Uses of Plant Analysis
Modern plant analysis is used primarily as a source of information on plant nutrient status and, ultimately, as a tool to aid in nutrient management decisions. For nutrient management of crops, analytical data are used in various tests designed to:
Diagnose existing nutrient problems
Predict nutrient problems likely to affect crop production between sampling and harvest
Monitor crop nutrient status for optimal crop production
There are also other, less common applications, such as crop-quality measurements, regional nutrient status evaluations, assessment of crops for animal and human nutrition, and environmental protection.
A plant nutrient, or essential element, is not considered essential unless:
(a) a deficiency of it makes it impossible for the plant to complete the vegetative or reproductive stage of its life cycle;
(b) such deficiency is specific to the element in question, and can be prevented or corrected only by supplying this element; and
(c) the element is directly involved in the nutrition of the plant quite apart from its possible effects in correcting some unfavorable microbiological or chemical condition of the soil or other culture medium.
(Arnon and Stout, 1939)
Diagnosis of Nutrient Status
Most designs for using plant analysis to assess nutrient status are based on the relationship between nutrient concentration and yield or growth of a plant or plant part. There are different ways to express concentration, but the most common are percent (%) and mg/kg (or part per million, or ppm). Percent is commonly used for the major nutrients – N, P, K, S, Mg and Ca – while ppm is used for the micronutrients.
There may be several fundamental relationships between yield and nutrient concentration in plant tissue, but the most common is shown in Figure 1. The curve comprises three basic segments: an ascension, where yield increases as nutrient concentration increases; a plateau, where yield is constant with increasing nutrient concentration; and a descent, where yield decreases with increasing nutrient concentration. These three segments generally represent zones of deficiency, adequacy (sufficiency) and toxicity, respectively.
Critical level or concentration is a term that is common in both soil and plant analysis. It is usually defined in plant analysis as the level that results in 90% of maximum yield or growth, which is also a reasonable division of the zones of adequacy and deficiency in the figure below. The critical level for toxicity may be similarly defined in the division of the plateau and descent (toxicity) in the same figure.
Relationship between nutrient concentration in plant tissue and yield or growth (Adapted from Marschner, 1995)
Concentration or ranges of the major elements and micronutrients in mature leaf tissue generalized as deficient, sufficient or excessive for various plant species (Munson, 1998)
Prediction of Nutrient Response
Once the nutrient status of a crop has been diagnosed, the information may be used to correct existing problems if time allows, or to help predict and prevent future problems. Prediction and prevention are especially important in perennial crops, in which current season nutrient problems have a high likelihood of occurring in subsequent seasons.
The relationship between nutrient concentration of a plant at sampling and ultimate yield may not always be clear-cut. There are factors that affect nutrient dynamics from sampling to harvest, such as redistributable nutrient stores in the plant, nutrient supply capacity of the soil, and nutrient requirements for growth and yield. Plant analysis performed during the season may provide information on only the first of these three factors.
The most common method of using plant analysis for predicting response to nutrient application after sampling involves the setting of nutrient predictive standards in the same way as for diagnostic standards, but correlating nutrient concentrations with final, rather than current, yield. Also, where enough data have been collected and assimilated, recommendations for fertilizer (especially N) application from plant analysis may be made by integration with other factors such as climatic risks and soil properties through the use of computer software and models.
Monitoring of Nutrient Status
Monitoring crop nutrition by plant analysis is very valuable. Monitoring usually involves periodic sampling of a crop to provide a continuous assessment of its nutrient status. The goal of monitoring is usually to keep nutrient concentrations from limiting crop performance and yield. Monitoring using plant analysis for perennial crops may involve sampling as infrequently as once per year, with sampling timing and procedures consistent from year to year. Monitoring for annual crops may require multiple samples during a single season. Where this is done, concentration standards need to be established for a series of growth stages. Monitoring of N status is of particular importance in some cropping systems, especially where N impacts crop quality or where leaching is a concern.
As with soil testing, an important phase of plant analysis is sample collection. Plant composition varies with age, the portion of the plant sampled, the condition of the plant, the variety, the weather and other factors. Therefore, it is necessary to follow proven sampling instructions. Most laboratories provide instruction sheets for sampling various crops, plus information sheets and directions for preparing and submitting samples. It is usually suggested that samples from both good and problem areas be submitted for comparison when diagnosis is the goal. Because experience and knowledge are vital in sampling plants correctly, agricultural advisors or consultants often do the job.
The four basic steps in plant analysis
02 Sample preparation
03 Laboratory analysis
Plant Analysis Sampling Instructions for Selected Crops (A&L Laboratories, 2013)
Collect the first fully developed leaves from the top of 15–20 plants. If the plant is less than 12 inches tall, collect all of the above-ground portion.
Corn Before Tasseling
Corn from Tasseling to Silking