IN THE GARDEN: What’s up with pH levels? Knowing your soil’s acidity can be crucial to garden health

Friday, May 17, 2019 - 10:16
  • Spring Creek Farm assistant production manager Phoebe Autry picks a turnip on Tuesday, Sept. 19, 2017. The six acre vegetable farm, owned by Alaska Pacific University (APU) is part of APU’s nearly 900-acre Kellogg Campus in the valley. (Loren Holmes / Alaska Dispatch News)

The role of pH in growing a healthy garden should not be overlooked.

In Alaska we generally have very acidic soils. The role pH plays in gardening is not talked about that much and is one of the most overlooked discussions in gardening. For the home gardener, pH is the key that can unlock the door to your garden success. This discussion will attempt to take a good look at why this is true.

Understanding pH and its relationship to a plant’s ability to grow is paramount. The pH scale runs from 1 to 14. Neutral is 7, below 7 is considered acidic and above 7 is considered alkaline. It turns out that plants, like the human body, are very particular about their pH. We know that some plants thrive in an acid pH, like azaleas and rhododendrons. Garden vegetables on the other hand prefer their pH to be around 6-7. Each plant does have a preferred range to grow its best, but as a general rule, this range works well to allow the nutrients to be taken up by the plant. You can look up your particular crop and find the pH it prefers pretty easily.

For a plant to be able to absorb their needed nutrients, the nutrients work best when dissolved in a water solution. If the plant’s soil pH is not right, the plant will lose some of its ability to absorb needed nutrients and will not grow to its potential.

You can buy pH kits at most all garden outlets and online and they are numerous. There are some good pH meters being made now and they are getting fairly accurate. The least expensive I’ve seen is $6. These kits tell you how to check your soil and give a pH color graph to compare with your results. This was very eye-opening for me when I checked my soil and found it to be very acidic at 4.5. It is no wonder that I could not grow certain plants in my yard!

To change your soil from acidic to neutral, you will do what is called, “sweetening” your soil. The most common way to raise the pH of your soil is to add lime. Calcium limestone is primarily calcium and dolomitic limestone is a combination of calcium and magnesium. Wood ash is composed of about one-third calcium and significant amounts of magnesium and potassium. Which amendments you choose will depend on your soil’s specific needs. Read the directions on the bag to be sure not to overshoot.

Alkaline soils require the addition of acidic organic matter such as pine needles, leaf mold and peat moss or wood shavings. Not only do these decrease your pH, they also help to improve soil content and structure. As we add store-bought wood chips to our soil, we need to realize and take into account that we are adding an acidic product to our soil and though it looks great, it is further decreasing the acidity of our already acidic soils. Yikes, guilty here for sure!

The best time to correct the acidity of your soil is in the fall. If you find an acidic soil in the spring however, you will want to deal with it at least three weeks before planting, as it takes a while for the lime to work into the soil. To change alkalinity, because these organic amendments tie up nitrogen while they decompose, it is best to add these in the fall when your plants are not actively growing.

Plants will let us know their nutritional problems through their leaves. For example, leaves showing iron deficiency turn yellow. Iron is only required in small amounts but it is one of a plant’s essential nutrients whose solubility and therefore uptake is affected by pH. In pH values over 7.0 only 50% of iron is available to the plants. As long as the pH is in the 6.5 range, 90% of the iron is available to the plant.

I have not paid much attention to the pH of my soil until just recently. I was surprised how huge my rhubarb grew last year with the addition of lime. Rhubarb I found likes a pH of 5.5 to 7.0 and over the years has been very forgiving of my ignorance of its pH needs, doing its best to survive despite me.

One of the reasons potatoes do so well in Alaska is that their pH requirements are 4.5-6.0. Carrots also like a lower pH of 5.5-7.0. Cabbage on the other hand, likes a pH of 6.0-7.0. Who knew that a plant could get so particular!

We learn as we go and this gardener needs to understand how and why plants in my garden excel or do not. It is all a science experiment really and I absolutely love it, so stay calm and garden on. You can do this!

Chris Wood is a master gardener from Eagle River. Email her at [email protected]

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