Primary Nutrients

The primary nutrients used by plants are nitrogen (N), Phosphorus (P), and Potassium (K). Without these minerals, plants cannot grow.

Nitrogen-deficient melon leaves

While all nutrients are important to plant growth, primary nutrients are used in the greatest amounts. Before you add more fertilizer, however, you need to find out what is already in your soil. Adding too much of a nutrient is just as bad, or worse, than not enough.

The only way to know about the nutrients in your soil is with a lab-based soil test. Over-the-counter soil tests are not accurate enough to be of any use. Soil tests are inexpensive and extremely valuable.

Each of the primary nutrients supports different aspects of plant growth and health. The more you know about what they do for your plants, the better you will be able to spot deficiencies and toxicities.

Nitrogen-deficient cabbage (ShoutWiki) CC 3.0

Nitrogen

Nitrogen is the single most limiting factor in plant growth. Nitrogen is used to make chlorophyll as well as plant enzymes and proteins. Nitrogen is responsible for lush, green, vegetative growth. Without nitrogen, photosynthesis cannot occur. 

Stunting and chlorosis are the two most common signs of insufficient nitrogen. Nitrogen is highly mobile within the soil and in plants. Too much nitrogen can be just as bad as not enough. Excessive nitrogen is seen as darker than normal leaves and more vegetative growth than fruit or flowers. Too much nitrogen can burn plants, and it can cause erratic or reduced budbreak. Too much nitrogen can also stimulate new  growth that may be vulnerable to pests, disease, and injury. ​

Phosphorus deficient corn (عمرو بن كلثوم) CC BY-SA 2.5

Phosphorus

Phosphorus is another primary nutrient, second only to nitrogen in plant health. Phosphorus helps plants use and store energy. Most important to the home gardener, phosphorus supports flower, seed, bud, and root growth. It is the reproduction nutrient, the in-between stage between growing and fruiting. 

Despite being so important, phosphorus is rarely found in a form plants can use. Mostly, phosphorus exists as phosphates. Organic sources of phosphorus include animal manure, urine, guano, fish emulsion, compost, blood meal, and bone meal. Phosphorus is commonly applied around seeds at planting time in a process called banding.

Phosphorus is a mobile nutrient. This means that deficiencies are generally seen in older leaves first, when they occur at all. Phosphorus deficiency is practically unheard of in California home gardens. Since phosphorus is an important part of genetic information transfer, deficiencies ultimately result in smaller and fewer leaves, and fruit set failure. You may also see leaves turn darker and more purple or red than normal, especially on the underside, with a shiny almost metallic appearance on the top surface. These same symptoms may indicate several other conditions, so get you soil tested.

Potassium deficiency in beans (Alandmanson) CC BY-SA 4.0

Potassium

Potassium (K) is the third primary nutrient and it dictates the size, shape, color, and sugar content (brix) of your fruit crops. It also boosts photosynthesis and respiration, helps plants stay upright, and promotes healthy root systems.

There’s a lot of potassium on Earth, but most of it is unavailable to plants. Plants can only use potassium that is in solution. As plant roots absorb mineral-rich water from the ground, potassium is pulled in and put to work. Potassium, also known as potash, is concentrated in leaves and growing tips. Also found in bat guano and wood ashes, potassium is a highly mobile element.

Potassium deficiencies result in reduced nitrogen absorption and a build up of sugars that can give leaves a burnt appearance. Other signs of potassium deficiency include wilting, brown spotting, and discolored veins. These symptoms move from older/lower growth to higher/newer growth.

Potassium is one nutrient that plants can absorb at levels higher than they can use, in an action called ‘luxury consumption’. If you see a white crust developing on leaf margins (edges), it is the sugar and potassium residue from guttation. 

Nutrient imbalances and high temperatures can interfere with nutrient absorption. Before you toss another bag of fertilizer at your plants, make sure they really need it. The only way to know for sure what your plants are working with is to invest in a soil test from a local, reputable lab. It will save you a lot of money in terms of replacement plants, reduced harvest, unnecessary soil amendments, and chemical treatments.

So just remember: nitrogen promotes lush, green growth, phosphorus helps plants prepare for reproduction, and potassium promotes healthy crops.

​And too much of a good thing can be a bad thing.