Chlorosis

No, it's not a new brand of mouthwash.

Chlorosis is the plant equivalent of a human gasping for air. It is the word used to describe the yellowing or bleaching of leaves frequently caused by insufficient sunlight. Or, it might be too much water or disease. Or it might be pest feeding. Or a nutrient imbalance.

Iron deficient plum leaves

Plants suffering from chlorosis are unable to produce chlorophyll. Since plants need chlorophyll to help them convert sunlight into energy, it’s a significant problem that warrants a closer look without delay.

If you notice chlorosis on any of your plants, consider these possible causes:

• Insufficient sunlight - Most fruits and vegetables need at least 6 to 8 hours of direct sunlight to stay healthy and productive.
• Too much water - Over-watering can make it hard for plants to breathe.
• Pest feeding - As aphids, mites, and thrips feed, they can cause leaf yellowing.
• Disease - Several diseases can cause chlorosis, including blights, mosaics, and rots.
• Soil structure - Compacted soil and soil with poor permeability can cause chlorosis.
• Soil pH - Soil that is too acidic or alkaline can cause yellowing.
• Nutrient levels - Too little iron, magnesium, or zinc, and excessive levels of other nutrients can cause chlorosis [See case in point below.]

​When I first bought my home, I sent soil samples to the University of Massachusetts Extension lab. [I think over-the-counter soil tests are a waste of money.] For the price of a bag of fertilizer, I learned exactly what was (and wasn't) in my soil. 

My soil test results told me that my soil had a superabundance of every nutrient known to humanity except iron. Since plants use iron to process nearly every other nutrient, the previous owner kept adding more fertilizer whenever her plants started yellowing. As a result, the soil had too many nutrients, interfering with the delicate chemical dance between microorganisms, plant roots, water, and nutrients.
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I sprayed my plants with liquid iron and applied ammonium sulfate. Over a few years, I was able to bring the excessive nutrient levels down a bit (which is harder than you might expect) and iron levels up. As a result, chlorosis was less common, my plants were healthier, and I saved money on fertilizer that I didn't need.

As soon as you correct the problem causing chlorosis, your plant's little energy factory will kick right in. Everything should start greening up pretty quickly.