By Alby Thoumsin

In my work as an arborist, a large portion of what I do is consult with tree owners and talk about the health and recommendations to grow healthy trees. Even if a large portion of what is ailing trees comes from foliar fungi, insects, root diseases, cankers and viruses — which are in the majority of the cases visible above grade — a fundamental reason why trees are not doing well has to do with soil quality. 

Before planting a new tree, I always recommend first matching the tree species with the soil type. Depending on where you live in Eugene and Springfield, the soil quality varies greatly from the best (River Road and Ferry Street Bridge areas with many parts of downtown) to the worst (South Hills, many parts of southwest Eugene and the Eugene wetlands.) 

A good soil includes a balanced amount of sand, silt and clay, high organic matter content, excellent microbiological activity that will benefit from good structure with great pore space to ensure natural drainage. These are ideal conditions for any plant to thrive, be more resilient to diseases or insects and if under attack, in a better condition to recuperate and resume good growth. 

In most urban settings, many soils have been heavily compacted, contaminated or scraped off, so any new planted tree already starts its life with an uphill battle. Unfortunately, very little is done for the soil to be improved. Having worked for a few landscape companies, I remember we were focusing more on production than long-range vision of a healthy landscape. Budgets are allowed for the conventional (lawn, a few shrubs, a couple of shade trees and irrigation) but never or very little for the essential: the soil quality. In many cases, we had a boring repetition of the same plant/tree species no matter what the soil type was! 

Checking for soil quality takes a little bit of work, and the ideal tool to have is a soil probe that will allow you to retrieve a core sample of your soil that you can directly analyze. Ideally and simply put, you want to have a good balance of moisture, dark chocolate color, worms and a good earthy smell. An anaerobic soil will have a dense clayish texture and often smell bad and a soil that is too sandy drains too fast and contains hardly any organic matter. 

Annual replenishing of bark-o-mulch can lead to problems also, especially in sunny areas. It takes a few years for mulch to properly decompose and become soil and since new mulch is virtually void of organic matter, a thick layer will be hydrophobic and the water coming from irrigation heads, especially spray nozzles, will simply bead off like rain on Gore-Tex! 

An easy trick is to check the drainage; if your yard turns into a pond after a good rain and it takes days for the water to percolate into the soil, you know for sure that drainage is an issue since an excess of water will make the soil unlivable for everything. 

So, what can be done to improve soil quality? Drainage is one way to help but it doesn’t fix the issue of the lack of organic matter. The affordable and fairly easy solution is called verti-mulching. The technique consists in either boring or blasting (under air pressure) holes into the soil to be filled with a good compost filled with organic matter and micro-biology that will literally inoculate the soil and start multiplying by colonizing it. 

The boring is usually done with a 2- to 3-inch wide auger, the blasting by a special tool called Air Spade® that is connected to a compressor and has the advantage to break apart the soil and increase pore spaces. I have used the latter technique and have seen amazing improvements on the tree we helped. Incorporating great soil into poor soil is a key to success, no matter the technique. If you DIY, be sure to call 811 before you drill. 

Just like humans, trees and plants need a great environment to live well and a healthy soil is the launching pad.

(Note from Kim: Sperry Tree Care offers the Air Spade service, or you can call around to other companies.)

Alby Thoumsin is a ISA certified arborist, RCA registered consulting arborist and TRAQ qualified through the International Society of Arboriculture. He can be reached at Alby@SperryTreeCare.com or 541-461-1737. Kim Kelly edits the Garden Palette.

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