David Robson Extension Educator, Springfield Extension Center, University of Illinois
Yard & Garden
Proper pH Prepares Soils for Planting
How to measure and adjust your garden’s soil pH
Last month we discussed a little bit about soil pH. Soil pH is one of the most important aspects of gardening, and often the least understood because it involves the word “chemistry” and that takes many back to their high school days.
Soil pH measures the acidity or alkalinity of the soil. Some people like to refer to it as the “sourness” or “sweetness” of the soil, but those terms can cause problems. They’ll work in many cases but not all situations.
An improper pH can tie up some the essential nutrients for plant growth. It’s not that the nutrients aren’t there, but the soil particles, due to the soil chemistry (there’s that word again) bind them so tightly that roots can’t absorb them.
There are some alternatives. We can inject trees with the micronutrients, but that creates a wound that allows for decay and insect invasion.
We can apply “chelated” products such as Miracid, which are formulated in such a way that soil particles don’t grab on to them. However, chelated forms tend to be extremely water soluble, meaning they may leach through the soil before the roots can absorb the micronutrient.
A soil test is the best bet to determine the soil pH. You can buy kits at some garden centers or nurseries. The University of Illinois Extension maintains a list on our Web site of current soil testing labs. (www.extension.illinois.edu)
As mentioned last month, the soil pH really isn’t something you want to disturb unless you are absolutely sure of its reading. If the soil is basic or alkaline and you add more limestone, you can do some drastic damage. Soil pH isn’t something that changes quickly or regularly. It’s something that is modified bit by bit.
Most native soils have a fairly consistent soil pH. Only when you start gardening does the pH start to change. Adding wood ashes to your garden raises the pH. Using excess fertilizer can lower the pH. Watering can affect the pH, but that would depend on the water’s pH, which can be determined by calling your municipal water source or testing your well.
Adding limestone year after year, because “grandpa always did it” will change the pH. Years ago, it’s possible that some of the prairie soils were acidic and needed limestone. Not today.
Once you have a soil test in hand, you can determine if the soil pH needs to change.
If the pH is between 5.5 and 7.0, there’s really no need to do much. Most of our garden plants will do just fine in that range.
On the other hand, if the pH is less than 5.5, you should consider adding garden limestone to the soil. The rate is approximately 150 pounds per 1,000 square feet. The amount seems unwieldy, but it’s relatively inexpensive. If you have only a few areas that need adjusting, buy smaller bags.
The finer the limestone, the faster it reacts.
If the pH needs lowering one pH point (for example, 6.5 to 5.5), add approximately 15 to 20 pounds of sulfur. Here, the quantity isn’t much. Make sure you use elemental or garden sulfur. Dusting sulfur is a fungicide and should NOT be used.
Some of the garden fertilizers will also acidify the soil. Ammonium sulfate, iron sulfate and aluminum sulfate will do the trick over time. Word of warning: aluminum sulfate may damage some plants and should probably only be used around hydrangeas to turn the flowers blue.
Apply the limestone or sulfur in the early fall, spreading evenly over the area. Fall rains and winter snows should start changing the pH by next spring.
Wait a couple of years and test the soil again. You can test the soil again to see the effects of your efforts.
Remember, the soil’s buffering action may resist further changes.
On top of that, remember that pH is based on multiples of 10. In other words, it took 150 pounds to make the soil more alkaline. It won’t take 300 pounds to move it another point, but 1,500 pounds per 1000 square feet.
All you can say about that is “Wow.”
David Robson is an Extension Educator, Horticulture, at the Springfield Extension Center, University of Illinois Extension, P.O. Box 8199, Springfield, IL 62791. Telephone: 217-782-6515.
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