Environmental Engineering Reference
In-Depth Information
remain large: theoretical yield potentials for wheat and rapeseed are, respectively,
19.2 and 9.2 t/ha.
New ways to increase yields will be necessary because there are obvious biophysi-
cal limits to further increases in HI values: minimum needs for indispensable struc-
tural (roots and stems) and photosynthetic tissues (leaves) will make it impossible
to increase them much beyond their current level. The most likely maxima for HI
in cereals are between 0.60 and 0.65, as it would be impossible to support more
than 65% of the total yield as grain on less than 35% of the overall phytomass.
However, values up to 0.80 may be achieved with some root crops (Hay 1995).
Future yield increases will thus have to come largely from increased production of
total phytomass, which is limited by the amount of intercepted solar radiation and
by the maximum rate of photosynthetic conversion, which is higher inherently
higher in C
4
crops.
Maxima of new phytomass production in unstressed crops are about 1.7 g/MJ
of intercepted solar radiation for corn, 1.4 g/MJ for rice, and 1.2 g/MJ for soybeans
(Smil 2000). In terms of actual yields, the i rst rate would translate to harvests of
up to 35 t/ha in the Corn Belt, the second one to yields of up to 25 t/ha of rice in
East China. Actual maxima, even for crops grown under optimum conditions, will
be considerably lower, but potential maxima can be judged by looking at the record
harvests: 22 t/ha for corn (in Michigan in 1977), 14.1 t/ha for wheat (in Washington
in 1965), and 13.2 t/ha for rice (in Japan in 1981). Of course, all of them required
exceptionally high applications of nitrogenous fertilizers.
What difference future yield increases of staple grains will make will depend on
the rate of population growth and on the progress, and eventual saturation levels,
of the worldwide dietary transition, that is, primarily on the future rates of increased
meat consumption. Although the UN has lowered every one of its regular long-term
projections of the world's population, and although there is a fairly good probability
that the global total will not surpass nine billion, the latest medium variant forecast
is for 9.15 billion in the year 2050, nearly a third above the 2010 aggregate of 6.9
billion (UN 2011). Virtually all of this growth will take place in low- and medium-
income countries: European and Japanese populations will shrink, North America
might add about 100 million people, but Asia's total will go up by at least one
billion, and so will Africa's population, nearly doubling to two billion.
Many worrisome scenarios can be constructed using these i gures. For example,
if Africa's average per capita meat consumption were to reach only today's Asian
mean (going from about 16 kg/year in 2010 to 28 kg/year by 2050), the continent's
