Everybody understands that soil is the 'stuff ' that covers most of our land surface, and acts as a medium for plant growth, but what exactly does soil consist of? All soils consist of particles of weathered rock and minerals, and particles of organic matter, all mixed together. It is varying proportions of all these constituents, which when combined, give rise to a particular soil type.
The main difference between a sandy soil and a clay soil, is the size of the mineral particles. We have all walked on a sandy beach, and although some sands are finer than others, when we hold sand in our hands we are aware of distinct particles which are easily seen with the naked eye. It feels rough and gritty to the touch, and there is room between the particles, which means when dry that there is plenty of air between the particles, and when wet, that it can drain easily. Now consider a lump of clay, which many of us will have come across at school. Here the mineral particles are extremely small and cannot be seen with the naked eye. They are closely packed together, allowing for little air to enter. When wet, clay does not drain easily, becoming sticky and malleable. Indeed, it is these properties that make it so useful for modelling.
Of course, beach sand and modelling clay are the extreme cases, but it is the above properties that give rise to the differences between a sandy soil and a clay soil, and now we will discuss each in turn. Sandy soils are well drained and light, and are usually easy to dig. They are quick to warm up in spring, but are generally not very fertile, containing little in the way of organic matter. They are usually acidic in nature, having a pH of less than 7. Their fertility is affected by the fact that they drain so well, any nutrients are easily washed away. The addition of bulky organic matter, such as home-made compost, will help a sandy soil retain moisture and nutrients. Sandy soils contain lots of air, and plant roots can grow relatively easily into it. Root vegetables, such as carrots and parsnips, can easily grow long and straight in sandy soils.
A heavy clay soil can be back-breaking to dig. As clay is poorly drained, it can often become waterlogged over the winter, and is slow to warm up come springtime. However, clay soils are usually very fertile and full of nutrients, and once improved, can give rise to bumper crops. Here the addition of bulky organic material helps to create space between the fine clay particles, allowing air in and excess water to drain. Digging in coarse sand or grit will also help with this process. The surface of clay soils tends to bake hard in hot summers, and this can hinder the watering of plants. Unless improved, only plants with very rigorous root systems will grow well in clay soils. Some gardeners will add lime to a clay soil, as this helps the very fine clay to aggregate into larger particles, again improving aeration and drainage.
Most garden soils will be neither an extremely sandy or clay soil, but will lie somewhere in between. One way of knowing which type of soil is found in your own garden, is by getting your hands dirty. Dig down a couple of inches and pick up some of the soil. Now squeeze your hand into a fist and compress the soil. A clay soil will retain the shape you have squeezed it into, and may even be worked by hand into a ball. If you do the same with a sandy soil, the soil will not retain its shape, and cannot be worked into a ball. An ideal soil, or loam, contains approximately 20% clay, larger mineral particles, and good amounts of organic matter and nutrients. Loam will retain its shape when gently squeezed, but cannot be worked into a ball, and can be easily crumbled.
Whether the gardener has a sandy or a clay soil, both types can be improved to aide plant growth. There are plants which grow well in clay soils, and plants which grow well in sandy soils, and as such soil type should be no hindrance to the creation of a beautiful and productive garden.