Applications of fertilizer N to boost grass production when the activity of the clover component in a perennial ryegrass-clover pasture is restricted by low temperatures, is confined to applications in autumn, winter and spring, seasons characterized by high rainfall. Of major environmental concern is the flow of N from the pasture into water reserves. The strategic application of fertilizer nitrogen, if managed wrongly, can therefore be regarded as an important environmental pollution hazard. The recovery of fertilizer N by plants is generally low and rarely more than 50-75% (Allison, 1966; Ball & Ryden, 1984). In order to reduce N losses from pastures, the application of fertilizer N should aim to remain within the steepest portion of the pasture's response curve to ensure rapid absorption and efficient N use by plants. The objectives of this study were to monitor the mineral N content of the soil after fertilizer N application to develop strategic N fertilization norms for soil and climatic conditions as experienced in the Western Cape Province that will ensure 1) maximum N response efficiencies and 2) minimum contamination of natural resources as a result of leaching. The dynamics of soil inorganic-N in reaction to a single, strategic application of fertilizer N was studied during the period 2000 to 2002 in an Oakleaf soil with a clay content increasing from 10 to 30.8 % with depth. Soil inorganic-N was monitored to a soil depth of 500 mm (0-100, 200-300 and 400-500 mm layers) over a seven week period (7, 21 and 49 days) following the application of 0, 50 and 150 kg N/ha in the form of LAN (28) applied as a once off application to a perennial ryegrass-white clover pasture during five different seasons (autumn, early winter, late winter, early spring or late spring). An autoanalyzer was used to determine the soil ammonium- and nitrate-N content and the total inorganic N calculated as the sum of ammonium- and nitrate-N. Inorganic-N content increased from 2000 to 2002 possibly as a result of biological N fixation by the clover. The application of 50 kg N/ha did not result in significant differences in inorganic-N content compared to the 0 kg N/ha treatments during almost the entire study and can therefore be regarded as a low risk treatment with regard to environmental pollution. The increasing effect of 150 kg N/ha on inorganic-N lasted for at least 3 weeks in the 0-100 mm layer and 7 weeks in the 200-300 mm layer with the highest inorganic-N content measured in autumn and early winter during years of average rainfall (2000 and 2002). Leaching of fertilizer N within one week after application to the 400-500 mm soil layers during the 2000 and 2001 emphasised the possible negative effect of application rates exceeding the absorption capacity of the pasture. The optimal N rate would clearly be the rate that results in sufficient N to last for only one regrowth cycle. This will require relative low rates of fertilizer N when pasture productivity is low and/or under high rainfall conditions and may be increased as pasture productivity increases in spring and water supply is managed by thorough irrigation scheduling as rainfall decreases with the onset of spring. References: (i) ALLISON, F.E., 1966. The fate of nitrogen applied in soils. Adv. Agron. 18, 219-258. (ii) BALL, P.R. & RAYDEN 1984. Nitrogen relationships in intensively managed temperate grasslands. Plant Soil 76, 23-33. Keywords: Inorganic nitrogen, perennial ryegrass, strategic nitrogen, white clover.