It has been a while since my last blog article, apologies for that but I have been working hard preparing a couple of papers for publication and I’ve recommenced playing the piano after so many years so lunchtimes are often spent in the university practice rooms at the moment. Today though I’ll be taking a break from the literature and the keys to discuss the ways in which soil can be affected by the type of plant that grows within it. This is a bit of a topic of fascination for me, although admittedly no longer my chosen research area, during my undergraduate years I studied the ecological and agricultural effect of plant on soil and it is good to come back to this now.
As you might expect, plant and soil interaction is a two way process. Farmers and gardeners will tell you that you can’t just put any plant anywhere, even if the climate is correct, the soil must also be correct for the plant to thrive. You can adjust the soil sometimes a little more easily and less expensively than you can the plant’s surrounding climate (unless you have access to greenhouses or other such structures). For example, a wheat crop will need a range of nutrients if it is to grow to its full potential and therefore the farmer will add fertiliser to the field. Nowadays, in the advent of precision agriculture, it is no longer necessary to spread the same fertiliser over all your fields. This could be considered wasteful and it certainly can be expensive. Soils are known to be very different from each other even at small scales (heterogeneity) so why should all soil be given the same treatment? Precision agriculture aims to readdress the balance between soil and additive, to ensure better uniformity of nutrient availability across the field and save the farmer time and money on fertiliser applications.
Agriculture and forestry has done much to change the landscape and the soils that form it. By selecting what plants will grow on a patch of soil, people have made long-standing changes to the chemical, biological and physical nature of the soils below. For example, let’s take a fictional pine plantation. Forty years ago, a patch of deciduous woodland was chopped down for timber and because they are fast growing and easy to mill, tall pine trees were planted uniformly in its place. Quickly the acidic and slow to decompose pine needles get to work on changing the chemical balance of the soil. The efficient roots of the pine soon reduce the diversity of plants below the forest canopy and therefore addition of other types of decomposing plant matter to the topsoil diminishes. Eventually a new topsoil will develop that is shallow, acidic and slowly decomposing. The soil type will be very different from how it was when the naturally occuring trees occupied the soil forty years ago.
A reverse example is the reclaimation of heathland by deciduous woodland in the UK. Heathlands are a man-made landscape, created due to continuous grazing by livestock of previously wooded often sandy soils. Today, heathlands are less regularly grazed and slowly they revert to their natural state as trees seek to reinstate their claim on the soil. Managers of heathland have the perennial problem now of preventing this recession to woodland and they do so by various methods; burning, manual removal of trees and reintroduction of grazing to name the most common. Depending on the usage of the heathland site, each of these methodologies have their own pros and cons. It is important to remove the trees at an early stage partly because of the effect that the trees have on the underlying soil type. A good heathland needs nutrient poor, sandy and well drained soils to establish long-lasting heather and other heathland vegetation. An example of this type of soil is shown in the below picture.
This was taken in 2008 on Chobham Common, an established heathland site in the South East of England. You can see there is a thin dark top layer and then a white-grey sandy layer. Below this is another shallow dark layer followed by gravelly sand.
The following graph shows a walk through a section of Chobham Common that is slowly reverting to birch woodland. The colour bars represent different ‘layers’ in the soil profile and the key thing to notice is that there are two different soil types here. The black-grey-black type (correctly called a ‘podzol’) as described above is represented by colour bars with light blue and yellow in them. A more uniform brown soil type with a thicker and lighter organic top layer is shown by bars that are missing the light blue and yellow.
The research showed that the birch saplings at 0-2m and 6-10m had already had a noticeable impact on the soil type despite being young trees (probably less than 5 years old). Established birch trees were positioned at 18 to 20m into the ‘transect’ walk. A later study by a consultancy company looked into this data alongside their own investigations and provided advice to managers of Chobham Common to help them manage their heathland effectively. Chobham Common is now in the midst of grazing trials and hopefully, reintroduction of animals to the site will help to keep the trees back and protect the man-made environment that has become a home for so many specialised plants and animals.
In my next article on Ground to Sky, I will talk a little about one of my current research articles which focuses on the effect of cattle on the greenhouse gas emissions from a peatland. I will discuss how the experiment was designed and some of the conclusions, which give you a little more insight into how land management can affect the soil and atmosphere.