How Much Soil Phosphorus is in Pennsylvania’s Lawns?

Indiscriminate use of phosphorus-containing fertilizer on runoff-prone turfgrass sites is thought to contribute to contamination of ground and surface water.
How Much Soil Phosphorus is in Pennsylvania’s Lawns? - Articles
How Much Soil Phosphorus is in Pennsylvania’s Lawns?

Consequently, several states have enacted laws restricting the use of phosphorus (P) fertilizers and others are considering similar legislation, including Pennsylvania. Although soil testing is primarily performed to assess nutrient status in crop and turfgrass systems, some researchers have used soil test summaries to examine trends in nutrient management practices and the status of soil P in cropland and lawns at the regional scale (Sims, 2000). Currently, little published data exists on trends in soil P concentrations for turfgrass sites in Pennsylvania. Such information would be beneficial to validate assumptions made for soil P levels in lawns and other turf areas.

Data from 68,328 home lawn soil samples submitted to Penn State's Agricultural Analytical Services Laboratory (AASL) between 1 January 2004 and 31 December 2015 were summarized in two 6-year increments (January 1, 2004 to December 31, 2009 and January 1, 2010 to December 31, 2015) to determine trends in soil P concentrations. Samples were submitted by homeowners and professional lawn fertilizer applicators via test kits distributed from county extension offices in Pennsylvania. The distribution of test kits was on a request basis, and no attempt was made to solicit soil samples from homeowners and professional applicators for this summary. Instructions for collecting soil samples are included in the kits and call for a sampling depth of between 2 and 3 inches, 12 or more cores per sample from each sampling site, and discarding all grass and thatch from cores.

All soil samples were processed and analyzed at AASL according to methods listed by Wolf and Beegle (1995). Phosphorus was extracted from soil samples using Mehlich-3 extractant, and P concentrations were determined via inductively coupled plasma emission spectroscopy (Wolf and Beegle, 2011).

What the data shows:

Summary results show that 40% of the 34,456 home lawn samples submitted to AASL between January 1, 2004 and December 31, 2009 were less than or equal to 45 ppm Mehlich-3 P (Figure 1). Similarly, 43% of the 33,872 samples for the period between January 1, 2010 and December 31, 2015 were less than or equal to 45 ppm Mehlich-3 P (Figure 2). Home lawn samples having less than 45 ppm Mehlich-3 P receive a P fertilizer recommendation from AASL. It should be noted that just because the Mehlich-3 P concentration of a soil sample falls slightly below 45 ppm does not necessarily mean a P deficiency is occurring in the turf. However, due to the variability of soil P in lawns, sampling irregularities, and possible P removal from the turf/soil system between soil testing episodes, Penn State agronomists recommend that P be applied to raise soil P above 45 ppm to insure a deficiency does not occur.


Figure 1. Frequency of Pennsylvania home lawn soil samples (n = 34,456) submitted to the Agricultural Analytical Services Laboratory from 1 January 2004 to 31 December 2009. Frequency bars are arranged in increments of 15 ppm from 0 to 705 ppm, and increments of 150 ppm from 706 to 1755 ppm.


Figure 2. Frequency of Pennsylvania home lawn soil samples (n = 33,872) submitted to the Agricultural Analytical Services Laboratory from 1 January 2010 to 31 December 2015. Frequency bars are arranged in increments of 15 ppm from 0 to 705 ppm, and increments of 150 ppm from 706 to 1755 ppm.

Of the remaining samples submitted between January 1, 2004 and December 31, 2009, 53% were between 46 and 200 ppm and 7% were above 200 ppm Mehlich-3 P. For the January 1, 2010 to December 31, 2015 period, 51% of the samples were between 46 and 200 ppm and 6% were above 200 ppm Mehlich-3 P. A concentration of 200 ppm Mehlich-3 P is an important indicator because it is considered the threshold above which the P Index should be used to estimate risk of P loss on agricultural soils in Pennsylvania (Sharpley et al., 2001). This threshold is based on runoff from agricultural sites that typically have greater potential for soil erosion than turf sites with extensive and dense ground cover. Currently, there is no known P threshold to estimate the risk of P loss in runoff for lawns in Pennsylvania. Proposed legislation in Pennsylvania states that P may not be applied to turf when a soil test indicates Mehlich-3 P concentrations greater than or equal to 200 ppm.

Although results show a very slight reduction (1%) in samples exceeding 200 ppm Mehlich-3 P between the two 6-year increments (January 1, 2004 to December 31, 2009 and January 1, 2010 to December 31, 2015), it's too soon to determine if a trend is occurring towards reduced P in lawn soils.

Conclusions

Sims (2000) indicated that soil-test summary data from public laboratories may be somewhat biased because they do not include information from private labs and may represent only those individuals who care enough about the state of their crops or lawns to soil test. However, Sims also stated that these data should not be ignored when striking trends emerge. The results of the present study show that the majority of soil-test P concentrations from home lawn soil samples submitted to AASL have adequate or below-optimum soil-test P concentrations for turfgrass growth, and that a relatively small percentage of lawns have P concentrations high enough to present a risk of P runoff.

Education efforts should be directed toward individuals engaging in practices leading to excessive P in home lawn soils. Legislators and stakeholders should also consider that a significant percentage of Pennsylvania lawns may benefit from P applications. Despite the fact that most fertilizer companies have eliminated or reduced P in their lawn maintenance formulations since 2010, Pennsylvania soil test data is not revealing a significant trend towards lower P in home lawns. Summaries of soil test data from Pennsylvania lawns will be monitored to determine if education efforts and reduced P in home lawn fertilizers influence trends in soil P concentrations.

Literature Cited

Sharpley, A.N., R.W. McDowell, J.L. Weld, and P.J.A. Kleinman. 2001. Assessing site vulnerability to phosphorus loss in an agricultural watershed. J. Environ. Qual. 30:2026-2036.

Sims, T.J. 2000. The role of soil testing in environmental risk assessment for phosphorus. P. 57-81. In A. N. Sharpley (ed.) Agriculture and phosphorus management: The Chesapeake Bay. CRC Press LLC. Boca Raton.

Soldat, D.J. and A.M. Petrovic. 2008. The fate and transport of phosphorus in turfgrass ecosystems. Crop Sci. 48:2051-2065.

Wolf, A.M. and D.B. Beegle. 2011. Recommended soil tests for macronutrients: Phosphorus, potassium, calcium, and magnesium. p. 25-34. In J. T. Sims and A. Wolf (eds.) Recommended soil testing procedures for the Northeastern United States. Northeast regional bulletin #493. 3rd edition. Agricultural Experiment Station, University of Delaware, Newark, DE.

This article was prepared by Peter Landschoot, Professor of Turfgrass Science, Penn State; John Spargo, Director of AASL and Ann Wolf, former Director of AASL, Penn State; and Jeremy Schwenk, Director of Grounds, Meadowood Senior Living, Worcester, PA.

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