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Potatoes require large amounts of potassium to get a high yield . fertilisers such as growmore and fish blood and bone have a relatively low nutrient content and would need to be applied in large quantities(8 oz Yd2)to apply sufficient nutrients for a good crop!.This would need to be suplimented with around 1oz Yd2 of potassium sulphate/Nitrate. Potatoes also have a relatively large demand for calcium which is best applied as Calcium Nitrate during bulking at around 1oz Y2.
Do a scientific literature search regarding potato fertiliser requirements and you will see that potassium is required in large quantities, more than any other nutrient. I followed the professional fertiliser recommendations this year and got double the yield of the standard 4oz yd2 growmore.The fertiliser I used was 12-11-18 +20%SO3+2.5%Mg+B+Fe+Zn+Mn, a professional fertiliser at£17/25Kg much cheaper per unit than Growmore/FBB.This was supplemented with 1ozyd2 of Calcium Nitrate and Potassium Nitrate at tuber bulking.The results speak for themselves, do some research and you may be surprised!.