The new leaf on your tomato plant have gone all washed out and pale yellow. All the veins are still green, so there’s a dramatic map of yellowing running up the leaf blade.
The first thing you’re likely to do: reach for bottle of general fertilizer and give the thing a shot of nitrogen. “It needs a kickstart,” right? Wrong (well, nearly always). No, this isn’t a request for nourishment; the plant is yelling that it can’t get at what’s actualy available in your soil.
How to Read Plant Leaves
This is the essence of the gardener’s frustration: they are feeding the soil but starving the plants because the soil chemistry is off. Where does the symptom occur on the plant? That’s a big clue to what’s missing, and it’s the best diagnostic clue you have.
How do you know if a mineral is mobile or immobile? A mobile nutrient, such as magnesium, can travels within the plant. Magnesium deficiency cause yellowing of lower leaves, because the plant extracts it from them as it needs more for young growth. An immobile nutrient (such as calcium or iron) cannot move inside the plant. What the plant can get at the moment of uptake is what it gets. If the plant cannot get any more, then the youngest leaf tips will turns yellow or die off first. The simple rule is that mobility are in old leaves and availability is in new ones.
According to the infographic, if you have high-pH soil, iron is often the most limiting factor. When the pH exceeds 7, the availability of iron plummets like a rock. This is called lockout: though there may be tons of iron present in your soil test, it isn’t available to your plants because the iron has changed chemically into a form that roots cannot see. Your soil test says you have lots of iron, but oh, my, do those plants looks like they’re drowning in a nutrient shortage.
Adding additional iron typically won’t help you. You must lower the pH to unlock what’s there. And that’s where folks screw up. Rather than change environment, they just treat the symptom with supplements.
With calcium, it’s different. Because it doesn’t move around; when there isn’t enough, it shows up at the growing tips. That may be as blossom end rot on the fruit, or as tip burn on the leaves. This is often confused with a lack of calcium in the soil; but it is usually a watering problem. Remember: Calcium moves with water. If the soil dries between waterings the calcium won’t move into the fruit. The plant can’t move it from somewhere else.
Sometimes just keeping it consistently moist are the answer, not adding calcium (usually in the form of gypsum). Magnesium is another common trouble spot in sandy soils because it washes right out. If you have some older leaves that is showing interveinal yellowing, it’s probably magnesium. Adding epsom salts can sometimes helps, but if the pH is low too, dolomitic lime is a longer term solution.
If you’re looking for signs of boron or zinc deficiency, these aren’t as prevalent (or obvious) but they show up if you know what to watch for. With a zinc deficiency, it’s called little leaf: New leaves will be clustered and stunted. Boron shows up as distorted growing points or hollow stems. Remember: These are micronutrients; we need them in small quantities. They are measured not by percentage (like calcium), but in parts per million (in tissue samples).
A little change in soil conditions can throw things off. Phosphorus levels that are too high can “lock up” zinc. Boron leaches fast from sandy soils.
Don’t guess. That’s the key. Examine the leaf. Old or new? Vein or tip? What’s the pattern? That’s the clue to the trouble. Amend too soon, before diagnosis, and you may load up another element and cause more lockouts. Magnesium gets blocked by potassium. Zinc by phosphorus. It is a delicate balance.
Begin with the pH. Half of the lockout issues dissolves when the pH is right. Next, check the consistency of moisture for the absorption of calcium. Last, review those specific leaf symptom for each of the micronutrients.
Adding more isn’t the issue. Ensuring that what’s already present can reach where it needs to go is. Once you know how to read the leaves, the soil becomes a friend instead of a mystery. The yellow leaves were simply a question in need of an answer.