What do nutrients do for plants?

If a plant decided to go on a diet, you know, to keep its plant-like figure, what do you think it would choose to be as healthy as possible?

It turns out that plants, like humans, rely on essential nutrients to maintain a healthy and balanced lifestyle. So, what’s the plant equivalent of a protein shake you might be wondering?

Probably an N-P-K shake comprised mostly of nutrients like nitrogen (N), phosphorus (P), and potassium (K). Then you can add some “shots” of smaller amounts of calcium (Ca), magnesium (Mg), and sulfur (S), which are essential for plant health.

A comparison of common nutrients and what they do for plants and humans. Source: Agronomy: Grow with It!, Ateh et al., 2016, page 73.

Nitrogen (N)

Nitrogen is used by plants to create amino acids, the building blocks of proteins. Proteins are essential for all our human and plant cells. Amino acids are needed to form protoplasm – the site for cell division. That makes nitrogen crucial to plant growth and development. All plant enzymes used in biochemical reactions are also made of proteins.

Besides the creation of amino acids and proteins, nitrogen is also a necessary component of the chlorophyll molecule. So, nitrogen influences the photosynthesis process. Nitrogen can improve the quality and quantity of dry matter in leafy vegetables and protein in grain crops. Nitrogen deficiency results in stunted growth, pale green or yellowing of older leaves, as the plant tries to scavenge nitrogen from older leaves to younger tissues.

You might be thinking, well I’m going to pour on the fertilizer, but hold that thought! Plants can also have toxic levels of a nutrient. Nitrogen toxicity also results in stunted growth with plants looking dark green. It can also result in vegetative bud formation (more plant leaves or stems), instead of reproductive bud formation (more flowers or crops), which isn’t very good for yield.

Phosphorus (P)

Another big player in plant nutrition is phosphorus. It is very important in plant metabolism. Phosphorus is used in plant photosynthesis and respiration as it is needed for energy storage and transfer. It’s also part of RNA and DNA, the stores of genetic information of living things. Seeds usually have large stores of phosphorus available for young cells in shoot and root tips where growth is rapid. If the plant lacks phosphorus, this would result in stunted growth as cell division gets compromised. Luckily, phosphorus can be mobilized in a plant and transferred to sites of new growth. However, if a plant does this, it causes older leaves to appear dark or blue-green, even purple in severe cases. Phosphorus deficiency thus causes slow development and low seed and fruit quality. But be cautious about applying too much phosphorus to a plant – excess amounts can cause iron and zinc deficiencies.

Potassium (K)

Potassium is a bit different to nitrogen and phosphorus. It doesn’t become part of any organic compounds in the plant. It’s more like the ultimate assistant to many processes happening in a healthy growing plant. For example, potassium assists in the regulation of water use in the plant by controlling the opening and closing of stomata, allowing the plant to cool itself. At sites of energy production, potassium maintains the balance of electrical charge. Potassium has even been shown to improve disease resistance of the plant, improve the size of seeds and grains and improve quality of fruits and vegetables. But too much potassium can also result in toxic levels in the plant leading to calcium, magnesium and nitrogen deficiencies.

Calcium (Ca)

Calcium is needed in smaller amounts in the plant but it’s not mobile in the plant. This means that if adequate amounts of calcium aren’t available, symptoms of deficiency will first appear on young leaves as growth is stunted. Growing points of the stem and roots also stop developing with notable deterioration of the root system before the above-ground parts of the plant. Again, on the flip side, high calcium in a plant can cause magnesium and boron deficiencies.

Magnesium deficiency in a canola plant leaf. Notice the vein area remains green, while the rest of the leaf is turning yellow. Credit: Ivan Izgagin

Magnesium (Mg)

Magnesium is an important component of the chlorophyll molecule and therefore needed for photosynthesis. It is mobile in the plant, therefore when there’s a deficiency, chlorosis first occurs on older leaves in the leaf tissue between the leaf veins. Leaves can start to look yellow, bronze or reddish while leaf veins remain green. High magnesium levels can cause a calcium deficiency.

Sulfur (S)

Finally, sulfur is used by the plant to create some amino acids and is essential for chloroplast growth and function (the part of the cell where photosynthesis occurs). Sulfur is not mobilized in a plant so symptoms of deficiency first show on new plant growth where there is a uniform yellowing of new plant tissue. Growth is stunted and maturity is delayed which lowers yield. Toxic levels of sulfur are hard to create so a plant likely won’t suffer from this problem.