Soil aeration and drainage
Over the years, I’ve experienced only minor problems associated with inadequate soil drainage. This surprises several of my gardening colleagues since I have frequently worked in regions where rainfall exceeds evaporation and contributes to an elevated groundwater. In retrospect, I was fortunate to work on private estates where adequate garden drainage was designed and installed, sometimes dating back to Elizabethan times. Of course, natural determinants of poor soil drainage can be sufficiently inherent to defy normal engineering attempts to resolve them.
Soil drainage properties can usually be explained in terms of the geographical location, surrounding physical topography, and the compositional structure defined within the various soil layers. For a physical scientist, drainage is evaluated with minimal reference to the effects on a soil’s fertility and its capacity to nurture healthy plant growth. Gardeners will appreciate that a vegetable like the potato can only be planted in soils with optimum drainage and aeration characteristics. Alternatively, there are well known species like the cranberry which succeed admirably in heavy, waterlogged conditions. As much as possible, I try to match individual plant species according to the aeration and drainage characteristics of specific garden locations. To assist me in this, I often depend on nothing more than general observation and some simple field testing.
Why aeration and drainage are important?
Good soil aeration is important for many garden plants. At the structural level, it is dependent upon the amount of pore space existing between the soil particles. There is also an effect determined by the physical composition of these particles, particularly their ability to form aggregates. Otherwise described as clods, the physical cementing of smaller particles provides a mechanical strength which enables pockets of air to be maintained within a soil despite heavy watering from above or the gradual rising of groundwaters from below. When reliable drainage is hampered, air pockets surrounding soil particles are replaced by water. This waterlogging restricts the availability of gaseous oxygen which is essential for healthy soil organisms and normal root development.
As the available oxygen decreases, the normal concentration of carbon dioxide may increase to a level which is toxic for many plants. Carbon dioxide is produced by living organisms and usually comprises around one percent of the gaseous substances held within soil. While carbon dioxide concentrations for well aerated soils are approximately ten times the normal atmospheric level, poorly aerated and waterlogged soils will exceed this by multiples. The extreme effects of restricted aeration and drainage can be observed in a peat bog. These will generally be colonised by bacteria capable of surviving with minimal oxygen. Because they possess limited abilities to decompose plant materials, extremely waterlogged soils tend to contain high quantities of unprocessed organic material. This problem is often due to poor soil structure combined with an exposed or depressed location that collects rainfall and condensation.
Checking your soil
When doubtful about a particular section of garden, I use a simple procedure to test the soil for adequate drainage. The first step is to dig a series of thirty centimetre deep holes approximately one metre apart. These holes are deliberately filled with water from the hose. The next step is to evaluate the amount of time required for each of the water filled holes to drain completely. Provided the water drains completely within several hours, the soil will suit the majority of garden plants. Should some of the holes still contain water after this period, the possibility of a significant drainage problem must be considered.
As mentioned earlier, most of the drainage issues from my own gardening experience have been relatively minor and were able to be solved with simple remedies. In the past, for example, I have treated damp clay soils with a mixture of gypsum and coarse river sand to improve their drainage characteristics. Gypsum provides a soil with the abundance of calcium ions which subsequently lowers the concentration of sodium. Apparently, it is high levels of soluble sodium which disperses additional clay particles and therefore contributes to the poor drainage observed in these difficult soils.
Occasionally, it will be helpful to undertake some structural solutions to a mild but persistent drainage issue. Structural solutions are suitable for small problematic sections of the garden, particularly when these would otherwise disrupt the natural flow and harmony of design. Raised garden beds are easy to manage and provide an effective remedy for inadequate drainage. A structural barrier is built to contain a growing medium which is rich in organic materials, and of a texture that supports a healthy mixture of air and water around soil particles. A base of gravel will enable excess moisture to be released from the bed, protecting soil and plants from the risk of becoming waterlogged. These are not always easy to build as some might claim. I usually advise novice gardeners to seek qualified assistance when designing and constructing their first elevated garden bed.
When it’s serious
Serious drainage problems can be difficult and expensive to correct. It is often expedient to consider practical steps and solutions before installing plumbing or undertaking structural improvements to land or property. Some soils drain poorly due to surface compaction and will benefit from a digging over with the incorporation of organic matter. If this fails initially, the top metre of soil can be removed in small sections to enable a ten centimetre layer of coarse gravel to be laid as a supporting foundation. The soil is then mixed with additional organic materials and carefully replaced.
Land drains can be installed to channel water away from sections of the garden. They may provide the best practical solution in regions with an elevated water table or those prone to periodic flooding. They are also appropriate whenever water threatens housing or the safe use of an area. Often requiring high levels of technical expertise, land drains should be designed with input from qualified landscape consultants, even a hydrologist in the case of serious drainage problems.