Lesson Video: Fertilizers Chemistry

Video Transcript

In this video, we’ll learn about fertilizers. We’ll learn why they’re important for plant growth and how we can make them both at the industrial and the laboratory scale. Fertilizers are something that we can add to the soil that plants are growing in that provides them with essential nutrients they need to grow. Some fertilizers are geared towards soil quality, but most fertilizers are aimed at nutrition. So that’s what we’ll focus on in this video.

There are more reasons to apply fertilizers outside of nutrition. Applying fertilizer to crops allows them to grow bigger faster. Another thing that fertilizer can help with has to do with the nutrients in the soil. Since these are nutrients that are essential for plant growth, plants will absorb them from the soil over time. So if you grow plants in the same area over and over again, eventually, those plants will remove all of the nutrients from the soil and the soil will be depleted. Applying fertilizer to areas where these nutrients have been depleted can restore those nutrients to the soil.

So we keep talking about these essential nutrients that plants need. But what exactly are these? Well, in order to make carbohydrates, plants need the elements carbon, hydrogen, and oxygen. And in order to make proteins, plants need carbon, hydrogen, oxygen, and the nitrogen. The element phosphorus is important for the growth of flowers, roots, fruit, and seeds. This is because phosphorus is found in both DNA and ATP, which is what cells use for energy. The element potassium is important as well, since it helps water and nutrients move through the plant. And there’s plenty of other elements that plants need, like iron, sulfur, calcium, and magnesium. But plants only need small amounts of these, so we don’t need to worry about supplying these elements through fertilization.

We also don’t need to supply the plant with carbon, hydrogen, or oxygen. That’s because the plant can get these elements from gases that are found in the air, like carbon dioxide, oxygen gas, and water vapor. Though nitrogen gas is abundant in the air, it’s unfortunately not in a form that plants can absorb. So we need to supply plants with nitrogen through fertilization. We also need to supply the plants with potassium and phosphorous.

So the three elements that you’ll find in fertilizers are nitrogen, phosphorus, and potassium. Since the atomic symbol for nitrogen is N, the atomic symbol for phosphorus is P, and the atomic symbol for potassium is K, fertilizers that supply all three of these elements are called NPK fertilizers, although not all fertilizers supply all three of these elements. If you’re ever at a gardening store and you happen to see a bag of fertilizer or potting soil that contains fertilizer, you might notice that there’s three numbers separated by dashes at the bottom of the bag. These numbers tell you the relative amounts of nitrogen, phosphorous, and potassium that are found in the fertilizer.

So now that we know what elements need to be in fertilizers, how do we go about making them well? Well, fertilizers don’t need to be anything fancy. Fertilizers can be something as simple as compost, animal waste, or bone meal. Since these kinds of fertilizers are biological in origin, they’re often called organic fertilizers. But most fertilizers that are going to be used on industrial scale are synthetic.

Any synthetic fertilizers don’t actually contain nitrogen, phosphorus, and potassium in their elemental forms. Rather, they’re made of various as salts that contain these elements. For example, nitrogen-containing fertilizers are often made of salts that contain the nitrate, or NO3− ion, or the ammonium, or NH4+ ion. Another option for a nitrogen fertilizer would just be the nitrogen-containing compound, urea. Phosphorous-containing fertilizers are often made with salts that contain the phosphate ion. And potassium-containing fertilizers are made of any potassium-containing salt.

So now let’s talk about how we would make fertilizers that contain these elements, starting with nitrogen. The creation of nitrogen-containing fertilizers starts with the Haber process. In the Haber process, nitrogen gas and hydrogen gas react to form ammonia. Now ammonia could be applied directly to crops as a fertilizer, but for ease of application, it’s used as a raw material to create other nitrogen-containing compounds. For example, we could react to the ammonia with nitric acid, which would give us NH4NO3, ammonium nitrate. Or we could react to the ammonia with sulfuric acid. This would give us (NH4)2SO4, ammonium sulfate.

Now let’s move on to phosphorus. The creation of phosphorous fertilizers starts with phosphate rock. Phosphate rock is mined from the Earth’s crust. It contains high amounts of phosphorous-containing minerals. It’s unfortunately insoluble in water, so we can’t use it directly as a fertilizer. So we’ll have to react it with something in order to obtain a soluble salt that we can apply as a fertilizer. For example, we can react the prostate rock with nitric acid, which would give us a mixture of calcium nitrate and phosphoric acid. The phosphoric acid is usually neutralized with ammonia to get ammonium phosphate. This particular reaction is convenient for the creation of fertilizers because we end up with something that contains both nitrogen and phosphorus. So if we wanted an NPK fertilizer, all we need to add is some potassium-containing salt.

We can also react to the phosphate rock with sulfuric acid. This would give us what’s often referred to as single superphosphate. This is a mixture of calcium sulfate and calcium phosphate. This reaction is particularly convenient because our reactants, phosphate rock and sulfuric acid, are both extremely easy to produce on an industrial scale. In fact, this exact reaction is the primary use of both phosphate rock and sulfuric acid on a global scale. We can also react phosphate rock with phosphoric acid, which would give us something that’s commonly referred to as triple superphosphate. Chemically, triple superphosphate is calcium hydrogen phosphate.

So now we know how phosphorous-containing fertilizers are made, so that just leaves us with potassium. The creation of potassium fertilizers starts with any potassium-containing salt. Unlike phosphate rock, potassium-containing salts are soluble. This means that we don’t have to perform any reactions on these salts in order to use them as a fertilizer. The only thing that we might have to do is purify them to remove other salts like sodium chloride. Now these reactions that we’ve looked at to create fertilizers might look a little bit different, depending on if we’re performing the reaction in a laboratory setting or an industrial setting. And that’s not because the reactions are going to be any different, just the process that we use to perform these reactions.

For example, let’s use this reaction that we looked at earlier. In this reaction, ammonia and sulfuric acid react to form ammonium sulfate. If we were to perform this reaction in a lab to create fertilizer, we would need to purchase our reactants, ammonia and sulfuric acid, from a chemical supply company. We would perform this reaction in standard laboratory glassware. And when we perform this reaction, we would bake batches of ammonium sulfate. If we want more ammonium sulfate, we would have to perform the reaction again to create another batch.

When we perform this reaction in industry, there’s no need to purchase the reactants from a chemical supplier. Instead, we can create them directly from the raw materials. As long as the plant where the fertilizer is being made is running, these fertilizers can be created continuously. There’s no need to create them in batches like we would if we were in a lab. But in order for this to happen, the plant needs to use expensive industrial equipment. And with that, we’ve hit on everything we need to about fertilizers and how they’re made. So let’s test our knowledge with a couple of problems before we wrap up this video.

Which of the following is not a reason why farmers use fertilizers? (A) Growing crops more quickly, (B) increasing the protein content in the crops, (C) growing larger crops, (D) restoring depleted minerals in the soil, (E) supplying essential minerals used by crops.

Fertilizers are something that we can apply to the soil that plants are growing in that provides them with essential nutrients they need to grow. In addition to supplying plants with essential nutrients, fertilizers also help plants grow larger faster. Also, as plants grow, they will absorb nutrients from the soil. If we continue to grow plants in the same area, over time, the nutrients in the soil will become depleted. And applying fertilizer to the soil in those areas can restore the depleted nutrients.

So now let’s take a look through our answer choices. We’re looking for the statement that is not a reason why farmers use fertilizers. Well, answer choice (E), supplying essential minerals used by crops, is the primary reason that we use fertilizers. So that can’t be the correct answer. As we discussed, allowing plants to grow larger faster is definitely a reason why fertilizers are used. So answer choices (A) and (C) aren’t correct either. And fertilizers do restore depleted minerals in the soil. So that leaves us with answer choice (B), increasing the protein content in the crops. Well, it’s true that applying fertilizers can allow plants to grow larger. They don’t change the plant in any way, so they wouldn’t increase the protein content.

Why is fertilizer A not as effective as fertilizer B at promoting plant growth? (A) Fertilizer A does not contain any sodium. (B) Fertilizer B inhibits seed germination. (C) Fertilizer B only contains nitrogen. (D) Fertilizer A does not contain any phosphorous or potassium. (E) Fertilizer A raises the pH of the soil. Fertilizer A is urea and fertilizer B is an NPK fertilizer.

Fertilizers supply plants with essential nutrients they need to grow. These essential nutrients are the elements nitrogen, phosphorus, and potassium. That’s what this NPK in the NPK fertilizer stands for, the atomic symbols of these three elements. So since fertilizer B is an NPK fertilizer, it contains all of the essential nutrients we need to supply plants through fertilization. If we look at fertilizer A, urea, we can see that it does contain nitrogen, but it doesn’t contain phosphorous or potassium. In this question, we’re looking for the reason why fertilizer A is not as effective as fertilizer B. And what we just discussed about the elements that each fertilizer contains probably has something to do with it.

But let’s take a look through our answer choices. Answer choice (A) says that fertilizer A does not contain any sodium. This statement is true. Fertilizer A doesn’t contain any sodium, but neither does fertilizer B. That’s because sodium isn’t a nutrient that we need to supply plants through fertilization. So this isn’t the correct answer. Answer choice (B) says that fertilizer B inhibits seed germination. Nitrogen-containing fertilizers do inhibit seed germination. So you should take care not to apply too much nitrogen-containing fertilizer too close to one seed to germinate. But both fertilizer A and fertilizer B contain nitrogen. So this isn’t correct either.

Answer choice (C) says that fertilizer B contains only nitrogen. This statement is not true. Fertilizer B contains nitrogen, phosphorus, and potassium because it’s an NPK fertilizer. Answer choice (D) says that fertilizer A does not contain any phosphorous or potassium. This is true. Fertilizer A doesn’t contain any phosphorous or potassium, which are two of the three essential nutrients that we need to supply plants. So we found the correct answer. But let’s take a quick peek in answer choice (E). This one says that fertilizer A raises the pH of the soil. Fertilizer A, urea, does actually change the soil pH. But it slightly lowers the pH of the soil, not raises it. So, as we said, answer choice (D) is the correct answer. Fertilizer A is not as effective as fertilizer B because fertilizer A does not contain any phosphorous or potassium.

Now that we’ve worked some problems, all that’s left in this video are the key points. Fertilizers supply plants with essential nutrients. These nutrients are the elements nitrogen, phosphorus, and potassium. Nitrogen fertilizers are primarily NO3− or NH4+ containing salts made from ammonia. Phosphorous fertilizers are mainly PO43− containing salts made from phosphate rock. And potassium fertilizers are any potassium-containing salt. NPK fertilizers contain all three of these elements.