"The threat of nuclear weapons and man's ability to destroy the environment are really alarming. And yet there are other almost imperceptible changes - I am thinking of the exhaustion of our natural resources, and especially of soil erosion - and these are perhaps more dangerous still, because once we begin to feel their repercussions it will be too late." (p144 of The Dalai Lama's Little Book of Inner Peace: 2002, Element Books, London)
Transcript
"The threat of nuclear weapons and man's ability to destroy the environment are
really alarming. And yet there are other almost imperceptible changes - I am
thinking of the exhaustion of our natural resources, and especially of soil erosion - and these are perhaps more dangerous still, because once we begin to feel their
repercussions it will be too late." (p144 of The Dalai Lama's Little Book of Inner
Peace: 2002, Element Books, London)
Soil erosion is the #1 source of pollution to surface water in most states. Each year rainstorms and snowmelt wash
tons of dirt off the land.
How could something so ‘natural’ be so bad? Soil erosion is natural after all.
However, when we change the landscape from forest to yards, streets, farm fields, shopping centers and roads,
we accelerate soil erosion.
•In the USA, soil is eroding at about seventeen times the rate at which it
forms.
Soil is naturally removed by the action of water or wind: such 'background' (or 'geological') soil erosion has been occurring for millions of years. In general, background erosion removes soil at roughly the same rate it formed.
But 'accelerated' soil erosion is a far more recent problem that results from overgrazing or unsuitable cultivation practices since the first European settlers arrived.
Accelerated soil erosion by water or wind may affect both agricultural areas and the natural environment, and is one of the most widespread of today's environmental problems. It has impacts which are both on site and off site.
More recently still, the use of powerful agricultural implements has, in some parts of the world, led to damaging amounts of soil moving downslope merely under the action of gravity: this is so-called tillage erosion.
The GLASOD project has produced a world map of human-induced soil degradation in three sheets at an average scale of 1:10M (Mercator projection). The map was digitized afterwards and stored in GIS format with attribute database and supplementary statistics on the extent and degree of degradation
Loss of soil usually means loss of topsoil (first). Topsoil contains the most Organic matter and nutrients, and has the most desirable soil structure.
Bad for the land and bad for the final resting place.
First concerns were post-WWI. Soil Conservation Service was formed in 1935 to study this problem.
Annual loss can be as high as 300 metric tons per hectare per year (2.5 cm), which means the whole plow layer would be lost in 6-7 years. (Geological erosion is about 0.2-0.5 t/ha/yr.)
Anything greater than 10 t/ha/yr is considered to be ‘serious’.
Despite the global nature of the problem, we do not have good information regarding the global extent or severity of erosion by water.
The GLASOD study estimated that around 15% of Earth's ice-free land surface is afflicted by land degradation. Of this, accelerated soil erosion by water is responsible for 56% (11 million square km) and wind erosion - 28% (5.5 million square km).
The area affected by tillage erosion is currently unknown.
Because soil is formed slowly, it is essentially a finite resource. The severity of the global erosion problem is only now becoming widely appreciated.
Environmental and Economic Costs of Soil Erosion and Conservation Benefits
excerpts from Science Magazine Vol. 267, February 1995
In the article, Pimentel et al say:
Soil erosion is a major environmental threat to the sustainability and productive capacity of agriculture. During the last 40 years, nearly one-third of the worlds arable land has been lost by erosion and continues to
be lost at a rate of more than 10 million hectares per year. With the addition of a quarter of a million people each day, the world
population's food demand is increasing at a time when per capita food productivity is beginning to decline.
"In the United States, an estimated 4 to 5 x 109 tons of soil and 130 x 109 tons of water are lost from the 160 x 106 ha of cropland each year. This translates into an on-site economic loss of more than $27 billion each year, of which $20 billion is for replacement of nutrients (50) and $7
billion for lost water and soil depth. The most significant component of this cost is the loss of soil nutrients."
Soil may be detached and moved by water, wind or tillage. These three processes, however, differ greatly in terms of:
•where and when they occur
•what happens to the area that is being eroded
•how far the eroded soil is moved, and
•if the soil is moved away from the place where it was eroded, what happens as a result.
Soil erosion by water is the result of rain detaching and transporting vulnerable soil, either directly by means of rain splash or indirectly by rill and gully erosion
Rain may move soil directly: 'rainsplash erosion'. The rain must fall with sufficient intensity on bare soil to detach and move soil particles a short distance. This is solely an on-site effect.
Because rainsplash requires high rainfall intensities, it is most effective under convective rainstorms in the world’s equatorial regions. Rainsplash is relatively ineffective where rain falls with low intensity, such as in the north USA or in northern Europe.
Detachment versus transport (or actual loss)
Well aggregated soil versus non-coherent sand
Erosion is a process that involves the
detachment of soil particles from within
the soil surface
FOLLOWED by the
transport of these
detached particles away from the site
of detachment.
You need runoff to have transport. Rainfall must exceed the infiltration capacity of the soil.
marked increase in destructive capacity. Faster flow and less infiltration.
Less common so of ‘less’ concern.
That fraction of the rainfall which does not infiltrate the soil will flow downhill under the action of gravity; it is then known as runoff or overland flow. Runoff may occur for two reasons. First, if rain arrives too quickly (i.e. with too high an intensity) for it to infiltrate. Second, runoff may occur if the soil has already absorbed all the water it can hold (i.e. because it is fully saturated, or frozen).
As runoff moves downhill, it is at first a thin diffuse film of water which has lost virtually all the kinetic energy which it possessed as falling rain. Thus it moves only slowly, has a low flow power, and is generally incapable of detaching or transporting soil particles.
The microtopography (i.e. small-scale pattern of irregularities) of the soil’s surface tends to cause this overland flow to concentrate in closed depressions, which slowly fill: this is known as ‘detention storage’ or ‘ponding’. Both the flowing water, and the water in detention storage, protect the soil from raindrop impact, so that rainsplash redistribution usually decreases over time within a storm, as the depth of surface water increases. There are, however, complex interactions between rainsplash and overland flow.
There are five factors of rainfall erosion:
1. Nature of the rainfall (frequency, intensity, seasonality)
2. Soil characteristics (infiltration, susceptibility to detachment and transport)
3. Steepness and length of slope
4. Cover
5. Soil Management Practices
A is the predicted average soil loss in metric tons/ha/yr
R is based on the number of heavy rains per year, including the total energy of the storm (size of raindrops, number of raindrops and total amount of water) and the maximum 30 minute intensity. Only storms with >1.25 cm.
Erosion increases as the length and/or steepness of the slope increases by affecting the volume and velocity of water flow.
The slope determines the total area for erosion.
As speed , infiltration , runoff , and velocity .
If velocity x2, water can move particles 64 x larger and can carry 32x more in suspension. Erosive power is 4x greater.
As length , concentration of water .
If length of slope x2, soil loss x 2.6, and runoff x 1.8
- expressed as t/ha per unit of rainfall erosion index.
- measured on Unity plots under clean till fallow conditions.
Inherent ability of the soil to erode.
Can be estimated using information in the Soil Surveys:
a. % silt and very fine sand (0.002 – 0.1 mm)
b. % sand, excluding very fine (0.1 – 2.0 mm)
c. % OM
d. Structural class
e. Permeability class
K Factor Data ( Organic Matter Content)
Textural Class
Average Less than 2
% More than 2
%
Clay 0.22 0.24 0.21
Clay Loam 0.30 0.33 0.28
Coarse Sandy Loam
0.07 -- 0.07
Fine Sand 0.08 0.09 0.06
Fine Sandy Loam
0.18 0.22 0.17
Heavy Clay 0.17 0.19 0.15
Loam 0.30 0.34 0.26
Loamy Fine Sand
0.11 0.15 0.09
…etc.
Crop Type Factor
Grain Corn 0.40
Silage Corn, Beans & Canola
0.50
Cereals (Spring & Winter)
0.35
Seasonal Horticultural Crops
0.50
Fruit Trees 0.10
Hay and Pasture 0.02
C – The crop/vegetation and management factor is used to determine the relative effectiveness of soil and crop management systems in terms of preventing soil loss. The C factor is a ratio comparing the soil loss from land under a specific crop and management system to the corresponding loss from continuously fallow and tilled land.
Tillage Method Factor
Tillage Method
Factor
Fall Plow 1.0
Spring Plow 0.90
Mulch Tillage 0.60
Ridge Tillage 0.35
Zone Tillage 0.25
No-Till 0.25
a. Amount and duration of cover they provide
b. Quantity and type of residue left on the field
c. Nature of tillage practices
P reflects the effects of practices that will reduce the amount and rate of the water
runoff and thus reduce the amount of erosion. The P factor represents the ratio of soil loss by a support practice to that of
straight-row farming up and down the slope. The most commonly used supporting
cropland practices are cross slope cultivation, contour
farming and stripcropping.
Table 5. P Factor Data
Support Practice P Factor
Up & Down Slope 1.0
Cross Slope 0.75
Contour farming 0.50
Strip cropping, cross slope
0.37
Strip cropping, contour
0.25
Management Strategies to Reduce Soil Losses
Factor Management Strategies Example
R The R Factor for a field cannot be altered.
--
K The K Factor for a field cannot be altered.
--
LS
Terraces may be constructed to reduce the slope length resulting in lower soil losses.
Terracing requires investment and will cause some inconvenience in farming. Investigate other soil conservation practices first.
C
The selection of crop types and tillage methods that result in the lowest possible C factor will result in less soil erosion.
Consider cropping systems that will provide maximum protection for the soil. Use minimum tillage systems where possible.
P
The selection of a support practice that has the lowest possible factor associated with it will result in lower soil losses.
Use support practices such as cross slope farming that will cause deposition of sediment to occur close to the source.
% slopeP for
contour tillage
1-2 0.60
2-7 0.50
7-12 0.60
12-18 0.80
18-24 0.90
P = ½ P for contour tillage alone.
Zero tillage
Strip plant
We used to plow the field to incorporate old plants, then disk the field to break up the
clods.
Seed drill
Must use pesticides
R, SL and K are inherent properties of your location.
e.g.: R = 100, SL = 1 and K = 0.6 (A = 60 metric tons/ha).
We need to adjust C and P to reduce A to <10 t/ha.
If we choose 4-yr rotation of 2 of row crops, 1 wheat, 1 clover with C = 0.25 (A = 15 t/ha)
Contour tillage P = 0.6 (A = 9 t/ha) This is acceptable.
