For a newly built apartment, when and how do I start developing what's basically a building site into a healthy soil for a variety of plants?

Question

I'm about to get the keys to my new, Dutch home in a few months. It's a newly built apartment which comes with approximately 30 square meters of backyard that's free for plants, and a bit of front yard as well. Right now, both the back and front yard are just a building site, with dry brown sand/dust everywhere. I know the natural soil state of the place is dry, acidic and infertile, because of the heath and pine tree forest that's just across the road. So I know I have work to do to turn this into more fertile soil and a lush green garden, but I have no clue how to start doing this, and when/which season is the best to start this, or when I can start putting in plants.

I have already determined I want a lot of plants, and preferably also a big variety of plants, in my small garden. I'm not exactly sure what kind of plants yet, but I've been thinking about edible things (blackberries, strawberries and some herbs), pretty things (I really like clematis and butterfly bush, but also bulbs like tulips and narcissus), and insect-friendly flowers ('weeds') with an insect hotel. Most of what I will eventually plant will probably depend on what kind of environment I can realistically create though.

I've found a source suggesting it can take over a year for 'ground life' (like rain worms) to reach 'healthy' levels, and to not plant anything before that had happened. But it wasn't mentioned how to recognize "healthy", though it makes sense that there need to be some worms in a healthy garden. What I can find about turning a building site into a garden with good soil quality seems contradictory and biased, probably because this is mostly from companies or people that make their money in gardening/landscaping.

Sources seem to be divided into two camps: the first suggests to dig up the top layer of ground, as it will be compacted by heavy machinery travelling across it, and contain a lot of small bits/pieces of (inorganic) building material. They argue getting rid of it because otherwise, water won't drain well through very compacted ground and result in flooded pavements and rotting plant roots, and it's a good way to get rid of all the inorganic debris. The other side suggests that just dumping a layer of compost and putting plants in is sufficient. Digging everything up would increase the risk of pavement tiles moving out of alignment as the soil underneath them settles. Sources seem to suggest that ~30cm of compost would be enough, and can be dumped on top without the need for digging out other ground first.

I would love to do as much of this process myself, though if it's really necessary I can get a professional with their equipment to help out. But right now, I have no clue where and when to start, and I'd love some (hopefully independent from money) advice. So: For a new apartment built in an area with originally dry, infertile and acidic soil, what are some good guidelines with regards to when and how to start turning the building site into a more fertile soil for a garden with a variety of plants, and at what point I can start planting plants?

Answer 1

In my opinion (and you can probably get as many opinions about this as there are posters on this site), what you do depends upon what the builders did. The big question is: Did the builders spread spoil from the apartment across the top of the topsoil that was already on the site?

If the builder did remove most of the spoil, gravel, and debris, then you can follow the second source's advice with the compost, but with a twist. Instead of just spreading the compost on top of the soil and leaving it to decompose, I suggest that you use a rototiller/cultivator/rotovator (?) to both mix the compost into the soil and to break up the compaction at the same time.

If the builder did spread the spoil on top of the soil, then you have upside-down soil, with the spoil (sub-soil and, typically, gravel, rocks and some building debris) on top of the topsoil that was on the site. This situation is probably what the first source you cited was talking about when it recommended removing the top layer of soil (and what does that source suggest the top layer of soil is? 10cm? 15cm? 20cm?). The more you remove, the more expensive the task will be and the more topsoil you will need to replace that top layer. In this case, I suggest you follow the compost-and-till method I recommended in the other paragraph. In this case, you'll also want to use a relatively fine rake to remove all of the larger pieces of rock and debris that are in the soil. You will occasionally be removing debris and gravel from your gardens for the next 10 years.

To summarize: spread a fairly deep layer of compost and till it into the top 20cm of the soil. This will give you a quick start to building good soil.

To continue the soil-building, after planting, spread a layer of wood chips from a tree cutter/arborist on top of the soil. This should be at least 7.5cm deep away from the plants, sloping down to 1mm (yes, mm, not cm) deep at the plants' crowns. In my garden, the chips are about 2.5cm between perennials and 7.5cm around shrubs.

As the wood mulch decomposes it will feed the soil, attracting the fungi that the plants need and the micro- and macroscopic animals needed to create a healthy soil ecosystem. This decomposition will take a few years, but you can plant without waiting for it to happen if you fertilize the plants in the meantime. Two great benefits of the chips are: they hold water in the soil and suppress weed germination and growth. You will want to add about 1-2cm of chips every year or two to replace those have decomposed.

You would not put the wood chips on any lawn areas - just till in the compost and plant grass seed.

A couple of things to avoid - do NOT put cardboard of any landscape fabric on top of your soil. These are terrible for your soil and will greatly retard, rather than hasten, any soil building.

Answer 2

You can't usually just "dump topsoil" on top of what is already there, because if you add enough topsoil to make any difference (i.e. 100 to 200mm) you will raise the ground level enough to cause problems. For example the new soil will get washed over existing paths and driveways, or cover the damp course layer in the house foundations. Note, I am assuming Dutch houses are built similar to English ones, where there is a "damp course" layer (visible as a black line in the mortar between the bricks) around the whole house just above ground level.

It is certainly worth digging the whole site as deep as is practical, to find out if the builders have buried anything which shouldn't be there at all. For example I know of gardens (in the UK) which had the plastic wrapping from pallets of bricks buried a few inches below the surface, and one garden had dozens of glass milk bottles buried under the lawn!

If you want to see something growing as soon as possible, I would plant a "green manure" crop. You let this grow for a few months, and then dig it into the soil to rot down and improve the soil quality. There are many plants which can be used for green manure. Check with a local garden center what they recommend for the type of soil you have. Some green manure crops (e.g. mustard or clover) will also give you a crop of flowers.

You could then add some wood chips (as suggested in another answer) when you bury the green manure crop.

You mentioned bulbs, and Holland is where the best bulbs come from, of course! The good thing about planting bulbs is that the bulb already contains everything the plant needs to produce the flower, so whatever the soil is like you are almost guaranteed to get a good display in the first year. If the soil is poor, you won't get such a good display in the second year, but of course you can fix that problem by buying more bulbs.

So to summarize: (1) This year, focus on fixing any obvious problems with the site and grow a green manure crop to improve the soil. (2) Plant bulbs to give you a display of spring flowers next year (and don't forget there are bulbs that will flower after the daffodils and tulips, right through the summer and into the autumn - e.g. alliums, lilies, gladioli, etc. (3) Don't start your "long term" gardening plans until next year. After you have lived in the house for a few months, your ideas about what you want to do will probably change anyway.

One last thing: if you want a lawn, I will repeat some advice I got from an "old school" professional gardener in the UK that I posted here in another thread. There are two ways to make a perfect lawn. The first way is to spend six months preparing the site before you sow the seed or lay the turf. The second way is to spend six years fixing the problems you created because you didn't do it the first way :)

Answer 3

If you make raised beds, filled with commercial garden soil (tuin aarde), you can start right away. You can find instructions online on how to make raised beds, for instance videos like here.