SAMPLE ANALYSIS

C. Owen Plank and R.N. Carrow¹

Conventional plant analysis laboratories analyze plant samples for the total quantity of 12 to 13 elements. Plant tissue samples previously dried, ground, and weighed are prepared for elemental analysis by destroying the organic matter using either wet chemical or thermal digestion procedures (Campbell and Plank, 1998). Wet chemical digestion involves the destruction of organic matter through the use of both heat and acids. Acids that have been used in this procedure include sulfuric, nitric, and perchloric acids, either alone or in combination. This is a rather time-consuming procedure and may require up to 16 hours for complete digestion. A relatively new accelerated wet chemical procedure for organic matter destruction utilizes pressure and high temperature to shorten the digestion process to approximately 1 hour. Dry ashing is conducted in a muffle furnace at temperatures of 500^o to 550^o C for 4 to 8 hours. Once the organic matter has been destroyed, the elements are dissolved in dilute nitric or hydrochloric acid, or a mixture of both such as aqua regia (Campbell and Plank, 1992; Campbell and Plank, 1998).

The recent developments in accelerated sample digestion coupled with new innovations in high-speed analytical equipment such as inductively-coupled argon plasma emission spectrographs (ICAP) and combustion apparatus for nitrogen and sulfur analyses make it possible for scientists to complete a plant analysis in 24 hours or less. ICAP instruments have the capability of simultaneously analyzing samples for 10 to 12 elements at the rate of one sample per minute and combustion units analyze samples for nitrogen and sulfur at the rate of one sample per 5 minutes. Although the instruments generate results rapidly, they achieve a very high degree of accuracy because they are calibrated against both known chemical and plant standards. Data are collected via computers and promptly returned electronically to clients. These advancements have made conventional laboratories very attractive to turf managers in the last few years for obtaining rapid and accurate analyses.

The Near Infrared Reflectance Spectroscopy (NIRS) procedure has been promoted by some commercial firms as a means to obtain rapid tissue analysis information for diagnostic or monitoring purposes using either an on-site NIRS unit or by shipping samples to a laboratory that utilizes NIRS (Carrow, 2000). Results can be obtained quickly because NIRS is a non-destructive procedure and precludes the digestion phase required with wet chemistry procedures. Sample preparation usually only involves drying and grinding the sample. Once the sample has been prepared for analysis, scanning or analysis time typically is less than 3 minutes. Elements analyzed include N, P, K, Ca, Mg, S, Zn, Cu, Fe, Mn, B, and Na. Although this technology has been used for several decades for determining N, total protein, carbohydrates, lipids, other organic chemicals, and moisture content in forages, grains, and oil crops (Clark et al., 1998; Foley et al., 1998; Masoni, et al., 1996; Stowell, 1995; Vazquez, et al., 1995) its application in turfgrass tissue analysis is still being developed.

The basis of NIRS is to determine reflectance of specific wavelengths over the infrared range (750 to 2500 nm) and relate the degree of reflectance to a specific compound or element. Infrared wavelengths are absorbed mainly by:

• C-H bonds; common in carbohydrates
   
• N-H bonds; common in proteins, amides, and amino acids
   
• O-H bonds; common in water

If the wavelength radiation matches the vibrational or rotational frequency of the chemical bond within a particular plant compound, it is absorbed.

Statistical procedures are used to correlate the reflectance of one or more specific wavelengths to the true level of a compound or nutrient as measured by wet laboratory methods. A regression equation is developed that estimates the quantity of a nutrient or compound based on the strength of reflectance from these wavelengths. This equation is then entered into the computer software for use by NIRS on future samples where wet laboratory analysis will not be conducted (Carrow, 2000). However, achievement of statistically significant equations with high correlation (R² or coefficient of multiple determination; R²>0.90; 1.0 is perfect) has not been demonstrated for nutrients except for N (Plank, unpublished data 1990; Stowell, 1995; Stowell and Gelernter, 1998; Rodriquez and Miller, 2000). Carrow (2000) noted that except for nitrogen, none of the nutrient elements are directly involved in a C-H, N-H, or O-H bond. Reflected wavelengths are, therefore, always indirectly related to a nutrient rather than directly. This results in lower correlations of NIRS nutrient values (except for N) versus wet lab values than achieved for organic components, which usually exhibit correlation coefficients of determination of r² >0.95 (1.0 is a perfect correlation) because they are directly involved in C-H, N-H, or O-H bonds.

This does not preclude the use of NIRS in certain turf management programs because it can be effectively used for monitoring potential excessive or deficient levels of nitrogen.  Nitrogen management is very important in turf because nitrogen influences numerous growth factors directly or indirectly. These include:

• Color
   
• Density
   
• Growth rate
   
• Root growth
   
• Thatch accumulation
   
• Disease and insect tolerance

The uses of NIRS in turf management could possibly be expanded through additional research that may improve calibration equations for the majority of elements currently being analyzed using this technology.

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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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