Whether you're growing in soil or you're using a soilless medium like a hydroponic system, the metals in your feeding program all help your plants taking in nutrients, producing chlorophyll, and helping to regulate your plants' metabolism. While it can be tricky for your plants to have a deficiency in their metals, too much or too little of vital metals means your plants won't be able to create and process chlorophyll which means your plants won't eat. Thankfully the fix for these elements is pretty simple, so let's get to it!
Boron & Iron Deficiencies
Boron and Iron: metals that both you and your plants need. Without these essential metals, our blood isn't as rich and we begin to be anemic. Similarly, your plants begin to drop off their production of chlorophyll, and without it, they can't grow. 
Boron (B) helps in cell development within your plant. Anything from trichomes to cell walls to chlorophyll, boron's one of the building blocks of cellular formation. What to Look For: Boron deficiencies, as seen up top, usually happen as a result of nutrient or pH problems (usually a nitrogen or potassium deficiency). Old growth will look thick at the tips and grow abnormally. New growth will have yellow or brown spots on their leaves. Old growth will have twisted leaves, whereas new growth will look wrinkled or curled. Stems and roots will become weak, and the stems might also become shallow. 
Iron (Fe), on the other hand, helps manufacture chlorophyll so your plant to take in sunlight. It also helps in other vital biochemical processes that allow your plants to eat and breathe properly. What to Look For: If you have an iron deficiency, your plants will have bright yellow leaves with green spines and brown marks on their leaves. That's because it's not able to create the cells needed to take in sunlight properly, and when that happens your plants won't be able to create food for itself. Iron deficiencies are also usually a nutrient deficiency (your nutrients don't have enough iron in them) or pH problems. The Solution: Both of these elements are vital to helping your plants synthesize chlorophyll to eat. Usually, the culprit comes from your water supply or nutrients (or nutrient availability in the case of soil i.e. what's actually in your soil), so to fix the problems you've got to look at how you feed your plants.
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If you use a reverse osmosis machine to clean your water, you might want to stop. Boron and iron are found in tap water, and though you may not think tap water is the best for your plants, it's always better to combat pH levels than to add iron or boron and hope for the best
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Adding a pH Up or Down Mix will help better than giving your water a huge spike in iron or boron that you'll still have to combat. You may have to fight even harder if you add iron/boron supplements because of the concentration of elements in it, so do not add more iron or boron into your mix
- Adjust the pH using natural or manufactured adjusters
You'll also want to check your watering levels and your humidity. Low humidity and underfeeding can also lead to these deficiencies. If that's the case, simply adding more nutrient-rich water to your feeding cycle and increasing the humidity in your grow can reverse this damage to your grow.
Magnesium & Manganese Deficiencies
Magnesium (Mg) is a Secondary Nutrient, which means it's a pretty important element in your grow. It helps regulate the uptake of nutrients, and because it's found in chlorophyll, it's also vital in making sure leaves get the dark green color they need to take in sunlight to create food for itself. What to Look For: Magnesium problems aren’t too severe, but magnesium is a mobile nutrient, meaning your plant can move it wherever it feels it needs it. That means if your plant has a magnesium deficiency and you don’t take care of it, your plant’s going to have to pull it from wherever it can: new growth, old growth, anywhere it can pull it. That’s why you’ll see yellowing around the veins and edges of your plants. The tips of the leaves can also look burned (though this is not nutrient burn). 
Manganese (Mn) is crucial in chlorophyll production. Without it, your plants aren't eating-period. And though you'd rarely run into one of these issues, if you do it can be bad news for your grow. What to Look For: Manganese deficiencies can be pretty dangerous. The leaves and their veins will become yellow and start to develop brown spots. These spots are dead patches, and the more they spread the worse your leaves are going to do: they can eventually start to break apart, shred, or fall apart altogether. Forget a dead crop- you know what lots of heat does to dried out plants, right? Your grow and home will go up in smoke (and not the good kind of smoke, either). The Solution: Thankfully these issues are pretty easy to resolve. pH is the name of the game, so check those levels.
- Magnesium deficiencies come from a low pH level at the roots
- Manganese deficiencies come when your pH is too high or there’s too much iron in your feeding cycle
- To resolve these issues start by flushing your plants with pH’s water
- Then make sure to keep levels where they need to be, adding a pH Up or Down to your solution should bring your plants back to life if you’re seeing these deficiencies
