CHANGES IN FERTILIZER AND MANURIAL PRACTICES DURING 1960-1990 - IMPLICATIONS FOR N AND P INPUTS TO THE YTHAN CATCHMENT, N.E. SCOTLAND

A suggested increase in the growth of macrophytic algae within the Yth an estuary (N.E. Scotland) over recent years has been linked to the in creased amounts of nitrogen in the form of NO3-N entering the estuary from the river. The increased NO3 concentration in the river has been associated with recent changes in farming practices in this predominan tly agricultural catchment. Terrestrially derived phosphorus is also c onsidered to contribute increasingly to eutrophication of fresh waters . Historical agricultural census data together with appropriate survey s of fertilizer practice were used to calculate the total quantities o f fertilizer and manure derived N and P applied annually over the whol eYthan catchment during the period 1960 - 1990. While the total agricu ltural land area has remained similar, significant changes in cropping practice have occurred. In particular, a greater proportion of land i s given to autumn sown crops while the area of grassland has declined. These changes in farming practice are associated with differences in both the total amounts and timing of fertilizer applied. The use of in organic N in the catchment has trebled since 1960 and is currently app roximately 6400 tonnes (104 kg N/ha). The use of P has decreased by mo re than a quarter to 1274 tonnes (21 kg P/ha) over a similar time peri od. There has been no obvious change in total quantity of N and P deri ved from animal manures, estimated to be 44 and 11 kg per ha, respecti vely, when averaged over the area of agricultural land. Cattle and she ep numbers have remained relatively constant and together account for approximately 80% of the manure N and 70% of the manure P produced ann ually. However, poultry have declined by 70% since 1960 while pig numb ers have increased six-fold. The average annual application rate of ma nure derived N over the whole catchment (44 kg/ha) is considerably bel ow that proposed at the farm scale in the EC Nitrate Directive (210-17 0 kg/ha). However, on a local scale difficulties may arise for large m anure producing concerns such as dairy or pig units.