The art of ground prep
Minimising ground preparation for maximum benefit
The forestry sector is seeing an increasing focus on sustainable ground preparation, preserving soils and minimising disturbance. Preparing the ground for planting is a critical stage in any woodland creation project. It often involves large machines and specialist operators working on site for extended periods.
There are a range of good reasons for carrying out sustainable ground preparation:
Breaking a pan – A plough pan or iron pan layer in the soil could restrict rooting depth and tree growth. If the pan occurs in the top 50cm of the soil it is likely to have a long-term effect on the stability of any trees planted.
Controlling surface vegetation – Weed vegetation can compete with trees for nutrients. Tall plants and grasses can fall over the trees and smother them during the winter. Bracken can also be a significant obstacle to planting if planters struggle to fight their way through.
Improving aeration – On soils that are prone to water-logging, increasing the amount of air in the soil will improve the rooting environment. This improvement is temporary, so it is still critical to select tree species which can deal with the underlying conditions.
Increasing fertility – Mixing the organic and mineral layers of a soil will improve nutrient availability. Again, the effect is temporary so the tree species must be suitable for the underlying soil fertility.
Clarifying the planting positions – Helping the planters to clearly see where the trees are to go makes the planting design easier to deliver. If future access for weeding operations is needed, a consistent planting pattern helps to locate the trees.
Different types of ground preparation
There are a range of techniques available to help foresters achieve these aims. Each one creates a different effect and has different advantages and disadvantages.
- Ripping – A shallow tine runs through the soil to break up a pan or compaction layer. The tine should never be deeper than 50cm as there is no benefit to disturbing the soil any deeper. This technique is not suitable for clay or peat soils where the tine lines would carry water.
- Scarifying – Specialised scarification machines rotate large discs through the surface layers of the soil. In most cases this creates a straight line with planters choosing a planting position in the exposed mineral soil.
- Mulching – A mulching head on a conventional digger is used to mulch and mix the surface vegetation and the topsoil layers. Tough mulching teeth are required if the machine is to break into the soil.
- Inverted mounding – An excavator is used to turn over the surface layers of the soil to a depth of about 20cm. The extracted material is placed back in the hole that it came from.
- Hinge mounding – An excavator is used to turn over the surface layers of the soil and place it next to the hole that it came out of. The topsoil layers are doubled over and a pit left beside the mound.
- Hand screefing – The planters select a planting position and clear a small patch of surface vegetation with a spade. This is done immediately before planting.
Mix and Match
The art of forestry is finding practical ways to implement the science in a complex, evolving ecosystem. However real world sites will have a variety of soil and vegetation types which would create different ‘right’ answers. Foresters must rely on our judgement to pick the ‘best’ option for the site.
There are other factors which influence the decision, such as the terrain and the accessibility of the site to machinery. Linear techniques are not likely to be suitable on steep sites where they might start to carry water. There are also sites where straight lines would be inconsistent with the landscape.
Where there are multiple suitable techniques, the scale of the site is important to determine whether it is financially sensible to bring more than one machine to site. It is also important to bear in mind how the site will develop through time. Exposing mineral soil creates a good planting position in the short term. Yet it also opens up a seed bed for weeds, including weeds which might be more troublesome to control than the original sward.
Choosing a sustainable ground preparation technique is a classic forestry problem, requiring us to balance a wide range of factors. Being clear on the goals we are trying to deliver helps to ensure we act with awareness of our impact. We aim to turn the traditional assumption that ground preparation is always required on its head. Instead, ground preparation should only be specified for a specific reason. By doing this we can be sure that it is the minimum intervention required.