In the garden | Put the dirt on your dirt | Gardener’s

As the gardening season winds down, it’s a great time to reflect on the past year and look to the future. Soil tests are a great way to put some data and information behind your decisions for next year. I often test in the fall, assuming my tests will take into account last year’s soil use and help predict what next year might be like.

This is a great way to take your ground pulse now or collect a data point for later reflection. Additionally, it provides a window into plant health, as soil health is often linked to many common plant ailments.

Interestingly, a lot of gardeners I talk to rarely test their soil or often have never tested their growing areas. However, many of these people regularly add fertilizer to their garden soil as a means of keeping plants healthy and productive, especially in vegetable gardens, as these plants often have high nutrient requirements.

Over-fertilization and the subsequent movement of excess nutrients into our streams and rivers is a major water quality problem nationwide. Although much of this non-point pollution is usually attributed to industry, in recent years some research has shown alarmingly high levels of nutrient runoff from urban areas. Therefore, it is still important that smaller scale lawn and garden applications stay within recommended limits.

One way home gardeners can address this water quality issue is by using soil tests and applying appropriate fertilizers based on the results. In many cases, the soil test results I see around central Illinois show more than adequate nutrient levels, especially for potassium and phosphorus, indicating a low need for most fertilizers. Vegetable production areas can deplete over time, but most landscape samples I see need very little in the way of fertilizer.

Perhaps a bigger question in landscape soil analysis is the pH level. Whether it’s trees and shrubs or perennials supported by these soils, or even lawns, I rarely see a sample that falls below recommended nutrient levels. However, I regularly see high pH which can affect the plant’s ability to absorb nutrients. In some cases, a nutrient may be abundant in the soil, but because of the pH level, it may be unavailable to plants.

The best example of pH-driven nutrient deficiency in landscape plants can be seen with iron chlorosis. Plants suffering from chlorosis show yellowing leaves that often retain their green color along the leaf veins, which is the classic indicator. In these cases, the plants suffer from an iron deficiency that results in yellowing of the leaves, a general decrease in vigor and sometimes death. However, iron is a major constituent of our soils. It is incredibly abundant but not available to the plant because the pH is high and the plant simply cannot take up the iron in the soil.

Iron chlorosis is an extreme example where iron is almost completely unavailable to the plant, but in other cases nutrient uptake may simply be limited to some degree by pH. Based on extensive research, we know the pH range for optimal uptake by most plants and soil nutrients. Because it is a range, there are times when the pH reaches the high (or low) end where the plant can get some of the nutrients it needs, but not enough.

We may not see as strong an indicator as chlorotic yellow leaves, but we can often tell the plant is struggling because of slow growth, smaller leaf size, minor damage, or other vigor indicators low. Many of us want to start looking for an insect or pathogen as the cause, but pH can be one of the underlying factors.

So, I often recommend a soil test when clients call our office looking for help with plant problems. They are relatively inexpensive, but provide ample data to better paint the picture of what is wrong with diseased plants.

If you decide to test your soil, be sure to send samples to an accredited soil laboratory. Home test kits simply do not provide accurate results, while laboratory tests are performed according to established protocols that maintain consistency and provide reliable results. These tests usually cost between $15 and $30, and there are plenty of local soil testing labs in our area. You can either take a sample in person or mail your samples to the lab.

For a list of accredited laboratories in Illinois and more information on soil testing, please visit UI Extension’s soil testing website at extension.illinois.edu/soil/soil-testing.

Ryan Pankau is a horticulture extension educator with University of Illinois Extension serving Champaign, Ford, Iroquois and Vermilion counties.