Almost every book, and most web site tells you that you should amend your soil before planting a new plant. This seems to make a lot of sense. Few of us have perfect soil and we don’t want to put our new expensive plant into poor soil. If we amend it, the plant should grow better? That’s a common garden myth.
In this post I will talk about trees (including shrubs) since they can be affected more by this process than a perennial, but the principles apply to all types of plants.
In a previous post called “Gravel at the Bottom of the Pot Helps Drainage“ I talked about the movement of water between two different types of soil. When different soils are next to each other, water tends to stay in one of them and not the other. Usually, water stays in the soil with the finer particles – say clay.
You are planting a new tree, and you want to do everything that you can to make it grow well. You dig a hole and examine your soil. It might be very sandy, or it might contain a lot of clay. You decide to add organic matter to ‘condition the soil’. You complete the planting process, and water well. What happens?
You have created a big hole in the ground. Around the outside of the hole you have your normal native soil. Inside the hole you now have a different kind of soil – it contains more organic matter and is your amended soil. You have created the same condition we talked about in the previous post, namely two types of soil in contact with each other. We know that water has difficulty moving between two types of soils. You have created one of two problems depending on the type of native soil you have:
1) you have created a hole that retains water. Excess water sits in the hole and does not move away, drowning the tree roots.
2) you water the area but water tends to stay in the native soil and does not enter the hole. Your tree roots are dry.
Neither is good for your tree.
You have also created another problem that is even worse for your plants. You have created the perfect situation to encourage girdling roots.
Think about your plants roots. They start to grow and find themselves in some fairly good soil. Lots of air, nutrients and hopefully water. All goes well and the roots continue to get bigger. One day they reach the edge of the hole. Roots don’t have a brain and can’t think through the process, but plants are able to direct root growth to areas where they are finding the best growing environment. Roots start to grow into the native soil. Here they find less air if the soil is high in clay, or they find it very dry and nutrient deficient if the soil is sandy. Neither is good for roots. The plant then decides to put its energy into growing roots in the better soil. Since the hole is a round shape, the roots start growing round and round in the hole.
This is no different from the roots growing in a pot. You have all seen a potted plant with an overgrown root system. Roots go round and round.
For trees and shrubs this type of growth will result in roots choking the plant. The roots of trees and shrubs get thicker ass they get older and so they start choking each other. They can even choke the main trunk of a tree. Trees can die from this problem ro at the very least grow poorly.
Have a look at the damage that can be caused.
Don’t Amend Soil
The solution is both simple and inexpensive. Dig the hole, and replace the soil. Don’t amend it. Less work, less expense, and better for the environment.
Tree and shrub roots will have to travel great distances during their life time. Most of that will be done in your native soil. They might as well get used to it as soon as possible.