Soils are the key to growing your own successfully and if you are able to excavate soils in the course of building your raised bed garden, then these should be treated with the care deserving of something that is actually close to 10,000 years old, and with an incredibly fragile architecture! It is very easy to loose soil through carelessness and to damage soil structure irreparably, but both can be avoided by the following.
a. Remove vegetation from soils before they are excavated. On existing lawns, remove the turfs as thinly as possible (no more than 40mm). If these can’t be reused they should be stacked upside down in a shady corner to rot down (covering these with black plastic will speed the process up). If you are replacing a shrub bed, make sure you dislodge all of the soils from the ‘root-balls’ before these are discarded.
b. After clearing vegetation, excavate the soil with care - ideally with a digging fork and breaking the soil down as finely as possible to remove any clumps of soil bound with roots. This is quite hard to do below turf because the thatch is so dense, but it’s worth persevering given the high cost and poor quality of most (but not all) bagged topsoil! Digging needs to be done when the soils are moist to achieve the best results with the least effort!
c. Try to dig only the upper dark brown / black upper topsoil layer which contains the most organic matter; this varies in depth but will often be 250-300mm thick. The underlying subsoil will be lighter in colour and will often contain more stone and be more compact.
d. Remove any obstructions identified in the course of digging - particularly stones, brick and large roots. Set any earthworms aside (you can re-home these later)
e. Screen soil using a 10mm x 10mm sieve. This is time consuming but really rewarding as it gives the soils a really light and friable consistency, into which it’s easy to mix compost
The demands on soils to produce a succession of high-quality crops are extremely high, and meeting this requires a high level of fertility. This is achieved most effectively by the addition of large amounts of compost.
Extensive guidance has been prepared by Ergrownomics on the types of compost and the quantity of compost that should be added to soils, in order to bolster these for use in your raised beds. The compost types and the ratio of soil:compost vary depending on your soil texture. Greater quantities of compost are required to achieve the workability of sands and clay rich soils, compared to loams. This guidance is available here, and includes an instructional video on how to mix the soils and to fill your beds. These demonstrate that we are not asking anybody to do anything we wouldn’t ordinarily do ourselves! The resource available also includes tables confirming how much soil you’ll need to make for each model of planter.
The guidance referred to above includes a ‘Buyers’ guide’ which should be essential reading if you have to fend for yourself in finding a good supplier of soils! This is not that easy, but is essential to ensure that the time and effort taken to design and build your raised garden is rewarded.
The backfilling of containers is the final stage of building a raised garden. This is straightforward, but again we’ve produced information with useful tips for getting it right.