Monitoring soil moisture as we enter Spring is common, however, soil temperature can be easily overlooked. Why are Spring soil temperatures important to look at in the first place? Root growth is typically initiated once soil temperature reaches 45° or higher. Coming out of Winter, trees are just beginning to utilize nutrient reserves left in the soil from Fall. These reserves typically aren’t enough to meet the demands of the tree. When it comes to Spring fertility, knowing when feeder roots are most active can give you a more accurate indication of when roots are likely to take up nutrients, allowing you to better time applications. Below you see 2 graphs showing the swing in soil temperature from early morning to late afternoon. Although we don’t see big swings from the daily lows/highs, you can see our soil temperatures begin to trend upward as we move into Spring.

 

 

 

While it can be worth your while to monitor Spring soil temperature, the same idea applies for summer. Here in Eastern Washington, it is not out of the ordinary to get weeks of temperatures soaring into the high nineties or the hundreds.  With sustained heat like that, it can become harmful to a tree’s roots. According to Washington State University researchers, optimal temperatures for apple roots range from 65°-77°F (rootstock dependent). While soil temperatures above that can stunt root growth, temperatures reaching around 86°F or higher can be deleterious to roots. It’s also important to note that temperatures of that degree tend to affect first-year plantings due to how shallow the rooting systems are, but a lot of these concerns can be easily offset. A few ways to mitigate high soil temperatures are through proper watering, cover crops, mow, and blow (blowing cover crop trimmings into the tree row), as well as establishing a healthy canopy.