Soil Erosion

I hope that I’ve convinced you by now that soil is important to the sustainable gardener/allotmenteer. Therefore I now turn to the process that would remove this precious substance from us: soil erosion. So why is soil erosion a problem? Well, in some cases it can be irreversable, meaning that agricultural land can be lost forever (and with a rising population is that sensible..). In less severe cases it degrades the soil quality, neccessitating more use of fertilisers and soil improvers. The soil washed away is deposited somewhere – fine on fields – but more normally deposited in rivers, causing eutrophication from high nutrient levels and clogging of important water ways. All in all, it’s bad news.

Soil erosion can occur through three mediums: water, wind and tillage. Water erosion is the use of water, through rain and or water flow, to remove soil. Wind is the removal of soil through – well – blowing it away. Tillage usually includes anthropogenic forces as well as animals moving soil or disturbing it so that it is more suseptable to wind or water erosion.

Soil erosion is affected by a number of factors. RUSLE (Revised Universal Soil Loss Equation) identifies: climate, cover and management, slope, erosion control measures and soils. Although soil erosion is sometimes portrayed as being beyond our control, each of these factors can be reduced. So, taking each in turn from the perspective of an allotmenteer:

Soil: Sandy loam and loamy clay soils are most prone to soil erosion. However, soil which is high in organic content is most resistant to soil erosion. It’s pretty difficult to change the essential nature of your soil, but adding organic content will always be beneficial.

Cover and Management: Bare soil is nearly always the biggest factor in soil erosion. If the soil is covered, it is able to resist erosion much better. So during the winter, keep crops in, cover the soil with carboard or a mulch or let some weeds grow. Anything except bare soil. Seedlings do not provide much cover. For instance, for maise, the soil loss ratio (SLR) varies from 0.77 to 0.35 depending on the stage of growth, so consider mulching around them with dried leaves or similar. Weedkillers are bad news for soil erosion as weeds cover the ground – any bare ground left by weedkiller will be prone to erosion so it’s best to avoid it. Similarly, tillage through digging up the ground also increases the likelyhood of soil erosion. No-till or mulch policies are much more efficient at dealing with weeds and preventing soil erosion. The sort of crop that you grow also effects the likely amount of soil erosion. MAFF (1999) identify vegetatables as a particular risk, so attempts to reduce all other factors are sensible.

Slope: Steeper slopes are more prone to soil erosion by a significant amount. Therefore avoiding using these areas for crops that are more susceptable to soil erosion, or even planting them up with trees might help this mildly unavoid able problem. (See erosion control measures.)

Climate: Interestingly, it isn’t the quantity of rain that effects soil erosion so much as the intensity and duration. For instance, in Mediterranean shrub lands, erosion initially increases with decreased rainfall up to 280 -300mm per year, then decreases with decreased rainfall thereafter (Kosmas et al, 1997). Subject to climate change however, there is not much you can do about climate. The UK is not especially prone to soil erosion compared to medditeranean or other dry regions, but still levels of soil erosion in the UK are often unessasarily high.

Erosion control measures: This factor usually includes things such as terraces, to stop water taking soil from slopes or mulches to provide a sacrificial top layer. If your allotment is on a slope, terraces of some description may be a good option to stop your soil migrating down. Mulching is also well worth while as it adds organic content to the soil which reduces soil erodibility (the propensity of soil to erode) as well as protecting it.

These are just some of the ways that erosion can be prevented. Erosion is virtually nil with natural vegetation in all but a handful of parts of the world. It’s just a matter of growing the things we want and need in as sympathic manner as possible.