Getting to know your site
When forest managers refer to site characteristics,
they are talking about a broad range of physical and
chemical properties. Once you know a site's characteristics,
you can determine what species will grow well (and those
that won’t!). In most cases, there will be a few
species that should do well on the site - think about
the many different species you find in most natural
forests. This gives you a chance to base your final
species selection on additional considerations like
long-term objectives, species availability and cost.
Although your site may have several unique characteristics
that separate it from others, only two, soil type and
drainage, influence how well one species will do compared
to another.
Soil type
Although you don’t have to be a soils scientist
to plant trees, knowing a little about soil is valuable.
Trees depend on soil to anchor them in place, provide
moistureand nutrients, and to act as a seedbed for future
generations. All soils are made up of four main ingredients:
mineral particles (the bulk of the material), air spaces
between the particles, water in varying amounts, and
some organic matter from plant and animal debris. Classifying
soils is primarily a quantification of the different
ingredients found within it.
Mineral particles range from boulders as large as
basketballs, to tiny particles so small that can’t
be seen without a powerful microscope. Soil texture
is the relative proportion of the individual particles.
A handful of dirt rubbed between your fingers will have
a certain amount of ‘grittiness’ to it.
The more abrasive the soil feels the larger the individual
particles; the smoother the soil feels the smaller the
particles. This grittiness (or the lack of it) is a
relative measure of three main soil particle sizes –
sand, silt and clay. Sand has the largest particles,
which feel "gritty". Silt has medium sized
particles that feel soft, silky or "floury”.
Clay has the smallest particles and feels "sticky".
The amount of sandversus silt versus clay within the
soil directly affects a tree species' ability to grow
on a site.
There are three broad textural classes: sandy soils,
loamy soils and clay soils. The term loam refers to
soils with more equal proportions of sand, silt and
clay. Although there can be many combinations of classes
such as sandy loam, loamy sand or even clay loam, it
is only really necessary to determine which of the three
general classes you have.
| Table 1 lists some common properties to look
for when assessing a soil texture class. Grab
a handful of soil - does it feel gritty or smooth?
Squeeze the soil in your hand. Does it form a
cast (clump)? Lightly wet the soil and try it
again. Try to make a soil ribbon (Fig. 1) by lightly
wetting the soil until it feels like moist putty.
Then, try to squeeze it between your thumb and
forefinger upwardly into a thin flat ribbon –
if a ribbon forms the soil contains clay. The
longer the ribbon, the more clay it contains. |
Fig. 1.
Making a soil ribbon (source: http://ltpwww.gsfc.nasa.gov/
globe/stories/clays.htm) |
Table 1. Soil
texture class assessment properties
Soil
Texture Class |
Visual
Appearance |
Reaction
when squeezed
in the hand |
Ability
to form a ribbon |
Dry
Soil |
Moist
Soil |
Sand
soils |
Granular with easily detectable
particles |
Will not form a cast, or cast easily falls apart |
Forms a cast that crumbles easily |
Cannot form a ribbon |
Loam
soils |
Low to moderately granular
Can form clumps/ clods when dry |
Forms a cast that can be handled relatively easily |
Forms a cast that can be easily handled |
Pure loam cannot form a ribbon
Loam with more silt and clay will form a fragile
ribbon |
Clay
soils |
Fine texture with very few large particles
When dry, forms hard clumps |
Forms a cast that can be handled freely |
Forms a cast that can be worked and is cohesive |
Forms a long, flexible ribbon |
Drainage
Drainage is the second site characteristic that needs
to be assessed before making a species selection. How
well your site holds water can have a dramatic impact
on the long-term survival of different species.
Drainage is influenced by soil texture. The smaller
the particle size the more water the site holds –
a clay soil can hold considerably more moisture than
a sandy soil. Soil depth, recent precipitation, topography,
depth to the water table, and the amount and type of
vegetation also influence drainage in one way or another.
Soil drainage can be classified into four different
categories.
Well Drained – water drains from
the site rapidly; water seldom pools on the site even
during a heavy rain or after snow melt
Moderately Drained – water may pool
but only for brief periods
Imperfectly Drained – water pools
on the site, sometimes for extended periods especially
during the spring or in wet years;
Poorly Drained – water drains slowly
from the site throughout the year; soil
may appear wet below the surface.
Determining the drainage of your site is relatively
easy and usually comes from observing what happens to
the site in the spring and fall, as well as during and
after a rainfall.
Site variability
The area you intend to plant may not be uniform especially
if there is variation in topography. You should assess
the entire site looking for differences that might affect
your choice of species. If your site differs in either
soil texture or drainage you may need to choose different
species for certain areas. |
