Homegrown Column June 13, 2009
Just ran a simple home soil test. It shows the pH is at 7.0 but phosphorus, potassium are high and nitrogen is low. What can I do to improve the quality of my soil for better plant growth and lawn?
The numbers you have are very typical for soils here in the valleys of western Colorado.
Our soils tend to have pHs in the 7.0–8.0 range, are high (very high in many cases) in phosphorus and on the high end in potassium while nitrogen is low.
They tend to have a clay texture with poor drainage and aeration.
When I talk to people about improving the soil, I’ll usually ignore the high pH and the high levels of phosphorus and potassium. Though most plants prefer a neutral to slightly acid pH, it doesn’t interfere with plant growth unless you’re trying to grow a very acidic demanding plant such as a rhododendron or a blueberry.
In addition, it’s extremely difficult to lower the pH in our soils quickly.
The usual material used to lower pH is sulfur. The problem with that is that we have tons and tons of calcium in our soils (it’s called “buffering capacity”), and the calcium combines with the sulfur to form calcium sulfate, also known as gypsum. Gypsum is slightly basic and is a salt.
The result is that the pH isn’t changed to any appreciable degree and the salts spike up.
Not a good thing.
Nitrogen is usually low in our soils because they’re low in organic matter.
The way to approach improving our soils is to incorporate good amounts of well-decomposed organic matter. This will improve drainage and aeration as well as provide the organic matter healthy soil needs.
The combination of adding organic matter, regular watering, growing plants and time will start to change your soil into a better, more manageable soil.
Now, this is all well and good if you’re preparing a new area in the garden or if you’re dealing with an annual flower or vegetable garden, but you mentioned a lawn.
You really can’t add organic matter to the soil efficiently without tearing up the lawn and starting over. What I’d do is core aerate the lawn once or twice a year. This will provide openings to the soil for organic matter to filter down. The soil will improve, but it will do so very slowly.
When fertilizing, the primary need of your lawn is nitrogen. I like slow release forms of nitrogen the best. I think it’s OK to add a little bit of phosphorus and potassium since they will deplete slowly over time.
You also want a good amount (3–4 percent) of iron in the fertilizer as well.
Our plants can go iron-deficient in our high pH soils, so adding some regularly will help with those deficiencies as well as start to drop the pH into more of an neutral to acidic range.
Can I put cedar mulch or chips around roses?
Cedar mulch makes a wonderful covering mulch around all plants, including roses.
It helps hold water in the soil so you don’t have to irrigate as often, it moderates soil temperatures, which roses especially benefit from, and it helps keep weeds down.
Commercial cedar mulch (the stuff you buy from a garden center in bags or in bulk) is primarily a mixture of the tree’s bark (that’s the stringy stuff in the mulch) and a bit of the light tan sapwood chips.
You might be a bit careful if you’re using cedar wood chips because it will have a lot more of that cinnamon brown heartwood. Though it rarely causes problems, the heartwood contains chemicals that can occasionally impede the growth of any nearby plants.
I’ve used cedar mulch many times on all kind of plants (including roses) with great success.
Dennis Hill is the nursery manager at Bookcliff Gardens, online at http://www.bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or e-mail info@