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.
Soil Amendments:
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.
Girdling Roots
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.
References:
For many years I was amending soil for perennials and bushes and sometimes moved some to different locations and never saw roots curled because of different soils. They were penetrating native soil with no problem and spreading beyond original hole. Native soil was clay but not very heavy. The more good soil I have mixed with native soil the plants performed better, bigger hole help too. The ones planted directly in native soil did not perform well at all and never grew to full potential regardless of how much I tried to fertilize them. Not disagree with bowl effect in really heavy soil but idea to gradually amend the soil as proposed by somebody or make sure that there is a proper drainage on the bottom works just fine. Also there is something called I think replanting disease which is not fully understood especially for roses and some sources suggest replacing the soil and adding mycorrhizae fungi to promote better root development. That is a problem I am dealing with now and I have no choice than to replace soil preferably in the biggest area possible. It will be a challenge to deal with compacted soil at the bottom of the pit 😀
Wow, what a great article! This makes so much sense. But help! I’ve just now gotten my own yard and went a bit overboard. I’ve already planted two peach trees, two hydrangeas, and a rose bush in holes that I filled up with super nice expensive vegetable garden soil hoping that would give them a good starting place. I didn’t even bother to mix the soil with native. I just wanted them to grow so I used the best thing I could afford. I planted them and over the last two weeks. But they will be in for a shock when their roots smack into the dense, sticky, red clay here in northeast alabama. I want to dig them up and redo it. Will I hurt them by doing that?
no – not after a couple of weeks.
I just planted a Jacaranda which prefers well draining sandy soil into our heavy clay yard. But I did get carried away with the amending(compost+cactus soil). I did make the hole(about 7 feet diameter) almost 4 times wider than the root ball and planted in a raised mound. Should I dig it back out and move it or just hope for the best?
If it was just planted, I’d move it.
I agree with your post 100%, but having moved to an area with very heavy clay and I have many fruit trees that won’t tolerate clay, I have a question for you: if amending soil creates a bowl effect, would placing a 1.5m square raised bed 30cm high and filling it with landscape mix create the same issue? Or in that instance is it best to amend the clay under the sand fill?
A raised bed is a container. As long as the plant grows in it, it will do fine, but once it grows into the native soil, it needs to grow there.
If you want to amend – amend the whole garden.
I filled up several five gallon buckets with compacted clay, then I took out a 6″ core and replaced with garden soil and planted some tree seedlings. the roots did not girdle, they grew into the clay. do you actually have proof that this occurs in nature? Becuase I see this kind of pseudo science opinion expressed a lot, but have found zero reserach that supports it. typically they show a photo of a burlaped steel wire tree that’s been damaged in this way. i’ve yet to see any photo of an acutal palnted tree with this issue due to soil type changes.
To be honest I don’t have any evidence that the clay will cause circulating roots.
I live in Central Florida, the soil is white sand with some top soil, about 6 inches from previous mulch and planting with top soil around the plant and adding cow power. Testing has shown soil deficient in phosphorus, nitrogen and potash in most areas, sometimes critically low. Would you still go with no ammendments. Soil drainage is no problem.
I’d amend the whole area, not just the planting hole.
Your work is interesting, and I’m glad you’re taking some form of research approach when studying the effects various actions have on shrub/tree health and growth. Serious lack of real soil understanding outside of agricultural science.
However, one myth seems to keep being repeated across sites, including yours. Soils of different consistency do still have water transport between the two layers. It’s just necessary that the break is gradual and not drastic to avoid a reduction in transport. And as far as saturated soils are concerned, there is no drop in transport between two saturated soils of disparate particle size since suction forces are no longer a factor. Suction / capillary action does hold more moisture in the finer grained soil in unsaturated soils, but we’re talking percentage points, not 50% differences between a silt and a fine sand. If you’re going to amend, just make some effort to gradually reduce the amendment as you get further from the root ball so you don’t create a hydraulic break. But even this break is eventually going to reduce by being filled in with finer particles due to soil movements during percolation and freeze thaw cycles.
Clay is another beast altogether, it’s main issue being that it is frequently extremely low permeability, both to water and oxygen. Especially if it is a combination of clay, silt, sand and larger sized particles like in tills. In these cases, it can be bone dry since no water is even able to gain entry due to lack of connected pores. So it will block moisture rather than cause an issue with lack of percolation between silt/sands to clay due to suction forces.
Clay particles also have cohesion, and a preferred orientation, which means over time they will compact and regain their low permeability. Not amending a broken up clayey soil is just asking for that process to occur much sooner, and reduce the available oxygen and pore space around your newly planted tree or shrub before it’s able to expand an even more healthy root system, probably stunting it’s growth. The roots growing in around themselves can likely be avoided by doing the gradual reduction in amendment as stated above..This shows why people are constantly saying two to three diameters out even if they don’t understand why. And if you can go further, while gradually reducing the amendment until the soil is almost a native consistency, even better. A root isn’t going to suddenly change direction if you start getting a percentage drop in moisture and oxygen. So I would just half your amendment for every diameter out you go.
Since you seem to have a keen interest in testing theories, I would encourage you to try some trial and error on what I discuss above about reduction of amendment. It doesn’t have to get any more complicated than reducing amendment by half each diameter, but who knows, maybe you will find some actual semi-statistical results based on experimentation (albeit a small sample size…but this is gardening, not pharmaceutical research) and be able to make better recommendations.
1) If you create a gradual change in soil particle size, then you longer have a sudden change, and the advice still holds. It is difficult for gardeners to make such a gradual change in a planting hole.
2) Many soils do have clay as part of the soil.
3) Trees may not grow as well in clay – but that is the soil they need to grow in their whole life – so they better get used to it.
4) The kind of testing you suggest requires fields and hundreds of trees and a lot of time.
This kind of testing has been done and that is why most reliable sources not recommend not amending the planting whole.
Does this “no amendment” apply to planting roses as well? I have rocky soil, with lot of sand. I just planted my roses (don juan, golden shower and zephirine droughin) without any amendment but did add a top dressing of compost. And water well. I am worried about how much water my rose roots will actually get.
why would roses be any different?