I use slow-release fertilizer, 25% N, 4% K and 4% P at a rate that will supply approximately 50 lbs N/acre (= 50 Kg/ha). I'm not sure that our clay soils ever become deficient in potassium or phosphorous so I buy the richest slow-release source of nitrogen (which just happens to be 25/4/4). Per 1000 sq.ft. (ca 100 sq.m) to get 50 lbs N from the slow release granules, I apply 4.6 lbs (ca 2.1 kg) annually, 2.3 lbs in the spring and 2.3 lbs in the fall. Slow release is easiest for me with varied irrigation rates and sometimes very heavy rainfall, but I have used ammonium sulfate (21% N) successfully. The ammonium form of N tends to exchange with other cations on the clay and compost that I use. I have calculations on a worksheet if anyone's interested. Roy Davis, California; Sacramento Valley, very hot dry summers , mild spring and fall; irrigation required (usually) from April through October.