-[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. [music playing]
Excellent video! Thanks for sharing
I will walk into traffic to never take another soil science class agian.
I had to watch it twice.
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