Often, just raising or lowering the pH to the ideal 6.5 will free up phosphorus for plant use. It isn't used up quickly, and adding too much will build excessive levels that run off, causing pollution. Deficiencies are indicated by purple leaves, brittle roots, skinny stems, and late fruit set and maturity. In early spring, deficiency symptoms on seedlings may disappear with warmer temperatures and increased microbial activity. Superphosphate, rock phosphate, and bonemeal are good sources of phosphorus.
Potassium is vital for stem strength, root growth, and disease resistance.
Many soils are naturally high in potassium, and it is readily available to plants. However, sandy and highly weathered soils can be deficient. Signs are irregular yellowing of lower leaves and poor root growth. Muriate of potash, greensand, and wood ash are good sources. Calcium is important for cell-wall integrity and root and leaf growth.