On-site effects of soil erosion

, and the reduced water-holding capacity of many eroded soils.
In affluent areas of the world, accelerated water erosion’s on-site effects upon agricultural soils can be mitigated by increased use of artificial fertilisers; however, this is not an option for much of the earth’s population.

Erosion’s removal of the upper horizons of the soil results in a reduction in soil quality i.e. a diminution of the soil’s suitability for agriculture or other vegetation. This is because the eroded upper horizons tend to be the most nutrient-rich. Also, because the finest constituents of eroded soil tends to be transported furthest, eroded soils become preferentially depleted of their finer fraction over time; this often reduces their water-holding capacity.

In other words, “Erosion removes the cream of the soil”. Increased use of artificial fertilizers may to an extent, and for a time, compensate for erosion-induced loss of soil quality where economic circumstances are favorable.

This is not usually feasible in developing countries however. Loss of soil quality is a long-term problem; globally, soil erosion’s most serious impact may well be its threat to the long-term sustainability of agricultural productivity, which results from the “on-site” damage which it causes.

Crops are particularly reliant on the upper horizons of the soil, which are the most vulnerable to erosion by water and wind.
In this sense, erosion removes “the cream of the soi”. Agricultural tillage also redistributes soil, resulting in thinner soils on topographically convex areas within a field.

The damaging on-site effects of erosion, in terms of decreased agricultural yields, are well known in the developing countries of Africa and Asia.
But even in the developed world there is cause for concern. Water erosion is locally severe in Australia, New Zealand, parts of the US, Southern Europe and Eastern Europe (often as a result of the former large state-controlled farms).

In erosion-prone areas of the more affluent countries, productivity may be maintained in the short to medium term by increased fertiliser input.
The effects of erosion are thus rarely acknowledged by farmers in richer countries. This strategy is however infeasible with regard to erosion in developing countries.

Off-site effects of soil erosion
In addition to its on-site effects, the soil that is detached by accelerated water or wind erosion may be transported considerable distances. This gives rise to “off-site problems”.

Water erosion’s main off-site effect is the movement of sediment and agricultural pollutants into watercourses. This can lead to the silting-up of dams, disruption of the ecosystems of lakes, and contamination of drinking water.

In some cases, increased downstream flooding may also occur due to the reduced capacity of eroded soil to absorb water. Movement of sediment and associated agricultural pollutants into watercourses is the major off-site impact resulting from erosion.

This leads to sedimentation in watercourses and dams, disruption of the ecosystems of lakes, and contamination of drinking water. Rates of erosion do not have to be high for significant quantities of agricultural pollutants to be transported off-site.

This is a shorter-term impact than loss of soil quality; in the more affluent areas of the world it can be the main driver for present-day soil conservation policy initiatives.
A more minor off-site effect can occur in situations where eroded soil has a decreased capacity to absorb water: increased runoff may lead to downstream flooding and local damage to property.

Another major off-site impact results from the agricultural chemicals that often move with eroded sediment.
These chemicals move into, and pollute, downstream watercourses and water bodies.Where inputs of agricultural chemicals are high – as in the more affluent nations – costs of removing such pollutants from drinking water can be considerable.

Therefore the on-site impacts of soil erosion are a present-day problem for many of the developing nations.
Such on-site impacts will be a problem only in the long term future for developed areas: as such they are outside the relatively short time horizon within which their policy makers work.

In the short term, however, erosion’s off-site effects can be a notable problem for developed nations. Off-site impacts may therefore be the major driver for policy changes in such countries. – soilerosion.net