Phosphate availability within the soil is affected by the pH, soil temperature, aeration and waterlogging. Typically in spring, we have cold, wet soils meaning that P-availability is low. Crops will start to grow at 4-5°C whereas phosphate isn’t available from the soil in any kind of useful quantities until it reaches 8°C and above. Consequently, there is a potential period when crops start to grow but aren’t able to access this key nutrient.
The last thing we want to do is stop the momentum at this key growth stage, just as the crop begins to grow, so by adding some fresh available P to the soil it will bridge the gap between the kick-off of growth and soil-P becoming available.
The phosphate source is particularly important and needs to be one that is NOT immediately ‘locked up’ (TSP, MAP, DAP based fertilisers), and gives season long, continuous supply.
Another reason for spring P is that that is when the crop demand is highest! 70% of P is taken up in a 4-8 week period March to May. P is liable to be precipitated out or adsorbed to other ions within the soil so applications carried out in the autumn won’t be available to the crop when the peak demand is there.