The current thinking is that sand beds will need more P than peat beds. The amount and availability of iron, aluminum and manganese are more important in my opinion. Phosphorus does not leach and would not be any more likely to go through a sandy than a mineral or organic soil. Runoff is a concern if a significant rain event quickly followed a fertilizer application. Yes, that is the right timing, but irrigation for frost protection is only peripherally involved.
P is released as soils begin to dry out but are still cool after the winter flood melts.
Once the soils warm P is not released as quickly. It is coincidental that frost protection ending and soils warming occur at the same time. See the table below. Phosphoric acid can be used as a foliar P source, but should not be applied during flowering or on fresh fruit plantings. Rock phosphate is almost insoluble and, in my opinion, is not a good P source.