Am I losing nutrients by over irrigating, my crop is yellow even after fertiliser?

Yes, this is a common and frustrating problem. When you put on heavy water, especially on light sandy soil, the water drains down fast and carries soluble nutrients like nitrogen below the root zone where the crop can no longer reach them, which is called leaching. So you applied fertiliser, but it washed away before the crop could use it, and the plant looks yellow and hungry. The fixes are to water less at a time and more in line with what the soil can hold, split your nitrogen into smaller doses given through the season instead of one big dose, and on light soils add organic matter to improve how much water and nutrient the soil holds. If you have drip, fertigation, which means giving small amounts of fertiliser with the irrigation, keeps nutrients in the root zone and cuts this loss a lot. Check the soil moisture before watering so you do not over apply. For your soil and crop, ask your KVK how to split the doses and the right water amount.