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INTERPRETATION
C. Owen Plank and R.N. Carrow1
Interpretation of plant analyses for turfgrass can be very complex although the quantitative association between absorbed nutrients and growth has been studied by several
investigators. Reliable interpretive data are lacking for a number of turf species, particularly at different stages of growth and for nutrient concentrations near or at toxicity levels. Other factors that make the
interpretative process complex are the effects that variety or hybrid, nematodes, and environmental factors such as soil moisture, temperature, light quality and intensity have on the relationship between
nutrient concentration and plant response.
Several different methods have been proposed and used to interpret plant analysis data. Initially, single-concentration values were used to identify nutrient sufficiency, but research
showed that ranges in concentration would better describe the nutrient status of the plant. Another method of interpretation is based on "critical values," the concentration below which a 10% reduction in
growth may occur. This does not imply that a severe deficiency exists as 90% of maximum yield is still possible. This approach was developed for row crops and forage crops where yield data is easily attained. For
several years this system of interpretation had a serious limitation since it defined only the lower limit at which a 10% yield reduction might be expected, providing no guidance when the concentration found
exceeded sufficiency. However, Ohki (1987) proposed the use of two critical levels, one defining the critical deficiency level (CDL) and one defining the critical toxicity level (CTL) with the nutrient levels in
between the two points being adequate. Defining the “critical value,” “CDL,” or “CTL” for turfgrasses is somewhat more complex than for most agronomic crops because of the various cultural practices
employed in managing certain turfgrasses. A more useful method of interpretation is based on sufficiency ranges- the optimum concentration range below which a nutrient is low or deficiency occurs, and above which a
nutrient is excessive or toxicity occurs. This system of evaluation is currently in use in the University of Georgia Plant Analysis Laboratory and most other government and commercial laboratories.
Ideally sufficiency ranges are developed by plotting yield or plant growth with nutrient concentration. However, sufficiency ranges have also been developed for some crops utilizing
survey data from large populations of normal appearing plants and establishing the upper and lower boundaries of sufficiency using the population nutrient mean plus or minus one standard deviation. Figures 1 and 2
illustrate two ways in which growth is related to nutrient concentration in plants.
Fig. 1. The relationship between nutrient concentration in plants and yield.
Fig. 2. The relationship between nutrient concentration in plants and growth or yield.
It is significant to note the differences in the slopes of the two curves on the left side; the slope in Fig. 2 is quite steep whereas the one in Fig. 1 is more gradual. The
curve in Fig. 2 more nearly typifies one for micronutrients and the one in Fig. 1 typifies one for macronutrients. This illustrates the importance of accurate analyses
and interpretations for micronutrients because at the low end of the sufficiency there is a small difference in nutrient concentration between sufficiency and a
severe deficiency. Fortunately, with most turfgrasses micronutrient deficiencies are not common occurrences. The graph in Fig. 2 illustrates that for most
macronutrients a greater change in concentration occurs between sufficiency and deficiency.
A defined sufficiency range may not apply to all situations or environments. In plants nutrient concentrations are not absolute with respect to sufficiency or
deficiency because nutrient uptake and internal mobility, nutrient ratios, as well as dry-matter changes, can affect the nutrient concentrations in plant tissues.
Consequently, nutrient concentrations are not static; they change during the growing season in response to environmental and management conditions. Figure 3
shows the fluctuations in nitrogen content of well maintained bentgrass greens during a growing season.
Fig. 3. Range in nitrogen content of 18 bentgrass greens during 1991.
Concentration and dilution occur due to the difference between plant growth and nutrient absorption as well as movement of the nutrients within and between plant
parts. Under normal growing conditions, nutrient absorption and plant growth closely parallel each other during most of the vegetative growth period. However,
if the normal rate of growth is interrupted, nutrient accumulation (higher than expected nutrient values) or dilution (lower than expected nutrient values) can
occur. Some of the factors that can result in nutrient accumulation include: extremes in temperature for the grass species; heat or moisture stress; stress due to traffic, or
other cultural practices; and stunting (reduced growth) due to a soil deficiency of a particular nutrient or nematode infestations. Nematodes can produce nutrient
deficiencies similar to those resulting from low soil levels. When elements such as calcium and phosphorus are deficient in the plant tissue, but soil pH and soil test
phosphorus and calcium are adequate, this is a good indication that nematodes are the cause of the problem. Factors that can result in dilution are growth factors that
stimulate rapid growth, which may include highly favorable climatic conditions, and rapid growth response to nitrogen applications (Carrow, 2000). As noted,
several factors can affect a plant analysis result and this is why it is important to supply historical information requested on plant analysis history forms when
submitting samples to plant analysis laboratories, and the need to have an expert practitioner interpret plant analysis results.
It is a good policy to maintain a record of soil tests, plant and water analyses and refer to them each time a lime and fertilizer program is formulated. Evaluate
upward or downward trends in soil pH and nutrient levels in both the soil and plants. Having this information, coupled with visual observations of the turfgrass
and knowledge of field conditions, you can adjust lime and fertilizer applications to maintain the nutrient content of the soil and turfgrass within the sufficiency range for the majority of elements tested.
The following tables are provided as guides for interpreting plant analysis results for some turfgrasses. They have been taken from various published reports and
modified based on plant analysis surveys conducted by the senior author. The interpretative guidelines are for use with plant analysis data from conventional
laboratories. They are not applicable for interpreting data generated by NIRS instruments.
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1Associate Professor and Professor, The University of Georgia, Crop & Soil Science
Department, Athens, GA 30602-7272 and Griffin, GA 30223-1797, respectively.
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