University of Maryland Extension

Nutrient management

Author: 
Neith Little, Extension Agent, Urban Agriculture


Urban Ag home | Table of contents 

Plants need nutrients, like nitrogen (N), phosphorus (P), and potassium (K), to build their bodies. Different soil amendments add different amounts and forms of nutrients to the soil.

Soil fertility amendments could be fertilizers, composts, manures, or other materials that contain plant nutrients. Fertility amendments for sale should come with a label that reports their nitrogen (N), phosphorus (P), and potassium (K) concentration, by weight. These three nutrients are needed by crops in large amounts and are sometimes referred to as NPK.

Usually the label will report the NPK concentration as three numbers. The first number is the nitrogen concentration, the second is the phosphorus concentration, and the third is the potassium concentration. Nutrients in commercial fertilizers tend to be concentrated and quickly available. Nutrients in compost and manures tend to be less concentrated, and in bigger molecules that become available over a longer period of time. Compost and manure also supply other nutrients and organic matter.

A fertility test can tell you what nutrients are already in your soil, and what nutrients you need more of. Soil and growing media use different test methods to measure fertility. Different tests should be used to measure the fertility of soils and growing media.

More accurately, in the US most fertilizer labels and recommendations are calculated as available nitrogen (N), phosphate (P2O5), and potash (K2O) instead of elemental nitrogen (N), phosphorus (P), and potassium (K). But for pronounceability’s sake the N-P2O5-K2O ratio is often referred to as the N-P-K ratio. Because we pretty much all use the same system, you should not have to convert between available phosphate (P2O5) and elemental phosphorus (P), so you don’t really need to worry about it. N-P2O5-K2O and N-P-K should mean the same thing in most situations where you will encounter them. Unless you go to Europe, or into a research lab. Then all bets are off.

In Maryland, if you farm and sell at least $2,500 worth of crops per year, you are legally required to have an official nutrient management plan. To learn more about this rule, and how to get a nutrient management plan, go to http://extension.umd.edu/anmp

Learning more about how nutrients cycle through the soil, atmosphere, plants, and animals can help you improve your crops’ growth and use purchased inputs most efficiently. To start with, know that nitrogen is particularly mobile in the soil, water and air, and that it exists in many different forms. This means that in places that get a lot of rain during the growing season (like the eastern United States), measuring soil nitrogen at any one moment will not tell you much about how much nitrogen will be available a month from then. So many soil tests will not report nitrogen measurements and will instead recommend adding nitrogen based on book values for how much nitrogen specific crops need.

Nitrogen is also often the nutrient that limits plant growth and is particularly expensive to purchase using organic amendments. So if growing using organic methods is important to you, you will need to learn about cover crops. Leguminous cover crops (beans and peas) fix nitrogen from the air and put it into the soil. Other cover crops can “catch” nitrogen left over in the soil at the end of the growing season, and help hold it until the next spring. To learn more about cover crops, a good book to start with is Managing Cover Crops Profitably by Andy Clark. A free digital version is available online from Sustainable Agriculture Research and Education: https://www.sare.org

Nutrient management can be an intimidating topic, because it involves chemistry and math. But if you keep learning more every year, you’ll find that understanding your crops’ nutrient needs, what your soil provides, and what you add to it will help you grow better crops and spend your money more wisely.

 

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