It's getting too much light. Harsh sunlight can cause chlorophyll to break down in leaves, especially if your plant prefers shadier conditions.
In cases of heat stress, leaves will yellow and burn in spots.
Plants exhibit several vital signs when they're getting too much light. The most obvious indicator is leaf burning. This typically causes yellow leaves at the top of the plant directly under the grow lights. While the inside veins stay green, the leaves take on a yellow, burnt look.
In absence of sunlight the green chlorophyll pigment gradually undergoes degradation and no new pigment molecule is formed, giving the leaves of the plant a pale yellow color.
Yellow Leaves + Fading to Green + or Bright Yellow =
These symptoms together mean that your plant is overwatered. Usually lower leaves drop first, although the whole plant may be affected. The solution = repot (to remove soaked soil) and water less, or let soil dry out and water less.
When you see an entirely yellow leaf, you should remove it from your plant using a sterile cutting tool. Removing the leaf will let the plant focus its nutrients on healthy leaves. A yellow leaf has lost its chlorophyll (pigment) and it can't turn green again even after you correct the problem.
Overwatering
Watering issues are generally the most common cause of yellowing leaves. When your plants are overwatered, the performance and vigor decrease. Oxygen is being pushed out of the soil, and the roots are simply “under aired” and suffocating. With little air, the roots will begin to drown and rot.
Poor drainage or improper watering
Water issues — either too much or too little — are the leading reason behind yellow leaves. In overly wet soil, roots can't breathe. They suffocate, shut down and stop delivering the water and nutrients plants need. Underwatering, or drought, has a similar effect.
To correct a deficiency, spread organic mulch beneath plants and apply potassium fertilizer, preferably slow-release forms such as potassium silicate or sulfur- or polymer-coated potassium products. Potassium sulfate may be used, and potassium will be held by organic matter and clay particles.
This isn't anything to worry about – in fact, it's part of your plant's life cycle. “As most plants age, older leaves will die off, turning yellow and eventually dropping,” Cheshire explains. “This is so the plant can put its energy into new growth.”
Iron is involved when a plant produces chlorophyll, which gives the plant oxygen as well as its healthy green color. This is why plants with an iron deficiency, or chlorosis, show a sickly yellow color to their leaves.
Telltale Signs of Too Much Light
The most apparent sign is leaf burning. This typically causes the yellowing of leaves at the top of the plant but the veins stay green, and the leaves take on a yellow or brown, burnt look.
Slow growth and uniform yellowing of older leaves are usually the first symptoms of nitrogen (N) deficiency. Nitrogen-deficient plants produce smaller than normal fruit, leaves, and shoots and these can develop later than normal. Broadleaf foliage in fall may be more reddish than normal and drop prematurely.
Generally, heat stress of a plant will show itself by wilting, which is a sure sign that water loss has taken place. If this is ignored, the condition will worsen, as the plants will eventually dry up, turning a crunchy brown before dying. In some cases, yellowing of the leaves may occur.
The fastest way to add nitrogen to soil is by applying a nitrogen-rich fertilizer. This includes certain all-purpose plant foods with a high portion of nitrogen, as well as fertilizers formulated for green plants (especially lawn fertilizers).
The richest organic sources of nitrogen are manures, ground-up animal parts (blood meal, feather dust, leather dust) and seed meals (soybean meal, cottonseed meal).
Nitrogen is fixed, or combined, in nature as nitric oxide by lightning and ultraviolet rays, but more significant amounts of nitrogen are fixed as ammonia, nitrites, and nitrates by soil microorganisms.
Iron chlorosis is a yellowing of plant leaves often caused by iron deficiency that affects many desirable landscape plants in North America. The primary symptom of iron deficiency is interveinal chlorosis, the development of a yellow leaf with a network of dark green veins.
Nitrogen deficiencies cause lawns and the leaves of plants to turn a yellowish or bluish-green color if the soil nitrogen is not consistently replaced through the application of fertilizer.
Leaf tips turn brown when that lost water can't be replaced for some reason. Ideally, water flows from plant roots through stems and waterways until it finally reaches leaf tips last. But when water's limited, other plant parts get served first; tip cells lose out and die from a kind of drought.
Plants do best when watered about three times a week, factoring in the rain. If the plants are seedlings, water them twice a day until established. But don't just water without thinking. Feel your soil!
Yellow is the color of light, hope, and encouragement. You see, leaves have long symbolised keeping things hidden – for the longest time, they symbolised the hidden dangers of the untamed wilds.
Compost made primarily from food byproducts is an excellent source of potassium. In particular, banana peels are very high in potassium. Wood ash can also be used, but make sure that you apply wood ash only lightly, as too much can burn your plants.