Gypsum, dolomitic, lime, sulfates vs. buffers and anti-fungal agents
Our soil in Chicagoland is dolomitic heavy clay, high in magnesium, pH above 8 in hard & sticky clay. While walking .. saw the neighbors' hybrid-tea roses are clean, despite this record-wet June. Theirs are grafted on Dr. Huey .... that root-stock likes alkaline clay.
My 40+ own-root roses are different: some behave like Dr. Huey, esp. French Romantics, which stay clean if I give dolomitic lime (has both calcium and magnesium). Kordes rose like Deep Purple black spotted in my heavy clay, but was 100% clean in fluffy & slightly acidic potting soil. Austin roses differ greatly: Jude the Obscure hated it when I gave it gritty lime. But Evelyn did well with dolomitic lime (pH 9).
I bought Pink Peace, grafted on Dr. Huey, just to do experiments. There's a hole previously occupied by a HUGE woody bush (over 10 feet tall). The soil there was fluffy, so I knew it was neutral pH. I didn't want to put cracked corn at pH 3.5, so I put alfalfa pellets (pH 5.8). What I didn't realize was alfalfa fermented or became "sour" when buried, and that brought the pH below neutral.
I tested the pH there: one Tablespoon at surface level, where I topped with pH 8 clay, plus mulched with cocoa mulch (pH 5.4), plus gypsum (pH 6.8). That came out neutral pH (clear in red-cabbage boiled with rain water). Few inches deeper, it became more pinkish, and at below four inches level, it's around pH 5, NOT good for Dr. Huey-root-stock.
I tested Carding Mill, own-root with black spots. That's was previously occupied by Firefighter (a heavy bloomer) .. so I know potassium is depleted. I topped that with crushed red-lava rock (pH 8.2), plus cocoa mulch (pH 5.4). I also put tons of alfalfa pellets, both in the planting hole, and on the surface. I got 1 tablespoon of soil at surface level, it came out acidic, light pink, despite a chunk of red-lava-rock soaked overnight in red-cabbage juice. Red-lava-rock DID NOT buffer, or neutralize the acidity of decomposing organics.
I test many soil samples. Found the hole fixed with pine bark (pH 4.5) became clear (neutral) in red cabbage juice. I found that decayed grass is slightly alkaline, versus VERY alkaline rock-hard clay (pH above 8).
Below is color-changes in red cabbage juice. Previously I used distilled water (pH 6.8) to boil red-cabbage, but my recent rest using rain-water (pH 5 to 6) corresponds best to the color- chart, posted in Cheapest way to test soil pH:
I also tested Schultz potting soil with red-cabbage boiled in rain-water. It was clearly acidic, with a light pink-tinge, but not as deep as the acidic groups: cracked corn (pH 3.5), birch bark (left of cracked corn), pine bark (pH 4.5), and brewer's yeast (below cracked corn):