(Describer) Title: ScienceOfAgriculture dot org. In an animation...

(host) Not all soils are created equal. And if it weren't for chemistry, we wouldn't be able to grow many crops here,

(Describer) Desert.

nor here, nor here.

(Describer) Delta.

(Describer) Forest.

It's like this: Soils are composed of sand, silt, clay, and organic matter. Some have more sand, others more clay. Each soil's 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 soil's 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 a thousand 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 when it rains and will not be available for plants. Remember that time when you rubbed the balloon on your best friend's hair, and you stuck it to a wall? Well, a similar phenomenon occurs in the soil. Through their electrostatic energy, nutrients cling onto 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.

(Describer) A wave rolls.

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 2, like calcium, 2 hydrogen cations are needed. The higher the positive charge,

[grunts] 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 1 will be the first to be released. A divalent cation having a charge of 2 will be released more easily than a cation with a positive charge of 3. 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 or sulfate, have a negative charge, they are unable to attach themselves to negatively charged particles and, as a result, leach 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 soil's 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 supersized, but others have a kiddy cup. Pouring too much will just cause a mess, but if you refill several times, you can still quench your thirst. Farming in 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. Accessibility provided by the U.S. Department of Education.

(Describer) Titles: Additional materials available at ScienceOfAgriculture dot org Principal Investigators: April Ulery PhD, Laura White PhD, Barbara Chamberlain PhD Executive Producer: Jeanne Gleason EdD Instructional Designer: Barbara Chamberlain PhD Studio Production Director: Pamela N. Martinez Project Manager: Adrian Aguirre Artists and Animators: Adrian Aguirre, Philip McVann, Rene Flores, John Sloan Script Work: Adrian Aguirre, Barbara Chamberlain PhD Editor: Amy Smith Muise Voice Talent: Timothy Black Copyright 2016 NMSU Board of Regents New Mexico State University Learning Games Lab United States Department of Agriculture Accessibility provided by the US Department of Education.

♪