Deep root fertilization is a recommended procedure by many arborists. Does it work? Is it the best way to fertilize trees? Do trees need to be fertilized?
Deep Root Fertilization – What is it?
Deep root fertilization for trees is a process where you stick a pipe down into the soil about 8-12″ and then, under pressure, squirt fertilizer into the ground. The theory is that since tree roots are deep down in the ground, the fertilizer would also need to be put deeper in the ground. Since this process requires special equipment, it is usually done by an arborist.
It is interesting that the International Society of Arboriculture (ISA) recognizes this as a gardening myth, and yet many ISA certified arborists still sell the service? This is what the ISA says:
“you don’t need to perform “deep root fertilization” to reach their root system-most
of the trees’ fibrous, absorbing roots are in the top eight inches of soil”
A recent review of available literature on tree fertilization in the USA by Daniel K. Struve (ref 1) concluded that “Little difference has been found among fertilizer application methods; broadcast applications are as effective as subsurface applications”.
Note added June 2014: One of the people adding a comment below suggested that deep root fertilization could be done as a DIY (do it yourself) project using a very simple device available from hardware stores. When I first wrote this blog I was only considering the application done by an arborist, which is very expensive. In this situation the original post is still correct. If done as a DIY project, the cost is much less and maybe it is no longer a waste of money. I hope to do a future post looking more closely at DIY deep root fertilization.
Most fibrous absorbing tree roots are found in the top 2-8″ of soil where water and oxygen are abundant. Fertilizer that is placed below this level does little for the tree and is in fact environmentally harmful.
The key nutrients required by the tree are nitrogen, phosphorus and potassium (NPK). Let’s look at each of these nutrients.
Nitrogen moves through the soil very quickly and is probably the nutrient your tree needs most. An easy way to feed your tree with nitrogen is to just spread it on the ground. It will dissolve in water and flow down to your roots.
Phosphorous does not move through the soil very quickly and so adding it lower down in the soil seems to make sense. However, most soils in North America have plenty of phosphorous in them. Unless a soil test indicates differently, or you know that the soils in your area are phosphorus deficient, you don’t need to add phosphorus.
Potassium moves through the soil relatively quickly, but not as fast as nitrogen. Most healthy clay soils have enough potassium.
Should You Fertilize Your Trees?
If you have healthy soil you probably don’t need to fertilize. The problem in our urban landscapes is that we remove the tree leaves each fall. The leaves are natures way to fertilize the trees and by removing them you are removing the food for future years.
Tree roots that are covered by grass add an additional problem for trees since the grass roots compete with tree roots for nutrients.
In either of these situations it does make sense to feed your trees, but deep root fertilization is a waste of money. Just take normal fertilizer and spread it on the ground. If you are fertilizing your lawn, you are also fertilizing your trees. Let nature move the nutrients to the roots.
- Top Seven Myths of Tree Care, by ISA: http://www.treesaregood.com/searchResults.aspx?q=deep+root&cx=010107291988267817406%3aanhvlksvvbw&cof=FORID%3a9
- A Review of Shade Tree Nitrogen Fertilization Research, by Daniel K. Struve.
- Photo Source: Larry D. Moore