Abstract

Fingerprinting crops to detect organic or inorganic fertilizer use can be done by determining nitrogen (N) stable isotope values. In previous research, crops grown with organic N had higher amounts of 15N while those grown with inorganic N were higher in 14N. This information may be useful to follow plant demands and N requirements of heavy N feeding crops like tomato (Solanum lycopersicum) and lead to more efficient N inputs. A greenhouse experiment was conducted with ‘Better Bush’ tomatoes using four soil fertility treatments consisting of a) inorganic fertilization [Miracle Grow®; 24N-4P-13K], b) organic fertilization [bonemeal (6N-8P-0K), bloodmeal (12N-0P-0K), liquid Earth juice (2N-1P-1K) and 25% vermicompost]; c) mixed fertilization [Miracle Grow® with 25% vermicompost] and d) control [no fertilization].

Plant vigor assessment ratings clearly showed significant differences with the applied N source (i.e., none, organic and inorganic). Nitrogen isotopes of tomato leaflets, fruit peels and juice differed among treatments. The 15N was highest in plants from organic fertilization with vermicompost treatment, lower in those from the mix of inorganic and vermicompost, and absent in tissues from plants grown with inorganic N fertilizer. Nitrogen isotopes can be easily determined from dry plant material for a low monetary cost, offer a means of screening for nitrogen fertilization source, and may assist with the organic certification process. For tomato, dried mature leaflets or fruit peels are recommended for nitrogen stable isotope analysis. Nitrogen isotopes may slightly vary dependent upon tomato varieties grown or with those that have been grafted, and further research needs to compare N isotopic values from different tomato varieties and/or grafting treatments to better highlight N isotopic patterns.

Keywords: Organic agriculture; Organic certification; Tomato; Vermicompost