Modern application of plant analysis has evolved from years of research and experience with individual crops. In most cases, research was not conducted for the sole purpose of identifying critical limits or sufficiency ranges. These values were extrapolated from research in which the primary purpose was to develop response curves for specific fertilizer application and soil test calibration. Equally important in developing this tool has been experience gained in interpreting plant results and observing response to fertilizer treatments. Extensive use of plant analysis in solving problems and managing healthy crops fosters confidence in this important management tool.
Plant analysis is the chemical evaluation of essential element concentrations in plant tissue. Essential elements include those that are required to complete the life cycle of a plant. The elements carbon (C), oxygen (O), and hydrogen (H) are supplied by the atmosphere and water and generally are not considered limiting. Agronomists place most emphases on essential elements supplied by soil or feeding solutions. Macronutrients — nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) — are required in greatest quantities. Micronutrients — iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo) and chlorine (Cl) — are required in very small quantities. Toxicities of micronutrients are equally important and yield limiting as deficiencies. Plant analysis is also effective in diagnosing toxicities of micronutrients. Cobalt (Co) is also essential for symbiotic N2-fixing bacteria associated with legumes.
The interpretation of plant analysis results is based on the scientific principle that healthy plants contain predictable concentrations of essential elements. A number of researchers have offered schematics showing the relationship between maximum yield and concentrations of essential elements (Ulrich and Hills 1967; Brow 1970; Dow and Roberts 1982). Chapman (1967) added interpretation ranges to these relationships (Fig. 1). Schematics of crop response and nutrient concentrations are based on general scientific principles and do not account for differences due to plant part sampled, tissue age, stage of growth, variety, and other factors.
Campbell has further expanded this relationship to include excess and toxic levels of nutrients along with an interpretation index (Fig. 2). The additional ranges allow agronomists and farmers to address excess and toxic levels of elements that may not only influence growth but increase risks to the environment. The interpretation index allows practitioners of plant analysis to become interpreters without extensive training and knowledge of sufficiency ranges for individual elements and crops.
Best indicator samples have been identified for most economically important crops. For crops receiving greater research support, indicator samples have been identified by stage of growth.
Plant analysis is generally associated with evaluation of leaf samples. In recent years, diagnostic tests and criteria have been developed for petioles of indicator leaf samples. These tests have generally served to fine tune the prediction of nitrogen status. Potassium and phosphorus have also been evaluated in petioles of important crops including cotton, grape, and strawberry. Nitrate nitrogen or petiole nitrate levels, as they are commonly referred to, indicate the current status of nitrogen by placing emphases on the mobile form of the element rather than the total that has been assimilated in the plant.
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| Figure 1. Schematic of yield and nutrient concentration (Chapman 1967). |
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| Figure 2. Schematic of yield or growth in response to increasing nutrient concentration and interpretation. |
There are three major methods of interpreting plant analysis results. They include the use of critical values, sufficiency ranges, and ratios. Most advisory services use sufficiency ranges for primary interpretation. Ratios and DRIS analysis are generally used as secondary and supportive evaluations.
- Critical Values
Critical values have been defined as the concentration at which there is a 5–10% yield reduction. The use of critical values for practical interpretation has limited value. It is best suited to diagnose severe deficiencies and has little application in identifying hidden hunger. Symptoms are generally evident when nutrient concentrations decrease below the critical value. Critical values play an important role in establishing lower limits of sufficiency ranges.
- Sufficiency Ranges
Sufficiency range interpretation offers significant advantages over the use of critical values. First, hidden hunger in the transitional zone can be identified since the beginning of the sufficiency range is clearly above the critical value. Sufficiency ranges also have upper limits, which provide some indication of the concentration at which the element may be in excess.
- Ratios
In simplest form, the use of ratios in the interpretation of plant analysis results involves the evaluation of two essential elements together recognizing the effect of one element on the other. The most commonly used ratio is N:S (Nitrogen to Sulfur). The ideal N:S ratio for most crops is 10–15. As the N:S ratio approaches and exceeds 18, sulfur is limiting in relation to nitrogen. In reality, the plant does not assimilate nitrogen well because sulfur is limiting. The N:S ratio can be high when both nitrogen and sulfur concentrations are within the sufficiency ranges for these elements.
Other ratios commonly used to support sufficiency range interpretation include N:K and Fe:Mn. Interpretative data bases for these ratios are available for a limited number of crops. In general, the N:K ratio should be 1.2–2.2. The Fe:Mn ratio should be > 1.
The most complex application of ratios in the interpretation of plant analysis results is DRIS (Diagnostic Recommendation Integrated System). This technique, which was developed by Sumner and others (Beaufils 1973; Walworth and Sumner 1987), places emphasis on the relationship among essential elements rather than absolute concentrations. In short, DRIS ranks the essential elements in their order of limitedness. Theoretically, if the most limiting element is applied then the second element becomes most limiting. DRIS evaluation compares ratios of essential elements in the unknown sample to ratios of these elements in high yielding populations. Modification of DRIS interpretation in recent years to account for the magnitude of limitedness has significantly improved this diagnostic tool. Previously, elements were listed in a descending order of limitedness even when the most limiting element was not a significant problem. Normal ratios of high yielding populations are available for a number of economically important crops.
Careful sampling ensures the effectiveness of plant analysis as a diagnostic tool. For major crops, best indicator samples have been identified by stage of growth. For young seedlings, the entire plant is sampled 2.5 cm above the soil level. For larger plants, the most recent fully expanded or mature leaf is the best indicator of nutritional status. As some crops, including corn, approach flowering and fruiting, the best indicator of nutritional status is the leaf adjacent to the uppermost fruit (earleaf). When unfamiliar with sampling protocol for a specific crop, it is generally acceptable to select the most recent mature leaf as the best indicator of nutritional status.
A very small amount of plant material is required for a laboratory test (< 1 gram), but reliable samples must include enough leaves to adequately represent the affected area. For crops with small leaves (azalea), 25–30 leaves are required for a good sample. Larger leaved crops, including corn or tobacco, require significantly fewer leaves for an adequate sample.
- Problem Solving
Diagnostic samples should be taken at the first indication there may be a problem. Generally, the earlier in the life cycle of the plant, the more reliable the sample. Samples taken prior to or at flowering are significantly more reliable than those taken in various stages of maturity. Comparative samples from good and bad plants allow a high degree of accuracy in identifying the most limiting element. Matching soil samples taken from the root zones of plants in each of the sample areas provide more complete information for problem solving.
When symptoms on plants are zonal and the most recent mature leaf appears normal, it is helpful to sample leaves showing symptoms in addition to the most recent mature leaf. Knowledge of the accumulation of elements in certain plant parts also helps in selecting additional samples that should be taken when problem solving. For example, bud samples provide additional confirmation of boron deficiency. Likewise, older plant leaves are important in diagnosing boron toxicity.
- Monitoring
The evaluation of healthy crops in fine tuning nutrient application requires consistent sampling. Ideally, monitoring samples should be taken the same time of day and from the same area in the field each sampling date. If there is wide variability in the field, it is desirable to take the sample from a relatively small area. Results can then be evaluated for that specific area. All other areas in the field can be compared to the standard sampling area.
Monitoring samples for intensively managed crops, including vegetables in greenhouses or fields, should be taken no less than every two weeks. Hydroponic crops should be sampled weekly. Less intensive field crops, including corn, should be sampled just prior to sidedressing and at flowering. Additional samples are taken as the need arises.
- Petiole Sampling
Petioles for nitrate nitrogen determination should be removed from the most recent mature leaf or trifoliate. Ideally, petioles should be removed at sampling to avoid further transport of nitrates. Values generally are lower when petioles are removed at the laboratory. Petiole nitrate monitoring requires sampling no less than every two weeks during critical development periods, including flowering and fruit development.
- Signs of Problems in Sampling
Chronic deficiencies or excesses of certain nutrients may indicate a sampling problem. Since calcium accumulates in lower leaves as cell walls develop, consistently low levels of this element when there are no symptoms may indicate the sample is being taken too near the growing point. Likewise, consistently high calcium and low potassium may indicate the sample is being taken too far down from the growing point. Comparative sampling of upper and lower leaves is helpful in identifying the best indicator sample.
- Sampling Containers and Laboratory Transport
Samples should always be shipped to laboratories in a paper container. Plastic containers that promote respiration and decomposition by disease organisms should never be used. Most laboratories provide a proper sample container. Samples should be packed loosely so that drying can begin in transport. Samples can be dried in ovens at 80° C before shipping to save shipping expense but valuable response time is lost.
- Environmental Conditions
Caution should be exercised when sampling crops damaged by disease, insects, drought and other factors. Comparative samples of good and bad plants help to neutralize the effects of some environmental factors. Environmental conditions should always be noted on the sample information form. Many times plant samples help to eliminate nutrition as a causal agent when other factors like disease or insect damage are suspected.
There are a number of important applications of plant analysis in research and production agriculture. Plant analysis is very effective in documenting response to nutrient applications. Leaf concentrations have, therefore, been correlated with yield and soil test values in calibration work. This data base provides the basis for problem solving and monitoring. Crop requirements have been well established using plant analysis. Nutrient uptake patterns, accumulation, and partitioning have been defined for many crops. Fertilizer efficiency, depending on placement and form, have also been effectively studied. Although plant analysis was first used in production agriculture to diagnose potential deficiencies, it now has developed into an important management tool in monitoring the nutritional status of healthy crops.
- Problem Solving
Comparative samples from good and bad areas of production fields are very effective in pinpointing the limiting element(s). Matching soil samples from the root zones of plants in each of these areas provide additional evidence of the problem and help determine the best corrective action. Comparative plant and soil samples from areas responding differently also help to isolate or neutralize the overriding influence of confounding factors including moisture, insects, disease, and other sources of injury.
- Monitoring
In recent years, plant analysis has become an integral part of managing healthy crops to enhance yield and quality while also maximizing efficiency and protecting the environment. As pressure has mounted to dispose of waste products on farm land, plant samples have provided a means for monitoring these sites to ensure maximum crop performance while avoiding excess application. Intensively managed vegetable crops with trickle irrigation and feeding require weekly sampling to guide nutrient management. With interest in precision agriculture and prescription fertilizer application, monitoring will become even more important in the future.
References
Beaufils ER. 1973. Diagnosis and recommendation integrated system (DRIS). Natal (South Africa): University of Natal. Soil Science Bulletin No. 1.
Brown JR. 1970. Plant analysis. St. Louis (MO): Missouri Agric Exp Stn. Bulletin SB881.
Dow AI, Roberts S. 1982. Proposal: critical nutrient ranges for crop diagnosis. Agron J 74:401–3.
Russel JS, Bourg CW, Rhoades HF. 1954. Effect of nitrogen fertilizer on the nitrogen, phosphorus and cation contents of bromegrass. Soil Sci Soc Am Proc 18:292–6.
Ulrich A, Hills FJ. 1967. Principles and practices of plant analysis. In: Soil testing and plant analysis. Part II. Madison (WI): Soil Science Society of America. (Special publication series; 2).
Walworth JL, Sumner ME. 1987. The diagnosis and recommendation integrated system (DRIS). In: Stewart BA, editor. Advances in soil science. Volume 6. New York (NY): Springer-Verlag. p 149–88.
Electronic Document Prepared by:
Catherine Stokes, Communication Specialist
Agronomic Division of the N.C. Department of Agriculture and Consumer Services. July 2000.