By Steven A. Tjosvold
Laboratory analysis of soil and plant tissue remains the standard method of determining soil and crop nitrogen status. Although accurate, these techniques can be costly and usually have a time lag between sampling and getting useable results to the grower. On-site soil and plant tissue quick tests for nitrogen analysis have been developed in the last several years for use with vegetable crops in California.

The tissue quick test involves the measurement of the nitrate content of fresh petiole (leaf stalk) sap by a portable, commercially available nitrate-selective electrode (Cardy meter). A tissue petiole test takes a few minutes and costs about 26 cents per sample. (See Tissue Quick Test Guidelines.)

The soil quick test involves the extraction of nitrate through a simple volumetric dilution method, with nitrate concentration measured by colorometric test papers (EM Quant nitrate test strip), similar to measuring acidity with pH paper. A soil quick test takes a few minutes and costs as low as 39 cents per test. (See Field Soil Quick Test Guidelines.)

Evaluation for cuts

The suitability of the quick test for monitoring soil and tissue nitrogen status was evaluated in field and greenhouse cut flowers. Results indicate that the test can be used effectively for many California cut flower crops and soil conditions. Examples are given in Figures 1 and 2 below that compare conventional laboratory tests and quick tests when used to monitor nitrogen status in greenhouse-grown gerbera.

Figure 1 shows a close relationship for measured nitrate-nitrogen soil analysis using conventional lab and quick test techniques. Soil tests are used to determine nitrogen levels that are readily available for uptake by a plant's roots. In commercial vegetable field production, nitrate nitrogen levels of 10 parts per million or below are insufficient and levels above 20 ppm are sufficient to meet immediate crop needs. Many field and greenhouse cut flower crops would be expected to have similar sufficiency levels.

Figure 2 shows the relationship between the nitrate-nitrogen in the petiole sap using conventional laboratory and Cardy meter quick test analytical techniques. Note that the nitrate concentration of the lab tissue is approximately three to four times that of the Cardy meter measured concentration.

The lab measurement is based on the dry weight of the plant tissue and the Cardy meter measurement is based on the liquid tissue sap. Therefore the lab-analyzed tissue has more concentrated nitrate nitrogen.

A grower using a Cardy meter would be interested primarily in the rise and fall of the nitrate concentration of tissue sap to help determine sufficient nitrate levels. A large decrease in sap nitrate concentration can indicate, before visual symptoms appear, that nitrogen deficiency will soon occur.

Sufficient nitrate levels using tissue sap techniques have not been determined for most ornamental crops. Practical nitrate levels could be determined for many flower crops by correlating nitrogen concentration data with observations of crop production and quality.

On-site nitrogen levels

In a survey of ornamental nurseries, we found that the soil nitrogen levels in greenhouse-grown crops were consistently very high. Nitrogen levels in field flower crops were quite variable. Most notably, some field crops were even found with no readily available soil nitrogen.

Flower crops may benefit greatly from nitrogen monitoring, particularly to eliminate nitrogen deficiencies that could adversely affect the crops. It is expected that integrating soil and tissue test results would be most beneficial to determine the nitrogen needs of the crops.

One potential drawback of the quick tests relates to their specificity for detecting just nitrate nitrogen. If the primary nitrogen form in the tissue sap is not in the nitrate form then a large portion of the potentially available nitrogen won't be detected. However, most cut flowers tested so far have high concentrations of nitrate nitrogen in their tissue sap. Roses are a notable exception of a plant that has little of its total nitrogen transported in the sap in the nitrate form.

If a grower applies primarily ammonium fertilizer, at least initially, there may be some undetected ammonium nitrogen being used by the plants. However, in warm soils the ammonium nitrogen is, within a few weeks, converted to the nitrate form and would be adequately measured by a nitrate-specific test. Nitrate nitrogen is usually the most abundant form of nitrogen found in the soil and the most readily available for uptake by plants.

The quick test can be used to help fine-tune a fertilization program through the regular analysis of nitrogen. It should be recognized that significant reduction of applied nitrogen may not be safely achieved unless irrigation water is applied uniformly. To ensure that nitrogen fertilizer is used effectively, there should be a strong emphasis on the need to improve irrigation system performance.

The author thanks California Cut Flower Commission and Kee Kitayama Research Foundation for supporting research evaluating quick nitrogen tests.

© 2001 Branch-Smith Publishing