“There's a wealth of real possibilities for change to a more sustainable and more human course.” – Bill McKibben, billmckibben.com on Plan B.
Chapter 8. Raising Land Productivity: Rethinking Land Productivity
After climbing from 1.1 tons per hectare in 1950 to 2.8 tons in 2002, the world grain yield has reached a level where it is becoming more difficult to sustain a continuing rapid rise. Much of the impressive gain in yields came as scientists boosted the share of photosynthate going to seed from 20 percent in traditional varieties to over 50 percent in modern high-yielding grains, close to the theoretical limit. Efforts to raise yields further are starting to push against the physiological limits of plants. In many countries, the rise in yields is slowing and in some it is leveling off. For example, yields have not risen much in rice in Japan since 1984, in wheat in Mexico since 1980, or in wheat in the United States since 1985.9
This loss of momentum is worldwide. While world grainland productivity rose by just over 2 percent a year from 1950 to 1990, it averaged only 1 percent annually from 1990 to 2001. (See Table 8-1.) And in the last five years from 1997 to 2002, the annual yield gain dropped to 0.5 percent.10
The rise in grain yields will likely slow further during this decade. In addition to the shrinking backlog of technology to draw upon, many farmers also must deal with a loss of irrigation water, and farmers worldwide are facing the prospect of record-high temperatures—all of which could make it difficult to sustain a steady rise in land productivity.
Although the rise in yields is slowing, there are still many opportunities for increasing yields, but in most situations the potential for doing so is modest. In Africa, for example, where fertilizer use is restricted by aridity and transport costs, the simultaneous planting of grain and leguminous trees is showing promise. The trees start slowly, permitting the grain crop to mature and be harvested. Then they grow to several feet in height. The leaves dropped from the trees provide nitrogen and organic matter—both sorely needed in African soils. The wood is then cut and used for fuel. This simple, locally adapted technology, developed by Pedro Sanchez, head of the International Centre for Research in Agroforestry in Nairobi, often enables farmers to double their grain yields within a matter of years as soil fertility builds.11
The magnitude of the challenge ahead is unmistakable. It will force us to think about both limiting the growth in demand and using the existing harvest more productively. On the demand side, achieving an acceptable balance between food and people may now depend on stabilizing world population as close to 7 billion as possible and reducing the unhealthily high level of consumption of livestock products in industrial countries. But we must also think more broadly about land productivity, considering not only the individual crop but how we can increase the number of crops harvested and how to use them better.
|Table 8-1. Gains in World Grain Yield Per Hectare, 1950-2001|
1Yields for 1990 and 2001 are three-year averages.
Source: See endnote 10.
9. Yields from USDA, op. cit. note 1; percent photosynthate to seed from J. T. Evans, Crop Evolution Adaptation and Yield (Cambridge: Cambridge University Press, 1993), pp. 242-44.
10. Table 8-1 from USDA, op. cit. note 1.
11. Pedro Sanchez, "The Climate Change-Soil Fertility-Food Security Nexus," summary note (Bonn: International Food Policy Research Institute, 4 September 2001).
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