Plants require a number of mineral nutrients in order to survive. These nutrients govern everything from growth to flower and fruit formation to metabolic regulation. Deficiencies in even a single nutrient affect plant function and create symptoms of disease. Several well-known deficiencies produce problems in tomatoes.
Calcium deficiency causes blossom-end rot in tomatoes, a disease where the bottom of the fruit darkens and becomes leathery in texture. Secondary infections may cause actual rotting of the entire fruit. Blotchy ripening occurs when plants receive inadequate potassium. Fruits have yellow or green patches and may have brown discolorations inside. Phosphorus deficiency causes purple-tinted leaves, with discoloration especially noticeable around the leaf veins.
In many cases, tomato nutritional deficiencies create unsightly but edible fruit. For example, unless afflicted with a secondary infection, the "rotted" end of blossom-end rot is not actually rotten. It can be cut off and the rest of the fruit eaten. Blotchy ripening causes unusual coloration but does not render the fruit inedible. According to the Texas A&M University AgriLIFE Extension, phosphorus deficiency may diminish yields but does not damage the fruits themselves.
Nutrient deficiencies in tomatoes often occur in conjunction with other cultural problems. For example, the calcium deficiency in blossom-end rot often strikes during dry periods, when calcium that moves with water is not being absorbed by the plant. Phosphorus deficiency occurs when plant roots are too cold. The Iowa State Extension notes several factors that worsen blotchy ripening, including inadequate light, wet soil, compacted soil, low temperature, and the overuse of fertilizer.
Maintaining optimal growing conditions for tomatoes eliminates most nutritional problems. To start, ensure that your soil has adequate nutrients by having a soil test done and correcting any deficiencies as part of your fertilization program. Follow best practices for growing tomatoes, such as planting at the correct time, mulching around plants, and providing plants with adequate light and water.
If your tomatoes are showing signs of nutritional deficiencies, first correct any cultural problems that may be contributing. For example, mulching around plants helps to preserve water and keep soil temperatures even; the Iowa State University Extension recommends mulching to solve blossom-end rot and phosphorus deficiency. If soil tests reveal deficiencies in your soil, your extension office should be able to recommend treatments that will restore soil fertility for future crops.