Leaves Turning Yellow? Identifying Nutrient Deficiencies


I recently noticed that some of the leaves on my plants were turning yellow, known as chlorosis.  I began to wonder what causes yellow leaves in plants.  I did some research and found out that a nutrient deficiency may be to blame.

So, what nutrient deficiency causes yellow leaves?  If the leaves on your plants turn yellow, the most common nutrient deficiencies are magnesium and iron.  It is possible that other nutrients may be deficient, such as nitrogen, phosphorus, potassium, copper, sulfur, zinc, molybdenum, or manganese.

Before you go dosing your plants with fertilizer and supplements, remember that there can be many causes of yellow leaves in general and nutrient deficiencies in particular.  This article will help you get to the bottom of the problem, one step at a time.  You can also skip to the table at the bottom to get an idea of what could be wrong.

What Nutrient Deficiency Causes Yellow Leaves?

First, we should define chlorosis, which simply means that plant tissues, including leaves, turn yellow due to a lack of chlorophyll.

chlorophyll molecule
The chemical structure of chlorophyll includes 55 carbon atoms, 72 hydrogen atoms, 5 oxygen atoms, 4 nitrogen atoms, and one magnesium atom.

Some common nutrient deficiencies that cause yellow leaves are magnesium and iron deficiency.

However, keep in mind that fixing the problem may not be as simple as you think.

Before you add a complete fertilizer or a nutrient mix to your solution, you need to find out where the problem lies.  First, you need to figure out if the problem is caused by a factor other than a nutrient deficiency.

If you can rule out these other factors, you will need to determine which nutrient is deficient.  Then, you can take the appropriate steps to provide this nutrient to the plant.

What Should I Check First to Troubleshoot Yellow Leaves?

Let’s start by eliminating the simple possibilities first.

Water Level

A lack of water is not likely in a hydroponics system.  However, it is possible.  If your plants are growing quickly and are using water faster than you can replace it, the leaves can turn yellow and become wilted.

If you are using a DWC (deep water culture) hydroponics setup, make sure that the level of nutrient solution is high enough so that the plant’s roots are in the solution and able to take up water.

Light Level

Too much or too little light can cause the leaves on your plants to turn yellow.  Some plants require a period of darkness.  For these plants, leaving the lights on all day and night can backfire, instead of causing them to grow faster.

LED lights
Some bright LED lights.

In addition to the duration of light your plants receive, make sure that the intensity is correct.  In addition to affecting plant photosynthesis and growth, light intensity can affect the temperature of your system.

Temperature

If the temperature is too high, the leaves of your plant may turn yellow.  In addition, higher temperatures cause less carbon dioxide gas to dissolve in water.  This results in lower levels of carbonic acid in your water, and thus higher pH.

Thus, you should make sure to adjust your temperature before proceeding to check the pH of your system.  Using a water chiller or heater can help to maintain a balanced temperature.

The Effect of pH on Nutrient Availability

Nutrient lockout occurs when a plant cannot absorb a nutrient, even though the nutrient is present in your hydroponic system solution.  A common cause of nutrient lockout is a pH that is either too high or too low.  Think of it like your assets at a bank being frozen; the money is there, but you can’t get at it.

An ideal pH range for most hydroponic plants is 5.5 to 6.5, slightly acidic.  Each nutrient is most readily absorbed within a specific pH range.  The ranges vary for every element, but there is some overlap.  To see the ideal pH ranges for various nutrients, check out this chart on the Research Gate website.

As you can see from the chart, most elements are highly available (thick blue lines) between a pH of 5.5 and 6.5.

However, when pH gets to 5.0 or lower, elements like phosphorus and magnesium start to become less available.  Similarly, when pH gets to 7.5 to 8.0 or higher, elements like phosphorus, iron, and boron start to become less available.

If you monitor your pH regularly and make adjustments as necessary, you should be able to stay within the ideal pH range.

If your pH happens to be too high (alkaline, or basic), use pH DOWN to adjust downward.  If your pH is too low (acidic), use pH UP to adjust upward.  Remember that these products are very strong, and a small amount has a large effect on pH, especially in smaller systems.

Start with a small amount (a fraction of a teaspoon), mix well, and let sit for about 30 minutes.  Then, measure your pH again, and see how much it changed before you add more pH DOWN or pH UP.

If you find that the water level, light level, temperature, and pH in your system are fine, then it’s time to consider the possibility of a nutrient imbalance or deficiency.

How Do I Know Which Nutrient is Deficient?

To identify which nutrient deficiency is causing yellow leaves in your plants, there are some factors to consider.

Which Leaves Turn Yellow First?

First, pay attention to which leaves turn yellow first.  There are a couple of cases to consider here.

1) If the bottom leaves (or old growth) turn yellow first, then the yellow leaves are caused by a deficiency in one of the following nutrients:

  • Nitrogen
  • Phosphorus
  • Potassium
  • Magnesium
  • Zinc
  • Molybdenum

A mobile nutrient is one that a plant can easily transport between its different parts.  Since the top growth is more important for absorbing light, a plant with a mobile nutrient deficiency will transport the nutrient from its bottom leaves to its top leaves.

In effect, the plant is sacrificing the old leaves for the sake of the new leaves.  The result is that the bottom leaves start to turn yellow first, and then the condition moves up the plant.  How quickly this occurs depends on the severity of the nutrient deficiency.

Magnesium is the most common mobile nutrient deficiency, since magnesium is part of the chemical formula for chlorophyll, which gives plants their green color.

2) If the top leaves (or new growth) turn yellow first, then the yellow leaves are caused by a deficiency in one of the following nutrients:

  • Calcium
  • Sulfur
  • Iron
  • Boron
  • Copper

An immobile nutrient is one that a plant cannot easily transport between its different parts.  A plant with an immobile nutrient deficiency cannot transport the nutrient from its bottom leaves to its top leaves.  In this case, the top leaves will start to turn yellow first.  Again, how quickly the yellow leaves spreads depends on the extent of the deficiency.

Iron is the most common immobile nutrient deficiency, and it frequently occurs when the pH of your hydroponic solution drifts too high (see the discussion above about pH).

iron chlorosis
Leaves with an iron deficiency.

Calcium is another common nutrient deficiency, although it does not usually cause yellow leaves.  Calcium deficiency is caused by a high pH or too much magnesium.

If you look at a periodic table, magnesium and calcium are in the same column (the alkaline earth metals).  This means that they share many similar properties, and in a sense “compete” with one another in a hydroponic system.

See the table at the end of the article for more information on how these various nutrient deficiencies can occur, and how to treat them.

Which Part of the Leaves Turn Yellow?

To further narrow down which nutrient is deficient in your system, look at which parts of the leaves turn yellow.  Sometimes, the entire leaf will turn yellow, including the veins.

In other cases, the veins of the leaves stay green, but the parts between the leaves turn yellow, known as interveinal chlorosis, seen below.

interveinal chlorosis
A leaf with interveinal chlorosis.

If you see leaves with interveinal chlorosis, it is a good bet that the cause is a deficiency of iron, magnesium, or zinc.

As stated before, if the bottom leaves develop interveinal chlorosis first, the cause is likely a magnesium deficiency.  If the top leaves develop interveinal chlorosis first, the cause is likely an iron deficiency.

Remember that using ultraviolet light to kill algae or other undesirable organisms in your system can cause a chemical reaction that causes iron to precipitate out of your solution.  This makes the iron unavailable to your plants, and can lead to a deficiency.

Can I Have Too Much of a Given Nutrient?

It is possible to have too much of a given nutrient.  This is known as nutrient toxicity, and this can cause yellow leaves in some cases, due to nutrient lockout.

In addition to actual levels of different nutrients in your system, you need to pay attention to ratios of these nutrients.  If there is too much calcium, for example, you may be limiting a plant’s ability to take up magnesium, even if there is plenty of magnesium in your solution.

How Do You Fix a Nutrient Deficiency?

For help with identifying nutrient deficiencies in your plants, check out the table on this page.

If you cannot fix the problem using the table, there is always the option of flushing out your system and using new water, properly calibrated with the proper nutrients and pH from the start.

Conclusion

Hopefully, the table and troubleshooting information above were helpful in diagnosing the problem that is causing yellow leaves in your plants.  If not, ask the experts at your local hydroponics or garden store.

If you have any helpful information or troubleshooting tips to share, please leave them in the comments below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

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