
N, P, and K Explained
Gardeners and gardening articles often talk about NPK, which stands for nitrogen, phosphorus, and potassium. The initials N, P, and K are the chemical symbols for those elements on the periodic table (yes, potassium is represented by K. It’s a long story). Much discussion revolves around using a fertilizer with more of one element than another to promote flowering, vegetative growth, or root growth. But just what are these elements, and how do they help our plants?
Nitrogen, Phosphorus, and Potassium are called the primary macronutrients. Besides carbon, hydrogen, and oxygen (supplied by water and air), they're the three most needed and used elements in plant growth. Calcium, sulfur, and magnesium are called secondary macronutrients and are the next tier of importance for plant health. As the name suggests, micronutrients are needed in small quantities, although their absence can still cause issues. Boron, copper, iron, manganese, molybdenum, nickel, zinc, and chloride round out the micronutrients. Of these 17 essential nutrients, 14 are supplied from the soil.
When gardeners are looking for fertilizer, we aren't often concerned with finding a supply of manganese or nickel. The primary nutrients, nitrogen, phosphorus, and potassium, are the three most likely to need supplementing. Of those, nitrogen is usually the problem, as it is very mobile in the soil and can easily leach away, moving out of the reach of plant roots.
For homemade fertilizers, or rather, home-gathered fertilizers like cow or chicken manure, composted kitchen scraps, and compost tea, the ratio of nitrogen, phosphorus, and potassium is unknown. General guidelines exist, but we usually apply them evenly and hope for the best. Purchased products are required to have the contents, called the Guaranteed Analysis, on the bag so we know what's inside. A detailed window with all percentages is usually on the back, but a quick look at the front will reveal the NPK ratio.
Reading the Numbers on the Bag
Look at a bag of fertilizer at the garden center, and you'll see three numbers, usually separated by a dash. Those numbers are a ratio of the big three primary macronutrients, and indicate the contents. A general purpose or balanced fertilizer might have a sequence of 10-10-10. You might see 0-5-2 or 5-2-4, or any combination, really. Each number stands for the percentage, by weight, of that nutrient in the product. So, a fertilizer labeled 5-10-8 would have five percent nitrogen, ten percent phosphorus, and eight percent potassium.
Notice that the above numbers don't add up to 100%. The rest, in the case of a 10-10-10 product the remainder would be 70%, consists of inert ingredients to help aid in application and delivery and may include other plant macro and micronutrients, typically adding up to only a percent or two.
You can use the NPK numbers, often called the NPK ratio, to choose between fertilizers and evaluate when purchasing. For example, comparing two products, each sold in ten-pound bags, a 15-10-10 bag would have more nitrogen, phosphorus, and potassium in the bag than a 10-4-2 product for the same weight of product.
If we use a ten pound bag as an example, a bag labeled 15-10-8 will contain 1.5 pounds of nitrogen product, 1.0 pounds of phosphorus, and 0.8 pounds of potassium. A ten-pound bag of 5-5-5 fertilizer would contain only half a pound each. This is why you'll often see high number NPK fertilizers cost more money–they have more nutrients per pound of product. That doesn't mean lower NPKs are bad. It just helps you be an informed buyer.
Nitrogen
Nitrogen is abundant everywhere on Earth. It makes up approximately 78% of our atmosphere. Nitrogen is found in nearly every plant structure, from chlorophyll in leaves to stems, flowers, fruits, roots, DNA and even enzymes. A lack of nitrogen causes yellowing leaves and stunted growth, but how could there ever be a lack when it is all around us, surrounding the plants, and even present in the pockets of air in the soil?
Nitrogen, as we know, is an element with the symbol N. In the atmosphere, two nitrogen atoms bond to form N2, a colorless gas we cannot smell. N2 is all around us, and we breathe it in and out with seldom thought about it. But, although abundant, it isn't in a form that plants can uptake and use. All that nitrogen in the air does a plant no good until it is transformed into different forms, mainly nitrate (NO3) and ammonium (NH4).
Nitrogen Fixation
Atmospheric nitrogen can be transformed into plant-usable forms via a process called nitrogen fixation, which can be biological or industrial. The industrial process is one way we get the nitrogen content in a bag of fertilizer or, more commonly, how we form large amounts of fertilizer for agricultural crops.
Biological or natural processes of nitrogen fixation include atmospheric deposition, decomposition of organic material (which had nitrogen in it, of course), and fixation by specialized bacteria called diazotrophs. Some bacteria form symbiotic relationships with plants, Rhizobium and plants in the legume family being the prime example.
Additional Sources of Nitrogen
The nitrogen component in the bag or box is often nitrate, ammoniacal, urea, or a combination of those. While the plant cannot tell what the source of nitrogen was–synthetic or natural–the disruption of the soil food web cycle caused by synthetic nitrogen fertilizer is worth considering. For natural sources, consider composted manure, blood meal, alfalfa meal, and good old-fashioned compost.
Phosphorus
Phosphorus in plant-available form is called phosphate. Plants need phosphorus for cellular respiration and energy transfer to make RNA, DNA, and other processes. We commonly hear that applying high rates of phosphorus will make flowers bloom more, but that isn't strictly true. If it was, commercial greenhouses would follow the practice, and they don’t. Plants need all three macronutrients, N, P, and K, to flower vigorously. Adding just one out of balance with the others won't increase blooms.
Most garden soils are not phosphorus deficient, although agricultural soils sometimes can be. A soil test will reveal whether or not additional phosphorus is needed. Excess phosphorus can cause problems with the uptake of micronutrients and run off into waterways causing issues.
Phosphorus deficiencies can often show up in seedlings started indoors. Most seed-starting mixes don't contain soil and their nutrients may be easily depleted as seedlings grow. Tomatoes are a prime candidate, and seedlings often show a purple tinge when phosphorus levels are low. Fortunately, they are easily perked up by fertilizing or transplanting in the garden.
Phosphorus does not leach easily out of the soil like nitrogen; the only typical loss factor is the plant material removed. Returning composted plant material to the garden alleviates some of this loss. In many fertilized garden soils, phosphorus levels actually build over time.
Sources of Phosphorus
Besides being a component in most fertilizers, other natural forms are available. Bone meal is a common additive to supply additional phosphorus, although it takes some time to have an effect. Chicken manure is also a good source. However, it cannot be applied "fresh" and needs to be composted. Again, compost made from your garden, leaf clippings, leaves and such is also a good source of phosphorus.
Potassium
Potassium (K) is the third macronutrient plants need. It's the last number in the N-P-K ratios found on the bag. Plants use potassium in processes including moving water and nutrients up and down the plant, disease resistance, growth, and respiration. In fertilizers, potassium is typically supplied as soluble potash.
Additional Sources of Potassium
Manure and compost both supply potassium to the soil. Wood ashes are also a source of potassium. Wood ashes can be added in small amounts to your compost pile or applied directly to the ground in winter (which also lessens the chance for smoldering embers to cause a problem). Go easy on the application. Only a few ounces per 10 square feet of garden is needed. Eventually, adding wood ash can begin to raise the pH of your soil.
Fortunately for gardeners, adopting a soil-health-first strategy of adding organic matter and compost, mulching, reducing tillage, and maybe a little aged manure if you can get it will go a long way toward providing a balanced nutrient profile for our plants in the soil. If applying commercially prepared fertilizers in the garden, get a soil test and follow the recommendations, then pick a product whose NPK ratio matches what is indicated by the soil test results.