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Plant Tissue Testing

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Soil testing can provide an estimate of plant nutrient availability in a soil. However, soil testing cannot predict the quantity of nutrients a plant or crop will actually use because many factors other than soil fertility levels are involved in plant nutrition. Only through plant tissue analysis can we assess the plantɫ��Ƶ���ؙs nutritional status and determine how well the soil is supplying the plantɫ��Ƶ���ؙs nutritional requirements. Plant tissue analysis cannot replace a good soil testing program; however, plant tissue analysis can provide additional information on plant nutrient status not obtained from soil analysis.

In theory, plant tissue testing is quite simple. Plant samples from a field are collected and the nutrient levels determined after the plant tissue has been digested or extracted in a solution. Generally, only those plant portions growing above ground are sampled, although underground parts are sometimes sampled. Frequently, only specific plant parts (leaves or petioles, for example) are sampled. After nutrient levels are measured, the plantɫ��Ƶ���ؙs nutritional status can be determined by comparing the measured levels with standard levels that have been previously determined through field research. Alternatively, when a field contains both healthy and unhealthy plants, samples can be taken from both and a comparison of nutritional levels can be made. Nutritional problems frequently can be identified by this process.

In reality, there are a number of factors that make plant tissue testing far more complicated than suggested. Plant nutrient concentrations are affected by plant age, plant part and sometimes by variety even in a healthy plant. These influences must be taken into consideration.

As a plant ages, the proportions of the various types of structures change. Young plants are very succulent, with a high proportion of water in the tissues. When the plant gets older, water content decreases, the proportion of cell walls increases and the plant may become woody. The different plant structures vary in plant nutrient content. Concentrations of some nutrients (N, P, K, Cu, Zn) tend to decrease as plants age, while the concentrations of others (Ca and Mn) often increase. Unfortunately, the rates in which these tissues change are difficult to quantify. Therefore, it is extremely important to know the plantɫ��Ƶ���ؙs growth stage (or age) when sampled if the composition is to be compared to ɫ��Ƶ���؜standardizedɫ��Ƶ���؝ levels. There are commonly recommended growth stages for sampling and standard nutrient stages. Samples taken from plants at different growth stages are not easily evaluated.

Just as plants of various ages differ in nutrient content, different plant parts may contain varying levels of plant nutrients. For example, the wood and the leaves of a tree contain very different nutrient levels. Similarly, stems, leaves, roots and fruits of nonɫ��Ƶ���ؓwoody plants may have distinct nutrient concentrations. Therefore, it is critical when taking a plant tissue sample that the plant structure collected is the one for which standard values are known. In small plants, the whole aboveɫ��Ƶ���ؓground portion of the plant is usually sampled. In older plants, the most common sampling method is to collect the youngest fully expanded (grown to its full size) leaf, or to take the petioles (leaf stems) associated with those leaves. For plants requiring leaf sampling, the petiole is usually not included. Petioles are often used for sampling waterɫ��Ƶ���ؓsoluble (nitrate and phosphate) rather than total nutrients because the petiole is the conducting tissue where nutrients travel from the stem to the leaf. The recommended plant part for sampling should be determined for each specific plant (see Table 1.)

If a field contains both healthy and unhealthy plants, these sampling guidelines are less critical. One can remove a sample from both healthy and unhealthy plants, making sure that the same plant part is taken in both. The healthy plant can be used as the standard value to compare against the unhealthy plant. This type of comparison may be less ideal than it appears because the physiological age of the two plant groups differ. It is not uncommon for an unhealthy plant to mature at a different rate than a healthy one. For example, an unhealthy plant may bloom much earlier than its healthy counterpart. Therefore, although two plants may have been planted at the same time in the same field, their physiological age, or stage of development, may not be the same. This can make direct comparison difficult. It is helpful if soil samples are collected from healthy and unhealthy areas when tissue samples are collected.

Plant tissue samples should be taken from plants representative of the sampling area. Dead or damaged plants, those with insect or disease problems, those at the end of rows or in edge rows, or plants that differ significantly from those in the rest of the planting should not be sampled. Plants that have been recently sprayed with foliar fertilizers should be avoided. It is important that at least the recommended number of plants is sampled to ensure that a representative sample is obtained. If the recommended sample size is 25 mature leaves, all leaves should be taken from separate plants. In addition, the sampled plants should be randomly selected from a field, not concentrated in one area.

Try to sample clean leaves. Plants analyzed for iron or aluminum should first be washed quickly in a mild (2 percent) detergent solution. Fresh tissue samples must be dried rapidly at 150° to 175°F until all water is removed (a kitchen oven on the warm setting will suffice). Drying at higher temperatures may destroy plant tissues; drying at lower temperatures will not stop biological activity. Tissue samples will dry best in open containers, cloth bags or opened paper bags. Samples should be dried immediately following sampling. If this is not possible, samples may be refrigerated for short periods of time prior to drying.

Tissue samples are ground to powder in the laboratory, then put into a liquid form for analysis. One of several methods may be used. Often plant tissues are digested in acid solutions. The tissue may be directly digested in boiling acid or it may be ashed in a furnace prior to acid digestion. Sometimes soluble nutrients are extracted from plant tissue in water or in salt or dilute acid solutions. Selection of appropriate methodology depends on the specific analysis to be conducted.

Results from analyses are most frequently compared directly to previously determined standard values. Standards are established for nutrient concentration ranges adequate for healthy plant growth; these are called sufficiency ranges. By comparing the results of a plant tissue analysis with these standards, the nutritional status of the test crop can be established. Sufficient nutrient concentration ranges for most crops grown in ɫ��Ƶ���� are presented in Table 2.

In some cases, the levels of soluble nutrients in petiole tissues provide more sensitive parameters for nutritional diagnoses than leaf analyses. Diagnostic values for petiole nutrient levels are given in Table 3.

Table 1. Recommended plant part and growth stage for selected crop plants.

¹ Whole top is the entire above ground portion of the plant.

² Most recently mature leaf is the youngest fully developed leaf.

Table 2. Nutrient sufficiency ranges for selected crop plants.1

¹Standard values are for plant parts and growth stages specified in Table 1.

Table 3. Sufficiency levels for nitrate, phosphate, and potassium in petioles and leaf midribs of selected crop plants.

¹ YML ɫ��Ƶ���ؓ youngest mature (fully expanded) leaf.

Nutritional diagnoses can give important information about the condition of a crop; however in the case of an annual crop, it may be too late to effectively remedy nutritional problems. Nevertheless, even when irreparable damage has been done, diagnostic nutritional information can be extremely valuable. If tissue analyses reveal shortages of nutrients routinely applied in a fertilization program (nitrogen, phosphorus or potassium), this may be an indication that the fertilization regime being used is inadequate for that crop. The next time the crop is grown at that location, fertilizer application rates should be adjusted. If tissue analyses reveal shortages of secondary or micronutrients, soil test information should be consulted and consideration should be given to various means of correcting the problem before the field is planted again. When dealing with perennial crops, adjusting fertilization practices can be made at almost any time. Action taken late in the season may not improve that seasonɫ��Ƶ���ؙs yield, but performance in subsequent years should be enhanced.

Information from plant tissue tests cannot replace that from soil tests; the two practices provide complementary data. By combining information from the two sources, one gets a clearer picture of the ability of a soil to provide adequate nutrition and of the crop to use nutrients. Both should be considered integral parts of a complete nutrient monitoring program.

References

The information contained in Tables 1ɫ��Ƶ���ؓ3 was derived from the following publications:

Dow, A.I. 1980. Critical nutrient ranges in Northwest crops. Western Regional Extension Publication No. 43.

Evanylo, G.K. and G.W. Zehnder. 1988. Potato growth and nutrient diagnosis as affected by systemic pesticide growth stage. Communications in Soil Science and Plant Analysis 19:1731ɫ��Ƶ���ؓ1745.

Gardner, B.R. and J.P. Jones. 1975. Petiole analysis and the nitrogen fertilization of Russet Burbank potatoes. American Potato Journal 52:195ɫ��Ƶ���ؓ200.

Geraldson, C.M. and K.B. Tyler. 1990. Plant analysis as an aid to fertilizing vegetables. In Soil Testing and Plant Analysis, ed. R.L. Westerman. Madison, WI: Soil Science Society of America.

Jones, J.B. Jr., B. Wolf, and H.A. Mills. 1991. Plant Analysis Handbook. Athens, GA: Microɫ��Ƶ���ؓMacro Publishing, Inc.

Kelling, K.A. and J.E. Matocha. 1990. Plant analysis as an aid to fertilizing forage crops. In Soil Testing and Plant Analysis, ed. R.L. Westerman. Madison, WI: Soil Science Society of America.

Kleinkopf, G.E. and D.T. Westermann. 1982. Scheduling nitrogen applications for Russet Burbank potatoes. University of Idaho Current Information Series No. 367.

MacKay, D.C., J.M. Carefoot, and T. Entz. 1987. Evaluation of the DRIS procedure for assessing the nutritional status of potato (Solanum tuberosum L.). Communications in Soil Science and Plant Analysis 18:1331ɫ��Ƶ���ؓ1353.

Redshaw, E.S. 1990. Plant tissue testing. Agriɫ��Ƶ���ؓfax, Alberta Agriculture. Agdex 100/08ɫ��Ƶ���ؓ1.

Sanchez, C.A., H.W. Burdine, and V.L. Guzman. 1989. Soil testing and plant analysis as guides for the fertilization of celery on histosols. Soil and Crop Science Society of Florida Proc. 49:69ɫ��Ƶ���ؓ72.

Sanchez, C.A., G.H. Synder, and H.W. Burdine. 1991. DRIS evaluation of the nutritional status of crisphead lettuce. HortScience 23:274ɫ��Ƶ���ؓ276.

Walworth, J.L., R.G. Gavlak, and J.E. Muniz. 1990. Effects of potassium source and secondary nutrients on potato yield and quality in Southcentral ɫ��Ƶ����. University of ɫ��Ƶ���� Fairbanks, Agricultural and Forestry Experiment Station, Research Progress Report No. 18.

Westfall, D.G., D.A. Whitney, and D.M. Brandon. 1990. Plant analysis as an aid in fertilizing small grains. In Soil Testing and Plant Analysis, ed. R.L. Westerman. Madison, WI: Soil Science Society of America.

Williams, C.M.J. and N.A. Maier. 1990. Determination of the nitrogen status of irrigated potato crops. I. Critical nutrient ranges for nitrateɫ��Ƶ���ؓnitrogen in petioles. Journal of Plant Nutrition 13:971ɫ��Ƶ���ؓ984.

Steven Seefeldt, Extension Faculty, Agriculture and Horticulture. This publication was originally prepared by James L. Walworth, former Soil Scientist, University of ɫ��Ƶ���� Agriculture and Forestry Experiment Station, Palmer.

Reviewed June 2015