Nitrous oxide, emitted into the atmosphere from natural and man-made sources, including biomass burning and fertiliser use, was 323.2 parts per billion in 2010 - 20%higher than in the pre-industrial era.

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Countries are issuing, or at least preparing, a long list of promises to help finance research into new seeds, build irrigation canals and spread fertiliser use among small farmers (seeds, irrigation and fertilisers were the main components of the first green revolution in the 1960s).

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Mr Zimba frets that he cannot afford enough seeds or fertiliser to make full use of his land.

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The pigeon houses riddling the rock faces, traditionally used to collect the birds' droppings for use as fertiliser, mostly stand empty.

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Eradication teams in Colombia talk of finding high-density coca fields in new areas, and more use of fertiliser by growers.

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Farmers in sub-Saharan Africa now use only about 9kg of fertiliser per hectare on average, compared with 142kg in South-East Asia.

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However, heavy machinery and herbicides are required to prepare a seedbed, and with several applications of fertiliser and sprays the field will see far more machinery use than any similar organic crop.

ECONOMIST: Organic matter

Although organic yields averaged 20% less than those from the conventional plots, the input of fertiliser and energy was reduced by between 34% and 53%, and pesticide use by 97%.

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Even the waste from the waste is put to good use -- the waste from the bio-fermentor is used as a natural fertiliser in the barley fields.

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All organic crop rotations contain three to four years of no ploughing because grass leys use the sun's energy to fix nitrogen for subsequent crops in place of oil-based artificial fertiliser.

ECONOMIST: Organic matter