Agriculture Requires Fertilizer Inputs, and That’s Good

Harvest time by joinash via Flickr.
Harvest time by joinash via Flickr.

On a brown, August-dry field in Eastern Washington, a farmer in a combine cuts a 24-foot swath across a field of wheat. The harvested grain then begins a journey, first to the storage bin, then to the local elevator, on rail to a flour mill, by truck to a bakery, by oven to bread, and by car to a home where it is eaten. This is good; our foremost mandate to agriculture is to produce food. However, with this successful export of food from farm fields to nearby and distant cities comes a problem: the nutrients in the bread, the nutrients that we need from food, and that plants need to grow, are now far from the field they came from. How do we replace them?
High yields worsen the problem.  A typical irrigated winter wheat field will yield 140 bushels per acre; about 5,600 loaves of bread. For a center pivot circle of 100 acres, the nutrients in those loaves amount to 182 pounds of N, 70 of P2O5 and 49 of K2O and smaller amounts of other essential nutrients that do not have to be replaced every year. All this ends up somewhere else (in people’s bodies or in sewage treatment plants); it will not be returned to the field1.
If we want agriculture to continue producing food, then replacement nutrients must be brought back to the field. With the possible exception of nitrogen (discussed below) farmers must then apply fertilizers, either synthetic or organic, to the field. This seems obvious, yet confusion on this point is widespread and often surfaces in statements declaring that modern agriculture is overly dependent on “expensive external inputs.” As I see it, there are two alternatives to being dependent on external fertilizers. The first is that we mimic natural ecosystems where only 10% of production is generally exported, eaten by migratory animals, moved by water, wind or lost through other processes. Typically a wheat harvest removes up to 50% of the field’s aboveground production (leaving wheat straw and chaff). If that were reduced to 10% the unsustainable result would include expensive food for some and starvation for others. The second option is to move people out of cities and disperse them across the countryside so that wastes, (both food and human), can be more easily recycled to farm fields. There are many reasons this would neither work nor be desirable.
There are, however, proposed biological solutions to this problem, often promoted by researchers studying plants in native habitats. Many plants, researchers find, have adapted an assortment of mechanisms and associations that allow them to better survive in low nutrient environments. These include mycorrhizal fungi, bacteria in root nodules (nitrogen fixing; discussed below), free-living soil bacteria, and proteoid roots. Others suggest using cover crops – buckwheat is known to make phosphorus more available – or compost tea, presumably full of bacteria and fungi that can get at nutrients that are unavailable to plants. One such solution is acclaimed (by a research institution) as “of great interest for farmers because bacteria-based biofertilizers constitute an alternative to conventional chemical fertilizers that are expensive and less sustainable from an environmental point of view.”
Are these really alternatives to fertilizers? I think not. Although these adaptations may help improve nutrient use efficiency of crops (that amount of the nutrient pool in the soil that crops take up), aside from legume nodules they fail as fertilizer alternatives due to conservation of mass, which here can be stated as “nutrients exported from a field must be replaced by an equal import of nutrients.” Nutrients are not created in the field through any mechanism, natural or not. Even nitrogen from legumes is imported from the air. None of these so-called alternatives to synthetic fertilizers create nutrients. They exist to help plants survive (not thrive) in the nutrient limited conditions found in natural ecosystems. Since farmers strive to eliminate nutrient limitations in their fields, these mechanisms are not so helpful, and they are often switched off when high levels of nutrients are available.
Using nitrogen fixation (by certain types of bacteria) in nodules on legume roots like alfalfa, beans, and peas, is another suggested practice to reduce fertilizer use. However, time constraints, and water and phosphorus use by legumes limit their usefulness. While legume crops do not need much nitrogen fertilizer themselves, they do not leave much nitrogen after harvest for the following crops. Legume cover crops however, could supply nitrogen to following crops. For legume cover crops, it takes time to fix enough nitrogen to both pay for the seed cost and make a significant contribution to the nitrogen requirements for the following crop. Unfortunately, this is time that is often needed to grow crops for food. For example, to make this strategy work in our eastern Washington wheat field, a legume has to be planted in late August, which only works after a few crops like wheat, early sweet corn, or green peas. The legume cover crop must survive the winter and be allowed to grow at least to mid-May. Here again, fields that will be planted with crops before mid-May, which includes most crops, are eliminated from using this practice. Although not a constraint in irrigated regions, water use by legume cover crops limits their use in dryland regions where the reduced water can reduce yields of the following crop, no matter how much nitrogen the cover crop provides. Finally, although growing a legume crops does not require a farmer to provide nitrogen, they still must provide these crops with significant amounts of phosphorus, which must be imported to replace that removed in harvest thwarting the replacement fertilizers.
How about the biodiversity strategy to increase the ecosystem function of maintaining soil fertility? Will it help to increase the diversity of plants grown on a field, either through a more diverse crop rotation or by adding cover crops and green manures? These too fail as alternatives to fertilizers, because they do not add new nutrients to the field. They may help scavenge nutrients that would otherwise be lost to the system (e.g. via leaching of nitrates), or they may improve the availability of nutrients already in the soil, like buckwheat does with phosphorus, but they do not bring in new nutrients. Without imported fertilizers, these solutions only help mine the soil of nutrients more effectively.
To avoid mining, we must, because of the conservation of mass, replace nutrients with inputs, and this is where those promoting certain “sustainable” systems have a problem. Unless we implement a large-scale return of biosolids (and more morbidly, nutrients from dead bodies) to agricultural fields, we must use other inputs, such as synthetic and organic fertilizers. The latter is often portrayed as more sustainable, but as I have argued before, and research has found, many of the nutrients in these materials come originally from synthetic fertilizers applied to fields, harvested in crops, fed to livestock, excreted in manure and recycled to fields. The problem is not solved, it just is harder to see the original source of these nutrients and so they look more sustainable.
Do farmers sometimes over-apply fertilizers? Yes. Do some nutrients from fertilizers end up in streams and lakes and the Gulf of Mexico? Yes. But the answer to these problems is not to ban fertilizer. We abuse antibiotics, but nobody is proposing that we solve this problem by banning their use. As WSU’s Craig Frear points out, we must improve both the fertilizers themselves and our management of fertilizer nutrients (and antibiotics) because they are required for our survival.
So then, agriculture is dependent on external inputs, and this is good because it means that agriculture is successful at providing food to people far from the farm. Attempts to avoid these inputs are not realistic. Back at that straw and chaff covered field, the farmer must figure out how to replenish the nutrients that were removed. It does not matter whether the farm is organic (organic fertilizers are also “expensive external inputs” due to the transport costs of their higher bulk and weight compared to synthetic fertilizers) or conventional, whether the bread was baked at home, sold at Whole Foods, or in a sandwich vending machine. If the nutrients are not replaced, then agriculture quickly fails to do what we most want from it, and that is produce food.
1 some biosolids do make it back to farmers’ fields, but there are challenges to increasing this, including the public’s general queasiness with the practice.
First posted here.


  1. This seems so obvious that I wondered for a second why it was written. Then I thought of all the folks advocating “natural” farming [no contradiction there] and realized that we need more articles such as this out there for the non plant growing majority to see.

  2. In other words: Matter can’t be created.
    I try to tell some of my friends this when they say we should be farming more efficiently. There’s limits because we humans are always pulling matter from this cycle. Instead, I’ll just point them to this article.

  3. Eric, you would not believe how many people I have come across online that think that “resting” the soil or growing cover crops replaces nutrients. It might make some nutrients already present a little more available, but if you keep taking out without replacing, no amount of resting the soil will maintain fertility. Thanks Andrew for writing this.

  4. I should also add crop rotation to that list. All crop rotation does is prevent one crop from selectively removing certain nutrients. Crop rotation just balances out the removal of nutrients, depending on the crops in the rotation. It does not replace nutrients, other than the limited nitrogen provided by legumes.

  5. Benjamin, What about my sandy soil? It is nutrient poor and holds water poorly. Wouldn’t it get “tired” and need more rest than a loam?

  6. Here are some examples of this online.
    From the Union of Concerned Scientists, who should know better:
    “Monoculture farming relies heavily on chemical inputs such as synthetic fertilizers and pesticides. The fertilizers are needed because growing the same plant (and nothing else) in the same place year after year quickly depletes the nutrients that the plant relies on, and these nutrients have to be replenished somehow.”
    They try to blame fertilizer use on monoculture, making it sound like one crop is going to use one set of nutrient and another crop another set of nutrients, but they all use N, P, K, just in different amounts.
    Also, Dr. Elaine Ingham, previously Chief Scientist with Rodale, promoted mining of the soil in an interview :
    “Nutrient cycling is another major issue. According to Dr. Ingham, there’s no soil on Earth that lacks the nutrients to grow a plant. She believes the concept that your soil is deficient and needs added phosphorous or nitrogen, etc. in order to grow plants is flawed, and largely orchestrated by the chemical companies, because it’s based on looking at the soluble, inorganic nutrients that are partly present in your soil.”
    “But the real nutrition your plants require actually is derived from microorganisms in the soil. These organisms take the mineral material that’s in your soil and convert it into a plant-available form. Without these bioorganisms, your plants cannot get the nutrients they need. So what you need is not more chemical soil additives, what you need is the proper balance of beneficial soil organisms.”

  7. I wonder what Dr. Elaine thinks the micro-organisms will live on? Chief scientist with Rodale? Wow, That organization just lost more respect.

  8. “But the real nutrition your plants require actually is derived from microorganisms in the soil.” Well, suddenly everyone is a microbial ecologist. Would be a shame to show them plant seedlings growing on sterile nutrient agar…

  9. There’s a little more truth to the Rodale comments than you guys think. Most agricultural soils (I don’t say all) do contain vast amounts of phosphorus and potassium in the rooting zone. Enough for several hundred years of modern cropping at least. In many areas it’s not readily plant available, but it is there. Soil micro-organisms can extract it. All they need to do it is decent pH, a little nitrogen and sulfur, and an ample supply of reduced carbon (root exudates and plant residues).
    Many fields in Eastern Nebraska have never seen any significant potassium fertilizer and yet they produce high corn and soybean yields year after year. The soil tests continue to show high levels of available potassium, despite 100 years worth of “mining.” Many of those same fields receive only low levels of phosphorus fertilizer, 25-50% of crop removal, and yet they show good levels of available phosphorus.
    I would venture to guess that most of the U.S. corn belt could be farmed on a three year rotation consisting of corn, soybeans, green manure/cover crops and maintain productivity and fertility with very little mineral inputs (other than lime to maintain pH). Certainly it could be done with less than 25% of the fertilizer currently used. There is a farm in the eastern corn belt that has been farmed like that for about 10 years, and the soil tests show available P and K continue to rise. Doing that does cut the total grain output by 1/3, of course, but if you can cut fertilizer use by 80%. . . Well, it just depends on the value society will place on fertilizer and grain down the road.
    Soil is far from a sterile medium, and the microbial community has more to do with plant nutrition than this article implies.

  10. Hey Paul, Your comment made me realize that I should occasionally remind folks that the teeny farm I grow on is in North Florida. Very Deep sand. If I break into a new area I can work and [slight exaggeration] not get dirty. The advice I have been given by several agronomists regarding microbes is to build up the soil and the microbes will follow. They do not think buying microbials is cost effective. My first thought when I started reading the articles by [usually] those selling microbes was that they just might starve in my soil. Further I cannot conceive of the microbe that can generate copper, zinc, manganese, Iron, boron, and magnesium from where there is very little. Still even considering that you may well be correct about the areas you speak of. Is it wise to mine nutrients other than in times of distress? It is not my soil and I would be the last one to get gov’t involved by anti mining rules. I just question not at least attempting to replace that which we ship to urban areas. .

  11. Hi Eric,
    Yeah, sand can be pretty close to the classic “sterile rooting medium” that many people envision soil to be. I agree completely with the agronomists you mention. Microbes can’t reproduce without food. They would starve in sand. You’ve got to provide the food first, and then maybe a few selected innoculates to increase diversity once you’ve got a steady food supply, but the food first. I know of corn growers in North Florida who plant rye-based cover crops in the winter, and then plant corn directly into the residue mat. That residue mat is nearly gone by corn harvest. They can produce very good grain crops, but they first grow several tons/acre of microbe food each year.
    It’s not that mineral nutrients can be generated by microbes, but rather that microbes can extract nutrients from the unweathered parent material. The soil dynamics are incredibly complex, and there is much to learn for years to come, but microbes really do control nutrient availability to a high degree. It’s not as simple as supplying plant-available NPK each year, those nutrients need to stay available throughout the growing season. The fertilizers we do apply get cycled through the soil community several times before the plants take them up.
    As for the wisdom of mining soils with robust microbes? Well, on the soils I’ve familiar with (relatively young, unweathered soils) it could last for several hundred years. Plus, you’re going to be mining the nutrients from somewhere. Is it really wise to burn huge amounts of fossil fuels doing it the way we currently are? That’s the way to frame the debate, I would think. Not, “should you mine nutrients”, but, “from where should you mine nutrients?”
    I don’t have a pat answer to that it, but perhaps it’s good to explore both routes simultaneously.

  12. Wow. didn’t know those l’il microbes can transmute P and K from S, H and C. And all this time, us foolish humans thought we needed 1.0e+9 deg C plasmas to do such similar tricks !

  13. Paul, I do agree that Dr. Ingham started with some truth. There is a lot of P and K in some soils, and microbes can make some of it available each year, but in most cases the amounts made available will not be sufficient to replace fertilizer (organic or synthetic) inputs if we want to maintain yields. The carbon inputs that you mention are needed for the organisms to do their work will also require either time, for cover crops to grow, and thus reducing time for food crops, or imports of plants grown on other fields (carbon mining?).
    My goal was not to say that fertilizers are the only answer, but that we cannot easily replace them with the alternatives being promoted, which all have downsides too, at least if we want to feed our current population, and a larger future population.

  14. We’re looking at the situation from opposite directions. You’re looking at the crowd that says we don’t need fertilizers at all, and I’m looking at the crowd that says pour the fertilizer to it. The truth is somewhere in between.
    At a conference last year, an Iowa agronomist stood up and said, in effect, “If a farmer is banding replacement P and K rates in strip-till, there’s no need to be soil testing.” Really!?! Just dump out replacement rates of fertilizer and never mind the soil. At a time when agriculture is getting blamed for every environmental problem out there, the idea of injecting replacement rates without soil testing seems like a particularly bad idea.

  15. So, like most issues local circumstances will determine the farmer’s correct decision. I can not mine what is not there. This subject has come up several times in the linked in sustainable ag. group. Several who advocate for microbials were also selling them and made what I consider to be unrealistic claims. Such as no pesticides would ever need to be used on such crops. I can understand why it might be necessary to buy a few types that do not naturally occur in my soil. But those might also show up in the mulch and manure I bring in. Also I understand that my situation is not the type of ag that Andrew is generally referring to. I can only do the mulching I do because I am tiny and sell direct retail. Paul, The growers you refer to in my area are going to additional expense to grow the microbial food you mention. They are also skipping time that those fields could be used for other cool weather crops if the nutrients were replaced with fertilizer. And they must still replace some of the nutrients each year. If the soil is anything like “normal” in this area. K, ,mg, mn, and others are usually found at very low levels.
    Benjamin, I was kidding about the tired soil. I suspect you know that. What was in my mind was a comment way back about how it was more natural to rest the soil and that I was driven by a desire to profit at “all costs”

  16. Almost forgot a side note. I do landscape inspections on FDOT highway landscape projects. Some of them require inoculants to be mixed in the backfill when planting palms and trees. One of my fellow inspectors observed shrink wrapped, palletized, inoculants on an asphalt parking lot in Florida. I suspect there was little alive when those were used.

  17. I am not sure how Rodale could lose any more respect. They employ homeopathic veterinarians to treat their animals and to try to convert other farmers to using homeopathy.

  18. Well Mike, I guess I’m glad I cancelled my Subscription many moons ago. Was not aware of the homeopathy use. So, you just caused them to lose even more respect. Apparently the proper method to cause loss of respect for them is to explain more about them.

  19. I agree, the actual situation is somewhere in between.
    With banded P and K, there is minimal risk of the applied nutrients moving out of the field unless there is severe erosion, which is should be avoided by using strip-till. So the agronomist may be right in saying that annual soil testing is not needed; depending on the rotation, a soil test for P and K may only be needed every other year, or maybe only one in five years. Testing for nitrogen is another story.

  20. It is true that the main purpose of agriculture is to produce food for our economy, curving food problems and so on. For over use to continue producing food our lands is being non-fertile day by day. As a results further crop production is more. The topic “Agriculture Requires Fertilizer Inputs, and That’s Good” is really helpful to know about the use of fertilizers more. For increasing soil fertility, we use green manure, compost and do practice of crop rotation along with planting of Legumes. in addition, for rapid demand of plant nutrients we use chemical fertilizers as Fertilizer Inputs in our Agriculture. But it does not always fruitful about what i want in Agriculture. Chemical Fertilizers have negative impact on soil such as ground water pollution, soil friability effect and destruction of Micro-Organisms. So, have a sharp look to avoid those negative impacts.

  21. What do you think of fertilizer from ocean harvested sources such as shellfish or/and seaweed farms? Particularly since these pick up nutrients from waste products and from runoff? If phosphorus and potassium get lost into the ocean this is going into the ocean to get it back. Along these lines, does extraction of these nutrients via the biproducts of desalination plants seem feasible?
    Also as for nitrogen it seems like the option of rotating with nitrogen-fixing cover crops is understated, perhaps even skipped? It seemed unclear how cover cropping was imagined to be done. Usually one does not simply wipe the entire farm clean and replant it all with a cover crop, one section is set aside while growing of cash crops continues on the other sections. Ideally I’ve heard it said that a 7 year rotation plan of multiple crop types (cash and cover, etc) is best. Yes you’ve taken land out of the loop for producing money in the short term, but you save land and nutrients in the long term. Moreover the benefits to the soil are more than just a fresh injection of new nitrogen.
    That phosphorus limitation does worry me though…

  22. I am not sure about the details of getting nutrients from the ocean, but I think we need to start thinking in this direction, recovering lost nutrient and reusing them.
    As far as nitrogen from legumes goes, yes, we should use them, but realize that nitrogen fixed by legume crops that we harvest do not add much to the soil; most of the fixed N is exported away from the field with harvest of hay, or beans. Cover crops, as you point out, are better at supplying N to the next crop, but it comes at a cost. As long as a cash crop is more valuable than the nitrogen from a legume cover crop, farmers will be reluctant to give up the profit as they must survive as a business.

Comments are closed.