Nutrient Cycling

In natural ecosystems, the nutrients cycle. Nutrients are collected from the soil by the roots of plants, contribute to the growth of stems, leaves, and fruits, and when the plants die are returned to the soil as the vegetation rots. A similar cycling supports animal populations: Nutrients, such as nitrogen, are ingested as the animals graze on grass and other plants, are partly returned in the excreta and urine, and partly returned when the animals eventually die and decompose (Figure 13.1).

As all farmers recognize, when plants are treated as crops and animals as livestock, the process of harvesting removes the nutrients from the ecosystem. Some soils are naturally richer in nutrients than others and can be mined, at least for a period, but

The nitrogen cycle in crops, livestock, and the soil

Figure 13.1 The nitrogen cycle in crops, livestock, and the soil.3

eventually for all soils the lost nutrients have to be replaced. Without nutrient replacement there is no agricultural sustainability.

Synthetic Fertilizers

Until the twentieth century, the only means of nutrient replacement available to the farmer was to apply composts or manures or, as the Romans recognized, to grow nitrogen-fixing legumes (such legumes contain bacteria in their root nodules that take up nitrogen from the atmosphere). Agriculture was transformed by the invention, at the beginning of the century, of the Haber-Bosch process for synthesizing ammonia from the atmosphere. Ammonia is the primary ingredient in synthetic fertilizers, and today, the manufacture of synthetic, inorganic fertilizers is highly efficient and, until recently, relatively inexpensive, relying as it does on atmospheric nitrogen and fossil fuels, such as methane and coal, for its basic ingredients. However, as I indicated in Chapter 1, rising energy prices have significantly increased the costs of fertilizer production and hence resulted in rising prices of cereals and other food crops.

The other key nutrients, phosphorus and potassium, are also fairly plentiful in mineral forms, as phosphates and potash, respectively. However, they are geographically narrowly confined (50 percent of the deposits are located in the Middle East and another 25 percent in South Africa), and there is controversy over whether the reserves of phosphorus are peaking.4 Estimates of these reserves and availability of exploitable deposits vary greatly. High-grade phosphate ores, particularly those containing few contaminants, are being progressively depleted, and production costs are increasing. One review concludes that within some sixty to seventy years about half the world’s phosphate resources will have been used up.5

The development of synthetic fertilizers opened up the potential for very high yields, far higher than were achieved by natural nutrient cycling. It was this potential that Norman Borlaug and his colleagues sought to exploit in the Green Revolution. Today about half of total fertilizers are applied to cereals, about 15 percent each for rice, wheat, and maize.6

In those countries that experienced the Green Revolution, fertilizer consumption is not much lower than in the developed countries.8 In China, fertilizer use is now higher than in the developed countries (Figure 13.2). But, there are considerable downsides to such heavy use. Crop plants rarely make efficient use of nitrogen, whether in synthetic fertilizer (i.e., produced by industrial processes such as Haber- Bosch) or in crop residue or manure. Of the 100 million tons of nitrogen fertilizer (N) produced industrially in 2005, only 17 percent was taken up by crops. Moreover, efficiency has been declining: between 1960 and 2000, the efficiency of nitrogen use for cereal production decreased from 80 to 30 percent.9

To make up for the losses, farmers tend to apply far more N than is needed, even taking into account the losses. A great deal is wasted. This is understandable. There is a massive variation in the response of crops to fertilizer applications—the soil type, the rainfall, and the kind of fertilizer all affect the efficiency with which the nutrients are taken up and converted to grain or other harvested product. It is difficult, even with sophisticated analyses, to determine the appropriate level of fertilization. The very high levels of fertilizer subsidy are a further incentive to overfertilize.

Nitrogen fertilizer overuse is ubiquitous. China now manufactures and uses a third of global nitrogen fertilizer. The percentage overuse in the Chinese provinces ranges from 50 to 100 percent, and it is estimated that China could halve N use without impacting yield or yield growth. But the government continues to push for high productivity and self-sufficiency and believes high N applications are essential.10

Growth of synthetic fertilizer use in arable land (nitrogen, phosphate, and potash)

Figure 13.2 Growth of synthetic fertilizer use in arable land (nitrogen, phosphate, and potash).7

 
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