Nitrogen in Plants
Photo courtesy of USDA ARS

Very few farmers or gardeners need to be told about the importance of nitrogen. Nitrogen (the N in NPK) is found in compounds making up as much as 50% of the dry matter of the protoplasm of plant cells. It is also a key ingredient in chlorophyll and amino acids.

Because plants contain so much nitrogen, it is little wonder that they require ample amounts. Without it, plants simply cannot grow.

Natural Sources

Animal manure is among the best nitrogen sources. This includes manure from all types of livestock, although poultry manure contains the highest levels of nitrogen. Bat guano and worm castings also count. (Please remember—no cat or dog manure in the garden, as it may contain dangerous pathogens.)

Other animal products frequently contain nitrogen, as well. For instance, blood meal, feather meal, fish meal, and fish emulsion are considered good natural sources.

Plant by-products that contain high levels of nitrogen include alfalfa, soybean, or cottonseed meal.

Finally, don’t overlook balanced compost or cover crops as a potential nitrogen source. In particular, consider a legume cover crop for an added nitrogen boost.


Nitrogen deficiency
Nitrogen-deficient corn (right) compared to healthy corn; photo courtesy of USDA ARS

Soil nitrogen levels can easily be depleted by plants during periods of rapid growth. Some plants, such as corn, are particularly heavy feeders.

Also, soil microorganism health is key to making sure that any nitrogen in the soil is readily available for plant consumption. Therefore, in some cases, a nitrogen deficiency may be caused by insufficient soil life rather than an actual mineral shortfall.

Symptoms of nitrogen deficiency in plants include:

  • Slow, stunted growth.
  • Purple stems.
  • Pale leaves with purplish undersides.
  • Wilting and loss of older leaves.
  • Reduced yield.

Plants suffering from nitrogen deficiency should be fed a high-nitrogen natural fertilizer.

To prevent future deficiencies, consider taking the following steps to raise nitrogen levels and promote microorganism health:

  • Improve soil drainage if necessary to avoid drowning beneficial microorganisms.
  • Reduce or eliminate tillage to avoid soil disturbance.
  • Rotate crops to prevent nitrogen depletion, preferably preceding hungry crops with legumes.
  • Add cured compost or rotted manure to the soil regularly.
  • Keep the soil covered at all times to keep moisture levels stable.
  • Use cover crop mixes containing legumes.


Nitrogen cycle
The nitrogen cycle

Nitrogen toxicity is typically the result of overly zealous fertilization. It’s a very easy mistake to fall into because just a little nitrogen makes such a big impact on plant growth. And if a little is good, more is better, right? Not necessarily.

The symptoms of toxicity are subtle, but include:

  • Wilting.
  • Burnt appearance of leaves.
  • Excessively bushy growth with little fruit.
  • Short roots.
  • Increased susceptibility to disease.
  • Increased insect pest activity.

If you inadvertently apply too much nitrogen to the soil, an efficient way to lower the nitrogen levels without wasting it is to plant something that can absorb a great deal without adverse effects. Lettuce is particularly effective here, but corn or Brussels sprouts will work, too.

Barring that option, your next best bet is to add ample amounts of high-carbon organic matter to the soil. Microorganisms use up a great deal of nitrogen when breaking down carbon, so use this information to your advantage. This is a great time to try out a mulch made of wood chips, tree bark, sawdust, or shredded cardboard.

Finally, be sure to water generously. Not only will this help flush the excess nitrogen out of the soil, but your plants will appreciate the extra hydration at this time. Nitrogen causes salts to accumulate in the soil, impairing the plants’ ability to stay hydrated.

Complete Series

Minerals in Plants

Minerals in Plants

Improving Your Garden Soil