Part of experiment rbk1

Broadbalk site and soil physical properties

The Broadbalk wheat experiment, established in the autumn of 1843, has been cultivated since at least 1623, and probably much earlier (Avery & Bullock, 1969). In his first Rothamsted paper, published in 1847, Lawes described the soil as "a heavy loam resting upon chalk, capable of producing good wheat when well manured" . Here, details are given of the site, the soil description and standard soil weights.

Site details

• Location: Rothamsted Research, Harpenden, Herts, AL5 2JQ, UK
• Latitude: 51 degrees 48 mins 36 secs North
• Longitude: 0 degrees 22 mins 30 secs West
• GB Grid Reference: TL123 136
• Gradient: A slope of 1 degree, West to East
• Irrigation: There is no irrigation

Plot area

When the experiment was established in 1843 most of the plots were very large. Most comprised an ‘a’ and ‘b’ half (each 3.77m wide) and were 320m long (the length of the field). Plots 21 and 22 are a little narrower. In 1894 the two halves were combined, giving a total plot area of 0.24ha. As the experiment progressed these large plots have been subdivided into different Sections (see Plans and treatments for more details), with corresponding changes in the area harvested. The current plot lengths vary depending on the Section and are between 15.24m (Section 0) and 28.04m (Section 1). The plots are now 6m wide (except plots 21 and 22 which are 4m wide) with 48 rows at 12.5 cm spacing. The harvested area is 2.1m wide. The harvested area is shown in most of the datasets.

Soil moisture and drainage

Soil Moisture Characteristics: With nearly 200 plots in the experiment, there isn't water retention information for all treatments. Water retention characteristics for contrasting plots can be found in Salter & Williams (1969). For more recent information, see Gregory et al, 2010, where retention characteristics have been fitted to the van Genuchten model.

Soil Drainage: Lawes described the soil of Broadbalk as having good natural drainage. However, as it was an experimental field, it was decided to improve the drainage, to allow greater access. Tile drains were installed under each strip in autumn 1849 (except plot 20). They were installed in the centre of the strip, under the furrow separating the 'a' and 'b' halves (see Plan 1852-1925). Tiles drains were installed under strip 2.1 (2A) in autumn 1884 before the first application of FYM. Tile drains were 5cm in diameter and 60-76 cm deep, and they discharged into a 10cm diameter main drain at the east side of the field.

Lawes and Gilbert realised that the drains could be used to measure losses of plant nutrients from the different fertilizer treatments. Small outlet pits were dug at the intersection of each strip drain and the main drain in December 1866 to sample the water draining from strips 2 to 16. This was not ideal, as there was the risk of the samples being contaminated. The drains from strips 17-19 were opened in November 1878. In spring 1879 the collection of drainage water was improved, with the drains from each strip discharging into their own pit, which overflowed into a separate, deepened main drain which was kept open. In 1897/98 this main drainage ditch was enlarged, the base concreted and the sides bricked. (Details from Johnston & Garner, 1969).

Soil description

• FAO Classification: Chromic Luvisol (or Alisol)
• U.S. Soil Taxonomy: Aquic (or Typic) Paleudalf
• Soil Survey of England & Wales Group: Stagnogleyic paleo-argillic brown earth (Avery, 1980)
• Soil Survey of England & Wales Series: Predominately Batcombe Series (Avery & Catt, 1995 - see Soil Map link on left hand side).

For more details of the Batcombe and other soil series, see Cranfield University 2018 Soils Guide.

Soil texture class: Clay loam to silty clay loam over clay-with flints. The soils contain a large number of flints and are slightly calcareous. Below about 2m depth the soil becomes chalk. The experiment is under-drained and the site is free draining.

Soil texture, 0-23cm (from Gregory et al, 2010)

• Sand (2000 - 63 µm) : 25 %
• Silt (63 - 2 µm) : 50 %
• Clay (<2 µm) : 25 %
• Particle density g cm ^-3 : 2.56 %

There is considerable variation in soil texture across the site, with clay contents ranging from 19 - 39% (Watts et al, 2006). The mean clay content for Sections 0 (straw incorporated since 1986) and 1 (straw removed, both continuous wheat) is 28.3% (Watts et al, 2006). Clay content increases with soil depth in the Batcombe Series (from Jenkinson et al, 2008):

• 23-46 cm : 30 % clay
• 46-69 cm : 50 % clay
• 69-92 cm : 49 % clay

Soil pH: The plough layer (0-23 m) is limed when necessary to maintain a minimum soil pH of 7.0 - 7.5. Broadbalk was first limed regularly from 1955-1967, with plots given the larger inputs of ammonium fertilizers receiving more lime than the controls. Liming stopped from 1968-1975, but began again in 1976-1992, with 3 or 4 sections being limed each year, with 2.9t/ha of chalk being applied each autumn to all plots of each section. From 2007 onward selected plots have been limed every 5-6 years based on soil pH measurements to maintain top-soil pH around 7.0-7.5. Selected plots were limed in autumn 2018.

• For information on soil organic carbon, see Soil Organic Carbon (%SOC) Open Access long-term changes in SOC from selected treatments, 1843-2015 (doi: 10.23637/KeyRefOABKsoc-02)
• For information on soil concentration of Olsen P (plant-available P), see Plant-available P (Olsen P) Open Access long-term changes in Olsen P from selected treatments, 1843-2010 (doi:10.23637/keyrefoabkolsenp).

Soil weights, Mkg/ha

The following standard soil weights should be used for Broadbalk soil, both continuous wheat and rotational sections. All weights are in 10⁶ kg/ha of oven-dry fine soil. To convert to g/cm3 divide by depth in cm (eg 23) and multiply by 10. Data prepared by A J Macdonald and P R Poulton, February 2014.

Broadbalk standard soil weights 10⁶ kg/ha, 0-23cm

Year	Inorganic a Plots 03-20	FYM since 1844 b Plot 2.2 (2B)	FYM since 1885 b Plot 2.1 (2A)	FYM 1968-2000 c Plot 01
1843 1865 1881 1884 1893 1914 1967 2000 2010	(2.88) 2.88 2.88 2.88 2.88 2.88 * 2.88 2.88	(2.88) 2.78 2.69 * 2.62 2.60 * 2.52 2.52	* * * (2.88) 2.81 2.75 * 2.52 2.52	* * * * * * (2.88) 2.52 (2.63)

Broadbalk standard soil weights 10⁶ kg/ha, 23-46cm

Year	Plots 03-20	Plot 2.2 (2B)	Plot 2.1 (2A)	Plot 01
All years	3.0	3.0	3.0	3.0

Broadbalk standard soil weights 10⁶ kg/ha, 46-69cm and 69-91cm

Year	Plots 03-20	Plot 2.2 (2B)	Plot 2.1 (2A)	Plot 01
All years	3.1	3.1	3.1	3.1

Topsoil data is the mean of comprehensive measurements of soil weight made in 1865, 1881, 1893, 1914 and 2000. Figures in brackets are assumed. * indicates no samples taken. 23-46cm is the mean of all plots measured in 1865, 1881, 1893 and 1914. 46-69cm and 69-91cm is the mean of all plots measured in 1865, 1881 and 1893. See Dyer, 1902 for 1865, 1881 and 1893 data. See Watts et al, 2006 for 2000 data. 1914 data is previously unpublished.

Notes:

^a Plots 03-20, receiving inorganic fertilizer only, no manure (FYM). This also includes plot 03, given no fertilizer, and plot 19, recieving rape cake/castor meal.

^b Assume no further change in bulk density/soil weight on plots 2.1 and 2.2 after 2000. However, BD may increase slightly on plots in rotation as FYM is NOT applied to the oats.

^c BD will increase on plot 01 as applications of FYM stopped in 2000.The increase is assumed to be at the same rate as the decrease in BD between 1968-2000.

For deeper soil layers, use the following weights, taken from Dyer, 1902:

Depth cm	Plots	Soil weight, 106 kg/ha	Soil weight, g/cm3
91-114 114-137 137-160 160-183 183-206 206-229	All plots All plots All plots All plots All plots All plots	3.21 3.18 3.20 3.22 3.37 3.37	1.40 1.38 1.39 1.40 1.47 1.47

Acknowledgements

With thanks to Andy Macdonald and Paul Poulton for help with compiling the information and text.

Key References