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.

Selection of the plant part for testing varies with the nutrient being assessed. With nitrate, it may be important that conductive tissue be selected so that the sampling represents the nutrient in transit to a site of assimilation and before metabolic conversions occur. However, potassium is not assimilated into organic combinations in plants; hence, selection of a plant part is of lesser importance than with determination of nitrate, and leaf petioles, midribs, blades, or other tissues can be used for potassium determination by quick tests or by laboratory measurements (50,51). Color of leaves can be used as a visual assessment of the nutrient status of plants. This assessment can also be quantitative in a quick test, and chlorophyll-measuring meters have been used to nondestructively evaluate the nitrogen status of plants (52). The meters have to be used in reference to predetermined readings for plants receiving adequate nutrition and at selected stages of development, which are usually before flowering and maturation. Correlations of readings with needs for nitrogen fertilization may not be good as the plant matures and flowers and as materials are transported from leaves to fruits.

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).