Our Unique Clay Soils in the Willamette Valley

Christine Pritts

February 4, 2024

A Best-Practices Management

Horticultural Hints – Christine Pritts

As a new McMinnville Garden Club member, I am aware that most of our members are seasoned gardeners who have wonderful established gardens. Yet, I think it can be interesting for new members and hopefully, even long-time members, to learn more about the benefits and challenges that come with our Willamette Valley’s unique soil.

My interest started because I decided to plant a long hedge along my fence line. When I looked at most plant tag recommendations, they usually read, “Does best in well drained soils”. Some online articles went further with dire cautions, such as “ There may be eventual problems with root rot in heavy clay soils.”

I definitely have heavy clay soil in my yard, so I dig a hole larger than usual, and I incorporate lots of organic material. Yet, I am still left with the question of how to improve the soil beyond the hole. Since planting a long hedgerow can be quite an investment, I decided to do some further research.

I went to the Oregon State University Extension Service department site, “Have a Question? Ask Extension!”. This is a handy site for a variety of horticulture questions. I submitted my hedgerow planting concerns regarding heavy clay soil.

The very next day! I received the following answer from Susan, one of the department’s Yamhill Master Gardeners.

Christine, thanks for contacting Ask Extension. We sure do have heavy clay soil! And it can be difficult! A couple of cautions: DO NOT rototill it, it only makes bigger clumps. DO
NOT add sand, you’ll end up with low-grade cement.

Do be patient. Honestly it may be a few years before you will be able to plant a hedge. One possibility may be to pile on a lot of top soil alternating with a pile of compost. That will
give you the equivalent of a raised bed. Susan, Yamhill County Master Gardener

Here’s a link for a website Susan described as “particularly good because it’s local”, (Corvalis).

It starts out with a query about something I have experienced, asking, “When you walk about your yard on a wet day, do your shoes stick in the mud? Could you make ceramic pots out of the soil in your garden? If the answers are yes to both, odds are you have clay soil, one of the biggest challenges for the home gardener.”

After this entertaining entry, it gets down to science-based information about clay soil challenges & benefits, while offering the history and best-practices management of our unique clay soils in the Willamette Valley. Ironically, the article is titled, “Like Diamonds, Clay Soils are Forever”. There I found an in-dept explanation for why Susan cautioned not to add sand or rototill clay soils. Basically, in order to avoid “forever diamond-hard soil”, or “low-grade cement” as Susan described it, the reader is directed to add amendments, such as bark, manure, leaf mold and compost TO THE SOIL SURFACE.

The article elaborates upon this surface technique, stating that these materials form a protective blanket that slows evaporation and reduces soil hardening. It cautions, “It’s common for people to want to rototill or dig these materials into their garden beds, but experience has shown that the best and easiest way is to apply two or three inches of organic materials to garden beds, without trying to mix them in.”

There you have it: no digging it in, just piling it on. So simple!

Now that I’ve saved you all that digging time, if you would like to take a deeper dive into clay soil recommendations, click on the above link for even more information on this topic. The fact is, clay soils have many benefits: just look around at the many lovely gardens in our area. And there’s a reason this area is a mecca for wineries.

Yes, it’s a challenge to garden in heavy clay soil; yet with patience and repeated applications of compost, you can build local soils that will foster amazing gardens and landscapes.

If you have a horticulture question that I can look into for our next Garden Club newsletter,
please send me a note below.

Stay Calm & Garden on,

Christine Pritts

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