All plants need nitrogen, potassium and phosphorous to thrive, plus various trace elements. Without nitrogen the leaves turn yellow and growth is spindly. Potassium generates good flowering and fruiting and helps ripen wood. Plants lacking potassium tend to have blue-, yellow- or purple-tinted foliage and a poor crop. Without phosphorous, which aids strong roots and shoots, leaves tend to be small and discoloured. The most common trace element deficiencies are manganese and iron, which cause yellowing leaves with brown edges.
In summer, raspberries fail to thrive, the crop is poor and the leaves have a general unhealthy look.
Pelleted poultry manure gives a quick boost of nitrogen, while organic compost gives a longer-term solution. Add bonemeal if your raspberries are short of phosphorous. In general, if you've added plenty of well-rotted compost to the soil to improve moisture retention, drainage and soil structure, you're unlikely to have too many problems.
Add sulphate of ammonia for nitrogen deficiency. Sulphate of potash cures potassium deficiency. For low phosphorous levels, apply super-phosphates. Top up manganese levels with Epsom salts.