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Pennsylvania Turfgrass – Max Schlossberg, Ph.D., Assoc. Prof. Turfgrass Nutrition/Soil Fertility, PSU Center for Turfgrass Science

As turfgrass managers, understanding the nutritional requirements of our turf and staying abreast of its nutritional status comprise important responsibilities. The methods by which we do so are varied, and generally coined fertility or nutritional assessment techniques. One of these techniques, soil testing, was the topic of a cover article I penned in the Fall 2016 Pennsylvania Turfgrass issue.

Tissue testing is the nutritional assessment of a plant through sampling of its vegetative tissue, followed by elemental analysis in an agricultural laboratory. The next step is to interpret the analysis results, which involves characterization of nutrient levels by keyword or index. Last is the recommendation step, typically comprising zero or more fertilizer treatment(s) recommended to reverse current or pending deficiencies.

Why is tissue testing an effective nutritional assessment technique? 

Reason 1. Because it is a direct analysis of the biomass we foster, revere, and defend. This morning’s clippings were yesterday’s canopy, so how wouldn’t they contain valuable information? Despite reading several popular twitter feeds, I’ve yet to discover a valid answer. If your health declined, and you sought medical help, which doctor would you prefer: the one who insists on examining you, or the one who insists on examining your living conditions? Relative to dicotyledonous angiosperms, grasses are simple plants that distribute acquired nutrients across vegetation per local concentration gradient. As a benefactor of robust research and reporting, our discipline maintains species-specific expectations for leaf concentration of fourteen mineral plant essential nutrients and arguably three additional elements. Ostensibly, the same cannot be said of current fertility assessments of turfgrass soil.

Reason 2. Because the full panel of turfgrass tissue nutrient concentrations readily implicate contamination, it intrinsically supports quality assurance and control efforts. Of course, discovering a clipping sample to be contaminated by fertilizer or soil is not a desired outcome. Yet even experienced scientists commit sampling errors and know well enough to jettison tainted observation(s) from the set. This is because agronomists recognize the irreplaceability of plant essential nutrients, and the likelihood of a faultily derived fertilizer recommendation consuming valuable time and resources, all while not remedying jack! And, yeah, irreplaceability is a word. I liken a full panel of turf clipping nutrient levels to a Figure-8 knot, the knot most used by climbers to secure their harnesses. Do you know why the Figure-8 knot is most used? Because its proper configuration is more readily confirmed than every alternative. Allegorically speaking, it is better to discover the error and restart than plow on oblivious and die. For those readers who took TURF 435 in residence, you may like that analogy as much as my photorespiration one. Perhaps not. Regardless, example tissue test results indicating sample contamination by source are shown (Figure 1). Full test results of single, stand-alone soil samples do not afford equivalent insight or indication of sample contamination.

Tissue Sampling

As with soil testing, the sampling step is an incredibly important component of the tissue testing process. Hence, the following guidelines to consider when sampling turfgrass tissue:

Collect a comprehensive sample set. If you have one or two fragile greens (out of 20) or one problematic field in a complex of four; by all means, tissue sample these suspect systems. However, be sure to ALSO sample tissue from one or two of your identically fertilized AND healthy systems. It is helpful to know the extent to which the healthy systems are responding to your fertilizer program. Securing a nutritional benchmark from healthy system(s) will help you determine whether the problematic system is suffering from a unique edaphic or cultural issue, or whether the healthier systems are steadily digressing to the problematic condition. 

Obtain a time array instead of a point. Growth rate is highly influenced by temperature, ET, and soil water. When plants are rapidly transpiring, they assimilate primarily mass flow nutrients, excluding those nutrients which move to the root by diffusion and/or biological facilitation. Furthermore, tissue growth can surge with N uptake and dilute less-mobile nutrients. Therefore, sub-sample on multiple dates (say Monday, then Thursday, then Saturday), homogenize the composite sample from that turfgrass system and submit as representative of the whole week. This approach is already used by golf course superintendents who recognize the pitfalls of over-irrigation, employ PGRs, and cannot obtain enough clippings in a single greens-mowing event. I know who you are and hope you never change! 

Avoid contamination.  For those continuing the long-honored tradition of assessing growth by clipping volume (Figure 2), kudos! The foremost benefit of #clipvol vigor assessment includes prompt receipt of clear results immune from soil/sand contamination. On the contrary, the accuracy of leaf clipping nutritional analysis is highly dependent on sample integrity. Why? Because dry leaf clippings have very low mass relative to sand/soil particles and/or fertilizer prills (Figure 1). While collecting clippings from recently dragged or brushed swards generally reduces the likelihood of particulate contamination, avoid collecting tissue samples within four mows of a topdressing event. DO NOT collect tissue samples immediately following a fertilizer application. Following a foliar fertilizer application, wait until either a significant rainfall/irrigation event has occurred or the turfgrass has been mowed twice. If a granular fertilizer has recently been applied, then wait at least a week to collect clippings for nutritional analysis (Figure 1). Fortunately, recent irrigation by low-quality effluent does not comprise a significant contamination risk. 

Processing. Dried clippings are most stable and analysis ready. Taking the extra time to air-dry your tissue samples, ideally in a dependably clean room, helps ensure quality results. If impractical, consider oven-drying the tissue at very low temperature. Research grade tissue drying is done at temperatures <180 F. Some nitrogen (N) forms volatilize at temps above 300 F, inciting under-reporting of tissue N. If you are in a hurry and must submit moist clippings, consider overnight mail and alert the lab of their condition. 

Submission. In the final stage of preparing the sample for shipment, shuffle the dry clippings on a 1-mm to 1.7-mm mesh screen (no. 18 to 14 sieve size) or on a clean, smooth, sloping surface. Avoid shoveling dry clippings into the mailer/bag; use multiple ‘pinched lifts’ to transfer the dry clippings instead. Most labs require 2 cups dry leaf clippings (yes, talking cooking measurements here) per full analysis report.

Tissue Analysis

Most labs will analyze tissue samples for most, or all, plant essential nutrients. The Penn State Ag. Analytical Serv. Lab (aasl.psu.edu) doesn’t offer tissue analysis of chlorine, molybdenum, or nickel; but includes sodium and aluminum (Al) at no extra charge. Reporting Al levels on a tissue test report comprises added value, despite it not being an essential nutrient. The reason? Aluminum is a common component of clay minerals, and elevated levels of leaf Al indicate sample contamination by soil (Figure 1).

Interpretation & Recommendation

Given a contaminant-free sample was analyzed, tissue test results routinely deliver valuable information. Tissue N level is particularly useful in assessing current fertilizer N availability as well as potential disease susceptibility. Penn State field research indicates creeping bentgrass systems having <4.2% leaf N in dry clippings are otherwise more susceptible to dollar spot. Similarly, leaf N in the 3 to 4% mass range of dry clippings collected from annual bluegrass greens indirectly relate to anthracnose susceptibility. Many overt soil testing proponents discount tissue testing as ‘a mere snapshot in time.’ Unconvinced that’s a bad thing, I encourage Penn State students and alumni to embrace high resolution technology in support of effective decision making, particularly June through August.

Another reason N concentration sits atop the tissue test report is accumulation of several other plant-essential nutrients interact with N fertilization. For example, potassium (K) level in turfgrass clippings often relates directly to N concentration, even when target soil K levels are maintained or exceeded. On the contrary, phosphorus (P) level in clippings often inversely relates to leaf N level, even across plots showing equivalent soil P availability. These interactions should be considered when interpreting your tissue test results.

In all fairness, deficiencies in tissue test results interpretation do currently exist. Many studies have shown different cultivars of turfgrass accumulate varied levels of nutrients, even when growing on common soil and receiving an identical fertilizer regimen. Thus, the extent to which phenotype governs nutrient uptake/accumulation remains an important objective of future turfgrass field research!

Summary. For those using/considering tissue testing as a tool for turfgrass management decision making, I approve and hope you found the content useful. Please do not hesitate to contact me to discuss the topic and/or methods further at mjs38@psu.edu.