Question: I have a problem with yellow leaves on my grapes and raspberries. Do you think dry spruce and pine needles as a mulch would help?
Answer: Yellow leaves is a common problem in eastern Idaho, not only on raspberries and grapes, but on many other plants including lawns. It is caused by our alkaline soil and water. The alkaline condition ties up iron, manganese, zinc and other nutrients so that plants are unable to get enough of them from the soil.
Shortage of iron is the primary cause of yellow leaves in our area. The typical symptom is light green to yellow leaves with dark green veins on the youngest leaves.
However, if the older leaves are yellow without green veins, the cause is most likely a shortage of nitrogen. This condition is much easier to cure by simply applying any fertilizer which contains nitrogen.
The best long term solution is to apply sulfur or supplements which contain sulfur and iron around the roots and cultivate them into the soil. This temporarily reduces the alkalinity so plant roots can obtain iron. However, irrigation with our alkaline water counteracts the change, so sulfur should be applied every year to sensitive plants. I also apply sulfur at least once every three years to almost everything. Whenever I plant something new, I mix sulfur into the soil. It requires at least one pound of sulfur per 100 square feet to make a difference. Three pounds can be used when mixing it with the top 6 to 8 inches of soil.
Another solution which gives more immediate results is to spray leaves and soil with ferrous iron sulfate or chelated compounds containing iron and other nutrients. The iron is absorbed through leaf pores before it can be made insoluble by the alkaline soil. Frequent reapplication is necessary—as often as once a week in extreme cases. Iron sulfate and iron chelate are usually only available from full service nurseries and garden stores.
Ammonium sulfate and most mixed fertilizers sold in our area contain sulfur. They will also have a limited impact on soil alkalinity. However, applying too much can have a detrimental effect on plants. One or two applications per season is sufficient.
Pine and spruce needles are also natural acidifying (alkaline reducing) agents. They can be incorporated into the soil when planting and used as mulch on top of the soil around plants. A thick mulch will also reduce weed growth.
Allen Wilson can be contacted at email@example.com.