A quarterly application of sulphate of potash to rose bushes is very easy and beneficial. It adds potassium to the soil, which may be lacking naturally and further diluted by animal manures.
Low potassium renders plants susceptible to disease, and its addition can help minimise black spot, rust and powdery mildew – all of which are much simpler to prevent than to cure. One of the most important of potassium’s many functions in plants is in the production of cellulose. This strengthens cell walls so that they are better able to resist fungal intrusion.
I buy sulphate of potash in a 20 kilogram sack from an agricultural supplier. It comes in the form of white granules the size of small pebbles, and should be kept dry and out of direct sunlight. It is a clean and safe product; however, eye and skin contact and dust inhalation should be avoided.
I apply it at the rate of 125 grams (4.4 ounces) scattered around the root zone four times per year. Here in Australia I do this in February, May, August and November, and I imagine this schedule would suit all rose growing climates in northern and southern hemispheres. A calendar reminder keeps the program on track. The total quantity needed can be calculated by allowing 500 grams per plant per year.
The task can be accomplished efficiently by preparing a dedicated jar. Place it on scales, “zero” the gauge, and then fill with 125 grams of sulphate of potash. Draw a line on the jar to serve as a measure as you scoop out the chemical for each application. I can do dozens of rose bushes in 15 minutes with this method. A quick job with great benefits.