So many of you have been in contact with us about how to improve clay soils that I will describe the problem and solution here.
Soils consist of mineral (clay, silt and sand/grit) and organic (humus) products in varying proportions.
In heavy, clay soils the balance is too far towards clay and silt with very low humus levels. The clay particles are able to hold strongly together with no interference.
There is nothing we can do about the soil "texture" related to the mineral content so we must work on the soil "structure" which is related to the organic or humus content.
The effects of increasing organic content is two fold. Firstly by coating the clay particles it physically disrupts the bonds between them. Secondly, one of the breakdown products of organic material as they rot is called Glomalin and has the opposite effect, it causes clay particles to bind into aggregates. However these aggregates greatly add to the friability of the soil as a whole.
The addition of organic matter is labour intensive but over time a stable, humic fraction will develop.
It is important that the organic matter you add has been composted. This is because the humified nature of compost makes it more resisitant to further decomposition. In this situation we are adding organic matter for soil structure reasons rather than nutrient supply.(in other words dung is not ideal for this task.)
What type of products are available?
There are three main leading products for this task:
1. The cheapest and longest acting product is composted bark. This is a by-product of the production of decorative bark. It is the fines from the grader. It has been composted and is ready for use.
2. The next product up in cost is green waste compost. This is recycled organic matter and carries a higher nutrient status than composted bark. You would be using this where you are looking for long term fertility increases. (for example in vegetable plots)
3. The most expensive, and now discouraged, is peat. This is very good to work with but the ready availability of bulk compost should make its use less common.
How much should you apply?
Of all these products you are looking to add 3-4" (75-100mm) depth. That means that a 1000L pack will cover 10 square metres.
You must then dig this into a depth of 8" (200mm). These products will only move very slowly through your soil profile and only if you have an active earthworm population. So mix and incorporate the material well when you apply it.
You should be repeating this process 2 or 3 times over the next few years to get to the stable levels you are looking for.
For the record, the product we recommend for soils pre-turfing is the composted bark. We do advise application more than one month before turfing to allow the initial settlement stage to happen naturally.
You can use a rotovator for the incorporation, however check you are getting sufficient depth.
