the fertilizer you apply this spring could affect groundwater quality 30 years from now. According to a USDA experiment completed in 1974 — and unearthed recently during a new study — fertilizer containing nitrate took 30 years to reach a 70-foot water table.

Soil scientists at the USDA’s National Soil Tilth Laboratory in Ames, Iowa, confirmed the nitrate originated from the old experiment by examining groundwater flow rates and ages, and by comparing the concentration’s depth with stream flow records.

The rate at which nitrate moves into groundwater depends on soil type, water table depth and climate, says Mark Tomer, one of the study’s authors. However, the study’s take-home message is clear: Be patient. Conservation practices that reduce soil nitrate levels may take several decades to greatly improve groundwater quality.