Agriculture Requires Fertilizer Inputs, and That’s Good

October 16, 2014
By Andrew McGuire
Harvest is an export of nutrients.  Photo: S. Bauer, USDA

Harvest is an export of nutrients.
Photo: S. Bauer, USDA

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, which we want, increase 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 physically hauled to the field. With the possible exception of nitrogen, discussed below, farmers must then apply fertilizers, either synthetic or organic, to the field. Although this is clear to see, 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 such 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, beyond the public’s general queasiness with the practice.

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