Written by s • Using Organic Material When Planting Trees & Shrubs

Back when I was a student of horticulture, the use of synthetic chemicals for gardening was widespread. Since then most home gardeners have moved away from them, and for many today they are ‘poison’. The value of organic matter has grown enormously for gardeners, and methods of growing based on compost and soil additives has become normal. I have written previously about organic matter on earlier blogs, but recently I have seen an increase in suggestions that we have gone too far. Nothing odd about that! We all tend to believe that, “if some is good, more is better” (an adage that causes more harm than good!)

So, I thought it would be a good idea to explore this issue, and try to find some clarity between the belief that organic material should always be mixed into the soil when planting trees, and the opposite instructions being increasingly given to plant directly into ‘native’ soil, without adding anything to it. ‘Native’ soil is simply the soil you happen to already have in your garden.

Buying Topsoil for a New Garden

Let’s get one issue out of the way. Despite it still be required in the specifications of garden designers, cities and property developers, there is no value, and plenty of potential harm, in buying truck loads of ‘topsoil’ for planting your garden. In fact, it isn’t ‘topsoil’ at all – no farmer is going to sell something so precious. It’s a blend of around 25% of some kind of compost, mixed with sand. If you don’t believe me, put some into a large jar, fill with water, shake and leave overnight. You will see the layers easily. After a few years the organic material will have mostly disappeared (as CO₂ and methane) and you will be left with sunken beds and poor, dry, sandy soil.

Far better is to work with the soil you have, unless you need to create raised areas. If you do need to work with bought-in topsoil, start off by tilling or digging the existing soil, and the work part of the new soil into it, adding it gradually and tilling as you go. Use plenty of walking and foot action as you add soil, to keep it compacted, and even use a compactor if the layers are thick. That way you won’t create a solid layer where the old soil begins – something that can, and does, cause drainage issues.

Start by Testing Your Soil

Having a professional soil test done on your soil – ideally from a few different locations if your garden is large – is always the best way to start a new garden. Besides the obvious value of pH in choosing plants, a clear idea of your soil ‘texture’ – sand, silt, loam, clay, etc. is very helpful. Levels of phosphate are also useful, since it is common to add some when planting. If those levels are good, save the money on bone meal and phosphate fertilizers. Don’t bother with nitrogen tests, as these are too variable with the seasons to be much use.

In writing earlier about this I neglected to include testing for organic matter, but I am strongly suggesting that you ask for an Organic Material reading. If you find you have 2% or more of organic material in your soil, then there is no strong reason at use any organic materials when planting trees and shrubs. There, that was simple! Levels of 5% would be better, but that is as much as you ever need for trees and shrubs.

What Happens to Organic Material in Your Soil?

No matter how much (or little) organic material you add to your garden beds, most of it disappears over time. It takes about 10 years for it all to go, which explains why beds made of ‘topsoil’ shrink  alarmingly after a while. Since 2% to 5% is ideal, the solution is adding more. The good news is you don’t need to dig it in. Instead, mulch your beds with something organic every few years (depending on how quickly the material you are using decomposes). A 2-inch layer is all it takes. Here I will be a bit more challenging, but don’t use landscape fabric or mineral mulches like gravels and stones, except for visual effect. Landscape fabric is a gift of the devil, in my opinion, and organic mulches, from wood chips to manure, are so much more effective than stones. Organic mulches break down over time, migrating into the soil and keeping the organic matter levels high indefinitely. It’s how it operates in natural places, as leaves and debris fall to the ground.

When Can Adding Soil Amendments be Useful?

Regular digging in of compost and other amendments is always going to be necessary for vegetable gardening, growing annual flowers and perhaps for perennials too. All of these plants grow rapidly in a short time, and need all the moisture and nutrients they can get. Even so, you absolutely don’t want more than 10% organic material in your soil, or problems can develop with your plants, and you will be contaminating the groundwater with nitrogen, adding to pollution. So don’t overdo it – more is not always better. . .

If you have very sandy soil, and your organic material readings are below 5%, then your soil can’t hold nutrients – they just wash straight through it with rain and watering. Organic material holds those nutrients, so I would always add some – 10 or 20% by volume – when planting into very sandy soils.

The second situation is with very heavy clay soil. The big particles in organic material open up the soil, improving water flow through it, so adding some to the backfill is usually valuable (but see lower down about planting).

Plant Properly!

Much of the controversy about adding amendments is not about the amendments at all – it is about the way they are used. Digging a deep, narrow hole in existing soil, hardened by time or construction activity, is a recipe for killing trees and shrubs, no matter what you mix or don’t mix with the soil you put back.

The issue being raised by critics of organic material is often called the ‘bathtub effect’. What happens if you dig a narrow, deep hole is that when you water, or it rains, the soil in the planting hole fills with water that then only slowly finds its way into the hard, less porous surrounding soil. (Google ‘perched water table’ for the technical explanation). This is made worse not only be the water retention in the amendments you add, but also by the soil in the pot the plant arrives in. It is 100% organic material, and holds a lot of water. As well, once the roots grow out, they soon encounter the hard sides of the hole, and spiral around, instead of spreading outwards. That keeps them trapped in what amounts to a big pot, and over time they will suffer.

The solution is to prepare beds, not planting holes. Till or dig the whole area of your beds, adding organic materials if needed, depending on your organic matter reading and type of soil.

For planting lawn specimens, or new plants into  old beds, dig a wide, shallow hole, just deep enough to bury the pot to the soil level inside it, but at least 3 times the diameter of the pot. You can dig the area where the pot isn’t going to sit deeper, but don’t disturb the soil directly underneath the plant. This is vital. Scrape and break up the sides of the hole, so that roots can penetrate it when they get there. Adding small amounts of amendments will help

In the end, most new trees and shrubs do badly, and maybe die, because they were either a) not watered enough, or b) planted too deep. While all these issues about soil amendments are true, these two things account for most of the trouble. So don’t sweat this amendments issue too much, and if you don’t use them, make sure you add them as mulch instead, every few years at least.