Leaching is the downward movement of a dissolved substance through the soil. Residual fertilizers in the soil that plants cannot use, leach through the soil along with rain or irrigation water. Pesticides too, can leach into groundwater, or they may vaporize into the atmosphere, or be broken down by microbial and chemical means into harmless substances. The quantity of an applied chemical that reaches groundwater depends on two factors, persistence and solubility. Chemicals with a long half-life remain active in the soil for a long time.
Solubility refers to how easily chemicals dissolve and mix with soil water.
Chemicals entering soil will adhere to soil particles or dissolve into the soil water. The combination of persistence and solubility measures a chemical's potential to contaminate groundwater. A very soluble chemical with a long half-life would have the greatest potential for contamination. On the other hand, a chemical with a short half-life and low solubility would not remain active long, and would be less likely to leach into groundwater. Slow release fertilizers remain in the soil longer for plant use than very soluble fertilizers.