Chemical fertilizers and the Magruder Plots
In the fall of 1894, Alexander Magruder’s fiancée returned from a trip in a buggy, during which a heavy rain occurred, chilling her so thoroughly that she developed a type of pneumonia referred to at the time as rapid consumption. She feared that death was close, and asked Magruder to marry her in case she soon died. He did marry her, and shortly after, she did die.
Figure 1—Alexander C. Magruder (1867-1924)
A sad story, indeed. Magruder was a relatively young man at the time—only 27. I don’t know if he thought of human death much before this, but I do think the death of a field had been on his mind for some time, for he began an experiment to determine just what it takes to kill a field, making it unable to grow crops. That experiment is still being conducted, and the field still hasn’t died. It’s one of the most interesting experiments ever conducted in the field of agriculture.
In the natural world where humans are not harvesting anything, soils will remain fertile in perpetuity because all the nutrients taken from the soil by plants and animals are returned to the soil in the form of decaying plants and animal manure. Nutrients are not removed, they are only recycled. It is a different story when plants are grown for human consumption, as the nutrients are removed at harvest and are then transported elsewhere. For instance, wheat may be harvested and consumed by humans, after which the nutrients leave the human as feces and are then treated at a sewage treatment plant. From here the nutrients may be released into rivers to be transported to the sea or be taken to a dump.
If you harvest crops every year without returning the nutrients removed you are basically mining the soil until its nutrients are depleted, at which point your soil is infertile. Magruder wanted to measure exactly how long it took to mine a soil completely, so he designed a rather simple experiment where a field was farmed but never fertilized. The experiment wouldn’t be of much use unless it was continued for a long time—a really long time. Fortunately, the plant scientists here at Oklahoma State University (OSU) are patient people. Magruder’s experiment that began in 1892 is still continued to this day, and the field still hasn’t died.(B2)
Let’s first take a look at the history of these plots. This video is an excerpt from an episode of Sunup, a television series produced by the agricultural college at OSU.
Video 1—Sunup segment on the Magruder Plots
Farming without fertilizer
Now let’s look at the results of the experiment. What happens if you keep harvesting wheat year after year with no fertilizer? The results might surprise you. During these 100+ years, yields have remained about the same, as the figure below shows. It would be like a person remaining the same weight despite eating no food for weeks and weeks. Every year, the wheat harvest removes nutrients from the soil without fertilizer to replace the nutrients. Yet wheat yields remain stable. How can that be?
Figure 2—Wheat yields on unfertilized plots
To answer that question we have to think about where plants acquire their nutrients. They needs water, of course, but that comes from the sky—so does carbon dioxide. The next most important ingredient is nitrogen, and when farmers apply fertilizer, nitrogen is usually part of the fertilizer. Yet even without fertilizer, nature has its own way of returning nitrogen to the soil.
The atmosphere is 80% nitrogen, but unfortunately plants can’t just grab it from the air like it does carbon dioxide (well, some plants called legumes can, but wheat is not a legume), but the article The fertilizers of our ancestors showed, there are bacteria in the soil that yank nitrogen from the air and store it in the soil in a form accessible to plants. Lightning also turns the atmospheric nitrogen into a plant-accessible form. While the plant scientists may not be applying nitrogen fertilizer to this experimental plot, bacteria and lighting are. Nitrogen is being returned to the soil, just not by people.
The other two main nutrients are phosphorus and potassium. Some of this might find its way to the field from the wind, but for the most part soils do not naturally replenish their supplies of phosphorus and potassium. The Nile river in Ancient Egypt was an exception, where annual floods would transport sediment and nutrients from upstream and deposit them downstream on the land adjacent to the river. Of course, here in the middle of Oklahoma we do not experience regular flooding. On the Magruder Plots, it just so happens that there was already lots of phosphorus and potassium. So every year, growing wheat is like mining phosphorus and potassium, and though the soil has been mined for over a century there is still plenty left to grow wheat.
Farming with manure fertilizer
Now that we know what happens when you don’t apply fertilizer, what happens if you do?
Up till 1898 the Magruder plots were simply planted in wheat, harvested, and then replanted with no fertilizer. Then the scientists decided to split the plots in half, leaving one-half the same but applying manure (the most frequently used fertilizer at the time) to the other half. Till this day they have maintained such plots, and the results show that manure has boosted yield, especially in the last half century.
In the last fifty years scientists have not only developed chemical fertilizers but new breeds of wheat specifically designed to utilize large amounts of fertilizer, so adding fertilizer today has a larger impact on wheat yields than fertilizer did in 1900. This demonstrates how technological innovations are seldom independent of one another. New plant breeds are created to take advantage of new technologies, and new technologies are created to get more out of those new plant breeds. It’s akin to smartphones and apps. Great apps today are great because of the invention of smartphones, and smartphones are so great because of the apps they can run.
Figure 3—Wheat yields with and without manure fertilizer
The age of chemical fertilizers
By the time the Magruder Plots turned forty-years-old scientists understood soil chemistry better, and new chemical fertilizers were available on the market. These were nitrogen fertilizers created in a factory using the Haber Process, and phosphorus and potassium that were acquired by mining deep within the earth. We call these fertilizers “chemical” or “synthetic” fertilizers, to differentiate them from natural fertilizers like manure and compost.
Video 2—The Haber Process for making chemical nitrogen fertilizer
Most of the fertilizers you buy for a farm, garden, or lawn contains nitrogen, phosphorus, and potassium. On the front of the bag will usually be three numbers, telling you the percent (by weight) of the fertilizer that is in nitrogen (the first number), phosphorus (the second number), and potassium (the third number). Plants do have other nutrient needs that require other fertilizers, and we will discuss that a little later.
Figure 4—Reading nutrient content of fertilizer from bag
Video 3—How phosphorus fertilizer is produced
Plants have other nutrient needs, but that discussion is relegated to another lecture.
As the chemical fertilizer industries were established in the beginning of the twentieth century it became evident that farmers would soon be able to expand acres beyond what manure could fertilize, would no longer need to leave lands fallow, would no longer need to plant cover crops (if the terms “fallow” and “double-crop” are unfamiliar to you see the article The fertilizers of our ancestors). If they double-cropped it would be to earn more profits off each acre or to break pests cycles, not to fertilize the soil.
In 1929 plant scientists at Oklahoma State University decided it was time to include chemical fertilizers in the Magruder Plots experiment. The plots were now split into the five different treatments below.
- No fertilizer of any kind
- Manure fertilizer
- Chemical phosphorus fertilizer
- Chemical nitrogen (N), phosphorus (P), and potassium (K) fertilizers
- Chemical N, P, and K fertilizer, plus lime whenever the pH of the soil dropped below 5.5.
Lime (processed limestone) is not exactly a fertilizer, but it is sometimes needed to make fertilizer work. As a farmer harvests her crop and replaces it with chemical fertilizer she is not returning the exact elements she removed at harvest, and thus the soil is changing, usually becoming more acidic. Many plants can only uptake phosphorus if the soil’s pH is between 5.0 and 7.5, and when it drops too low the application of lime will raise it to an acceptable level. One can think of lime as heat in cooking. Heat itself is not food, but it makes the nutrients in food more available to the body.
Limestone does add calcium and magnesium to the soil, so in that regard it is a fertilizer, but I don’ formally call it a fertilizer because it is often applied when the soil already has sufficient calcium and magnesium.
Since 1930 OSU scientists have gathered yield data from these five plots, yielding the results in the figure below. To conserve space only one of the three chemical fertilizer treatments are shown.
Figure 5—Wheat yields with no fertilizer, manure fertilizer, and chemical fertilizers with lime
It is obvious that fertilizer of some kind beats no fertilizer. The chart also asserts that manure is almost as effective as chemical fertilizer with lime. This shouldn’t be surprising. Manure contains not only N, P, and K, but also all the other nutrients crops need, and is thus more of a complete fertilizer. The reason manure is slightly inferior is that it is impossible to apply manure as precisely as one can chemical fertilizer, as the actual nutrient content of manure varies from one ton to the next, whereas the nutrient composition of fertilizer is almost perfectly uniform. The fact that manure is a complete fertilizer didn’ matter much in the data because the plots have maintained high levels of nutrients other than N, P, and K, so an incomplete fertilizer is not [yet] necessary .
Magruder was not the only person to come up with such a simple, yet novel experiment. A nearly identical one has been conducted on the Broadbalk fields in England, also using wheat—the results are nearly the same. Fields can continue to produce crops for decades without manure, but yields are much higher using organic fertilizers like manure and chemical fertilizers. Moreover, on the Broadbalk fields organic fertilizers could be just as good or better than chemical fertilizers.(E1)
The similar performance of chemical fertilizers and manure suggests that organic farms (which cannot use chemical fertilizers) can achieve similar yields as conventional farms—assuming no pest infestation and easy access to manure. (It should be noted that much of the manure organic farms obtain are from animals fed non-organic grain, meaning the animal feed was produced using chemical fertilizer.) If manure is equally productive, why do the vast majority of farms use chemical fertilizers?
Money is the main reason. Chemical fertilizers are just cheaper. Since 1900 the price of nitrogen fertilizer has fallen 90%.(R2) Sometimes manure is cheaper than fertilizer, but that is only in cases where the manure source is close to the farm, as manure is bulky and thus costly to transport. So when we compare farm productivity according to yield per dollar spent of inputs, conventional farms are usually, but not universally, more productive. If this were not the case, wouldn’t conventional farmers be using manure and compost instead?
Raising crops sustainably requires looking beyond nitrogen, phosphorus, and potassium. There will come a time when micronutrients become equally important, and these are discussed in the next article.
Figures
Figures 1-3 and 5 come from files and data made available by William Raun and Hailing Zhang, plant scientists at Oklahoma State University, in 2013.(R3)
Source for Figure 4 is in the figure itself
