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A century ago, there were only Natural Fertilisers - plenty of manure from the horses, cattle, pigs, and all the horses in the towns and cities too. There were two aspects to using it in the fields:-
1. The animals wastes added to their straw bedding made manure, which put nitrogen and organic matter back into the fields when it was ploughed back in.
2. The stuff had to be disposed of somehow - you couldn't dump it in the rivers without poisoning the drinking water of the people downstream
This was hard work ! In Victorian times, large quantities of guano (bird droppings) were imported to use as concentrated fertiliser, and many other 'natural' by-products are also used as fertilisers, now mostly by gardeners rather than farmers. These include bone meal, hoof and horn, and dried blood, which all have various short- or long-term advantages for the gardener. The farmers now seldom make manure - the slurry from the animals is usually disposed of by spraying it on the fields, where it has some fertilising effect.

The principles of Artificial Fertilisers were already well known 100 years ago - there are 3 basic chemicals involved which are highly beneficial for plant growth:- Nitrogen (usually supplied as a nitrate), Potassium (as K2O), and Phosphorus (as P2O5). In 1928 and 1978, the relevant annual usages in the UK were:-

  N K2O P2O5
1929 50,000 tons 50,000 tons 140,000 tons
1978 1,115,000 400,000 400,000

Production has increased greatly, and also many new lines of plant have been bred which give proportionately better increases in yield for increases in fertiliser usage. However, the energy usages are:-
In 1975, about 80 x 109 MJ of fuel were used in agriculture, ( 1,900,000 tons of oil equivalent)
and, about 110 x 109 MJ of fuel were used in fertiliser production.( 2,600,000 tons of oil equivalent)(ref. 1, p 21)

Industrial Fixation from Microsoft Encarta 96 Encyclopedia, ref 3.
The principal industrial nitrogen-fixation process today is the production of ammonia by passing a mixture of atmospheric nitrogen and hydrogen over a metallic catalyst (see Catalysis) at 500°-600° C (932°-1112° F). Ammonia is then oxidized to form nitric acid, which is in turn combined with ammonia to yield ammonium nitrate, used primarily in explosives and fertilizers (see Fertilizer). In another method, cyanamide, which is used as a fertilizer or in the production of cyanides, is produced by passing atmospheric nitrogen over heated calcium carbide in the presence of a catalyst.

Current Situation. Artificial fertiliser usage is clearly economic, otherwise farmers would not use it, however, it will tend to lead to a reduction of organic matter in the soil, which may lead to soil erosion, as it is not 'held together' as well when the organic content of the soil reduces.

Future. World fertiliser production seems likely to increase, however, it seems likely that there will eventually be shortages of fuel to power the chemical processes, and that the economics of this will change. In this sense, the current situation does not seem sustainable. Hydroponics will probably increase. From ref 2, p75, it has been necessary in Holland to introduce a system of quotas for pig-keeping (the quotasare tradeable) and a system of contracts for the disposal of slurry to ensure that every Dutch pig breeder either has sufficient land to spread his slurry, or sells it to someone who does, to ensure compliance with European nitrate regulations

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