As Spring is here and those who like to grow their own are preparing to do so, I had a question from a client about planting vegetables in raised beds. It appears she, like a good Girl Scout, did everything by the book: organic potting soil and fertilizer and a drip system for watering. Yet her veggies grew with yellow leaves.
While it’s not as bad as having slugs in your garden it is annoying. So what’s a gardener to do?
Yellow leaves on vegetables usually mean they’re not getting enough nitrogen. The problem may well be that you filled your raised beds with “organic” potting soil and not a good quality amended top soil.
It’s come to my attention that some manufacturers of bagged organic soil are using wood products that have not had adequate nitrogen added to them in order to make them rot. Since they’re “organic”, they can’t add sulfate of ammonia, a cheap source of nitrogen, to the shavings because sulfate of ammonia is not a natural source of nitrogen.
Raw, un-rotted, un-nitrolized wood shavings will rob nitrogen from the soil in order to rot and this action turns veggies yellow from lack of nitrogen. To correct the problem you will need to add more nitrogen.
Blood meal is a strong organic source of nitrogen, but many gardeners don’t want to use it because it comes from feed lots. Alfalfa meal is also good but alfalfa tea gives quicker results. (There is a recipe for alfalfa tea on page 119 of “Pat Welsh’s Southern California Organic Gardening, Month by Month.”) The good news is that your problem will eventually clear up. Once the raw shavings in the soil have fully rotted, they will release the previously stolen nitrogen into the soil and plants will get a huge boost.
If you filled your raised bed with potting soil instead of topsoil you might consider adding top soil the next time the season changes and you plant new crops. Because, as the wood products in the potting soil rot, they decrease the volume of soil in your raised bed.