Island gardening

My friends in neighbouring villages often complain about how hard it is gardening here - they push a spade into the soil, and hit hard stone just below the surface. But my experience is one of deep soft soil, in which I have been able to plant 26 trees - and all the tree species currently there seem to flourish. I wondered why the difference? On the map I see we live on a raised area of land at the head of the streams that run down to the Dordogne and Vézère Rivers. Maybe that explains it.

We moved here at the right time. Had it been 100 million years earlier, we would have found ourselves submerged in the warm shallow sea which then covered most of the area now called France.

That was the age of the dinosaurs, who waded through the marshy landscape. In Angeac, 100 km north-west of us, excavations over the last 12 years have yielded the bones of the largest dinosaurs to be found in Europe - estimated to have been up to 33 metres in length and weighing up to 85 tonnes.

The dinosaurs walked the land for the next 34 million years, in the period known as the late Cretaceous, whilst in the shallows microscopic animals lived and died and left their calcium rich remains on the beds - solidifying to form layer upon layer of limestone. This scene was brought to a catastrophic end 66 million years ago, when a huge meteor struck the earth, where Mexico now is, resulting in the destruction of most of life on earth - known as the 'Cretaceous-Paleogene extinction'. The dinosaurs disappeared (with the exception of some of those which had evolved feathers, and went on to evolve into a diversity of birds) but there survived primitive flowering plants, insects, amphibians and small reptiles and mammals.

Continued movement of the earth's crust and its collisions caused areas of Europe to be lifted up, including a broad flat plateau now known as the Massif Central. Erosion of these lands formed sediments which, during the Eocene period 56 to 34 million years ago, were washed into the shallow seas of Perigord and blanketed the limestone. France began to have mass and take shape, and on this the flowers, birds and mammals flourished and radiated.

The earth grew cooler, sea levels fell, and erosion continued. Rain washed sediments towards the sea and cut into the soft limestone forming rivers, cliffs and caves - leaving between them plateaus known as causses.

Away from the reaches of the rivers and streams, the surface layers were not washed away. A look at a geological map shows that the area between the Dordogne River to our south and the Isle River to our north is a patchwork of Eoecene sediments, surrounded by Creataceaous limestone. We live on an island of sediment at its southern edge - an island of soil for gardening - with limestone below. 

I decided to look at the composition of our soil. I first analysed soil from the edge of the orchard which slopes down to the road, where I have planted my plum trees, which seems sandier than elsewhere. Maybe years of rain and erosion have leached the finer soil particles out and down the hill, or sand from the higher part of the orchard has been washed down (as clay particles can bind more tightly and resist erosion). The soil in the upper part of the orchard is indeed heavier, being more moist and even waterlogged in winter. I collected a trowel full from a mole hill, shook it up with some detergent in a jar of water and put marks where the sediment settled after 1 minute (sand), 2 hours (silt) and 48 hours (clay). A simple calculation told me that this soil is 85% sand, 12% silt and 3% clay.

The United States Department Of Agriculture triangle can be used to clasify soil texture. It shows this soil to be a loamy sand - a light soil which drains quickly and is therefore low in plant nutrients. I will need to watch the trees here during droughts, and add fertilizer as required. I guess the soils in the higher and main area of the orchard, and the rest of the garden, are probably sandy loams or sandy clay loams - good well balanced soils which avoid the extremes of hardening from clay and infertility from sand, and are regarded as the best soils for gardening.

Another important feature of soil is its pH - a measure of its acidity or alkalinity on a scale that goes from 0 (most acidic) to 14 (most alkaline) with 7 being neutral. Here is my simple understanding of pH: water molecules contain two hydrogen atoms and one oxygen atom (H2O). These molecules randomly bump into each other resulting in some losing the 'postive' part of a hydrogen atom to become negative -HO, whilst others gain a postive part of a hydrogen atom to become positive +H3O. Acids are molecules that are postive, and can donate hydrogen; alkalines are those which are negative, and can accept hydrogen. In pure water these charged molecules are in equal concentration, neither acid nor alkaline - a neutral pH of 7. Rain water is not neutral. As it falls through the atmosphere carbon-dioxide, among other chemicals, dissolves into it and increases the proportion of H+ by forming carbonic acid - decreasing the pH of rain to around 5.6, medium acidic. The reaction of the chemicals in the soil with rain water will also affect pH, and this will determine the amount of elements in a form available for absorption by plants. For most plants a pH just below neutral is optimum for the availability of the elements they need to grow and reproduce.

I tested the pH of the soil, and I interpret the colours on the wick on the far left as representing a pH of around that of rain water at 5.6. This might be expected for a sandy soil, as it is very porous and small particles which might increase alkalinity will be leached by rain. The measure here is basically the pH of rainwater around large particles. Maybe the heavier soils in the main part of the orchard have a higher pH - which would be ideal. 

I will analyse soil from other areas of the garden in Spring. In the meantime, I'll be content gardening on my Eocene sediment surrounded by my Eocene flowers, birds and mammals, looking down at my Cretaceous dinosaur neighbours in the valley below.