How To Prepare Soil For Planting Trees, Shrubs & Hedges

This article is primarily about preparing soil before planting trees (ornamental or fruiting), hedging, roses, and other shrubs.

Things are a bit different when considering vegetable beds, where investing more resources and energy might be appropriate to improve the top layer of soil, and to create a raised bed, perhaps using No-Dig methods.

Soil preparation is so important before planting in order to help plant roots establish quickly.

Like so many things in gardening, there is not one right way to do it, but there are some wrong-ish ways. 

In most locations, you will be working with one of three general soil categories:

• Clay soil: Heavy, not very dark, cracks and crusts in dry weather, feels smooth when moist & you rub it between your fingers, you can make it into a sausage shape that won't crack when you prod it.
• Good garden topsoil: dark and forms little crumbly clumps when moist. When you roll it into a sausage shape, it will crack apart when you prod it.
• Poor soil (sandy, chalky, or rocky): if your soil is light, gritty, and won't form a sausage maybe unless it's soaked, and the shape will just fall apart when you prod it.

Why is Soil Preparation Necessary?

The core reasons for soil preparation are:

• To break up soil compaction, thus opening the way for new roots to penetrate deeply. 
• To remove weeds, grass, other roots, and large stones.  

Digging in well rotted organic matter (like compost or manure) is optional.
Adding organic matter to the planting hole is helpful in many cases, unnecessary in some, and in the case of heavy clay, it is not really a good idea.

Is Soil Preparation Always Necessary?

When making new gardens, beds, or otherwise growing in uncultivated land, good soil preparation makes a huge difference.

• If your garden soil is already rich and well cultivated, then there is often no need to add anything extra: just dig it over to break it up.

In your garden, soil preparation & improvement should always be feasible for your ornamental or fruiting plants where it's needed. They are significant investments, and you want to see them have the best start. 

• In large scale field & forestry planting, soil preparation is usually not practical.

For planting large scale rural hedges & forestry, it is normal practice to kill the weeds with herbicide, then plant the hedging using the slit planting method demonstrated in our Country Hedge Planting Film.

But I was Told that Digging in Organic Matter was Essential!

The most important organic matter you add to the planting site is not what you put in the hole before planting, it is what you spread over the soil after planting, and every year or two thereafter: mulch. 

Mulch can be almost any green waste, and unlike the well-rotted organic matter that you would add into the planting hole, mulch does not need to be decayed at all.
You only need to shred or chip woody material into little pieces.

What are the best tools for Soil Preparation?

The essential hand tool for digging over soil prior to planting is strong digging spade. A fork is very handy: you use the spade to move the soil aside, and the fork to break it up and mix it, especially at the bottom of the hole.
Many gardeners prefer a mattock to a spade on rougher ground with lots of roots.

If you have the strength for it, a pickaxe is also excellent, even on light soil free from stones. 
A pickaxe can penetrate deeply and lever up sections of soil like no other hand tool; when using it this way, sink the head into the soil, then lift the end of the handle with your arm straight, acting as a hook, pushing with your legs, as if you were trying to step forwards and stand up tall at the same time.

Soil Preparation to improve Heavy Clay Soils

• Soil compaction tends to be worse on heavy clay, so breaking up the soil to a good fork's depth at the bottom of the planting hole is often very beneficial, as is working over a larger area than strictly necessary for the planting hole. 
• However, clay soil should not be worked when it is wet and sticky in Winter, partly because it's very difficult, and partly because you force all the air out, and then it turns into a brick when it dries. In general, avoid walking on clay soil beds in Winter.
• So, ideally prepare it early (Autumn or early Winter during dry weather) by digging over a good size area around the intended planting site.
  If you miss the opportunity and have to plant in Winter with no prior preparation, it is better to simply dig a hole big enough for the roots and pack the clay back in, without struggling to work over a larger area. 
• An empty planting hole in clay is like a sink that will fill up with rainwater. If you add a lot of organic matter to the backfill, you are creating a sort of sponge that most plant roots (roses are a prime example, which love clay soil) won't like: it's too wet, and the soil may not be firm enough to support the roots in windy conditions. It can also create a situation where the roots fill the improved soil in the hole as if it were a pot, and don't penetrate into the surrounding soil, which is bad!

The Difference Between Improving Planting Holes in Clay VS an Entire Bed

We advise against adding organic matter of any kind into planting holes or trenches on clay soil.

Instead, (unless you are using a mulch fabric) apply mulch over the soil after planting, and every year thereafter: let the worms mix it down for you.

If you are improving an entire bed or other large area, especially with the help of machinery, then it makes sense to dig organic matter into the whole section, as it won't be trapping the moisture in one pocket as it would in a single planting hole or trench.
This is a common practice for making vegetable beds on clay soil, where the aim is to raise the whole bed several inches in order to improve drainage, and help the bed to warm up in Spring. 

There is also no point in using clay improvers for clay soil on a small scale: it's a waste of your money. 
Clay improvers are either gypsum based for neutral to alkaline soils, or you would use lime for acidic clay soils. 
As with digging in organic matter, it might make sense to use these for an entire raised bed where you will grow prize vegetables, but otherwise they are either unnecessary or plain pointless.

Mixing in grit and sharp horticultural sand (not smooth building sand) is neither practical nor recommended. You have to use tonnes of the stuff to have any effect out in the garden, and it makes the soil prone to erosion.
Grit and sharp sand are useful for turning heavy clay into a potting medium for containers - one half clay, one quarter small grit & sharp sand, and one quarter compost makes a good, inexpensive potting mix. 

Soil Preparation to improve Good Garden Soils

• Soil compaction tends not to be a big issue on good topsoils due to all the soil life, so less effort is needed to work it over.
• Moving the topsoil to one side as you dig, you will typically reach the lighter coloured (which indicates less organic matter) subsoil somewhere between 30-60cm down (if you dig down more than about 75cm, and it's still all lovely dark topsoil, great! You don't have to add anything to such good soil).
• This subsoil level is where you want to use the fork or pickaxe to break up the compaction, and then mix in some organic matter.

As always, after planting, apply mulch around the area, and then yearly (unless you are using a mulch fabric).

Soil Preparation to improve Poor, Dry, and Chalky Soils

• Soil compaction is often somewhat of an issue on poor soils devoid of much soil life, but unlike clay they are easy to dig over at any time of year.
• These are the soil types where it really pays to mix in plenty (up to half of the backfill) of well rotted organic matter, as wide and deep as feasible. 

As always, after planting, apply mulch around the area (unless you are using a mulch fabric).
You will often find that poor soils seem to "drink" mulch: it vanishes down into the soil in a matter of months, and so a twice yearly application may be necessary

 

Key Takeaways

Soil preparation is primarily in order to break up the soil and to remove weeds & other roots.

Digging in organic matter is not recommended for clay soil, optional for good garden topsoil, and recommended for poor, dry, sandy or chalky soils.

Clay soil should not be worked when it is wet in winter, so try to prepare it in advance of winter planting.

Whatever you do, the most important soil improvement is not really what you do at planting time, but the mulch you apply afterwards, which can be a mulch fabric, or an application of almost any organic matter one or twice a year.