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Rainfall also affects soil pH. Water passing through the soil leaches basic nutrients such as calcium and magnesium from the soil. They are replaced by acidic elements such as aluminum and iron. For this reason, soils formed under high rainfall conditions are more acidic than those formed under arid (dry) conditions.
If soil isn't in the proper pH range for growing onions, it can be adjusted. Amending soil with organic substances, like old leaves, will lower its pH and increase its acidity. Some fertilizers, like ammonium sulfate, will also boost acidity.