Answer: pH is the measure of alkalinity or acidity of the soil. Neutral soil pH is about 7, anything below that is considered acid, anything above that is considered alkaline. Most garden plants will adapt to neutral or slightly acidic or alkaline soils. but some plants have a specific requirement for very acidic or very alkaline soil and when the soil pH is too "acid" (low pH) or "alkaline" (high pH), nutrients present in the soil become locked-up or unavailable. Correcting the pH has the same effect as applying fertilizer since it "unlocks" plant nutrients already present.
Plants with a requirement for specific soil pH are usually indentified as such - blueberries, rhodies and azaleas, for instance, require acidic soils; lilacs prefer alkaline soils for best performance. If your soil's pH is different than the plant requires, it will fail to thrive, regardless of the amount of fertilizer you apply. That will be your first indication that you need to check your soil's pH.
Since you're only treating the rootzones of individual plants, you shouldn't worry about the lime or the coffee grounds leaching too far away from where you apply them. Give each plant enough room to grow comfortably without crowding and neither should be affected by soil treatments that you've applied to the other.
Hope this clarifies things for you!
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