Why Organic Fertilizers are Not the Solution to the Dead Zone in the Gulf of Mexico

By Steve Savage
November 12th, 2009, Environmental & Climate Science

From the comment streams and emails I’ve been getting about recent posts, it is clear that many people believe things that are not actually true about the environmental profile of organic fertilizers.  I don’t mean to minimize the challenge we face when it comes to fertilizers, particularly nitrogen fertilizers.  They take energy to make, have the potential to generate the potent greenhouse gas, nitrous oxide, and can lead to the pollution of ground and surface waters.  With a “rap-sheet” like that I understand why people are concerned, but there is a catch – without fertilizers we don’t eat much.

Still, there is a widespread belief that “Organic” fertilizers are the solution.  I’ve already blogged about why organic fertilizers are dramatically worse from a greenhouse gas point of view.  Today I want to talk about the water pollution issues and why “Organic” fertilizers are actually a much worse problem from that perspective as well.

Why Nitrogen Fertilizers Can Pollute

The reason that ALL nitrogen fertilizers (synthetic and Organic) are a water pollution threat is that they at some point convert to the nitrate ion (NO3-).  That particular form of nitrogen is very water soluble so the nitrate can move down into ground water or sideways into surface water.  The “Dead Zone” or “Hypoxia zone” in the Gulf of Mexico is driven in some large part by nitrate coming from farms.   There are ways to manage this issue, but first I need to talk about the fundemental challenge of crop fertilization.

The Fertilization Challenge

As a crop grows it starts off with a limited need for nitrogen because the plants are small and they are getting some which was stored in the seed itself.  Then they go into a period of very rapid growth where they need quite a bit of nitrogen (and other nutrients).  During the period of “grain fill” there is very high demand for nutrients.  After that, the plant does not really need or take up nutrients over weeks prior to harvest.  The challenge is to supply as much nutrient mix as the plant can use during those periods of peak demand.  A farmer has a built-in incentive to try to match demand and supply as closely as possible, particularly when fertilizer prices are high (they are tightly tied to energy price).  The ideal system is where the nutrients are supplied through the irrigation system.  They can be “spoon-fed” to almost exactly fit the need.  This is only something that can be done in irrigated crops which are mostly high value (fruits, vegetables, row crops in certain areas).  It is no wonder that Organic growers of these crops were motivated to use the supposedly Organic liquid fertilizers that could be delivered in this spoon feeding mode that turned out to be spiked with “synthetic” nitrogen for many years over much of “organic” production.

Ways to Optimize Fertilization

For the hundreds of millions of acres of row crops that are “rain-fed,” the spoon feeding approach is not feasible.  Instead a grower has economic incentive to try to match the nutrient supply with the nutrient demand as well as possible.  For the “conventional” grower there are a lot of options to do that.  The fertilizer can be precisely placed where the crops roots will be growing at key times in the season.  The fertilizer rate can be varied across a field based on soil and yield maps (via GPS systems).  There are nitrogen stabilization products that can be used.  The farmer can use “split applications” to better time the nutrient availability.  The grower can use no-till methods so that erosion is almost eliminated (and with it the movement of nitrate into surface water).  The farmer can plant a winter “cover crop” to scavenge the nitrate that is still in the field after harvest. (Fortunately there is a sort of Renaissance of cover cropping starting in ag today and more growers would like to use them if they had the time).

Issues with Organic Fertilizers

Organic growers do tend to plant cover crops which helps, but the other solutions described above are mostly not options for the Organic grower because of the Organic rules.  Organic farmers are limited to “natural” fertilizers which for non-legume, large acreage, crops boils down to manure or composted manure (Organic growers are not the only ones that use manure, but the problems occur for all). Some of the nitrogen molecules in an “organic” fertilizer are exactly the same chemicals as in “synthetic” fertilizers – urea and ammonia.  Some of it is in more complex biologically formed molecules like amino acids, nucleic acids and a variety of intermediate metabolites and structural molecules.  That is why organic fertilizers are “slow release” forms of nitrogen.  Over time, soil microbes convert those more complex forms into exactly the same nitrate ion that comes from a synthetic fertilizer –  the nitrate that plants can use (and which can become a pollutant of the water or atmosphere).  The problem is that the conversion process does not match the crop demand.  To achieve good yields, Organic growers need to apply very high amounts of total nitrogen so that enough is available when the crop needs it.  Much of this nitrogen continues to be turned into nitrate well after the crop is using it and so it is well documented that this form of fertilization leads to water pollution issues.  This is why farmers around the Chesapeake Bay are being payed to truck manure far away from the waterways that drain into the bay.  Slow-release nitrogen sounds a lot better than it is.

Organic fertilizers also have the problem that they contain more phosphorus than is needed if they are used at the rates that make sense from a nitrogen point of view.  Growers using manure or compost pick rates based on nitrogen, but that means that phosphorus is over-supplied.  This too leads to water pollution.

Fortunately only around 5% of US cropland is being fertilized with “organic” fertilizers (and little of that is USDA Organic).  I’d like to see that number go to nearly zero and instead see that manure being turned into energy.  Hypoxia in the gulf is a very real and important issue.  There are solutions, but they don’t involve the use of organic fertilizers.