Plant analysis, also called leaf analysis, is a technique for determining the elemental contents of tissues of particular plant parts. It plays a major role in diagnosing mineral nutrition problems in the field and involves a series of steps (Figure 12.1):

• Sampling • Sample preparation • Laboratory analysis • Interpretation

Taking as samples the mature leaves exposed to full sunlight just below the growing tips on main branches or stems just before or when a plant begins its reproductive stage of growth is the preferred technique. In some cases, sampling may be done earlier or during the growth cycle of the plant if the intent is to collect leaf tissue of the same maturity. The important components for proper tissue collection are:

• Plant parts taken from a specific location on each plant • Stage of plant growth or specific time of sampling

If the prescribed sampling directions are followed, the sampler should achieve reasonable statistical reliability. Sampling instructions are very specific in terms of plant part and stage of growth, since a comparison of assay results with established critical or standard values or sufficiency ranges is based on clearly identified plant parts taken at specified times. When specific sampling instructions are unknown, the general rule is to select upper leaves. Avoidance criteria are also crucial; plants to be avoided:

• Have suffered long-term climatic or nutritional stress • Have been damaged mechanically or by insects • Are infested with disease • Are covered with dust, soil, or foliar-applied sprays unless the extraneous substances

can be removed effectively • Are border row plants or shaded leaves within the plant canopy • Contain dead tissue

Sampling two different populations of plants for comparative purposes, a highly desirable diagnostic procedure, poses a difficult problem, particularly when the type of stress produces substantial differences in plant growth. For example, when two or more sets of plants exhibit varying signs of a possible nutrient element insufficiency, collecting tissue for comparative purposes may be difficult because of the effect of the nutrient element stress on plant growth and development. Therefore, it is important whenever possible, to obtain plant tissue samples when the symptoms of stress first appear rather than waiting until substantial differences in plant charactistics are noted.