Factors Affecting Soil Erosion

It is impossible to play-down the importance of preventing soil erosion, but like many problems before you can address it you first have to understand it.  There are several factors which effect how rapid a soil will erode.  In this broadcast I would like to share these factors with you and briefly discuss them.  I will also be limiting this broadcast to only discuss soil erosion caused by water. 

Soil erosion includes the processes of detachment of soil particles from the soil mass and the subsequent transport and deposition of those sediment particles.  This simple process is the number one source of pollution in the world.  Erosion is the source of 99% of the total suspended solids loads in waterways in the United States and undoubtedly around the world.  Somewhat over half of the approximately 5 billion tons of soil eroded every year in the United States reaches small streams.  This sediment has a tremendous societal cost associated with it in terms of stream degradation, disturbance to wildlife habitat, as well as direct costs for dredging, and reservoir storage losses.  This does not even begin to account for the cost of losing productive soil from agricultural land.

Soil erosion is a very complicated problem to solve, because there are so many factors, which affect the rate of erosion.  These factors include: rainfall, soil type, landscape, crops, and farm management.  There is an equation to predict the soil loss from fields using these five factors.  I would like to discuss each of these factors with you and explain how they effect soil erosion. 

Rainfall is the first soil erosion factor that I will discuss.  As rain falls from the sky it has tremendous force and as it impacts the soil it can break away small portions of soil and make it erosion likely.  This is why rainfall is an important soil erosion factor, but the amount of rain, type of rain, and the distribution of rain are what really need to be looked at. 

Where I live we receive about 100 cm of rain annually, other areas can receive much more (up to 200 cm) in these areas rainfall will be a much more important factor controlling soil erosion.  The rains in my area tend to come as gentle to heavy down pours.  Some areas only have light rain, even though they may receive rain every day the rain generally does not fall with enough force to detach much soil.  When considering rainfall it is important to look at when it generally rains.  Some areas receive heavy spring rains and just occasional summer rains while others may receive most of their rain in at 2-3 month period.  If the rainfall generally comes when fields are being prepared there is a much higher likelihood for erosion to occur and special care should be taken to protect these fields.  The rainfall factor to predict soil erosion in my area is 230; this will be different depending on where you live.  

There are thousands of different soil types around the world and each of them has properties, which make them unique.  One of their properties is their soil readability.  Some soils are just much more susceptible to eroding than others.  I am currently working with a farmer who has a field with only 2% slope, but this field is eroding at a rapid rate.  As I investigated the soil I learned that the soil was primarily silt.  Silty type soils tend to be the most erosive.  In much of the world there is a layer of silt, which was deposited by wind.  This wind blown silt, which is referred to as loess, is probably the most erosive soil.  Soils that have a relatively high content of clay tend to be the least erosive soils.

As you all know soils have a mixture of sand silt and clay in them, and in many soils the ratio is very similar.  However, even with soils with similar ratios of sand, silt and clay may have drastically different soil erodability properties.  Can you think of what may also change the soil's erodability?  The two, which first come to my mind, is soil structure, and rock content.  Soil with good soil structure will allow more water infiltration and thus reducing runoff water and erosion.  This is also true of rock soils.  They also tend to have greater water infiltration rates.  The numerical value to describe a soil erodability ranges from 0.2 – 0.4 in my area. 

Slope and Length of slope is critical factors in determining soil erosion.  As you know there are fields with all kinds of slopes.  Flat fields with 1-2 percent slopes may not be very prone to erosion, but fields with slopes of 10-15 percent slopes will likely be very prone to erosion.  However, you really cannot just look at the slope of the land, you must also look at the length of the slope.  Long fields with a constant slope of 2 percent may erode severely, because as the water runs off the field it builds momentum and the faster the water runs the more energy it has for transporting soil.  In my area the soils have been studied extensively and there have been numerical values assigned to describe the length and slope of field.

Let me give you an example.  A field with a slope of 1 percent and a length of slope equal to 100 meters has a Length/Slope Factor of 0.15.  This compares to a field with a slope of 5 percent and a length of slope equal to 100 meters has a Length/Slope Factor of 0.81.  This may not sound like a huge difference.  However, if all other factors were constant the second field would have erosion rates 500 percent greater than the first field.

Another factor, which as a great impact on how much erosion takes place, is the crop being grown and how it was planted.  Lets first talk about different crops.  As you know the crop in the field protects the soil from the impact of raindrops.  However, different crops provide different levels of protection.  Let us compare and contrast wheat and soybeans.  Wheat is planted in the fall of the year and it covers the soils in the fall, winter, and spring before it is harvested.  Soybeans on the other hand are planted in the spring and harvest in the late summer.  Therefore the field has little protection in the fall winter and early spring.

As I mentioned earlier it also makes a difference how the crop was planted.  Let us take soybeans for an example.  Soybeans planted on completely plowed fields in 1 meter wide row is much worse than soy beans planted on fields with previous crop residue left on the soil surface in 0.5 meter rows.  Let me give you a few figures for you to compare.  Soybeans planted with spring plowing have a crop factor of 0.35; wheat planted with fall plowing has a crop factor of 0.10; corn planted with spring plowing has a crop factor of 0.25. 

The last factor is call farm management. This is a difficult factor to explain.  However there are some management practices a farmer can undertake to reduce soil erosion.  The simplest one I can describe to you is called cross slope planting.  This is when the farmer plants his crops in rows perpendicular to the slope of the land.  By doing this, the farmer slows down runoff water and in urn reduces the energy contained in this runoff water.  Slower water is not able to transport as much soil as rapidly flowing water.  Later on I will ignore this factor when calculating soil loss.

Of these factors which effect soil erosion rates there are some, which you can control and there are other which you cannot control.  In life, just like in preventing soil erosion there are some factors, which you can control.  But in life there are also many, which you cannot control.  The good news is that you can control your eternal life.  This reminds me of a Bible story where Jesus was walking and He asked his disciples a momentous question: "Who do men say that I am?" (Matt. 16: 13). The disciples reported what others said as to Jesus’ identity (vs. 14). However, Jesus placed the focus on them, "But who do you say I am? (vs. 15). Peter then confidently answered: "You are the Christ, the Son of the living God" (vs. 16). Jesus then commends Peter and confirms Peter’s answer (vs. 17). Jesus then enunciated: "And I say also unto you, that thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it" (vs. 18).

What does "upon this rock I will build my church" mean? The word "rock" obviously stands for foundation. A solid foundation is vitally important. A rock foundation was the ultimate, lasting foundation. Notice that this rock was to be the foundation for the church Jesus would build. Since the church is no better than its foundation, the right foundation was imperative.

Jesus, the Son of God, is the foundation. The context of Matthew 16: 18, the testimony regarding Christ’s identity, involves Jesus being "the Son of the living God" (vs. 16). The Jews knew that Jesus’ claim to be the Son of God necessarily meant Jesus partook of the same nature as his Father – He was himself divine (Jn. 10: 36). Hence, they sought to stone him for blasphemy.  Jesus is the proper foundation for the church. The Father was well pleased with Jesus (Matt. 3: 17), we are to hear Jesus (Matt. 17: 5, Acts 3: 22, 23), and Jesus is the mediator between God and man (I Tim. 2: 5). Moreover, Jesus is the perfect sacrifice (Heb. 9: 27, 28), great shepherd (I Pet. 5: 4), and the sinless one who offered perfect obedience (Heb. 4: 15, 5: 8, 9). "Upon this rock," then, identifies Jesus as the foundation of the church, not man, any man! Jesus is also the head of the church and the savior of the body (Eph. 5: 23).

This is truly the good news of the Bible.  If you will only believe and place your faith in Jesus you will have the foundation, which even the gates of hell shall not prevail.

I would like to share a few example soil erosion examples with you.  The values that I use are common to where I live, but may not be representative to your area.  However, the process will be the same.  In order to calculate soil loss all you have to do is multiply the soil erosion factors.  Listen to the following equation:

Soil Loss = Rainfall Factor X Soil Type Factor X Length Slope Factor X Crop Factor X 2

Where soil loss is in metric tons of soil loss per hectare.

Example 1.  A field of soil beans is planted on a field with a slope of 5% and a length of slope of 100 meters.  The soybeans were planted in 1-meter wide rows on a soil with a soil factor of 0.34.  The rain factor is 250.  The soil loss is as follows:

Rainfall Factor (250) X Soil Type Factor (0.34) X Length Slope Factor (0.81) X Crop Factor (0.35) X 2 = 48 Metric Tons/hectare

How could soil erosion be reduced from this field?  Where we cannot change rainfall or the soil type, but we may be able to reduce the length slope factor and the crop factor. Installing terraces can change the length slope factor.  This generally does not change the slope of the land, but it does drastically change the length of the slope.  However, depending on how the terraces are constructed the slope of the land can also be reduced.  The crop factor is probably the easiest factor to work with in order to reduce soil erosion.  This particular field may not be suited for soybeans lets try planting wheat.

Rainfall Factor (250) X Soil Type Factor (0.34) X Length Slope Factor (0.81) X Crop Factor (0.10) X 2 = 14 Metric Tons/hectare

Just by changing the crop we have reduced soil erosion from 48 to 14 metric tons per hectare.  Suppose we now also install the terraces as mentioned earlier.

Rainfall Factor (250) X Soil Type Factor (0.34) X Length Slope Factor (0.15) X Crop Factor (0.10) X 2 = 3 Metric Tons/hectare

This applications have greatly reduces soil erosion even further.  The practice, which I have ignored, is farm management.  This factor can ever reduce soil erosion even more.  In the next radio broadcast I want to share with you more about a topic called conservation tillage and crop residue management.  As you look at your farm and at you life I challenge to look at what you can change and what you cannot change.  I want to encourage you to make the changes on your farm to reduce soil erosion and to make changes in you life and place Jesus as your foundation.