Plant tissue test
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.
Comparison to soil tests
Although soil testing is widely used, it does not always accurately estimate the nutrient status of the crop. Even if soils have the recommended level of nutrients, the crop can still be nitrogen deficient if it has a nutrient uptake problem. Only tissue tests tell growers the actual nitrogen status of the crop.
Another drawback of soil nutrient tests is that they do not take account of factors such as soil structure or biological activity, which affect the rate at which nutrients leave the soil. These factors mean that the concentration of nutrients in the soil can rapidly deviate from the results of a soil test.
Another reason why soil tests do not always guarantee the best management of nutrient treatments is that cultivars vary in their ability to scavenge soil nutrients. Some cultivars can produce their optimum yield when grown in soil with less than the recommended levels of nutrients. Directly testing the nitrogen status of the crop helps growers adjust fertilizer applications to the specific needs of that crop.
Most useful times
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;
- For monitoring the nitrogen status of the crop throughout the growing season. Soil tests are commonly performed before planting.
- In highly controlled environments, such as hydroponic production in greenhouses, crops require a constant feed of nutrients in their water supply. Even a transient lack of nutrients can reduce yields. In these controlled environments, soil testing is unlikely to be sufficient to manage crop nitrogen status. Soil testing is more suitable when growing crops in slow-release composts and manures.
- When there is a risk that the nutrient applications are toxic to the crop, such as during the application of poultry litter that contains micro nutrients such as copper.
- To guarantee that nitrogen levels in the crop do not exceed a certain limit. High concentrations of nitrates has implications to human health because nitrates can be converted into nitrites in the human digestive tract. Nitrites can react with other compounds in the gut to form nitrosamines, which appear to be carcinogenic. Crops contain high concentrations of Nitrate when excess fertilizer is used. This is an issue in crops with high levels of nitrates, such as spinach and lettuce.
Disadvantages of traditional tests
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
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.
Chlorophyll content meters
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. Researchers at the University of Nebraska concluded that a producer’s ability to manage nitrogen is enhanced by using a Chlorophyll Meter in combination with the researcher's new technique to fine-tune N-management during the growing season.
Peer-reviewed studies have concluded the ccm-200plus Chlorophyll Meter accurately measures leaf nitrogen content. For instance, researchers at the Universidad Politécnica de Madrid used a CCM-200plus to measure a Chlorophyll Content Index (CCI), and concluded that leaf chlorophyll content performed well as an indicator of nutritional status in sugar beet. The CCI was responsive to differences in nitrogen availability and was able to discriminate between treatments. The CCI parameter was concluded to be quick, easy and non-invasive.
Researchers at the University of Vermont assessed the ability of the ccm-200plus to estimate Chlorophyll Content Index (CCI) and nitrogen content. The N-content was laboratory analysed, and was found to have a significantly linear relationship with CCI (P < 0.001). The CCM-200plus was concluded to be an effective tool for estimating relative chlorophyll and nitrogen content.
Chlorophyll fluorometers are designed to measure variable fluorescence of photosystem II, or PSII. With most types of plant stress, this variable fluorescence can be used to measure the level of plant stress. The most commonly used protocols include: Fv/Fm, a dark adapted protocol, Y(II) or ΔF/Fm’ a light adapted test that is used during steady state photosynthesis, and various OJIP, dark adapted protocols that follow different schools of thought. Longer fluorescence quenching protocols can also be used for plant stress measurement, but because the time required for a measurement is extremely long, only small plant populations can probably be tested. NPQ or non-photochemical quenching is the most popular of these quenching parameters, but other parameters and other quenching protocols are also used.
Another test protocol based on fluorescence is the OJIP test   This method analyses the increase in fluorescence emitted from dark-adapted leaves when they are illuminated. The rise in fluorescence during the first second of illumination follows a curve with intermediate peaks, called the O, J, I, and P steps. In addition, the K step appears during specific types of stress, such as N-deficiency. Research has shown the K step is able to measure N-stress.
- Strasser, R. J. "Analysis of the Chlorophyll a Fluorescence Transient"