DOI: 10.23637/rbk1-bksoils-01
title: Dataset: Broadbalk soil chemical properties 1843-2021

description: This dataset consists of the soil chemical properties of individual plots (Section and Strip combinations) of the Broadbalk Wheat Experiment from 1865-2021, and estimated values for when the experiment began in 1843. It includes percent soil organic carbon (SOC) and total soil nitrogen (N), soil pH, Olsen phosphorus (P) and exchangeable cation concentrations. Also included are stocks (t ha<sup>-1</sup> 0-23 cm) of SOC and total N. Topsoil (0-23 cm) was sampled every 10-20 years, from 1865-1987, then every five years from 1992 onwards. Subsoils were sampled less frequently. Not all plots and Sections were sampled on each occasion. Also included are details of the fertiliser and manure treatments applied.
description: Soil samples have been taken for chemical analysis and archiving since 1865. From 1865-1987/88, samples were taken at irregular intervals in topsoil (0-23 cm) and subsoils. Since 1992 a systematic sampling plan for topsoil has been adopted, so that all 10 sections are sampled once in any five-year period. Subsoils were also sampled in 2000 and 2005/7 from selected plots. Samples were usually taken after harvest and before ploughing. 

In autumn 2004 it was apparent that parts of the field had been ploughed slightly deeper than 23 cm as subsoil clay was visible in random patches across the field. Thus in 2005 and 2006 all plots were resampled to create a new baseline, if necessary, for soil chemical properties. The sections were then sampled systematically from 2008 onwards.

**Soil sampling methods and positions of samples:** There were two main sampling methods:<br>
*1) A few large holes*. From 1865-1914 between four and eight large holes were taken from each treatment Strip. Samples were taken with an open-ended metal box, 23 cm deep and usually 15 x 15cm across. In 1865 there were eight holes, positions unknown, which were bulked together to give one sample for each of the 17 treatments strips. In 1881 there were six holes for each treatment Strip, positions known, so they can be related to the current 10 Sections. In 1893 there were four holes, positions unknown. Data is given for each hole, and also for the mean of all four holes. In 1914 two holes were taken from the top half of the experiment (top1 and top2) and two from the bottom half (bottom1 and bottom2), positions within each half unknown. In 1944 two holes were taken from each plot (Section x Strip combination) from areas corresponding to each of the 10 modern Sections, using a spade to sample the topsoil, and a semi-cylindrical auger to sample the subsoil (23-46 cm). 
*2) Several small cores*. In 1936 and from 1966 onwards, 10-20 cores were taken from each plot (Section x Strip combination) with a 2 cm semi-cylindrical auger, and bulked to give one representative sample per plot. In 2000, subsoils (23-46, 46-69 and 69-91 cm deep) were sampled from more than half of the plots. To prevent carry over of topsoil to deeper layers, the topsoil was first removed with a 3 cm auger and discarded, before sampling the lower layers with a pneumatic sampler fitted with a 2 cm diameter auger; 10 cores were taken per plot and bulked to give one sample for each of the three subsoil layers. <br>

Broadbalk has been sampled many times, but because the sampling method has changed, and the experiment has been divided, initially into two halves, then into five Sections (I-V) and finally into 10 Sections (0-9), it can be difficult to directly compare one sampling with another. <br>
**See soilsamplingdates.pdf for more information.**
description: The Excel file, 01-BKsoils.xlsx, contains the Broadbalk soil chemical data, fertilizer and manure treatment details and cropping information, for 1843-2021. Frictionless CSV files are also provided, with the same data. The Excel worksheet fields_metadata contains column descriptions for all the resources. For all other information refer to the landing page or machine readable metadata.json

Also included are three pdfs: 
- Soil sampling dates and positions - soilsamplingdates.pdf
- Broadbalk Section names - BKsectionnames.pdf 
- Broadbalk Strips showing the different fertilizer and manure treatments - BKstripsummary.pdf
description: All soil samples are air-dried and sieved to &lt;2 mm. Chemical properties are given for air-dried soil (approximately 98% dry matter). Standard soil weights and bulk density are given on an oven-dry basis (i.e. 100% dry matter). No adjustment has been made for the slight moisture content in the analysed soil, as this is considered negligible. Concentrations are expressed as percentage for SOC and N and as mg kg<sup>-1</sup> (i.e., parts-per-million) for Olsen P, exchangeable calcium (Ca), potassium (K), magnesium (Mg) and sodium (Na). 

In 2000-2004 archived soil samples from selected plots sampled in 1865, 1881, 1893, 1914, 1936 and 1944 were re-analysed for soil pH, Olsen-P, exchangeable cations, %N, %SOC and CaCO<sub>3</sub>-C. These values are used in this dataset, except for %N and %SOC from the 1865 samples, which were very different to the original data and the 1881 and 1893 re-analysed soils, so the original 1865 data was used. Soil samples taken in 1966 were re-analysed for exchangeable cations in 2019. Total C was not measured in 1966 but was calculated as the sum of SOC and IC. SOC was measured by corrected Walkley-Black. CaCO<sub>3</sub>-C was analysed in 1971 from selected plots, for another purpose.

**Total soil N %** 
*1865*: Original soda lime analysis for total N (Johnston, 1969) multiplied by a factor derived from the comparison of soda lime and LECO analysis values for 1881 and 1893 samples. <br>
*1881-1994; 1992 onwards*: Combustion analysis, based on the Dumas method, using a LECO combustion system. Finely ground soil (&lt;0.355 mm). <br>
*1966, 1987-8*: Kjeldahl digest method for total N (Bremner, 1965). Digest analysed colorimetrically with a Technicon continuous flow analyser. Finely ground soil (&lt;0.5 mm). 
**Soil organic C %** 
*1865*: Derived from modified soda lime analysis for total N and C:N ratios for 1893 for organic carbon (Dyer, 1902). <br>
*1881-1944, 1992 onwards*: Combustion analysis, based on the Dumas method, using a LECO combustion system. SOC determined as total C minus CaCO<sub>3</sub>-C, measured by a calcimeter. Finely ground soil (&lt;0.355 mm). <br>
*1966*: Chromic acid titration method (Walkley and Black, 1934). Correction factor of W-B x 1.3 used, which is equivalent to organic C by Tinsley or total C by combustion minus CaCO<sub>3</sub>-C. But see also Johnston (1969). Finely ground soil (&lt;0.5 mm). <br>
*1987-8*: Dichromate digestion, modified Tinsley (Kalembasa and Jenkinson, 1973) to measure organic C. Finely ground soil (&lt;0.5 mm). <br>
**Inorganic C % (IC%) also known as CaCO<sub>3</sub>-C**. <br>
*1865-2012*: Soil CaCO<sub>3</sub> measured by a calcimeter. CO<sub>2</sub> is liberated from CaCO<sub>3</sub> by treating with HCl in a closed system. Converted to percentage inorganic C by dividing by 8.333. Not measured in 1987-88, as %SOC was determined directly by Tinsley analysis. <br>
*2013 onwards*: Measured by an automated Skalar Primacs inorganic carbon analyser. Soil is treated with phosphoric acid and heated to 135 degrees C. Inorganic C is released as CO<sub>2</sub> gas from CaCO<sub>3</sub>, which is determined by the NDIR detector. The instrument measures CaCO<sub>3</sub> and data is presented as %IC, by dividing by 8.333.<br>
Multiply IC% by 8.333 to convert to %CaCO<sub>3</sub><br>
**Soil pH**: <br> 
In water, with a 1:2.5 soil:water suspension, mean of two readings. Soil pH was not routinely measured in subsoils, except 23-46 cm in 2000 and 2005-2007. <br>
**Olsen P (plant-available P)**: <br>
All samples were analysed by the Olsen method: soil extracted with a solution of 0.5<i>M</i> NaHCO<sub>3</sub>, buffered at pH 8.5 (Olsen et al, 1954). The extracts were analysed by continuous segmented colorimetric flow analysis. <br> 
**Exchangeable cations - Ca, K, Mg and Na** <br>
All soils were extracted with 1<i>M</i> ammonium acetate (NH<sub>4</sub>CH<sub>3</sub>CO<sub>2</sub>) solution, after the method of Metson (1956). The extracts were analysed by ICP-OES (Inductively Coupled Plasma - Optical Emission Spectrometer). 
**Soil weight/bulk density** <br>
Measured standard soil weights, 10<sup>6</sup>kg ha<sup>-1</sup> (Mkg ha<sup>-1</sup>) of oven-dry fine soil for each soil depth, are used, based on comprehensive measurements of soil weight made in 1865, 1881, 1893, 1914 and 2000. Soil bulk density (g cm<sup>-3</sup>) is calculated as soil weight divided by depth (eg 23 cm) x 10. The soil weights are assumed to remain constant for the plots <b>not</b> given organic manures, at 2.88 x 10<sup>6</sup>kg ha<sup>-1</sup>, equivalent to a bulk density of 1.25 g cm<sup>-3</sup>, 0-23 cm. <br>
The FYM applied to strips 1, 2.1 and 2.2 lead to a decline in topsoil weight/bulk density with time until 2000. In the years when soil weight was not measured, these were calculated, assuming a linear decline with time. Soil weight/bulk density is assumed to remain constant on Strips 2.1 and 2.2 after 2000. However, bulk density may increase slightly in the Sections in rotation, as no FYM was applied to oats 1996-2017, and no FYM is applied to beans from 2018. 
Bulk density will increase in strip 1 after 2000 as applications of FYM stopped in 2000. The increase is assumed to be at a similar rate to the decrease in bulk density between 1968-2000. <br>
**Calculation of stocks (t/ha) of SOC and total soil N:** There were two methods<br>
*Stock in sampled depth*: calculated as percentage total N or SOC x soil bulk density (g m<sup>-3</sup>) x sampled depth (e.g. 0-23 cm). <br>
*Stock in the original soil weight (2.88 x 10<sup>6</sup>kg ha<sup>-1</sup>, 0-23 cm)*: Adjusted for observed decreases in soil bulk density in topsoil (0-23 cm) in Strips given FYM (Strips 1, 2.1 and 2.2 only), by including the appropriate amount of subsoil N or SOC directly below 23 cm, to ensure that the soil weights were comparable over time. After the method of Powlson <i>et al.</i>, (2012). Calculated as total N or SOC in sample depth, plus additional weight of subsoil, with appropriate subsoil percentage N or SOC. Subsoil percentage N and SOC was measured in some years and calculated in others by assuming a linear change with time. Stock calculated for topsoil only. 

See **Additional Information** for estimated starting values of %SOC, %N, etc when the experiment began.
description: This dataset is derived from measurements made by the Analytical Chemistry Unit, Rothamsted Research, Harpenden. Once the data were entered into e-RA, they were independently back-checked against the original data sheets. The Analytical Chemistry Unit follows the Joint Code of Practice (JCoPR) and participates in European Quality Assurance programmes. All performance is strictly monitored using certified external standards alongside in-house standard materials. Standards and check samples are monitored and recorded. **Estimated starting values when the experiment began in autumn 1843.** 
It is unlikely that all treatment strips started at the same value. 
*% SOC = 1.00 +/- 0.05 (0-23 cm)* estimated from later changes; <br>
*% SOC = 0.58 (23-46 cm)* based on strip 5 in 1865; <br>
*% N = 0.110 +/- 0.005 (0-23 cm)* estimated from later changes;<br>
*% N = 0.074 (23-46 cm)* based on strip 5 in 1865;<br>
*Olsen P = 10 mg kg<sup>-1</sup> (0-23 cm)*, based on a measured value for the nearby Exhaustion Land site in 1856; <br>
*Exchangeable K = 95 mg kg<sup>-1</sup> (0-23 cm)*, estimated from 1881 value for strip 3, and trends for the other strips;<br>
*Exchangeable Mg = 45 mg kg<sup>-1</sup> (0-23 cm)*, estimated from 1881 value for strip 3, and trends for the other strips; <br>
*Soil weight = 2.88 x 10<sup>6</sup>kg ha<sup>-1</sup> (0-23 cm)*, based on comprehensive measurements of soil weight made in 1865, 1881, 1893, 1914 and 2000.