Elizabeth Guertal and Joe Kemble

A growing plant can take up and use nitrogen (N) in two forms: nitrate (NO₃) or ammonium (NH₄). Inorganic N fertilizers supply that N form from materials such as potassium nitrate (KNO₃), ammonium nitrate (NH₄NO₃), or calcium nitrate [Ca(NO₃)₂]. With organic fertilizers, such as cow manure or poultry litter, much of the nitrate and ammonium becomes available through decomposition of organic nitrogen. Regardless of the source, plants must have nitrate or ammonium forms of N available so they can grow and reproduce.

Some plants prefer the ammonium form of N, and others like to take up nitrate first. The majority appear to show no real preference, or research has not been completed that shows a preference exists. In vegetable and strawberry crop production it is a long-held belief that N fertilizers high in the ammonium form of N are less suitable, as high-ammonium N fertilizers might produce ammonium toxicity, softer fruit, and enhance other diseases, such as blossom-end rot. However, the research that leads to this conclusion appears to have been conducted under hydroponic (no soil) or pure sand environments, growing conditions that do not really match those used in the field or home garden. Thus, an AAES study was conducted to examine the effect of N fertilizer sources [ammonium nitrate, potassium nitrate, urea, urea ammonium nitrate (UAN), and calcium nitrate] on tomato yield and quality.

The two-year experiment was conducted using black-plastic-mulch-covered raised beds with drip fertigation (drip irrigation through which each N fertilizer was supplied). A total of 180 pounds of N per acre was applied with each N source, with 25% applied preplant/premulch and the remaining N applied as 10 weekly applications of 13.5 pounds N per acre. If an N source contained Ca (calcium nitrate) or K (potassium nitrate) that amount was applied to all other N sources (preplant and fertigated) as potassium chloride (KCl) or calcium chloride (CaCl₂). Collected data included plant height, leaf N concentration, and yield. Harvested tomatoes were separated into marketable and non-marketable segments based on fruit size and quality.

Different N sources had varying and inconsistent effects on yield and quality of fruit. Although plant height and stem diameter of tomatoes from UAN treatments were always smaller than those from other N sources, this effect did not lead to decreased total marketable yield. Differences in N concentration of tomato leaf tissue were not consistent with N source, and were not related to differences in tomato yield. There were few differences in yield and quality of non-marketable fruit due to N source (see table).

Total Marketable and Nonmarketable Yield of Tomato as Affected by N Source

N Source	1995		1996
	Marketable	Non-marketable	Marketable	Non-marketable
	lbs./acre	lbs./acre	lbs./acre	lbs./acre
Calcium nitrate	31,734	4,841 (13.2)*	12,761	4,324 (25.3)
Potassium nitrate	31,237	4,138 (11.7)	25,360	7,506 (22.8)
Ammonium nitrate	35,199	3,061 (8.0)	23,854	6,033 (20.2)
Urea	31,684	4,186 (11.7)	24,357	5,919 (19.6)
UAN	30,745	3,790 (11.0)	14,494	4,470 (23.6)
* Numbers in parentheses are non-marketable yield expressed as the percent of total yield (marketable + non-marketable).

In this limited study (one site, two years) yield and quality of field stake tomatoes grown on raised beds with drip irrigation were only slightly affected by the type of N source applied through the drip irrigation line. Although plants grown in UAN were shorter and had less early yield (1996 only) than plants receiving other N sources, total yield of tomato was not affected by the UAN treatment. There were few differences in yield and quality of unmarketable fruit due to N source. In this study, it appears that any of the N sources--calcium nitrate, UAN, urea, potassium nitrate, or ammonium nitrate--would make suitable N sources for tomato production when total marketable yield is a primary concern.