Maine Forestry 2007 - All rights reserved

Soils

Chapter 4 - Soils 

by Michael Rochester

 

A Christmas tree farmer decided to plant an open field he had purchased with balsam fir for the production of Christmas trees.  The trees grew very slowly and the needles appeared very yellow in color.  He tried fertilizer to help improve the growth rate and color.  Nothing seemed to improve the growth and appearance of the trees.  There was something that the farmer did not know.  Before he bought this land, a previous owner had stripped and sold off the topsoil.  This left behind the mineral soil with very little organic material left in it.  Balsam fir do not prosper very well in this type of soil.  If he had planted white birch, the trees probable would of done much better.  But then, you can’t sell white birch as a Christmas tree.

 

Soils will play an important role in your decisions in managing your woodlot.  Soils are a thin skin of material that are part of the most outer layers of the earth.  They are composed of minerals, air, water,  countless micro-organisms, and the decaying remains of once-living things.  Soils are looked at differently depending on why you are looking at them.  Farmers and woodlot owners need to know what the soils are composed of so they can maximize the soils potential by planting and managing the right crop.  Someone who wants to build a house needs to know if the soils are suitable for that purpose.  You do not want to build a house on a site where the soils are poorly drained.  A good soil survey is invaluable to determine if the site is capable of supporting the desired outcome.  There is a whole science about soils so I will just touch on the basic characteristics and how they affect tree growth.

 

Soils are broken down into different layers.  If you dug a deep hole with an excavator, you could see and identify the layers.  There is a wide amount of variation in the layers and some soils have more layers than others.  A common soil profile would include an organic pad, A horizon, B horizon, and C horizon (parent material).   There are more horizons identified but I will just explain the main ones.

 

Organic Pad (O):  The organic pad consists of the organic matter on top of the mineral soil.  It contains leaves, needles, twigs, and branches.  If the pad is less than 4 inches, the decomposition is proceeding normally and nutrient release is taking place at a normal rate.  If the organic pad is deeper than 4 inches, this probably indicates a slower release of nutrients.  This deeper pad is often associated with wet swampy sites.   The organic mat helps protect the soils below the mat and also controls water infiltration and holding capacity.  The organic mat also holds various seeds vital for regeneration and is home to many species very important to the health and functioning of the forest.

 

"A" Horizon :  The A horizon is the top layer of the soil horizons or 'topsoil'.  This layer has a layer of dark decomposed organic materials, which is called "humus".  The technical definition of an A horizon may vary, but it is most commonly described in terms relative to deeper layers. "A" Horizons may be darker in color than deeper layers and contain more organic material.  Most of the biological activity occurs in this zone.

 

"B" Horizon:  The B horizon is commonly referred to as "subsoil", and consists of mineral layers which may contain concentrations of clay or minerals such as iron or aluminum oxides or organic material moved there by leaching.  Tree and plant roots penetrate through this layer, but it has very little humus. It is usually brownish or red because of the clay and iron oxides washed down from A horizon.

 

"C" Horizon:  The C horizon (parent rock) is simply named so it comes after A and B within the soil profile.  It forms the framework of the soil. The A and B layers are formed by this layer. The C horizon forms as bed rock weathers and rock breaks up into particles.  This layer is affected very little by the soil forming process known as "weathering".

 

Soils also have a texture that is determined by the different size of the particles that make it up.  You can determine the soil texture by moistening a small amount of soil of the soil and kneading it to the consistency of putty.   Use the following definitions to identify the texture: 

 

·        Sands:   Gritty, does not stick together.

·        Silts:      Floury or talcum powder feel when dry, slightly sticky when wet.

·        Clay:      Very floury when dry and extremely sticky when wet.   

 

The depth of the soil is very important and has a great amount of importance on stand management.  The depth of the soil determines how deep the tree roots can grow and how much wind firmness the stand will have.  A beautiful thinning operation can turn tragic if the wind blows the rest of the trees over due to the roots growing shallowly.  The depth of the soil can be determined by measuring the combined depth of the organic pad and the soil above the parent material.   

 

·        Shallow soil:       Soil has a total depth of less than 10 inches.

·        Moderate soil:    Soil has a total depth of 10 20 inches.

·        Deep soil:           Soil has a total depth of greater than 20 inches.

 

The soils may also place limitations on what equipment can be used on them.  Slopes, rocks, swamps, etc. will have an influence on the equipment that can operate on your woodlot.  Swamps should not be entered for harvesting purposes unless the soils are frozen.  You will want to limit the amount of damage that can be caused to your woodlot.

 

Summary 

 

In summary, investigate what type of soils you have on your woodlot before you start to make management decisions.  You can do this manually by digging holes.  You can also look where soil cuts are present such as where an excavator or bulldozer has built a road or made holes.  You can also visit your local Natural Resource Conservation Service (Soil Conservation Service).  They can help map your woodlot and tell you which trees grow best on certain soils and the wind firmness of each soil type.  They can also tell you if the soils are good for road building and any equipment limitations the soils may have.  There is also a fairly new website called the "Web Soil Survey".  On this site, you can make maps that will show you the soils under your property and you can read about the description of each.  There is a little learning curve you must overcome to use this site.  The N.R.C.S. field office (Soil Conservation Office) can probably offer you some assistance with the site.  Search for USDA Web Soil Survey on the web.