Our Indispensable Soil - Soil: Reflections on the Basis of our Existence - page 2

What is Sustainable Agriculture?
All great civilisations have built their prosperity on a long tradition of farming fertile
soil. Along the River Nile, rich volcanic silt washed by rainwater from the moun-
tains of Abyssinia enabled farmers to cultivate the surrounding flood plains for thou-
sands of years of agriculture without loss of soil fertility. The silt deposits came from
once-forested slopes cleared by poor Abyssinian shepherds to create pasture for their
animals. By destroying the forests, the shepherds exposed the earth to erosion and
inadvertently ensured the viability of agriculture in Egypt. To this day, millions of
tonnes of rich sediment accumulate on the Nile's flood plains and deltas every year.
Sustainable agriculture means ensuring that nutrients removed from the land
during harvesting are replenished in equal measure. This can be through the addi-
tion of fertiliser or by flooding, as in the case of the Nile. Sustainable farming
also requires that soil humus is not depleted because it is important for maintain-
ing the soil's structure and ability to retain water. Humus decomposes at a higher
rate when soil is cultivated but will increase if compost is added or plants with
large underground root systems are grown. If no plant matter grows on the soil, it
becomes vulnerable to erosion by wind and rain.
Inexorable soil depletion from agriculture. Soil fertility declines if the nutrients lost at harvest time
are not replenished. Since 1843, scientists at Rothamsted research park in the UK have been study-
ing how the presence or lack of nutrients affects crop yields. The bright yellow rows have had no
nitrogen added to them, resulting in a yield of only 10 % of that produced in the fully fertilised rows