Why Nitrate is the Most Efficient N Source
1. Most crops grown on the Californian Central Coast prefer nitrate nutrition
Most plants are able to use both nitrate and ammonium. Some plants have very specific nitrogen requirements (such as blueberries adapted to very acid soils). However, many of the common crops grown in this region belong to the “Solanaceae” (including tomatoes, peppers and potatoes) “Rosaceae” (including raspberries, apples, peaches, almonds) plant families and they have a strong preference for nitrate nutrition. Other crops, such as lettuce, also show a preference for nitrate, and even have a greater “crunch” when fertilized with nitrate, compared with ammonium (1).
2. Farmers have a choice of nitrogen fertilizers containing various mixtures of nitrate, ammonium, and urea. These three sources have their place in crop production, but they will all be converted to nitrate in the soil.
(a) Nitrate is immediately available for crop uptake and can move with the soil water. With proper irrigation management, nitrate remains in the root zone and makes an excellent source of immediate plant nutrition.
(b) Ammonium can be taken up by plants if it is close to a growing root, but has limited movement in the soil. Ammonium will be converted to nitrate (nitric acid) by soil bacteria. Nitrate fertilization avoids this soil acidification.
The conversion to nitrate is largely controlled by the soil temperature. In Monterey County, average soil temperatures at the 4-inch depth range from 49o in January to 63o in September.
This means that nitrification of ammonium fertilizer may take between 4 weeks (summer) to 12 weeks (winter) for the process to be complete. During this extended period, it is uncertain what the N availability will be to the crop and how the newly formed nitrate might behave during irrigation events.
We are aware now that during the conversion of ammonium to nitrate, a small, but very important, amount of nitrogen is released to the atmosphere as nitrous oxide (N2O), a potent greenhouse gas.
Effect of soil temperature on nitrate formation (2)
Ten-year average soil temperature at the 4-inch depth in Monterey County, CA during the year
(c) Urea requires two steps before it is converted to nitrate. The first step (hydrolysis) is generally quick and is complete within a few days. During this time, urea is mobile in the soil and moves freely with irrigation water. The ammonium that results from urea hydrolysis will slowly be nitrified over an unknown period, the same as if ammonium had been initially added.
Use caution to avoid leaching urea to lower parts of the rootzone and then having the later-produced nitrate continue to move below the roots. Some plants are capable of directly using small amounts of urea for their nutritional needs.
The choice of a nitrogen fertilizer source should consider what is best for each situation. Many crops perform best with a nitrate source of nutrition. All sources of added nitrogen eventually transform to nitrate in Central Coast soils and can be leached below the rootzone. However, it is more difficult to predict and manage the nitrogen that comes from mixed and organic sources, since their release, movement, and uptake are dependent on several unpredictable environmental factors.
Regardless of what nitrogen source you select for your crop, it requires careful attention to management. All nitrogen fertilizers are susceptible to loss if good irrigation practices are not followed. Nitrate-based fertilizers provide many advantages over other nitrogen sources, but they all must be used properly. Don’t let the word “nitrate” scare you from doing what is best for your crop and for farm stewardship.
(1) https://doi.org/10.1081/PLN-100103667
(2) Adapted from Frederick, L.R. and F.E. Broadbent. 1966. Biological interactions. p. 198-212. In M.H. McVicker et al. (ed) Agricultural anhydrous ammonia technology and use. ASA, Madison, WI.
(3) https://www.istockphoto.com/photos/soil-profile (source of the tree illustration)
Contact your local Yara representative to discuss nitrogen sources for your crop.
