|
(Conservation Currents,
Northern Virginia Soil and Water Conservation District, Fall
2006)
Anyone who has put a few drops of plant food into a watering
can knows that dissolved nutrients such as nitrogen, phosphorous,
potassium, and iron are essential for plant growth, but many
people do not know that a nutrient must be dissolved in order
to facilitate growth. Roots are the conduit through which all
nutrients from the soil enter into a plant, but roots take up
only one thing: water. Because of this, nutrients must be dissolved
before they can be used by the plant.
Yet, dissolved nutrients are also a pollution threat. Stormwater
can run off the surface or percolate through highly fertilized
soils to contribute dissolved nutrients to our streams and groundwater.
This abundance of nutrients disrupts the natural ecology of
streams and creates well water that is unsafe to drink. Luckily,
as long as fertilizer is not over-applied, soil can handle the
influx of nutrients thanks to some unique chemical characteristics
of clay and organic matter.
Clay and organic matter are both known as colloids; tiny particles
in the soil that carry a small electric charge. Both come about
their charge in very different ways. Clay is a crystal, made
of repeating sheets of aluminum and silicon oxides. Aluminum
and silicon are both cations – positively charged atoms
– and both carry a fairly strong charge. Oxygen, on the
other hand, combines with the aluminum and silicon to make the
clay oxides and is an anion – a negatively charged atom.
The positive charges of the aluminum and silicon and the negatively
charged oxygens cancel each other out. However, during the lifespan
of a particle of clay, dissolved cations in the soil water will
migrate into the clay’s crystal structure and replace
some of the aluminum and silicon. These cations, such as iron
and calcium, do not carry as strong a charge as aluminum or
silicon. Thus over time, as more and more substitutions are
made, clay develops an increasingly negative charge.
Organic matter does not have an ordered crystal structure like
clay. Rather, it is a chaotic and variable mess of carbon chains
with hydrogen, oxygen and other atoms bonded to its tips. Oxygen,
as stated before, is an anion. Hydrogen is a very weak cation.
As the environment in the soil changes due to fluctuations in
pH, moisture and other variables, the hydrogen cations leave
and return to the bonding sites on the tips of the carbon chains.
This two-way migration causes small negative and positive charges
to appear and disappear on the surface of organic matter, kind
of like twinkling lights on a Christmas tree. However, like
clay, the total sum of the organic matter’s charges is
usually negative.
So clay and organic matter tend to be negatively charged, but
how does this relate to nutrients? Well, most nutrients are
cations and one of the most basic rules of physics is that opposite
charges attract. Positively charged nutrients will be attracted
to the negatively charged clay and organic matter. This attraction
is strong enough to pull many of the nutrients out of the water
and bond them to the negatively charged surfaces, but not strong
enough to make the bond permanent. This means that soil can
hold onto nutrients strongly enough to prevent them from leaching
into surface and groundwater, but will also release the nutrients
slowly enough that plants will not lack them.
The ability of a soil to hold onto nutrients is measured by
its cation exchange capacity, or CEC. Soils with an abundance
of clay or organic matter will have a very high CEC, while soil
with lots of sand or little organic matter will have a low CEC.
To increase the ability of your soil to retain nutrients, the
best thing to do is add organic matter such as compost. Organic
matter has a higher cation exchange capacity than clay, and
adding compost to your soil will have the added benefit of improving
structure and aeration, reducing density, and increasing nutrient
content. Adding clay also will increase a soil’s nutrient
retention, but the secondary effects are almost all negative
– such as decreased water infiltration. So if you want
a soil that can fight pollution and nourish your plants, look
no further than compost.
| 
