After light and water, nitrogen (N) is considered a key factor for plant growth. Whether you subscribe to the BCSR (base cation saturation ratio), MLSN (minimum levels for sustainable nutrition) or SLAN (sufficiency level of available nutrients) soil-testing method for fertilizer recommendations, the overall goal for the turfgrass is to achieve maximum nitrogen uptake — kinda like my goal when I eat Sunday dinners at Momma Fidanza’s house!
For plant nitrogen uptake, though, it all comes down to the roots, and how those roots function in that rhizosphere environment. The rhizosphere (specifically the root zone) is where biological, chemical and physical properties influence root growth and function. Thinking of that zone as the “soil-root interface,” is it possible turf roots could take up nitrogen more effectively when there’s proper air-water balance in the rhizosphere, and therefore would a soil surfactant be of any help? Research at Clemson University (Clemson, S.C.), led by Dara Park, Ph.D., conducted what is perhaps the first study utilizing radiolabeled 15N to attempt to answer that question.
In this greenhouse study, PVC pipe columns (4-inch diameter × 18-inch length [10 centimeters × 46 centimeters]) were packed with three different wettable soils (not hydrophobic) consisting of sand (0% organic matter, pH 7.3), sandy loam (4.1% OM, pH 6.8), and sandy clay loam (2.2% OM, pH 7.0). Tifway 419 bermudagrass washed sod was established onto the columns, and all columns were irrigated the same with deionized water, so no additional nutrients were added.
Four treatments were applied to the bermudagrass columns: (1) soil surfactant (10% alkoxylated polyols and 7% glucoethers) + 15N urea in a tank-mix, (2) soil surfactant (10% oleic acid esters of a block copolymer) + 15N urea in a tank-mix, (3) water + 15N urea, and (4) water only (used to determine ambient 15N). All treatments were applied once, and various measurements were taken over the next 28 days. Each treatment was replicated five times, and the experiment was repeated twice to ensure accurate and consistent data collection.
The results were statistically similar for both soil surfactant treatments. Bermudagrass treated with either soil surfactant grown in the sand, sandy loam or sandy clay loam root zones had greater amounts of leaf nitrogen and soil nitrogen detected as compared with the urea-only treatment. Specifically, it was an average of 16% more leaf nitrogen in sand, 25% more leaf nitrogen in sandy loam, and 65% more leaf nitrogen in sandy clay loam, and, coincidently, there was an average of 144% more nitrogen retained in sand, 107% more nitrogen retained in sandy loam, and 25% more nitrogen retained in sandy clay loam, compared with urea applied alone in any of these soils.
Also, with bermudagrass, the nitrogen collected in the leachate for either of the soil surfactant plus urea-treated columns resulted in an average of 46% less nitrogen leached in sand, 37% less nitrogen leached in sandy loam, and 23% less nitrogen leached in sandy clay loam, as compared with the water and urea-only treatment. And, applying either soil surfactant with urea resulted in significantly higher soil volumetric water content (in sandy loam and sandy clay loam soils only) and higher bermudagrass clipping yield (in all three soils) compared with urea applied alone. Interestingly, this response occurred in soils that were not water repellent.
Biologically speaking, 40% to 60% of applied nitrogen in the form of a fertilizer is taken up by the plant. What nitrogen is not taken up by the plant remains in the soil, volatilizes, undergoes denitrification, or leaches into the groundwater. This study at Clemson University showed that when either of those two soil surfactants was applied with nitrogen in the form of urea fertilizer, to three wettable soils, there was an increase in turfgrass nitrogen uptake, more nitrogen remained in the soil, and less nitrogen was leached compared with applying urea alone.
These results are pretty darn impressive — a soil surfactant application improved nitrogen-uptake efficiency and reduced nitrogen leaching — a win for the turfgrass and a win for the environment. What is going on in the rhizosphere to account for these effects? Look for more insights to emerge in the next few years.
Source: Abagandura, G., D. Park, W. Bridges and K. Brown. 2021. Soil surfactants applied with 15N labeled urea increases bermudagrass uptake of nitrogen and reduces nitrogen leaching. Journal of Plant Nutrition and Soil Science 184(3):378-387 (https://doi.org/10.1002/jpln.201900162).
Mike Fidanza is a professor of plant and soil science in the Division of Science at the Penn State University Berks Campus in Reading, Pa. He is a 20-year member of GCSAA.