Problem with Purple Leaves Caused by Phosphorus Deficiency

I had a very strange problem develop in my garden this spring. Around the edges of my vegetable garden, I planted marigolds and globe amaranth. Everything looked good, the veggies were growing, the seeds were sprouting, and the sun was shining! But when I went out to check on the garden one morning, the leaves of my marigolds and globe amaranth had turned a dreadful shade of dark purple!

What Causes Purple Leaves?

Purpling of leaves can have a variety of causes, but the most common one is a deficiency in phosphorus. Phosphorus is a nutrient essential for plant growth that is particularly important for rooting, flowering, and fruiting.

If phosphorus is deficient – or if conditions make it difficult for plants to absorb the phosphorus that’s there – plants will stop, or severely restrict, growth. The older leaves start turning a dull, dark green, which then turns to purple. The “purpling” begins at the tips or undersides of the leaves, and moves its way across the entire leaf.

In most cases, phosphorus deficiency isn’t actually caused by a lack of phosphorus in the soil, but by some other problem that keeps the plants from absorbing nutrients properly. Adding phosphorus won’t help, and high phosphorus chemical fertilizers are very polluting.

So before adding phosphorus supplements to your soil:

• Do a Soil Test: A soil test will tell you the usable phosphorus levels, as well as other conditions that can affect phosphorus absorption. If your soil test does indicate low phosphorus levels, you can gently raise them using rock phosphate, bone meal, superphosphate, manure, or compost.
• Consider the Weather: Cold, wet soil can keep plant roots from absorbing phosphorus properly. Signs of phosphorus deficiency are frequently seen in early spring, if plants are set out a little too early. If cold is the culprit, the plants may recover when things warm up.
• Check Soil pH: Highly acidic soil can also affect phosphorus uptake.
• Look at Iron Levels: Soils high in iron can bind up phosphorus in a way that’s difficult for plants to absorb.

As for my plants, I decided that a late spring cold snap was the likely culprit that caused the problem with phosphorus absorption. The flowers were the only plants affected. The nearby vegetables were growing like weeds, and the surrounding grasses were healthy and strong, which makes me think the problem wasn’t in the soil. As further evidence, I had a few extra plants still in their store pots (not in my soil), and they turned purple, too.

Rather than rush out for tests and supplements, I decided to adopt a wait-and-see approach, keeping the flowers watered until the weather turned warmer, then continuing with my original feeding plan with a mild, balanced organic fertilizer.

If the problem becomes worse or starts to spread to my veggies, I’ll conduct a soil test and proceed from there.

Further Information

• Fertilizer 101 (article)
• Recognizing Nutrient Deficiency Symptoms (Cornell University)
• Phosphorus Fertilizer in Your Lawn or Garden (article)
• The Debate over Organic vs. Chemical Fertilizers (article)