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? They are important to look at because 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 may not always be enough to meet the demands of the tree. When it comes to applying fertilizer in Spring, knowing when feeder roots are most active can give you a more accurate indication of when roots are likely to take up nutrients. Below you can see 2 graphs showing the swing in soil temperature from night to day. Notice that we only see a big swing in soil temperature on the top 4-8 inches of soil while the other sensors stay fairly level. This is important to note because most feeder roots are found within the top 12 inches of soil.

While the change in soil temperature from 4-8 inches won’t affect root health much in the Spring months it can have a great effect come summer. According to Washington State University researchers, optimal temperatures for apple root, 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. Therefore being able to monitor soil temperature through the entire growing season can be of great benefit to any grower.