My Tomato's Leaves Are Yellow With Purple Underneath

Phosphorus

• Along with nitrogen and potassium, phosphorus plays an important factor in the health of your tomato plants. Perform a soil test prior to planting a garden to determine and correct any nutritional deficiencies. Tomatoes require deep loamy soils with pH levels between 6.2 and 6.8

Damage

• A phosphorus deficiency in the soil tends to slow the growth of tomato plants while they are still young. The lack of this nutrient causes the leaves to develop purple tones on the undersides and turn a pale shade of green or yellow the upper surface. The veins on the upper surface may also appear purple against the off-colored surfaces.

Prevention

• The best way to promote healthy, tomato growth is to correct nutrient deficiencies prior to planting. Amending the topsoil with about 3 lbs. of fertilizer per 100 square feet of garden soil will help prevent leaf discoloration and poor growth due to phosphorus insufficiency. Choose a complete fertilizer with a nutrient ratio of 5-10-10 or 5-20-20, incorporating it into the garden soil about two weeks before you plant the tomatoes. Applying a nitrogen fertilizer after the fruit begins to appear also serves to promote healthy growth in these plants.

Considerations

• As a warm-season plant, tomatoes grow best in climates with long summers. Starting them indoors in cool climates ensures adequate time for ripening and harvesting. However, setting the small seedlings outdoors too early can result in yellowed leaves with purple undersides, regardless of the amount of phosphorus in the soil. This condition occurs when low temperatures interfere with phosphorus uptake in the young plants. Avoid this by waiting a couple of weeks after the final frost to transplant tomatoes outdoors. Excess aluminum in the soil can also inhibit the assimilation of phosphorus.