A comparison was made of the effect of N and P in fertiliser and in organic manure used in agriculture, on eutrophication of ground and surface waters.
It was shown that losses from soil organic matter and organic manures often are higher than from fertiliser. This is for nitrogen mainly due to mineralization of organic matter by bacterial activity outside the growing season, especially on tilled land. Thus for tilled land the effect on dry matter production of an annual application of 100 kg total N.ha-1 in farmyard manure was estimated to be about 50% of the effect of 100 kg N applied as fertiliser in spring. This difference is mainly caused by higher leaching losses of nitrate in autumn and winter and evaporation of ammonia.
The P-losses in agriculture are small and will range from 0-0-5 kg P.ha^.y-1 if soil erosion is absent. However, an average leaching loss of 0*22 kg P.ha ‘.y-1 was found to be dangerous to 35% of the lakes and permissible to only another 35%. Although movement of inorganic phosphorus into the soil profile is a very slow process, these results indicate that in the long run this penetration process may yet be a dangerous one, especially on light soils.
In organic manure, however, P-corapounds are found, which are not easily adsorbed by the soil profile and therefore will move downwards much faster than the inorganic compounds from fertiliser.
Another drawback of organic manure in general is that the ratio of plant nutrients in the manure is not in agreement with crop requirements. This easily leads to a lack or surplus in plant nutrients and in the latter case to increased losses to the environment.
In spite of these drawbacks organic manure will be indispensable in farming practice in controlling soil fertility, and maintaining or increasing soil organic matter content and soil structure.
The conclusion is that an improvement of water quality might be possible to a certain extent by better fertilisation practices. This, however, will result in much extra costs for the farmer and therefore in higher food prices for the consumer.