When planting seeds or transplants it is important for the backyard grower, community gardener or urban farmer to keep track of soil temperatures, as the soil is where the seed will germinate and where the roots are located. It is not uncommon to have a large difference in soil temperature in relation to air temperature, especially early in the season. A grower can get fooled into thinking the time is right for planting based on a warm sunny spring day when the soil temperatures are not actually at the point ready to plant. I see tomatoes planted each year in April or early May that have been planted too early.
When planted before the soil warms up, phosphorus is not able to be taken up by the plant, even if enough phosphorus is present in the soil. This manifests in a purple discoloration of the tomato leaves, especially the older leaves near the bottom of the plant.
The purple coloration is from anthocyanin, the same pigment that gives blueberries its color. Phosphorus is a critical macro-nutrient that assists seedling growth and vigor. These tomato plants are at risk of further disease or insect damage due to being stressed from nutrient deficiency.
To remedy this, you need to deliver phosphorus to the plant. The best way would be to use a foliar feeding of a water soluble fertilizer containing phosphorus applied to the leaves, top and bottom. This should be done early in the morning before the tomatoes stomata close. (the pores on the leaves). Once the soil warms up the tomato will start to take up phosphorus if enough nutrient is present. You may need to foliar feed until this occurs.
It is important for the backyard grower, community gardener and urban farmer to monitor soil temperatures to ensure that seeds and transplants go into the ground at the correct time to maximize production success and minimize potential problems.
Ohio State maintains a website with soil temperatures at all of the OARDC stations. Click here for the link to check soil temperature in your part of Ohio.