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SOILSGetting to the Bottom of Things

he underlying geology has a huge impact on the soil, andevery area is different. In most cases, your soil is derived from the rocks nearby or directly underneath, though there has been

much mixing, mashing, and movement over time. In our northern areas, wesee the impact of glaciers on our surface soils. Other areas show the move-ment of massive amounts of water, wind, erosion, lava, or other factors thatgive us our soil. The farming, logging, or fire history will also have impacts.The soil has a great impact on the forest. Soil texture, nutrients, and

drainage affect the species composition of the forest, growth rates, regenera-tion success, and access. We’ll look at each of these, but first, we will look atthe layers in a soil profile.

LayersIf you dig a test pit in your forest, you will notice various layers of soil, differ-entiated by color, texture, moisture, and nutrients. The soil layers are calledhorizons. There are all sorts of things going on in each layer, and of course,this will vary greatly in different regions. Water, air, soil organisms, and allsorts of chemistry are at work. Many forest soils have a humus layer at thetop. This is organic material: leaves, twigs, and plant parts in the process ofdecay. Nutrients are cycled through this layer and there are bacteria, fungi,and other organisms at work. In some cases, there is little humus because ofvery active decomposition (such as in tropical areas), the effects of forestfires, or previous agriculture that has mixed the organic material into lowerlayers. The recycled nutrients can move into the lower layers. Normally, the first layer under the humus is topsoil: mineral particles

mixed with organic matter. This is usually dark color and might be a fewinches to several feet thick. Most of the tree roots will be in this area, espe-cially the fine roots that take up nutrients and water. Of course, deeper top

by Robbo HolleranWe walk through the forest looking up,almost all the time. That’s why we fall down more than most hikers. And that puts us in contact with

our subject today: Soil.

T

SOILSGetting to the Bottom of Things

soils will be better sites to grow anything. The subsoil will be mostly mineral particles with lit-

tle organic material and some nutrients. In welldrained soil, there may be tree roots in these layers.Surprisingly, most tree roots are in the top foot or twoof soil, unless it is a very dry site. The drainage ofthese lower layers is important to tree growth, nutrientcycling, and limitations on forest management and har-vesting. Under the subsoil may be actual bedrock, oradditional layers of material such as gravel, loose rock,glacial till, or compacted material.

TextureThe texture of the minerals is important in each ofthese layers. This refers to the size of the particles.The main components are sand, silt, and clay. We allknow what sand feels like. Clay is fine enough to feelsmooth between your fingers, and silt is intermediatebetween the two. But soil may also contain gravel,larger cobbles, or giant boulders. The mix of these par-ticles has a huge impact on the drainage of the soil,and the ability to holdmoisture and nutrients.Drainage is good, and larg-er particles drain better.Sand and gravel drain reallywell, but they do not holdmuch moisture or nutri-ents. Smaller particles likeclay (and silt to a lesserextent) hold nutrients wellsince they have so muchsurface area. But clay soildoes not drain well, andholds moisture excessively.Here is another conun-drum: drainage or nutri-ents? Loam soils are gener-ally ideal with a mix ofsand, silt, and clay. Theyhold nutrients and mois-ture, and have gooddrainage. The diagramshows how the percent ofclay, silt, and sand combine to define the soil texture.The bedrock and any other underlying materials

contribute to the soil texture and nutrients available inyour soil. Bedrock breaks down into smaller particlesby water, wind, soil organisms, and chemistry into soil.Each type of stone has certain nutrients. For example,granite has some potassium, but tends to break downinto larger particles for excessively drained sandy orgravelly soil. Limestone and marble break into finerparticles and will have calcium, which buffers acidityand provides a higher soil pH. Calcium is an importantnutrient for many trees. Phosphorus and other nutri-ents are found in various types of rock. Plants and

trees grow best with a mix of nutrients, and some areaswill have an abundance of some things, and lack ofothers. This will impact the tree species that grow beston your land, both because of nutrients and soil tex-ture.

DrainageDrainage is our next category, and you already knowthe wet spots on your woodlot. Poor drainage can becaused by clay soils, or impermeable layers likebedrock under the topsoil. In our glaciated areas, wesometimes have a packed subsurface layer called hard-pan. In any case, poor drainage limits the growth ofroots, since roots need both moisture and air to grow.Trees will be very shallowly rooted, and particularlysusceptible to wind damage. The soil on top can berich in nutrients, and growth of young trees can beprolific. But most species will not grow well to fullsize. Species that tolerate wet feet include red maple,black ash, larch, black gum, and bald cypress. Poordrainage also limits the ability of the soil to hold up

heavy equipment.Excessive drainage is adifferent problem. Thesesites are dry by definition,but they also tend to below on nutrients.Nutrients can be dis-solved in the soil water, sothey also leach out if thereis too much drainage.Trees like oak and pinecan grow on dry sites,with low nutrients. While every species

grows best on that perfectloamy soil, what we findin nature is that eachspecies is found where ittolerates the conditions,and grows “better” thanother trees. Pitch pine isfound on pure sand, notbecause it “likes it.” It

survives, grows, and regenerates with low moisture andnutrients. Likewise, larch is found in bogs and wetareas. These species occupy these sites because theygrow better than anything else might. If you plantlarch or pitch pine in good soil, they grow remarkablywell, but not as well as ash or sugar maple. The bettersites simply have more options for species and man-agement. Whatever has started there will grow well.Species like sugar maple, white ash, black walnut,Douglas fir, and redwood are usually found on bettersoils. These tend to be higher-value species.Most forest soils do not have 3 feet of loamy topsoil,

but don’t be discouraged. We have sites where the

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average soil particle seems to be the size of a refrigera-tor, and they still grow trees. If you know your land,you probably already know where the good and bad,dry and wet areas are. And you are probably not goingto go and dig a series of test pits and consult a soilsexpert to interpret the horizons. But most areas havesoils maps already available through the NaturalResources Conservation Service (NRCS), a division ofthe U.S. Department of Agriculture. This primarilyserves farmers, but is an important resource for forestmanagement. Understanding what you have will helpyou manage your forest to its potential. Soils mapshave their own language, and your local NRCS officecan help you interpret them. The maps are availableonline at soils.usda.gov/survey and other sites.There are plants that grow in the forest which indi-

cate the site quality. Getting to know your ferns andother plants is helpful. Here in the Northeast, espe-cially rich hardwood sites usually have maidenhairfern, blue cohosh, and Dutchman’s breeches. Certainother plants will indicate poor sites, wet sites, etc.Each area will have its own indicator species, whichyou should be familiar with.Specific ferns are often goodindicators.

Height GrowthDiameter growth in a forest islargely influenced by crowding,but height growth is determinedby the soil and site quality. Youhave noticed that trees on top ofa ridge will be short, but in thevalley are much taller. Nutrientsare mobile in the soil moisture,and get leached away. As soilminerals decompose, new nutri-ents are released from the par-ent rocks, so there is a cycle.The tops of hills or ridges tendto lose nutrients over time, andthey accumulate at the base.One way to assess site quality isto standardize the height growth of dominant healthytrees over a specific period of time. In the East, we use50 years. So if ash trees get 90 feet tall in 50 years onone site, but only 50 feet tall on another site, the first issuperior, for ash. We call this the “site index.” Somewestern regions use 100 years as the standard age, andsouthern pines use 25 years. But you seldom have treesexactly 50 years old to measure. So we use a chart(below) to track the age and height to determine siteindex. For example, an 80-year-old tree that is 60 feettall shows a site index of 50. The chart above shows theearly height growth on all soils, leveling off sooner onpoorer soils. On a good site, height growth continuesbeyond 100 years.

Each site will have species that will survive andgrow better than others. On better sites, we can grownutrient-demanding species (which are often highervalue) to full maturity and large diameter. On poorersites, we have to set our objectives to less-desiredspecies, smaller diameters or shorter rotations. Here inthe Northeast, wetter sites might be best occupied byspruce and fir, drier sites with pine or oak. Rich hard-wood sites can grow sugar maple and ash, but poorersites might grow red maple and birches. Knowing yoursoil’s potential will help you to determine your speciesobjectives, which should influence crop-tree selection,and your silvicultural system. The site quality will also affect the regeneration

potential. We often find sugar maple or white pinegrowing on post-pasture sites where they do not seemto regenerate. With maple, it seems the site is notquite good enough to establish new maple stands, andbirches, beech, or softwoods will eventually replacemaple. Loss of calcium from acid deposition may be afactor here. Pine often grows into abandoned pastureson very rich soils, and was able to start because of

competition with grasses, or partialgrazing. But as the pine forestsmature, it is difficult to establishyoung pine since the native hard-woods will grow more aggressively. We don’t have many options for

changing the soil. We can impairour soil productivity by erosion orcompaction. Erosion usually occurswith exposed soil on steeperslopes. Heavy logging equipmentcan cause soil compaction and dis-turb soil drainage patterns.Displacing air from compactedsoils affects the soil organisms,damages roots, and impairs rootgrowth. Proper layout and mainte-nance of skid trails is essential,along with re-vegetating disturbedsoil. On the other hand, there havebeen some successes with adding

either fertilizer or lime to improve forest soils. This isdone on intensively managed forests such as southernpine plantations and maple sugar bushes with promis-ing results. It is important to understand the limitingfactors to tree growth on these sites. Stocking levelsare important, and soil testing should be done to deter-mine which nutrients are lacking. Next time you falldown in the woods, take a closer look. n

Robbo Holleran is a private consulting forester helpinglandowners meet their goals in Vermont and adjacent areas.His work has him outdoors about 150 days each year, plusplay time. He is one of the authors of the new SilviculturalGuide for Northern Hardwoods in the Northeast.

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