Yes, applied IMMEDIATELY right now. Acidic rain and snow slowly dissolve Langbeneite so it can leach down to root level. Langbenite has 10% magnesium and 20% potassium. Magnesium is what makes clay sticky when wet and rock-hard when dry. Acidic rain/snow will break down magnesium to release potassium. But it takes time.

Magnesium is hard to leach down like phosphorus. Per U of CA at Davis study, rose tissue analysis showed 3 part nitrogen (fast to move down), 2 part potassium (medium-slow), 1 part calcium (medium leaching), and only 1/10 phosphorus and 1/10 magnesium (both are very slow to break down).

This month heavily rain collapsed both my 1 foot tall brick border, and my fire-pit, the heavier the rain, the faster it breaks down anything.

Individual soils vary in mineral content - always sample your soil and have it analyzed before undertaking a regimen of pouring on soil mineral sources as a maintenance routine. Because it is possible to overdo it and poison your soil. With the aforementioned phosphorus being a leading example of a mineral that has been overapplied by consumers buying high phosphorus fertilizer products.

@strawchicago z5

Thank you for your confirmation, straw! I will do that tomorrow.

@Embothrium

Tbh, I have given up. I took several tests to the local extension last fall but they never sent the results. It was a pain to collect all the samples and to find the place...then I waited and waited, I emailed them - nothing.

So I gave up.

I am not overdoing the amendments so I hope it's OK.

Artist-FKA-Novice Zone 7B GA It's easier to see which INDIVIDUAL rose is lacking, rather than measuring (via soil test) the entire acre of soil. One's soil is NOT the same across the garden in the amount nutrients nor pH level.

Plus each plant is different from each other in requirements. Zillion petals roses need more calcium than single petal. Deep cup roses need more potassium than shallow-cup roses. Cluster blooming roses need more phosphorus (for branching) than one rose per stem.

With red cabbage juice pH testing, I can test 20+ samples of soil across my garden, and they are different in pH, depending on what topping I used, how much organic matter I dumped, and how many rocks in each soil sample. That's why I started below thread in Organic rose:

How to tell mineral or nutrient deficiency in plants and soil (houzz.com)

@strawchicago z5

Thank you, straw! I will save this link as the soil testing seems very troublesome to me. After all, how many gardeners used to have access to scientific facilities that tested their soil?