Fall/Winter Rain and Raised Bed Gardens
Will anticipated heavy fall/winter rains deplete my raised bed garden soil of its essential nutrients? If so, should I cover the garden box with plastic? Perhaps I should spread a 2" layer of compost over the bed followed by a layer of worm castings and then cover the entire box with plastic or tarp? Thank you so much in advance. It is a wonderful service that you provide. (-:
Placer County California
Thank you for your question about helping your soil over the winter! Whether winter rains deplete your soil of nutrients depends on the type of soil you have, and if you added topsoil and compost to your raised bed structures, there will be less 'leaching,' as this is called, than with native soil composed primarily of sand. That's because the compost added to the topsoil or native soil decomposes slowly, primarily through the work of microbes (bacteria, fungi, etc.). Adding plastic harms the microbes in two ways: it reduces the amount of moisture that reaches them (and they all need moist soil) and it depletes the oxygen getting to them, which they all need. Adding a layer of compost, as you suggested, will provide them additional 'food' over the winter. Worm castings--unless they have come from your worms--are rather expensive, and I'd recommend adding them in the spring when you are planting seeds and/or transplants. Work them into the top few inches of your soil, near the roots. Adding them now won't serve any particular purpose. This UC article has lots of information about soils and especially organic supplements.
Some people do cover their garden beds with either cardboard or coffee bean sacks, which are made of burlap and allow both water and air to penetrate them. But, if you want to do something that will both protect your soil from getting compressed by the rain, as well as add nutrients to the soil: a cover crop. Here's a link to a great UC Davis Extension article on the topic, and your timing for using this information is perfect!
Hope these are helpful for you. Good luck!