Temporal measurement of penetrometer resistance for 20 grassland sites on 4 contrasting soil types in South-West England

This dataset is comprised of penetrometer (soil resistance) data for 20 sites in South-West England. The sites are physically close but are on four different soil types. At each site a dipwell was sunk to ~85 cm and water levels measured. Penetrometer measurements (10 replicates) were made with a cone and with a bespoke metal plate designed to replicate cattle grazing the land. Sites were visited through Autumn, Winter and Spring at approximately 10-day intervals, or more closely when significant weather events occurred. Measurements were taken between 15/9/1983 and 24/12/1985, with a one-off additional measurement of penetrometer data on 24/1/1989. Sites 17 to 20 were only sampled from 2/10/1985 onwards.
Grazing of improved pastures is an important land-use in South-West England and more widely. Poaching is the compaction, marking and smearing of the topsoil by the hooves of grazing animals. Agronomically it is recognised as a form of soil degradation, which inhibits subsequent grass growth, as well as providing sites for weed species to enter the sward. There can also be hydrological consequences through enhanced runoff and sediment transfer. Different soils vary in their vulnerability to poaching, particularly due to their texture (particle-size and organic matter content) and hydrological properties. Wet soils have a lower soil strength and are more at risk of poaching, and therefore poorly draining soil types pose a higher risk of soil damage. Ill-timed traffic by farm machinery has similar effects to poaching by livestock.
The soils in this study include both freely draining soils and those more prone to waterlogging. The measurement of soil strength over two annual cycles on similarly managed grass fields demonstrates the varying risk of poaching on different soil types and their speed of recovery after being wet.

Location of sites
A total of 20 sites were sampled across two farms near Yeoford, Devon, UK, that were within 2.5 km of one another. No detailed land use data exists for the sites, but all were grazed pasture on dairy farms with additional herds of youngstock. This area was chosen due to the variability in soil type, and 4 sites were chosen per soil series according to soil maps, with the soil series at each site confirmed using a hand auger. Each site was a least 50 m from any neighbour. Sites were identified by securely fixing small (30 cm diameter) metal plates to ensure that repeat sampling occurred in the same location each time.
The soils were as follows, with soil series information taken from Cranfield University (2022). Sites 1 to 4, Halstow series (Avery, 1980 subgroup 4.21, Typical non-calcareous pelosols; World Reference Base [WRB] Endoleptic Stagnic Cambisols), a clayey material over lithoskeletal shale, and intermediate in character between the Crediton and Hallsworth series. Sites in the vicinity of SX784968 (50o 45’28.86N, 003o 43’31.29 W) and SX 786980 (50o 46’07.84’’ N, 003o 43’22.52’’ W).
Sites 5 to 8, Hallsworth series (Avery subgroup 7.12, Pelo stagnogley soils; WRB Clayic Eutric Stagnosols), in clayey drift with siliceous stones. Sites in the vicinity of SX784982 (50o 46’14.16’’ N, 003o 43’32.96’’ W) and SX785983 (50o 46’17.48’’ N, 003o 43’27.98’’ W).
Sites 9 to 12 were all freely draining brown alluvial soils, but of slightly different soil series. All are WRB Eutric Fluvic Cambisols, and all are described as light loamy river alluvium. Sites 9 and 10 were on Alun series (Avery subgroup 5.61, Typical brown alluvial soils). Site 11 was on Ty-Gwyn series (subgroup 5.62, Gleyic brown alluvial soils), distinguished from the Alun series by having slight gleying low in the profile. Site 12 was on Walford series (subgroup 5.61) and is a red variant of the Alun series. Sites in the vicinity of SX783989 (50o 46’36.74’’ N, 003o 43’38.90’’ W) and SX 786987 (50o 46’30.49’’ N, 003o 43’23.35’’ W).
Sites 13 to 20, Crediton series (Avery subgroup 5.41, Typical brown earths; World Reference Base [WRB] Eutric Chromic Endoleptic Cambisols), a reddish loamy soil from lithoskeletal sandstone, known to be freely draining and stony. Sites in the vicinity of SX 791995 (50o 46’56.76’’ N, 003o 42’58.78’’ W).

Dipwells
At most of the sites, 10 cm diameter dipwells were augered to >85 cm depth to enable recording of soil water levels and covered with a metal disc. Due to the Crediton series being characterised by rapid draining and good winter trafficability, only one dip well was sited among those soils.

Soil strength
Soil strength was measured using a hand-held Farnell Soil Assessment Cone Penetrometer. The rate of soil penetration should be 25 mm/s (ref operating instructions), and deviations from this can affect the reading. While a hydraulic or electrically operated device (which wasn’t available to the researchers at the time) can therefore be more accurate, a hand-held device has benefits in the speed of use (allowing more replicates). A single operator conducted all the assessments to avoid inter-operator variability.
At each site, soil strength was measured in two different ways. The ‘cone’ measurement was with the manufacturer supplied ‘CBR’ (California Bearing Ratio) cone tip, 129 mm2. A ‘plate’ measurement was also made with a flat, purpose-made circular plate with an area of 507 mm2 and a thickness of 5 mm, designed to be similar to the effect of standing and grazing cattle. The cone was inserted into the soil to a 10 cm depth, the plate to 5 mm. Measurements where the probe touched stones were rejected. At each site and on each date, 10 penetrometer measurements were made using both the cone and the plate within 10 m of the 30 cm cover plate. These cone and plate readings were not taken from exactly the same locations – 10 random plate measurements were taken around the cover plate, then the cone was fitted and the routine was repeated. Readings were taken using the method given in the operating instructions regarding the calibration of the equipment and the rate of downward pressure required.

Rainfall data
There are no rainfall data available with this dataset, but the nearest gauge at the time of sampling was at Barnstaple Cross [OS grid reference SS805014], 2.5 - 5 km to the northeast. During the spring of 1984 there was no effective rainfall through April and into May, so that penetrometer measurements reflect increasing soil strength as the soils dried, without the complication of intermittent rewetting by rain.

The penetration resistance force data are given as a cone index, which is the ratio of the force to the base area of the cone (Upadhyaya, 2005). In the spreadsheet Cone_index_conversion.csv, the cone indexes have been converted to penetrometer resistance (kg/cm2) by using the kg force as measured on a newly purchased spring balance, and the area at the base of the cone or plate (129 and 507 mm2 respectively).

At the time of taking the samples, there were thoughts that a cone index of 50 on the plate (~ 2.04 kg cm-2) was approximately the loading of a standing cow (Patto et al. 1978), but we are unable to access this reference to verify this.