Let us focus on fertilizers, and on nitrogen fertilizers in particular. I choose these, as they are the largest energy input into US corn production (and into much of agricultural production world wide, as we'll see) at present (ignoring the sun!).


Fertilizer inputs in general have increased greatly with the advent of the green revolution. The high-yielding varieties require large inputs of fertilizer to achieve their yields. Supplemental inorganic fertilizer is now used in 40 - 60% of the world's food production [Vital Signs 2011].

In addition, as the population has grown, cropland area has rougly stabilized, and soil has eroded (taking nutrients with it) fertilizer requirements in agriculture have increased. Basically, farmers are attempting to boost yields on smaller and smaller per capita acreage and soil that has lost much of its inherent fertility through erosion (and other forces ), by using more and more fertilizer.

This tradeoff between land and fertilizer is illustrated clearly by contrasting trends in per capita crop land and per capita fertilizer use (globally), which are illustrated in the following figure:



Figure 6. (Modified from World Watch Institute documents)

Crop land per person decreased by nearly 1/3 between 1950 and 1984 (and by nearly 1/2 between 1950 and 1993), as we've already seen.

However, fertilizer use per capita quintupled (from 6 - 26 kg/person) over that same 1950-1984 interval.

In addition to increases in fertilizer use per capita, there were steep increases in total fertilizer use (of course!); total fertilizer use increased 10-fold between 1950 and 1990, an increase from 14 up to 143 million tons, as illustrated below. (FAO reporte that 142 million tons were used in the year 2002.)



Figure 7. (Modified from "Vital Signs 1997," World Watch Institute; an updated version is in Course Documents on Blackboard). As of 2002, world fertilizer use was up to about 142 million metric tonnes and by 2012, up to about 178 million metric tonnes(FAO).

Basically, as land got harder to come by, and its natural productivity declined (in part because of faulty agricultural practices, as we'll see [check organic matter or land degradation for some insights]), farmers have tended to substitute energy in the form of fertilizer for land.


The trend of increasing fertilizer use changed temporarily early in the 1990's. Per capita fertilizer inputs declined globally, from 28 kg per person in 1989 to less than 23 kg per person in 1993, a 19% drop (see Figure 7, above). For example, in N. America, Western Europe and Japan, after rapid increases, growth in fertilizer use actually leveled or declined slightly through much of the 1990's (in part because of diminished returns for additional inputs -- see below). Similarly, in the former Soviet Union, market reforms that eliminated subsidies for fertilizers led to a decrease in use.

However, 1995 and 1996 and subsequent years saw global increases again, following five straight years of decrease. In 2007, world fertilizer use increased by ~ 5% (WorldWatch March/April 08).These increases were largely attributable to China, where high grain prices made additional fertilizer inputs economically feasible. In addition, in 1995, there were increases in US corn acreage (7.5%), as farmers returned land to production that had been set aside to decrease corn surpluses. Demand also increased in India. As interest in producing fuel ethanol from corn increases, we are likely to see still more acreage planted to corn, with a likely associated increase in nitrogen inputs. Global fertilizer use fell somewhat in 2008, owing largely to a spike in fertilizer prices, driven by strong demand; a return to increased use occured, however, as described above [Vital Signs 2011].

The US is no longer the world leader in amount of fertilizer used. US farmers in the mid 1990's actually used less fertilizer than they did in the early 1980's, and that trend seems to be continuing! US farmers are now matching fertilizer applications more closely to crop needs, rather than applying "luxury" amounts. This has come about for a variety of reasons, both economic (when returns are marginal, costs are cut wherever possible) and ecological (for example, concerns about damage to soils and water pollution ). Advances in precision agriculture have helped as well -- with the help of GPS (Global Positioning System) technology, farmers can map nutrient content of their soils precisely, and can then use GPS coupled with on-tractor GIS (Geographic Information Science) maps to enable delivery of various amounts of fertilizer to various parts of their fields, as needed, rather than applying a set amount over an entire field.

Fertilizer use in developing countries surpassed use in developed countries for the first time in 1992. Eastern Asia is now the world leader, with Southern Asia in second place and North America in third place (~ 39%, 19%, and 12%, respectively; Vital Signs 2011]. This is a big change -- in 1960, the developing world accounted for only about 12 % of world fertilizer use, whereas by 2002, that percentage had increased to ~ 60 %. China alone used ~ 36% of the synthetic fertilier applied globally; in fact, it is estimated that fertilzer application rates in much of China could be reduced by 50% or more without affecting yields! [Science 12 Feb '10].

Another driver of decreased fertilizer use in some of the more developed nations, including the US, is that yield responses to increasing fertilizer inputs are decreasing. For example, 20 years ago, each additional ton of fertilizer used in the US corn belt resulted in a 15-20 ton increase in harvest, but today that additional ton only adds 5-10 tons to the harvest. Similar trends are being observed for other crops, such as rice in some third world nations, however the diminishing returns are most apparent in agriculturally advanced nations, where fertilizer inputs have (until recently) been highest.

The yield response situation is analogous to other cases of diminishing returns, in which the initial response to some new input (energy, time, or other resources) is for the response variable (profit, yield, etc.) to increase steeply at first. However, at some point, the response to still more inputs is less dramatic, finally reaching a plateau, above which additional inputs elicit no additional response. (The curve would resemble the logistic , s-shaped, or sigmoidal curve that we discussed when we were talking about population dynamics.)

Why is this happening?

It can be explained by the concept, familiar in ecology, of limiting factors. Organisms are usually limited in their growth by some factor or factors in the environment that are in short supply, and they will only be able to grow as much as that factor or factors will allow.

By analogy, if you have a ton of sugar and a ton of flour and a ton of baking powder, but only one egg, you can only bake one cake; the egg is your limiting factor.

As another example, if the growth of a plant is limited by the amount of sunlight it is receiving, you can add all of the fertilizer you want and it won't do any good.

This seems to be what is happening with regard to crops and agriculture, for agriculturally "advanced" systems. The green revolution, high yielding varieties being grown under modern style agriculture have all of the fertilizers (at least N, P, K -- the big three) that they can use, and something else seems to be limiting production. Hence, adding more fertilizer will not enhance yields under present conditions.

In fact, it seems to many plant breeders and agriculturists that these varieties are now being limited by photosynthetic constraints. Similar to an engine, plants can only be so efficient at utilizing energy -- solar inputs -- and the green revolution varieties, grown under intensive agriculture, may simply be reaching the biological limits to their productivity. In this case, adding more fertilizer will do no good.

This encounter with photosynthetic constraints is not limited to the developed nations, such as the US. Many of the developing countries are now using fertilizers, irrigation, and high-yielding varieties, so yields there, as in the corn belt of the US, are basically limited by natural factors such as rainfall, daylength and solar intensity. These are fundamental environmental limits that we can't change.

While debatable, it seems likely that biotechnology doesn't hold much promise for improving yields per ha to a significant extent. Conventional breeding exploited most of the options available for re-allocating photosynthate , and remaining options for conventional breeding and molecular biotechnology are relatively small, and center on improving plants' ability to tolerate stresses, such as drought or soil salinity, to resist pests, or on enhancing plant nutritional quality. Basically, a ceiling in productivity potential seems to have been reached in many agriculturally advanced systems.

As of summer, 2006, however, a new research effort IS working to improve the photosynthetic capacity of rice (Science 28 July '06). An analogy is useful here -- past breeding efforts have focused on giving rice a "new body" (short and sturdy with relatively heavy allocation to grain); the new effort will "go under the hood to try to supercharge the engine" (i.e., the photosynthetic pathway). Without going into detail, some species of grasses are "C3" grasses, and use a particular photosynthetic pathway that results in almost immediate respiratory losses of a significant fraction of the photosynthate that they produce (via photorespiration). Other species of grasses (including corn) are "C4" grasses, and use a photosynthetic pathway that doesn't result in as much losses to photorespiration. It appears that C4 grasses have evolved from C3 species at various times in the past. The effort will focus on screening rice varieties to try to detect whether some actually already have photosynthetic pathways that are "heading towards" the C4 route. If these were found, breeding could be used to accelerate the transition, perhaps, and perhaps some genetic engineering would also be used to supplement with genes from C4 species. Highly theoretical at this point, but researchers claim that if rice could be converted from C3 to C4, its yields could increase by as much as 50%!. Note, also that this is a great example of why it is important for us to retain as many different genetic varieties of crop plants as possible -- they may contain genes that will be useful to us in the future!

Thus, while there is always uncertainty in trying to project trends, it will likely be difficult to resume the rate of increase in per capita grain production that held from 1950-mid '80's or so and current rates because:

There are real shortages of arable land and water

The cost of oil and natural gas is projected to increase

The old tricks don't necessarily work any more, as evidenced by the drop in returns for additional fertilizer input.

Many nations are losing far more nutrients via erosion of top soil than they can afford to apply as fertilizers. This is particularly true for many of the poorer lesser developed countries, where farmers and the government are both struggling financially, and there is much political instability. You can read more about potential limits to agricultural production in "Feeding Nine Billion," "Forecasting Agriculturally Driven....," and "Crop Scientists Seek...," all of which are assigned reading for this point in the course. Numerous articles on the list of supplementary readings for this portion of the course also address these challenges -- and some potential solutions!


Of the major nutrients needed for plant growth, nitrogen (N) is the most commonly limiting, at least in terrestrial systems. This is the case, despite the fact that the atmosphere is about 79% N (actually N2 gas). Plants, however, can't use atmospheric nitrogen directly. They can only use it after it has been converted to other forms such as nitrate (NO3-) or ammonium (NH4+).

So, it is important to make N available for crops, and there are basically three ways of accomplishing this:

(1) Soil microbes decay organic matter in soils (dead organisms, leaves, manure and so forth) and, in so doing, eventually release biologically available forms of N such as nitrate and ammonium.

(2) Nitrogen-fixing organisms (e.g.,bacteria) "fix" atmospheric N2 into biologically available forms of nitrogen, such as ammonia (NH3), by combining nitrogen with hydrogen, oxygen, or carbon. These organisms can be free-living or living in close association with roots, as in the Rhizobium bacteria that live in the roots of legumes. (Maybe when pulling your pea or bean plants, you've noticed the bumps or nodules on their roots? These contain the nitrogen-fixing bacteria. I'll show you some in class from my cover crop of fva beans.)

(3) Apply nitrogen as inorganic fertilizer. Here's where the energy input comes in. Nitrogen fertilizers are manufactured by combining atmospheric N2 with hydrogen (from methane) to produce ammonium nitrate, ammonium sulfate, or urea. The process is very energy intensive, as it must be done under high temperature and pressure.

(4) Probably relatively minor in terms of total input, hence my not including it one of the "big three" but lightning aslo fixes molecular nitrogen (N2 gas) into nitrate (NO3) which is available for biological uptake -- it takes a LOT of energy to combine N2 with O2 in the reaction that leads to No3).


Humans, largely through agriculture but also through burning of fossil fuels, have had a huge --and growing -- impact on the nitrogen budget of Earth. This is a topic of much recent concern, research, and modeling (see related references on the Study Guide for this unit on agriculture). The global rate of terrestrial nitrogen fixation has increased greatly as a result of human activities (estimates range from a human-induced increase of 1.5 to 2 times the natural amount) in the past few decades. (Marine sources are difficult to estimate, so are not included in this reckoning.)

About 57% of anthropogenic nitrogen fixation results from the manufacture of nitrogen-containing fertilizers, 29% from cultivation of nitrogen-fixing crops, and 14% from burning fossil fuels (some sources put the fossil fuel contribution at 25%).

Between 1995 and 2005 alone, human production of nitrogen fertilizers by the Haber-Bosch process increased by 20%!!! This production largely was used to facilitate production of cereal grains and rapidly increasing meat production. During this same period, cultivation-induced biological nitrogen fixation (largely by legumes) also increased by about 26%!! (Science 16 May 08). As these examples illustrate, human production of reactive nitrogen compounds is increasing at a truly incredible rate. ("Reactive nitrogen compounds" include all biologically, chemically, and radiatively active forms of nitrogen, many of which we'll talk about here in the context of fertilizer, and some of which we'll talk about later in connection with tropospheric ozone pollution [NOx's] and global climate change [N2O])

The important questions are:

1) where does all this "extra" N go, and

2) what are its consequences for natural and other ecosystems?

More and more attention is being focused on these questions, and some scientists believe that the consequences of this human input to the global nitrogen cycle will be nearly as significant as our influences on climate will be. Atmospheric transport is believed to be the dominant process of movement for these nitrogen compounds (Science 16 May 08). For example, some forms of nitrogen are produced directly or indirectly as a result of fossil fuel combustion, and include nitric oxide (NO, which contributes to acid deposition and is involved in reactions that produce ozone pollution in the troposphere [the lower part of the atmosphere]) and nitrous oxide (N2O; a "greenhouse gas" and depleter of stratospheric ozone, that we will hear more about later this term. Nitrous oxide is also prouced by reactions that start with nitrogen-based fertilizers on soils. Pat predicts that we will hear a LOT more about nitrous oxide in coming years (written in 2014).....

Other forms are volatilized (given off in gaseous forms), as is ammonia from fertilizers and from animal wastes and then returned to the surface with precipitation or in dry forms. About 25-35% of volatilized nitrogen compounds are estimated to go into rivers (particularly as nitrate), where they affects the rivers themselves as well as contribute to artificial "enrichment" ("dead zones" or eutrophication) of coastal waters such as Chesapeake Bay. (Despite the clearly visible effects of these riverine inputs into coastal waters, atmospheric deposition of nitrogen into oceans contributes much more than these in total -- atmospheric transport is the dominant means of movement of N-compounds, so much obviously ends up in the oceans.) In addition, various bacteria ("denitrifying bacteria") acting on nitrate convert it to other forms of nitrogen, some of which cause problems in their own rights (see my prediction about nitrous oxide just above...).

What are the likely consequences of this boost in fixed nitrogen? Fertilization of coastal waters and freshwater is virtually certain, but the magnitude (and desirability) of these effects is uncertain. Other possible effects include fertilization of terrestrial ecosystems and those effects intimated above (enhanced depletion of stratospheric ozone, accelerated climate change, exacerbated acid deposition and ozone pollution). However, models are currently best at predicting the fate of fixed nitrogen transported by rivers, and until we know where the non-riverine transported nitrogen goes, we can't really predict its consequences. They are likely to be major, however. For example, nitrogen is one of the nutrients most commonly limiting plant production, hence additional fixed nitrogen may be important in affecting forest growth. However, some soils have apparently become so saturated with nitrogen (that is, they contain more nitrogen than plants can take up or than can be held on exchange sites in the soil) that nitrate is being leached out of them with water, carrying other essential plant nutrients, such as calcium and potassium with it. This has serious implications for maintenance of soil fertility. On the other hand, if forest growth in some areas is stimulated by added nitrogen, then this growth might help sequester atmospheric CO2, which could be useful in mitigating the rising concentrations of that "greenhouse gas" in the atmosphere.

Oceans are being fertilized both by these nitrogen inputs and by added CO2 inputs. This could be a "good" thing, from the perspective of helping to avert global climate change: if ocean productivity increases, then oceanic uptake of atmospheric CO2 will probably increase too -- this would diminish the warming potential of the atmosphere. However, enhanced inputs of nitrogen into oceans may also increase the flux of N2O from the oceans to the atmosphere, which would be "bad" because N2O is an important greenhouse gas. The relative balance between these two responses is uncertain (Science 16 May 08), but likely to tend towards the "good" rather than the "bad" side.

We will talk about controls over various nitrogen gases in the atmosphere when we talk about tropospheric ozone pollution and global climate change, later in the term.

(See related articles on the supplementary reading list for this unit including those entitled "Human contributions to terrestrial nitrogen flux," "Human alteration of the global nitrogen cycle: sources and consequences," "Transformation of the nitrogen cycle....", and "Toxic Fertility" for more information. The article on this topic by Vitousek et al. is also available as an "Issues in Ecology" paper, published by the Ecological Society of America (ESA), the professional scientific society to which most practicing ecologists in the US belong. The paper is available by visiting the ESA's Home page and following the appropriate links. However, to read the article on the web, you'll need to download the free Adobe Acrobat Reader – instructions for doing so are given in the ESA home page section on the Issues papers. If you visit this site from here and want to return, click the "BACK" button in your browser's toolbar.)

The next section (click ">>" at the bottom of this section to move there now) introduces some problems with green revolution style agriculture, with more to come later in lectures on pesticides and land degradation .

Page maintained by Patricia Muir at Oregon State University. Last updated (partially) Jan 23, 2014.