Agricultural Productivity in relation to Population 
observed in nature, or in agricultural laboratories ? 
Algae have received a lot of attention—they have the inci- 
dental advantage that they can use up sewage—but they rarely 
have a yield of more than 20 grams dry matter per square 
metre per day. In July, the warmest—and often the wettest— 
month of the year in England, potatoes and sugarbeet grow at a 
rate of 30 grams of dry matter per square metre per day or more. 
In the tropics, rates of 50, so long as heat and water are avail- 
able, have been shown by sugar cane, elephant grass, and 
water hyacinth. The latter is a most troublesome weed which 
grows in waterways, but Pirie has very sensibly suggested that 
it should be grazed by hippopotamus, manatee or other marine 
animals, which yield good meat. Experiments in the agri- 
cultural laboratory at Oxford have shown that, under carefully 
controlled conditions, the familiar radish and broccoli can grow 
at 40 to 45 grams of dry matter per square metre per day. In 
open air in England the corn cockle has been found to grow at 
57—which is why the farmers find it such a dangerous weed. 
If we settle for 50 grams per square metre per day, then the 
growth of each person’s requirements of food and fibre requires 
only 27 square metres. This is the sort of information which may 
be required in the future by the designers of space ships and 
artificial satellites. Even this figure may be reduced further, as 
we come to learn more about the exceptionally complex and 
tricky process of photosynthesis. The theoretical maximum 
rate of photosynthesis has been calculated at seven times our 
figure of 50 grams per square metre per day. 
Malthus thought that human populations always tended to 
increase up to the limits of available subsistence, after which 
their growth was checked by ‘‘vice and misery”. Historical, 
geographical and anthropological evidence—most of which, to 
be fair, was not available to Malthus—alike show how untrue his 
proposition was. Primitive hunting and fishing peoples, who 
are able to make virtually no provision for the sick and infirm, 
suffer such high mortality that even reproduction at the maxi- 
mum rate biologically possible only just maintains their num- 
bers. When eventually the population rose so that adequate 
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