Types of Soils - If we were to talk to soil scientists, you would learn that there are thousands of types of soil. Soil scientists study each of the many different characteristics of each soil and put them into very specific groups and have many different names for soils. Let’s consider a much simpler model that considers just three types of soil. This will help you to understand some of the basic ideas about how the particular climate of an area produces a certain type of soil, but there are many exceptions to what we will learn right now.
Let’s consider the type of soil that would form in a region of the world where there are forests of trees that lose their leaves each winter, called deciduous trees. In order for trees to grow here, there needs to be lots of rain, at least 65 cm of rainfall per year. Wherever there are trees, there is enough rain to help them grow! The type of soil that forms in a forested area is called a pedalfer and this type of soil is common in many areas of the temperate, eastern part of the United States.
The word pedalfer comes from some of the elements that are commonly found in the soil. The element aluminum has the chemical symbol Al and the element iron has the chemical symbol Fe. These two symbols are combined ‘-al’ and ‘-fe’ to make the word ped –al –fe r. This type of soil is usually a very fertile, dark brown or black soil. It is rich in aluminum clays and iron oxides. Because it rains often in this type of climate, most of the soluble minerals dissolve and are carried away, leaving the less soluble clays and iron oxides behind.
Another type of climate related soil, called a pedocal, forms in drier temperate areas where grasslands and brush are the usual types of vegetation. It rains less than 65 cm per year in these areas, so there is less chemical weathering for these soils. With lower amounts of rainfall, there is less water to dissolve away soluble minerals, so more soluble minerals are present here but fewer clay minerals are produced.
With lower rainfall, there is also less vegetation here, so the soils have lower amounts of organic material, making them slightly less fertile types of soils than a pedalfer. A pedocal is named for the calcite enriched layer that forms. Some water begins to move down through the soil layers, but before it gets very far, it begins to evaporate away. Soluble minerals, like calcium carbonate, concentrate in a layer that marks the lowest place that water was able to reach before it evaporated away. This layer is called caliche.
A third type of soil called a laterite forms in tropical areas, where rainfall is so intense that it literally rains every day. The tropical rainforest is an example of this type of region. In these hot, wet tropical regions nutrient poor soils form due to intense chemical weathering. So much weathering happens here that there is practically no humus. All soluble minerals are removed from the soil and all plant nutrients get leached or carried away. What are left behind are the least soluble materials like aluminum and iron oxides. These soils are often red in color from the iron oxides. These soils bake as hard as a brick if they are set out in the sun to dry.
You can probably very quickly name many climates that have not been mentioned here. Each climate will produce a distinctive soil that forms in the particular circumstances found there. Where there is less weathering, soils are thinner but soluble minerals may be present. Where there is intense weathering, soils may be thick but nutrient poor. In any case, soil development takes a very long time. It may take hundreds or even thousands of years to form a good fertile top soil. Soil scientists estimate that in the very best soil forming conditions, soil forms at a rate of about 1mm/year. In poor conditions it may take thousands of years!
Soil Conservation
Soil is only a renewable resource if we carefully manage the ways in which we use soil. There are natural cycles of unfortunate events like drought or insect plagues or outbreaks of disease that negatively impact ecosystems and also harm the soil. But there are also many ways in which humans neglect or abuse this important resource. One harmful practice is removing the vegetation that helps to hold soil in place. Sometimes just walking or riding your bike over the same place, will kill the grass that normally grows there. Other times land is deliberately cleared to make way for some other use. The ‘lost’ soils may be carried away by wind or running water. In many areas of the world, the rate of soil erosion is many times greater than the rate at which it is forming. Soils can also be contaminated if too much salt accumulates in the soil or where pollutants sink into the ground.
There are many ways that we can protect and preserve our precious resources of soil. There are many ways to help to keep soil in good condition. Adding organic material to the soil in the form of plant or animal waste, like manure or compost, increases the fertility of the soil as well as improving its ability to hold onto water and nutrients. Inorganic fertilizer can also temporarily increase the fertility of a soil and may be less expensive or time consuming, but won’t provide the same long term improvements as organic materials. Agricultural practices like rotating crops, alternating the types of crops planted in each row and planting nutrient rich cover crops all help to keep soil more fertile as it is used season after season. Planting trees as windbreaks, plowing along contours of the field or building terraces into steeper slopes will all help to hold soil in place. No till or low tillage farming helps to keep soil in place by disturbing the ground as little as possible when planting.
EmoticonEmoticon