A primer on 3 things plants crave: N, P and K

Special to the Idaho StatesmanMarch 14, 2014 

Just as humans require a variety of nutrients to stay healthy, so do plants. If you look at a package of fertilizer, you'll see three numbers, such as "10-15-0" or "5-20-5." These refer to the percentages of nitrogen, phosphorus and potassium the bag contains. Those three elements are the main diet of plants.


Plants use a lot of nitrogen, taking it from the soil into the roots and up into the leaves. Nitrogen is part of chlorophyll and is a big part of the photosynthesis process. When plants are starved for nitrogen, they can't efficiently create the food they need to survive. Fertilizers made for use on lawns usually contain only nitrogen.


This element is also used in photosynthesis. Phosphorus promotes root growth and flower production and helps plants endure the stress of hot weather. If you look at the NPK ratio on products such as Miracle-Gro Quick Start fertilizer and Miracle-Gro Bloom Booster, you'll see that the phosphorus content is much higher than the other two elements.

In the vegetable garden, producing more blooms means a bigger harvest.


Photosynthesis also requires potassium. Potassium helps plants fight off diseases and produce quality fruit and seeds.


Ten other mineral nutrients and three nonmineral nutrients are needed by plants.

• Plants use a lot of calcium (Ca) to help build strong cell walls in order for the plant to stand up straight and get maximum sunshine. Our soils in the West tend to have plenty of calcium. Caliche, or hardpan, is known to scientists as calcium carbonate and we have an abundance of it!

Magnesium (Mg) is essential for photosynthesis. Some fertilizers have trace amounts of this element. A lack of magnesium in the soil will cause leaves to discolor.

Sulfur (S) helps with chlorophyll formation, growth, seed production and resistance to cold. Sulfur is found in organic materials, one reason why compost is so good for our soils. Sulfur helps lower the pH of our alkaline soils.

Other trace minerals are sometimes found in fertilizers, but most soils have enough of these trace elements to keep plants healthy. They are boron (B), copper (Cu), chlorine (Cl), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn). These elements help plants metabolize other nutrients, regulate plant growth, form chlorophyll and sugars and develop fruits and seeds.

The three nonmineral nutrients that plants need are hydrogen (H), oxygen (O) and carbon (C). Plants get these elements from the air and from water. They use these elements, plus the sun, to produce starches and sugars using the process called photosynthesis.

The pH of the soil is not an element, but it's important for plant health. If the soil's pH is too high (alkaline) or too low (acidic), plants can't use the nutrients in the soil. It's as if high or low pH acts as a gate and won't let the necessary nutrients to pass into the plant's roots. Ideal pH is 6.0 to 6.5, but many Western natives are adapted to our higher pH soils.

To find out if your soil has the proper nutrients and pH for optimal plant growth, you can have your soil tested. Both the University of Idaho and Western Laboratories (as well as others) do soil testing and have information on their websites on how to gather a soil sample. To have the university test your soil, go to the Ada County Extension Office (5880 Glenwood St.) and pick up a soil test kit. You'll receive instructions. Find information about testing by Western Laboratories at www.westernlaboratories.com.

Questions? Email Elaine at highprairielandscapedesign@yahoo.com. And read her column and others at IdahoStatesman.com/gardening.

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