Phosphorus

Phosphorus, this Nutrient gives strength to plants, making them robust and good producers, it is the key nutrient to encourage a good root system, hence all root crops prosper when copious quantities are applied.

Best applied early on in the growth cycle as it traditionally a slow release nutrient, when planting your potatoes it is a good idea to lay some high Phosphorus fertiliser in the bottom of the trench, cover with a bit of  compost and then lay your seed potatoes on that.

In conventional farming Super Phosphate is used, which is ground Phosphate Rock with Sulphuric Acid added to make the P more available.

 Rock Phosphate on its own is very slow to break down and get to a plant available form. However Super Phosphate is very acidic and harsh on soil microbe's, so what to do?

Which ever you use mix both with copious amounts of Compost, it will soften the Super Phosphate harshness or help the Rock Phosphate become plant available quicker particularly if you mix the two together, wet and leave for a month or so.

Other Organic alternatives are Bone Meal and Wood Ash (which has a fair bit of Potash as well)

Hope this helps.

Be Happy

Barry