Chlorophyll meter

The nutrient content of a plant can be assessed by testing a sample of tissue from that plant. These tests are important in agriculture since fertilizer application can be fine-tuned if the plants nutrient status is known. Nitrogen most commonly limits plant growth and is the most managed nutrient.

Tissue tests are almost always useful, since they provides additional information about the physiology of the crop. Tissue tests are especially useful in certain situations;

Traditional tissue tests are destructive tests where a sample is sent to a laboratory for analysis. Any laboratory test (soil or tissue test) performed by a commercial company will cost the grower a fee. Laboratory tests take at least a week to complete, usually 2 weeks. It takes time to dry the samples, send them to the lab, complete the lab-tests, and then return the results to the grower. This means the results may not be received by the grower until after the ideal time to take action. Nitrogen tissue tests that can be performed quickly in the field make tissue testing much more useful.

Another issue with laboratory tissue tests is that the results are often difficult to interpret.

Non-destructive tissue tests have advantages over traditional destructive tests. Non-destructive tissue tests can be performed easily in the field, and provide results much faster than laboratory tests.

To non-destructively assess nitrogen content, one can assess the chlorophyll content. Nitrogen content is linked to chlorophyll content because a molecule of chlorophyll contains four nitrogen atoms.

Nitrogen deficiency can be detected with a chlorophyll content meter. Many studies have used chlorophyll content meters to predict N-content of leaves, and generally a good correlation is obtained. The meters determine chlorophyll content by shining a light through a leaf inserted in a slot and measuring the amount of light transmitted.

Chlorophyll meters use different units of measure. For instance, while Minolta uses "SPAD units", Force-A uses the Dualex Unit and ADC uses a Chlorophyll Content Index. All measure essentially the same thing, and conversion tables are available.

While traditional absorption instruments have been very popular with plant scientists and have proved to work well with broad leaf species, they do have limitations. Limitations of absorption meters:

There are therefore samples which are not suitable for the absorption technique, these include small leaves, most CAM plants, conifer needles, fruit, algae on rocks, bryophytes, lichens and plant structures like stems and petioles. For these samples it is necessary to measure chlorophyll content using chlorophyll fluorescence.
In his scientific paper Gitelson (1999) states, "The ratio between chlorophyll fluorescence, at 735 nm and the wavelength range 700nm to 710 nm, F735/F700 was found to be linearly proportional to the chlorophyll content (with determination coefficient, r2, more than 0.95) and thus this ratio can be used as a precise indicator of chlorophyll content in plant leaves." The fluorescent ratio chlorophyll content meters use this technique to measure these more difficult samples.

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Wikipedia