metal mobility. It is measured on a 0 to 14 scale, with 7 being neutral. Soil nutrients are most available when the soil pH is between 6.5 and 7.5, and as a result, many garden plants grow best when the pH is around 6.5.

However, the proper soil pH can only be determined based on what you are trying to grow. For example, most vegetable plants will perform their best when the pH is between 6.5 and 6.8, whereas blueberries require an acidic soil between 4.5 and 5.0. Though not entirely precise, garden soil test kits can help determine if the pH is within a reasonable margin for the crops being grown, and whether lime or sulfur need to be added.

Of course, one thing that home garden soil test kits conspicuously lack are tailored fertilizer and amendment recommendations. After conducting its analysis, the UNH soil testing service sends a report summarizing the test results and providing recommendations. This allows you to customize fertilizer and lime applications for your plant’s specific needs.

Following those recommendations helps prevent the many problems that can arise from nutrient deficiencies, over-fertilization or a pH that is too high or too low. If you are relying on the results of a home test kit, you should follow the fertilizer recommendations suggested in the following UNH Extension fact sheets:

Fertilizing Vegetable Gardens

Fertilizing Fruit Trees

Fertilizing Trees and Shrubs

It is important to note that many NH soils are already high in phosphorus and more should not be added unless indicated by a soil test. Excess phosphorus in the soil has the potential to run off into nearby surface waters where it can contribute to serious water quality issues. Phosphorus-free fertilizers are available online and at most garden centers.