When is best to apply fish blood and bone meal on my allotment?

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Is now the time to put FBBM onto the soil or should I leave it until early in the New Year?

Apply when you want plants to grow actively, so not (noramlly) during the Winter season

Generally speaking, in the Spring/Summer apply to the soil 2 weeks before sowing / planting, and hoe/rake in to "incorporate" into the soil.  Don't worry about watering in if applied before the plants/seeds are planted/sown.

For things that are already planted (e.g. overwintered) which you want to give a boost to apply in spring once the weather has warmed up and the plants start to make new growth.  Again try to incorporate into the soil, although you may need to avoid damaging plant roots, particularly for anything very shallow rooting, and keep all fertilizers away from the stem of the plant (otherwise they can "burn" the plant).  If not rain within a few days after application best to water in.