I have noticed over the last few years a “yellowing” of the leaves on the raspberries and on the affected plants a general unhealthy look.
This normally occurs around early summer. My first reaction was that this was some form of lime-induced chlorosis but as Rosemoor soils are generally acidic it presented a puzzle. This led me to investigate further.
Lime-induced chlorosis shows as a yellowing of the leaves usually the younger leaves are affected first but not affecting the veins, these stand out as dark green.
When the RHS first took over Rosemoor in the late 1980’s it had a soil survey done for the entire site. The part of the site that later developed in 1995 into the Fruit and Veg Garden had a pH of 5.5 . This is low – the ideal for a veg garden would be around pH 6.5 – 6.7.
Since I have worked in the F&V garden (2001) we have made no regular liming of the soil except occasionally to add lime before a Brassica crop but in 2007 we had various parts of the F&V garden tested and the PH varied between 7 and 7.5. So in the short period of 12 years the pH had risen from 5.5 to 7.5 with no significant addition of lime!
What has been added in large quantities is organic matter in the form of FYM and or our own garden compost made on the site. It is known that some manures and garden compost can have a high pH. In 2005 we had our garden compost tested and the pH was pH 8.05 - an incredibly high figure. But I understand this is not unusual for garden compost.
The problem of the chlorosis on the raspberries has been getting worse and we have had to change from using bought in decorative mulch with a pH of 4.5 to mulching with our own compost. More research is needed but it seems very likely that our garden compost is the main cause of this problem.
Interestingly the soil scientist at Wisley is currently carrying out trials on the long term effect of adding different organic mulches to the soil as they have had problems of high pH with organic mulches.
Garry Preston, Fruit and Veg Gardener