The first Verdure column of the year is traditionally dedicated to turfgrass science history. So, let’s look “back-in-the-day” when the water drop penetration test (WDPT) was developed and then utilized as a reliable test method in turfgrass
First, what’s the WDPT? The WDPT measures the time it takes for a water droplet to infiltrate into a soil sample. Thus, instead of the “water drop penetration test,” it should have been called the “water drop infiltration test.”
To start, remove a soil core (0.75-inch/1.91-centimeter diameter) or soil profile section the length of the turfgrass root zone. Next, using an eyedropper or similar device, place a single droplet of water down the length of the soil sample. Start at
the thatch/soil interface, then place another water droplet every 0.5 inch/1.3 centimeters. Use a stopwatch to measure the time (in seconds) it takes for each droplet to completely infiltrate and disappear into the soil. If the soil is hydrophobic
(or water-repellent), the water droplet will just “sit there” on the surface and not infiltrate. If the soil is wettable, the water droplet will infiltrate quickly within a few seconds. It is also advisable to allow soil cores to bench
dry for a few days before doing the WDPT.
But where did this WDPT come from? It started with John Letey Jr., Ph.D., (1933-2014), son of Italian immigrant parents who began his career in 1959 at the department of irrigation and soil science at UCLA. At that time, research on water-repellent soils
was being conducted on soils of citrus crops and with sites affected by wildfires (those soils become really hydrophobic as a result of severe heat from the wildfire). Letey was the first to explore how the liquid-soil contact angle indicates the
degree of wettability or hydrophobicity of the soil. A few years later, UCLA phased out of agriculture, and Letey transferred to nearby UC-Riverside when that campus became the “ag school.” He earned the status of distinguished professor
of soil physics for his work in plant-soil relations, water quality and irrigation in agriculture, and water-repellent soils.
Leonard DeBano, Ph.D., (1930-2017), also of Italian heritage, was a soil scientist with the U.S. Forest Service in California and then the University of Arizona. He studied soil erosion and other fire-related problems of soils, including water-repellent
soils. In 1965, DeBano published the first description and use of the WDPT as a way of translating the concept of Letey’s contact angle to a simpler method to quantify soil hydrophobicity that did not require sophisticated equipment. In 1968
at UC-Riverside, DeBano and Letey organized the first global research conference on water-repellent soils. My former turfgrass soils professor, Donald Waddington, Ph.D., (Pennsylvania State University; https://tinyurl.com/4exf2wdn), participated in
In 1998, a three-day international conference on soil water repellency was held at the University of Wageningen (Netherlands). Soil physicist Louis Dekker, Ph.D., organized the event, and although he was not Italian, he was a good friend, and we certainly
enjoyed Italian red wine during many dinners. At that conference, Dekker formalized the WDPT scale as follows: 0-5 seconds = wettable; 5-60 seconds = slightly water-repellent; 60-600 seconds = strongly water-repellent; 600-3,600 seconds = severely
water-repellent; ≥ 3,600 seconds = extremely water-repellent. This scale and method of conducting the WDPT has become the accepted standard in research and in practice.
There are more advanced and detailed laboratory methods and instruments used to measure soil hydrophobicity and a soil’s wettable/nonwettable properties. Those methods are expensive and time-consuming and certainly have their merit. But thanks to
those soil scientists/soil physicists — Letey, DeBano and Dekker — the WDPT remains a valued and trusted method to measure soil hydrophobicity in the field. And as the late Paul Harvey always said at the end of his radio broadcasts: “…
and now you know the rest of the story.”
- Letey, J., J. Osborn and R. Pelishek. 1962. Measurement of liquid-solid contact angles in soil and sand. Soil Science 93:149-153.
- Krammes, J.S., and L.F. DeBano. 1965. Soil wettability: A neglected factor in watershed management. Water Resources Research 1:283-286.
- DeBano, L.F., and J. Letey. (Eds.) 1969. Water-repellent soils. Proceedings of Symposium on Water-Repellent Soils. University of California, Riverside; May 6-10, 1968, 354 pages.
- Dekker, L.W., C.J. Ritsema, K. Oostindie, D. Moore and J.G. Wesseling. 2009. Methods for determining soil water repellency on field-moist samples. Water Resources Research 45(4):W00D33.
Mike Fidanza, Ph.D., is a professor of plant and soil science in the Division of Science, Berks Campus, at Pennsylvania State University in Reading, Pa. He is a 21-year member of GCSAA.