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Soil composition dictates its ability to retain nutrients through electrostatic attraction, a fundamental chemical process enabling plant growth and agriculture across diverse environments.
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-[narrator] Not all soils are created equal. And if it weren't for chemistry we wouldn't be
able to grow many crops here, nor here, nor here. It's like this, soils are
composed of sand, silt, clay, and organic matter. Some have more sand, others more
clay. Each soils unique blend determines its color, texture, and storage capacity
for nutritious chemicals. Although incredibly small, nutrients still need
their space and by space we mean the area surrounding the soils tiny
particles. Keep in mind that surface area is not the same as particle size. For
example, clay particles are tiny compared to sand, but they have more than 1,000
times as much external surface area as the particles in an equal volume of sand.
However, if a nutrient just sits there unattached
it will likely leach out from the soils and grains, and will not be available for
plants. Remember that time when you rub the balloon on your best friend's hair
and stuck it to a wall? Well a similar phenomenon occurs in the soil. Through
their electrostatic energy, nutrients cling on to clay particle surfaces.
Nutrients like calcium, magnesium, potassium, and ammonium are all
positively charged chemicals or cations. And as it turns out, most clay particles
and organic matter in soil are negatively charged. So, many nutrients are
positive and particles are negative. Perfect! In chemistry, as in romance,
opposites attract. Good! No more leaching! But like the
balloon on the wall, the nutrients are only temporarily held. In fact, there's
actually a shell of water molecules that forms around the cation. Preventing it
from bonding permanently. This shell is often called a hydration sphere, but
that's a whole other video. So, back to cations. Basically if a plant wants a
nutritious cation like potassium, it will need to exchange it for another cation
or cations of equal charge. Luckily, plants produce hydrogen cations
that they can exchange. One hydrogen cation for one potassium cation, easy
enough. But for nutrients with a positive charge of two like calcium, two hydrogen
cations are needed. The higher the positive charge, the harder it gets to
exchange or trade cations. That's because a cation with high positive charge and
small size is preferentially held by the soil over those with lower charge or
larger size. Meaning that a large cation with a positive charge of one will be
the first to be released. A divalent cation having a charge of two will be
released more easily than a cation with a positive charge of three. Whether they
are held tightly or not, the nutrients are available to the plant in exchange
for other cations. Not all nutrients are cations, however. Some are actually
negatively charged compounds or anions. Since anions like nitrate are sulfate
have a negative charge they are unable to attach themselves to negatively
charged particles, and as a result leech out when watered. Of course, all soils are different.
There are soils in the tropics, for example, that have positively charged soil particles.
And in that case, it's the anions not cations that are held
temporarily and then exchanged with other anions. Most soils, however, have
negatively charged particles.
The more negatively charged the soil is and the
more surface area a soil has, the more cation exchange capacity it has.
This is such an important factor for plant growth that scientists measure a soils
cation exchange capacity, CEC, in order to help farmers determine how much and how
often fertilization is needed. That's because CEC is sort of like a cup
size at a fast food joint. Some soils are super sized, but others have a kiddy cup.
Pouring too much will just cause a mess, but if you refill several times and
still quench your thirst. Farming and low CEC soils works almost
the same way. Even though the soil has lower capacity, you can fertilize more
often using smaller amounts and the plants will grow healthy and strong. And
it's a good thing too! Otherwise, we'd have very little land to farm. So the
fact that farmers can grow crops almost anywhere kind of seems like superhero
powers. But really it's just knowing about chemistry.
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