Q. I have 4 Hawaiian tomato plants in containers, all producing tomatoes. On one plant the leaves on lower branches are turning yellow. The plant appears to be healthy having several flowers and small fruit forming. I fertilized all 4 plants 2 weeks ago with Miracle Gro and was planning to fertilize in 2 week intervals as suggested on product container. What can be done to prevent the yellowing of leaves on the other 3 plants and resolve the present problem with the one plant in question?
A. There could be several things going on. My first reaction was a lack of nitrogen fertilizer until you told me you were using a fertilizer on a regular basis. When plants don’t get enough nitrogen fertilizer the older leaves can turn yellow and die.
You can also look at how much new growth there is since nitrogen is also responsible for stem and leaf growth. If it is not growing and putting on new growth then this can also be an indicator of a lack of nitrogen.
Watering too often and keeping the soil too moist can also cause yellowing of foliage. So if you have mulched the soil and watering frequently then this might also be a potential problem.
Some soil amendments if they have not broken down completely can cause yellowing but this is usually compensated by using fertilizer high in nitrogen. Also poor grade composts can cause yellowing so try to avoid the very inexpensive composts and soil amendments. Good soil amendments are expensive unless you make your own.
If the soil is native desert soil that has been amended with compost or soil amendments, salts can cause yellowing of leaves. There are lots of natural salts in desert soils, frequently at levels too high for most of our landscape and vegetable plants. If your soil is fairly new, it make take a couple of years of compost and growing to get it into good condition.
Salts are removed by leaching or watering the soil with lots of water in and letting it drain over and over in repeating cycles. Salts dissolve easily in fresh water and the draining water carries dissolved salts from the soil to a depth below the roots of the plants. If you use composted native desert soils you should always leach the soil prior to planting.
So look at your soil modifications, how you are watering and whether you have leached the soil or not. If after this you think there is still a problem you might consider replacing the soil.