Plant tissue testing is a technique for rapid determination of the nutritional status of a crop and is often conducted on the field sites where crops are grown. The test generally assesses the nutrient status by direct measurements of the unassimilated fraction of the nutrient in question in the plant. For example, determination of nitrate in leaf petioles, midribs, or blades or in roots is often a chosen tissue test for assessment of the nitrogen status of a plant (37-40). Nitrate in these plant parts represents an unassimilated form of nitrogen that is in transit to the leaves and often shows greater variations in response to soil nutrient relations than determinations of total nitrogen in plant parts, although some research indicates that total nitrogen concentration in the whole plant gives the best index of plant nitrogen nutrition (41). Generally, in a tissue test, the sap of the tissues is extracted by processes such as crushing or grinding along with filtering to collect liquid for testing (34). Testing of a component, such as nitrate in the sap, is often done by semiquantitative determinations with nitrate-sensitive test strips (37,40,42,43), by hand-held nitrate-testing meters (44), or by quantitative laboratory measurements (45). In tissue testing, ammonium determinations are used less often than nitrate determinations because accumulation of ammonium can be an artifact of sampling and analysis (46).
An exception to the direct determination of an element to assess deficiency was the corn (Zea mays L.) stalk test of Hoffer (47). This test was based on the observation that insoluble iron compounds appeared at the nodes of corn plants under stress of potassium deficiency (48). The corn stalk test provided only a rough indication of the potassium nutrition of the plant but had a fair agreement with other tests for potassium deficiency and had some application to crops other than corn (34). Similarly, Leeper (49) noted that manganese-deficient oats (Avena sativa L.) accumulated nitrate in stems.
Leaf canopy reflectance (near-infrared or red), as employed in remote sensing techniques, can be used to assess the nutrient status of fields. Reflectance has been shown to be related to chlorophyll concentrations and to indicate the nitrogen status of crops in a field (53).
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