Vegetables in Raised Beds: Problems with Yellow Leaves

Question:

I am growing vegetables in raised beds filled with a very good potting soil. I have fertilized regularly with organic fertilizer according to package directions and I use a drip system for watering. What causes the yellow leaves and how can I correct the problem? The drainage is excellent. If anything, it’s too good! Also the plants seem to need more water than the drip system provides. I have to add extra water from the hose or there are dry spots. The beds are in full sun.

Answer:

The problems you are having come from using potting soil in your raised beds instead of filling them with good quality amended top soil. Raised beds are not like pots. They are more like a terrace, large enough that they can be treated just as if you were planting in the ground. Yellow leaves on vegetables usually mean they do not have enough nitrogen.  One current problem I have recently discovered is that some manufacturers of bagged organic soil amendment are using wood products that have not had adequate nitrogen added to them in order to make them rot. Since the word “organic” is printed on the bag, they can’t add sulfate of ammonia, a cheap source of nitrogen, to the shavings. Adding sulfate of ammonia would have prevented the problem, but sulfate of ammonia is not a natural source of nitrogen. It is a synthetic or chemical source of nitrogen, so you can’t call it “organic” as that word applies to farming or gardening.  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 another good organic source of nitrogen. You can also make alfalfa tea, which 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.

Since you filled your beds with potting soil instead of topsoil you might also want to purchase some top soil and the next time you change seasons and plant new crops, add some topsoil into the bed to raise the soil level,  since as the wood products in the potting soil rots it will decrease in volume. The topsoil also will aid in water retention and your drip system will work better.