Garden Word of the Day
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Nitrogen is the single most limiting factor in plant growth.
There is far more to tell about nitrogen than we have time or space for here, but I hope that this summary will give you a better understanding of what makes nitrogen so important in the garden, and encourage you to learn more.
What is nitrogen?
Nitrogen is an element, like hydrogen or oxygen. The Earth’s atmosphere is 78% nitrogen, but it is in a form that plants cannot use. Nitrogen is the first number you see on a bag of fertilizer. It is the “N” of NPK. Since pure nitrogen boils away at -320 degrees Fahrenheit, you won’t be buying a bag of pure nitrogen at your local garden center. [If you’ve ever had a dermatologist “freeze” off a wart or precancerous area, they are often using nitrogen.]
How plants use nitrogen
Nitrogen is a fundamental building block for chlorophyll and plant enzymes and proteins, including a plant’s DNA. Without nitrogen, photosynthesis cannot occur. Some crops use more nitrogen than others. Cucurbits, such as melons and squash, are relatively light feeders. Heavy feeders include sage, artichoke, potatoes, onions, lemongrass, and corn. If you are growing plants in containers or straw bales, plants should be monitored closely for signs of insufficient nitrogen.
Not enough nitrogen
Stunting and chlorosis are the two most common signs of insufficient nitrogen. Nitrogen is highly mobile within the soil and in plants. Nitrogen deficiencies are frequently seen as a pale area down the middle of each leaf, with older leaves affected first. This happens because the plant pulls nitrogen from older leaves to feed newer leaves. Nitrogen deficiencies in peach and nectarine tend to show as red areas on leaves (where photosynthesis is no longer occurring properly). Nitrogen deficiency and sodium toxicity are common in San Jose, California. Our heavy clay also reduces nitrogen levels in the soil.
Too much nitrogen
Too much nitrogen can be just as bad as not enough. Excessive nitrogen is seen as darker than normal leaves and more vegetative growth than fruit or flowers. Too much nitrogen can burn plants, and it can cause erratic or reduced budbreak. Too much nitrogen can also stimulate new growth that may be vulnerable to cold weather, thrips, leaf spot, Verticillium wilt, aphids, and scale. This is why the timing the use of fertilizer is so important.
Types of nitrogen
The Nitrogen Cycle refers to the conversion of atmospheric nitrogen into chemically reactive forms that attach themselves to other elements, creating ammonia or nitrate based fertilizers. Crops that prefer more acidic soil, such as blueberries and potatoes, seem to prefer ammoniacal nitrogen based fertilizers over nitrate based fertilizers. As plants absorb nitrates, they increase the soil pH, making it more alkaline. California soils are already more alkaline than many plants prefer. When plants take up ammonium, the soil becomes more acidic.
Nitrogen - a fleeting plant nutrient
Nitrogen is quickly used up by nearby plants. It also deteriorates rapidly and is leached out of soil by rain. This deterioration is largely a function of moisture and temperature. As temperatures rise, there tends to be less organic matter in soil. As moisture increases, so does organic matter. This is why it is so important in our hot, dry California weather to regularly add compost to our gardens and landscapes.
Native Americans used the Three Sisters Method of growing corn, beans, and squash together. Beans, being a legume, are host to bacteria that convert atmospheric nitrogen into forms usable by plants. Planting them all together provided the corn and squash with extra nitrogen early in their growing season. Some tribes added dead fish or eels when planting, which provided even more nitrogen. Fish emulsion is a mild source of nitrogen. According to study by the Washington State University Extension Office, coffee grounds contain 10% nitrogen after brewing. Blood meal, cottonseed meal, alfalfa meal, and feather meal are all good sources for nitrogen. Urea and urine both provide high levels of nitrogen.
Finally, if you are like many gardeners who plant marigolds to deter pests, you may want to plant them away from any legume crops. It is rumored that the same chemicals that make marigolds beneficial can also interrupt the nitrogen-fixing capabilities of legumes, such as peas and beans, though I have not yet found any scientific proof.
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