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Phosphorus: A Limited Resource

By Wendy Taheri, Ph.D.

Soil is a living, breathing ecosystem. Just as you and I breathe, soil too re­spires, and we measure that respiration rate as an indicator of microbial activity in soil. While there are large, non-mi­croscopic organisms living in soil such as worms, insects and small mammals, none of them exist by the billions in just a handful of soil except the microbes.

There are many scientific classifica­tions for microbes in soil, but from the farmer’s perspective only two catego­ries are relevant. Good microbes (major­ity) and bad microbes (small minority). Good microbes enhance plant growth, and bad microbes cause disease in plants. Of course, things are never quite so clear-cut in nature. Some things can be good under some circumstances and bad under other circumstances. So keep in mind this is a simplification of what are, in reality, very complex interactions.

Our management practices should be refined to support the good (most of the time) microbes and suppress the ones known to cause diseases in crop plants. Diseases are not always caused directly by organisms. Sometimes the balance of the system gets thrown off and something ordinarily not a prob­lem finds a new niche and can become problematic.

Weak plants may also be susceptible to organisms in the envi­ronment that normally would not have much impact on them. For instance, a nutrient deficiency might weaken a plant and lead to susceptibility. The good news is, of the thousands of microorganisms identified in soil thus far, only a handful of those really fall into the bad category. The good far outweigh the bad, and with a little thoughtful management, you can keep it that way.

In the case of good microbes, we can take this a step further and narrow our focus to the most crucial organisms within this group, which are those that provide the macro and micronutrients plants require for growth. The most limiting of these nutrients is typically phosphorus.

Nitrogen can play a close second in the nutrient race, but in most soils phosphorus is the most limiting nutrient, often occurring in quantities a thousand times lower than other miner­als. One of the reasons for this is the high reactivity of phosphorus. It tends to bind to soil particles and complex with metals in the soil. This makes it unavailable to plants even if it is present in the soil.

Phosphorus is an element, meaning there is a phosphorus atom. It can be nei­ther destroyed nor created. The amount that exists on this planet is all we have. When we remove crops from the field we remove the phosphorus those plants took up. It becomes part of the food we eat.

Nitrogen can play a close second in the nutrient race, but in most soils phosphorus is the most limiting nutrient, often occurring in quantities a thousand times lower than other minerals.

Much of that phosphorus is literally flushed down the toilet in our urine, re­moving it from our agricultural systems, although some wastewater treatment plants are now producing fertilizers.

The phosphorus used in the manu­facture of fertilizers comes from phos­phate rock, which is mined and then processed to make phosphoric acid. Phosphoric acid is used in turn to make fertilizer. A by-product of this process is phosphogypsum, which is highly radio­active. For every 1 ton of phosphoric acid produced, 5 tons of radioactive waste is also produced.

Once we have our phosphate fertil­izer we have a new problem. We have to pour enough into the soil to saturate it because it immediately begins binding to particles. This means only a fraction of what we put into the soil will ever make it into the plant. Furthermore, when plants detect high levels of phosphorus they reject their symbiotic partners, ar­buscular mycorrhizal (AM) fungi.

We are giving the plants for “free” what they normally have to trade carbohydrates to AM fungi for. When plants are colonized with AM fungi, the fungi become an extension of the plant’s root system. This greatly increases the volume of soil that is available to the plant for nutrient up­take.

When plants reject their AM fungal symbionts, they lose that extension to their root system. This in turn, reduces how quickly they can uptake phospho­rus. The longer the residence time that the fertilizer spends in the soil, the more time it has to bind and become unavail­able, runoff into streams, or leach into water tables. The efficiency of the system declines dramatically.

The issues of chemical fertilization don’t end there. The phosphorus that winds up in our waterways causes eu­trophication of lakes. Algae and aquatic plants can utilize the phosphate. This causes them to reproduce rapidly, often producing algae blooms. These blooms can reduce the clarity of the water, caus­ing the death of plants that grow on the bottom of lakes by filtering out the sun­light.

The algae may bloom faster than the environment can support and much of it too will begin to die. The dead algae and plants are a food source for bacteria, which in turn have their own bloom. These bacteria utilize a lot of oxygen. The reduction in oxygen can be severe enough to kill off fish, clams and any­thing else that needs oxygen to survive in the water. This is also a good example of a circumstance where something that is normally a beneficial member of the microbial community, happily recycling nutrients, has suddenly caused a prob­lem. Not because it’s a “bad” microbe, but because we have unintentionally thrown the ecology of the system out of whack.

Unfortunately, so much phosphate has run into our oceans over the years that they now have massive dead zones, which has a negative impact on the fish­ing industry. Eutrophication of our lakes can also have a negative impact on their recreation value.

The trade-off for these problems is maximizing food production in a world with over 7 billion mouths to feed, and that number is going up. The global population is expected to reach 9 bil­lion by 2050. Food demand is expected to increase with water and energy needs doubling by then. We are clearly overpopulating the world. Like all populations in nature, we are subject to the carrying capacity of our environment. In the case of human populations that is the entire Earth, but it is still a finite resource.

And we should ask ourselves if we really want to create a world where every living thing has been displaced or extinguished in order to attempt the support of unsustainable population levels.

Ironically, it was the advent of com­mercial fertilizers that allowed us to increase food production which in turn allowed our global population to in­crease to these levels. The exponential increase we see in our population is a common theme in ecological systems where a population rises until it exceeds the carrying capacity of its environment, utilizes all of a resource crucial to its sur­vival, and then crashes. We have been on a steep population rise for decades. Only time will tell if we will have the foresight to manage that population, or will con­tinue to place demands on the environ­ment until it can no longer sustain our numbers, at which point our population too could crash. Thus far, technology has helped us maximize our potential, but even technology has its limits. However, many scientists believe our world popu­lation will stabilize around 2050 near the 9 billion mark. Historically, industrial­ization has led to a rise in population followed by stabilization. This tends to follow increases in literacy, particularly of women, and access to birth control. If our population stabilizes at 9 billion, we will need to increase our agricultural production by 70 percent to keep that population fed. And we need to find a sustainable means of doing so.

Global Phosphorus Supply

Phosphorus is a limited resource. Mining phosphate rock to produce fer­tilizers requires a significant amount of energy. Fuel is manufactured from oil, which is also a limited resource. As phosphate supplies dwindle we will try to tap less accessible deposits, increas­ing the energy required to reach those deposits. We will also begin processing lower-quality phosphate rock with more impurities, increasing processing costs.

Much of the corn we grow is processed for biofuel production, and with corn prices high, farmers are encouraged to spend more on increasing their yields, often applying high rates of fertilizer because even a small increase in yield is worth the cost when commodity prices are high enough. This sets the stage for increasing demand and escalating prices.

Phosphate rock deposits are not evenly distributed world­wide. Many countries, realizing a shortage could occur in the future, are now putting high tariffs on phosphate exports to discourage it leaving their country. By far, the vast majority of the world’s phosphate deposits lie in Morocco. This small country controls roughly 75 percent of the world’s phosphate reserves, and it is all virtually under the control of one person, King Mohammed VI.

In 1973, when OPEC imposed an oil embargo on the United States, oil prices quadrupled. Much like our dependency on oil, our dependency on phosphate fertilizer puts our future food security in the hands of a foreign country. Prices for phosphate spiked in 2008, partially driven by biofuel demand. As a result, prices jumped 800 percent. That is double the price increase of oil that resulted from the embargo.

There has been a flood of phosphate reserve esti­mates in an attempt to determine how long the world supply can be expected to last. Estimates of when we will reach peak phosphorus range from now to over 100 years from now. Prob­ably the most accurate estimate reported thus far is that of Lindstrom, Cordell and White. They used a Bayesian statistical method based on the IFDC’s (International Fertilizer Develop­ment Center) reported worldwide reserves. Their analysis sug­gests we may reach peak phosphorus somewhere between 31 to 80 years from now.

Regardless of whose estimate you put your faith in, there is one thing all the scientists agree upon, that we will run out. It’s just a matter of time. And like oil, long before we run out, prices will rise substantially. This will lead to an increase in food prices, and countries that already suffer food insecurity will be hit the hardest.

The Solution

We need to increase our efficient use of resources such as phosphorus. We really can’t afford to waste it on saturating our soils and killing fish. This will extend our supply, help keep costs under control, and give us the time we need to reestablish the natural nutrient cycles agriculture once depended upon.

Those natural cycles have been decoupled by more than 50 years of intensive chemical fertilization. Without any fertilizer, natural ecosystems produce more biomass than our cropping systems. Yet, prior to the development of the first fertilizers, our agri­cultural production was a fraction of what it is today.

We must examine the salient features of natural ecosystems that allow them to be so productive and adopt management practices that exploit those features, integrating the ones that practicality allows into our cultural practices.

This story was published in the August 2012 issue of Acres U.S.A.

Dr. Wendy Taheri is a mycorrhizal ecologist.