Dearest Aziz, this have something to do with photoperiodism. Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals. Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of light and dark periods.the nutrient accumulation which was high in early sowing date may be due to the length of day period of photosynthesis compared to late date which may have long nights and short days.

Nutrient uptake depends very much on rooting pattern, root depth and available soil water.   In dryland sown crops, there is usually more soil available water early in the season thus allowing greater exploration of the soil for nutrient uptake.     Other factors to consider are soil temperature and vapour pressure deficit both of which have considerable influence on nutrient uptake.      The time to first square is likely to be quite different between early and late sown crops; our experience is that nutrient uptake declines after squaring due to remobilisiing of nutrients within the plant.

I agree with the explanation of  Dr Campbell. It may be related with water availability.

Length of the cropping season has much to do with the nutrient uptake apart from the other factor described in earlier answers. A prolonged crop stand in the filed would require more nutrients as long as the vegetative and reproductive development of the plant goes on. Since cotton plant is perennial , it can grow as long as you irrigate and apply nutrients. Moreover, the fruit bearing capacity and fruit load is more in the early sown cotton crop, therefore nutrient uptake is more.

thanks dr. syb