No, I’m not talking about Medieval medical practices!
Leaching describes how water-soluble plant nutrients are washed away with rain and irrigation.
While your plants certainly need water, too much of a good thing can be a bad thing. As nutrients are washed down from the topsoil and into the groundwater, they can cause contamination and algae blooms.
Our Bay Area clay soil holds far more water than sandy soils, but every soil has a carrying capacity. Once that limit is reached, gravity will pull the water downward into underground waterways where it will ultimately flow to lakes and oceans. As it flows away, the water carries nitrogen, salts, fertilizer, pesticides, fungicides and whatever else was in your soil with it - leading to a potentially dangerous chemical soup that can wreck environmental havoc. Leaching also moves valuable nutrients out of reach from your plants roots.
To avoid leaching, it is always a good idea to water only as much as is needed. You can see for yourself where your irrigation water is going simply by digging a hole next to the plants you intended to water (but not too close). Many people are surprised to discover that the water intended for their tomato plants actually went in another direction due to hardpan, sandy pockets, or poor soil structure. Improving soil structure with compost can improve drainage and help prevent leaching.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.