One of the first questions to address relates to your soil and its nutritional status. Do you know the pH and the P/K indicates?
If the answer is no, then it is advisable to soil sample every field going into maize. The soil pH is fundamentally important to nutrient availability, affecting not only N, P, and K, but also micronutrients. Also, do not overlook sulphur, which for maize, as for other crops, is essential for nitrogen utilisation. The best advice is to accurately establish the pH and P/K indices, to record the amount of muck and slurry applied and to produce an accurate nutrient management plan which takes all of this into account.