Trees need a variety of nutrients to stay healthy and grow. One important nutrient is nitrogen, which helps to produce chlorophyll and other vital compounds. However, nitrogen is only readily available to trees when the soil pH is between 6.0 and 7.0. When the soil pH is higher than 7.0, it becomes harder for trees to access the nitrogen they need. This can result in stunted growth, yellow leaves, and other symptoms of poor health. If you suspect that your tree’s soil has a high pH, you can have it tested by a professional. Once you know the pH of your soil, you can take steps to adjust it if necessary. For example, adding sulfur to the soil can help to lower the pH and make nitrogen more accessible to your trees. By taking care of your tree’s needs, you can ensure that it stays healthy and thrives for years to come.

High Soil pH: What does that mean for the health of your trees?

You have probably heard me talk about our naturally high soil pH at some point or another. This can be a problem for many of our trees in Sioux Falls. It is a principal reason this is called the prairie along with our natural weather patterns.

Understanding pH

pH is the measure of the concentration of Hydrogen Ions (H⁺) in a solution. The more Hydrogen the more acidic; the less H⁺, the more alkaline. pH is also a logarithmic measurement scale. It goes from 0 to 14 with 7 being neutral. What that means, is the amount of H⁺ ions in the solution or not in the solution, doubles (or 10-fold increases) the further away from 7 the pH is on the scale. For example, the alkalinity of 7.2 is double that of 7.1. And 7.3 is double of 7.2.

Why does this matter?

High soil pH (Alkalinity) changes the chemistry in the soil and binds up many micro-nutrients, making them unavailable for plants not naturally used to high soil pH. Trees and plants need those micro-nutrients like iron and manganese to make chlorophyll and other basic plant functions. It is a common misconception that Iron Chlorosis is an iron deficiency in the soil. When in actuality it’s absent in the plant because of the soil pH.

As a matter of fact, a similar situation is true on the Acidic side of the pH scale as well. In most of the United States (and North America) the soil pH is acidic (5.5 – 6.5). Therefore, most plants have evolved to deal with and thrive and grow in acidic soils (mostly trees). But, there are challenges there as well; Most food crops and forbs (flowers, clover, small annuals, and perennials that are not grass) prefer a more neutral or slightly alkaline soil to perform well. That is why you always will hear in most gardening books to add lime to your soil. That helps to raise soil pH. Sulfur will help lower soil pH.

If you look at an overall map of what naturally grew across the continent, you could roughly map out where the high soil pH and low soil pH are. Now I’m going to admit, that this is very grossly oversimplified. But, the point I’m trying to show, is that to have healthy trees in an ecosystem that historically was all grass, it has its challenges.

What can you do?

The first thing to remember is (due to a lot more chemistry explanations), that we can almost never permanently change a soil’s pH. We can move it up or down ways for a short time, but it will naturally balance back to where it traditionally has been. It’s called buffering capacity. And the soil buffering capacity in our Sioux Falls region is very strong.

But all is not lost. There are a few tricks and simple tips that you can follow to help your trees thrive in a prairie landscape.

• Ammonium Sulfate (AMS) – This is a fertilizer used all the time in farming. It is made up of 21% Nitrogen, 0% Phosphorus, 0% Potassium, and 28% Sulfur (21-0-0-28S). For the life of me, I don’t know why more hardware stores and Garden centers don’t carry this in 50lb bags for use in flower beds and in yards. So it is hard to find locally, but if you go sweet talk at the co-op, you can get it there. And it is not expensive.
  1. Apply to trees under the drip-line at about 1-2 lbs per inch of trunk diameter. Mix it in if you can, but watering it in lightly can work.
  2. For flower gardens and lawns apply at about 20lbs per 1,000sq ft.
  3. This is best applied in early spring or late fall.
  4. Apply about once a year or every other year depending on your soil. Be careful not to add too much! Conduct a soil test periodically to make sure you are on par with what you are adding!
• Add in organic matter to the soil! This is why you always hear me talk about mulch around your trees.
  1. Your soil test will tell you what percent of organic matter you have. Ideally, there should be around 7-8% organic matter in your soil.
  2. Adequate organic matter will result in less need to fertilize.
  3. Organic matter naturally buffers soil pH closer to neutral.
• Be careful how you water.
  1. Too much water can raise a soil’s pH. This has been very prevalent this year with the high amounts of rain we have been getting.
  2. Trees prefer long waterings with large spaces in between. Lawn irrigation schedules are often too frequent and promote higher soil pH’s.
• Last but absolutely not least, choose a tree that can handle the soils you have! The reality is we can try all we want to grow some types of trees here, but long term they will struggle because of how our soils are (function, structure, and pH).

In Closing

I think the largest message I wanted to get across with this article, is to help you understand why we have such limited variety of trees to choose from and why so many trees that people try and plant, struggle. So much of it is out of our hands because of where we live. But that doesn’t mean we can’t have nice plants, it just means we have to be smart and sometimes work a little harder. We need to carefully choose what we are planting and where we are planting it. pH is a huge reason why we have plant health issues, but with a few simple tweaks and considerations, it should be a major road black to healthy plants.

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