When garden crops aren’t thriving because of a nutrient, the nutrient is most likely nitrogen. Plants need nitrogen to produce many of their complex molecules, such as amino acids and protein, hormones, pigments, and complex phytochemicals. Nitrogen is also an essential part of every animal’s diet.

Deficient bean plant

Nitrogen deficient bean plant (note pale green color, yellow leaves, plus plants lack of fruits even though the plant has been flowering)

How will you know that a plant is deficient in nitrogen? The squash plant at the top, and the bean plant to the left, both show signs of nitrogen deficiency. When a plant’s green color starts to pale, that may be the first sign of a nitrogen deficiency. In the next stage, the oldest leaves will turn yellow and fall from the plant – some may turn brown and shrivel, but the yellow leaves are the key sign. In general, a nitrogen-deficient plant will not grow well, and often looks stunted. In the picture at top, the flowers of the squash plant look unusually large because the leaves are smaller than normal.

Excess nitrogen on pepper plant

Nitrogen excess on pepper plant (note lush, dark green color and lack of flower buds, flowers and fruits)

How will you know that a plant has an excess of nitrogen? Because excess nitrogen causes a plant to look deep green and succulent, the plant may look good to the eye. However, despite the deep color it is still relatively unhealthy. Too much nitrogen limits the plant’s reproductive growth: it will flower less and produce fewer fruits. This is bad news for plants such as tomatoes, watermelons, peppers, roses, fruit trees, and ornamental bedding plants. Some crops, like sweet potatoes and melons, will produce a poorer quality harvest.  Plants with too much nitrogen will produce leaves that are large, succulent and rich green. In fact, the entire plant tends to be very succulent. The plant may look tender and fleshy, but also somewhat weak. Plants with excess nitrogen are not winter hardy. Because of the excess liquid, such plants tend to be more susceptible to attack by insects and mites.

If you need to apply nitrogen to your plants, you’ll probably either do it with a chemical fertilizer or organically. What’s the difference? These two materials actually create different forms of nitrogen – fertilizer makes nitrate, and organic matter makes ammonium. Nitrate and ammonium have very different effects on the environment. Fertilizer nitrogen tends to leach into water, becoming a poison. Organic forms of nitrogen tend to bond with soil particles, so they don’t have the same impact.

There are several ways that plants get nitrogen naturally. Decomposing organic matter releases nitrogen, and some bacteria manufacture nitrogen that plants can use. These bacteria create the type of nitrogen that doesn’t wash away. Such helpful bacteria can be found on the roots of legumes, which is why raising a legume crop is a great way to improve your soil.

Another surprising source of nitrogen in soil is lightning strikes. In places that get a lot of lightning, the strikes fix nitrogen from the air into the soil. North America gets so much lightning that we typically “wash” 200 pounds of nitrogen out of the air per acre, per year.