Fertilizer, lesser-nutrients, and the nutrient content of food

Ancient China once found itself a vassal of the Mongolians, paying tribute because it could not defeat Mongolian horsemen in battle. The Mongols had stronger, faster horses, while Chinese horses were not strong enough to be ridden. Two or more could be used to pull a chariot, but they could not be ridden by a mounted warrior. Why? According to historians this is because the Chinese soils were deficient in the lesser-nutrient selenium, a nutrient needed for strong bones, while the Mongolian soils contained an abundance. The Mongols were thus superior in battle because their soils were superior in nutrients.^(H1) Sometimes, it is the soil that makes the nation. For the Mongolians, their soil was their secret weapon, even if they were unaware of its source.

Most of the time the world “fertilizer” refers to plant available nitrogen (N), phosphorus (P), and potassium (K), yet there are many other nutrients essential for plant and animals. There is carbon, oxygen, and hydrogen that plants acquire from the air or water. Other elements, most of which are acquired from the soil, are ...

• calcium
• magnesium
• sulfur
• boron
• chlorine
• copper
• iron
• manganese
• molybdenum
• zinc^(T2)

These nutrients other than N, P, and K which are acquired by the soil I refer to throughout this lecture as lesser-nutrients. This is because they are needed in lower amounts relative to N, P, K, carbon, and hydrogen.

There are some lesser-nutrients needed by few if any plants, but are necessary for animal growth. Examples are selenium and iodine, both of which plants take from the soil and thus present in vegetables, even if they do not need it. Plants can be healthy and provide large yields, even if they grow in selenium- or iodine-deficient soils. This means that just because a plant is in good health doesn’t mean that eating it will make you healthy.

Consider iodine. Mothers with low iodine levels in their womb have children who score worse on literary tests, so just because you don’t hear much about iodine doesn’t mean it isn’t a problem.^(R1) Look at the label of the salt you buy and you will probably see that it also contains iodine (unless it is Kosher or pure Sea Salt). Since we all use small amounts of salt each day, the best way to ensure we consume small amounts of iodine is to add it to salt. Some individuals do not like the taste of iodine in salt and choose Kosher salt instead. The Nutrition Diva Monica Reinagel says this is okay, as, “Vegetables can be a source of iodine, depending on the iodine content of the soil in which they’re grown.” Iodine may sound like a trivial concern, but recent research suggests that even with iodized salt pregnant women may not be receiving enough.^(C1,R2)

If a soil is being depleted of important lesser-nutrients, we could suffer the consequence. Many people in developing countries where iodine is not added to salt, and whose soils are lacking in iodine, suffer the consequences in the form of malnutrition from iodine deficiency. This is important, because the health of the plant doesn’t tell us everything about its nutrient composition, which means a farmer can harvest a healthy crop without realizing it is deficient in iodine. Some minerals important to human health are not actually used by the plant, so the plants could be remain healthy even if its nutrient content for humans is in decline.^(E1)

Although they don’t get much attention in the U.S., lesser-nutrients are important. We don’t hear much about them because they are usually so abundant in the soil that they pose no problem. One day, though, they might, so we should study them now instead of waiting until their absence causes a health problem.

Critics of chemical fertilizers rightly point out that N, P, and K (the nutrients covered in the lecture The Magruder Plots) aren’t everything a plant needs, and farming without returning many of these lesser-nutrients like zinc to the field means you are basically mining the soil.

How long can we continue to mine the soil? Some estimates for some specific locations suggests there are enough lesser-nutrients for hundreds, sometimes thousands, of years.^(W3)

Sometimes a lesser-nutrient problem exists even though it is prevalent in the soil. This is because not all nutrients are available to plants. North Dakota soils have large amounts of iron but much of it is unavailable because the soil’s pH is too high. The solution is not to apply more iron but to lower the soil’s pH, or to apply a chelate, which helps transport iron from the soil onto the surface of plant roots.^(N1)

We have an answer for what will happen when a soil becomes deprived of lesser-nutrients, because in a few areas it has already happened. In the last twenty years some wheat farmers in the state of Washington noticed that applying more nitrogen did not increase yields, which suggested the wheat’s growth was limited by the absence of two other nutrients: chloride and sulfur. How did Washington farmers respond? Fertilizer companies developed a market to profit off this need, and so farmers today easily acquire inexpensive chloride and sulfur and apply them to the field, restoring high yields.^(S1) When some other lesser-nutrient becomes scarce in the soil, fertilizer companies and farmers will respond in the same way. In fact, in areas like Oklahoma where lesser-nutrients are not a problem there are fertilizer salespeople trying to sell fertilizer supplements. When agronomists test the performance of these supplements they usually find they have no impact on yield.^(Z1) At one point agronomists at Kansas State University were concerned that their experimental wheat fields were lacking sufficient zinc. They had no trouble purchasing zinc to apply, but it did not seem to increase yields.^(S2) Now, just think how many salespeople would be marketing copper, iron, and boron, and zinc in Oklahoma and Kansas when it is actually needed!

Fertilizer and the nutrient content of food

Some people have observed that the nutrient content of many foods has fallen over time, and they attribute this to a lack of lesser-nutrients in the soil. Is our reliance on chemical fertilizers causing our food to be less nutritious?

Research suggests the blame mostly belongs to new crop varieties. With the rise of chemical fertilizer also came higher yielding crop varieties. These more efficient varieties of grains, fruits, and vegetables are subject to the genetic dilution effect, a concept describing the tradeoff between yield and nutrition. These improved varieties of plants achieve a higher yield in two ways: by taking more nutrients from the earth and by packing less nutrients per unit of food. Thus, the new crop varieties are probably the major source of nutrient loss in foods today.^(D1,D2) This was illustrated nicely on the Broadbalk fields in England, where experimental plots have been maintained since 1843. Between 1843 and the 1960s the concentration of lesser-nutrients in the wheat harvested remained steady, but then began falling, giving the impression that the soil was running out of zinc, iron, copper, and magnesium. However, the actual amount of [plant available] lesser-nutrients in the soil had remained steady or increased, leading researchers to conclude it was the choice of wheat varieties planted that reduced the nutrient content of the wheat, not soil deficiencies.^(F1)

Of course, the nutrient content of food is less important than access to total nutrients, and over the last century the U.S. has increased its per capita production of almost every nutrient (and for the nutrients that are less available today, the decrease is small).

Figure 1—Nutrient content of food (per capita) today compared to a century prior

Greater nutrient availability doesn’t necessarily mean greater nutrient consumption, if patterns of food waste are changing at the same time more nutrients are produced. Studies of nutrient consumption in the UK find that per-person intake of some lesser-nutrients like magnesium, iron, zinc, and copper has fallen over time, and in some cases is insufficient for a person’s daily nutrient needs. While there are nutrient deficiencies in the U.S. they have not changed much since 1999.^(F1) The purpose of the above figure is not to argue that there are no lesser-nutrient deficiencies in the U.S., but that the persistent use of chemical fertilizers does not seem to pose a nutrient problem.

Another way to inquire whether chemical fertilizers affect the nutrient content of food is to compare non-organic food to organic food. Most of the time, non-organic food is raised using chemical fertilizers, whereas organic food must use other sources. Many scientific comparisons have been made, but the overall results suggest organic food is equivalent to conventional food in terms of nutrition.^(B1,D3,W1) When grocery stores in the United Kingdom tried to market organic food as being more nutritious, it was ordered by the government to stop, because it was considered false advertising. The stores could not refute the government’s accusation, so they ceased advertising organic as nutritionally superior. Organic food does have less pesticide residues, but we defer this issue to the lecture on pesticides.

Can the Magruder plots teach us anything about the relationship between fertilizer use and lesser-nutrients? Yes. The chart below shows the concentration of seven lesser-nutrients on the plots receiving manure and chemical fertilizer, relative to the plots that have received no fertilizer since 1892. If the number says -10%, that means its concentration of a lesser-nutrient is 10% lower than those observed on the unfertilized plots.

One might think that the plots receiving chemical fertilizers would have fewer lesser-nutrients. After all, more wheat is harvested from those plots every year than the unfertilized plots, and therefore are removing more and more lesser-nutrients each year. Of the red bars, five are positive and two are negative, which means that the plots receiving chemical fertilizer had larger amounts of five lesser-nutrients but smaller amounts of two lesser-nutrients, relative to the unfertilized plots. If chemical fertilizers do not contain these lesser-nutrients, how can this be the case? One explanation lies in the fact that we are not measuring absolute levels of calcium or zinc, but the plant-available calcium / zinc. Remember when we said that using lime to achieve the proper pH level makes it easier for plants to access nutrients? That is probably what is going on. It is also remarkable that although the levels of zinc and magnesium fall the reductions are not large.

Figure 2—Lesser-nutrients on the Magruder Plots

For plots receiving manure fertilizer, the lesser-nutrient levels (compared to unfertilized plots) are considerably higher for calcium, zinc and boron, and are slightly higher for magnesium and copper. Only in the case of sulfur and iron are the less nutrients on the manure plots. The rather 10% decrease of sulfur is surprising, since one would expect manure to contain all levels of lesser-nutrients.

All things considered, lesser-nutrients just don’t seem like a big problem in U.S. agriculture today, at least in most regions. That doesn’t mean we can keep ignoring them, but for now, there is little reason to think that lesser-nutrient levels will hinder agricultural output. Moreover, blindly applying lesser-nutrients, simply on the assumption that one is supposed to, could backfire. It is easy to apply too much of a lesser-nutrient. When their levels do become a problem, we anticipate that fertilizer companies and scientists in agricultural colleges will quickly identify feasible solutions for dealing with the problem, just as they have already one with the macronutrients N, P, and K.

Figures

(1) Numbers are calculated as the percent change in average, per day, per capita nutrient supply over the period 1997-2006 relative to the average over the period 1909-1918. Source: Economic Research Service. Nutrients (food energy, nutrients, and dietary components [dataset]. United States Department of Agriculture. Accessed July 30, 2013 at http://www.ers.usda.gov/data-products/food-availability-(per-capita)-data-system.aspx#26715.

(2) Original chart created from data made available by William Raun and Hailing Zhang, plant scientists at Oklahoma State University, in 2013.