February 2021 Research Review No. 95 Analysis of top and subsoil data from the High Speed 2 (HS2) rail project Work package 3 London to Leamington Spa (Section C): Clay vales and ridges Stephen Heming1 1Reading Agricultural Consultants, Gate House, Beechwood Court, Long Toll, Woodcote, Reading RG8 0RR This review was produced a six month project (21140072) that started in July 2020. The work was funded by a contract for £2,700 from AHDB. While the Agriculture and Horticulture Development Board (AHDB) seeks to ensure that the information contained within this document is accurate at the time of printing, no warranty is given in respect thereof and, to the maximum extent permitted by law, the Agriculture and Horticulture Development Board accepts no liability for loss, damage or injury howsoever caused (including that caused by negligence) or suffered directly or indirectly in relation to information and opinions contained in or omitted from this document. 2 Contents 1. Executive Summary .......................................................................................................... 5 2. Methodology, Land Use and Parent Material ................................................................... 13 3. Soils on Charmouth Mudstone ........................................................................................ 19 4. Soils on Whitby Mudstone and Ironstone ........................................................................ 25 5. Soils on Limestone and Clay ........................................................................................... 30 6. Soils on Glacial Deposits................................................................................................. 35 7. Soils on Oxford Clay ....................................................................................................... 41 8. Soils on Kimmeridge and Ampthill Clay ........................................................................... 49 9. Soils on the Chilterns ...................................................................................................... 55 10. Soils on London Clay .................................................................................................... 63 11. Soils on Alluvium ........................................................................................................... 72 12. Phosphorus overview .................................................................................................... 78 13. Potassium overview ...................................................................................................... 83 14. Magnesium overview .................................................................................................... 90 15. pH overview .................................................................................................................. 95 16. Overview of soil organic matter levels ......................................................................... 101 17. Total Nitrogen overview............................................................................................... 109 18. Short-range variation in soil nutrient levels ................................................................. 119 A. Appendices – Multiple Regression .............................................................................. 127 A3 : Charmouth Mudstone ............................................................................................. 128 A4 : Whitby Mudstone and Horsehay Sand ................................................................... 133 A5 : Limestone-and-Clay ............................................................................................... 134 A6 : Glacial Deposits ..................................................................................................... 135 A7 : Oxford Clay ............................................................................................................ 138 A8 : Kimmeridge Clay .................................................................................................... 144 A9 : Chilterns ................................................................................................................. 147 A10 : London Clay ......................................................................................................... 151 AG : General correlations .............................................................................................. 154 A17 : Total Nitrogen ...................................................................................................... 159 3 WP A : East Midlands to South Yorkshire white WP B : Leamington to Crewe and Nottingham grey WP C : London to Leamington red 4 1. Executive Summary This data set if from 409 sampling locations, 185 arable, 148 grassland , 28 amenity grass and 48 woodland, taken along a corridor from London to Southam at density of 1 per 3 ha with some cluster samples (up to 5 per ha). This gives a representative coverage of soils on most of the geological sequence from London Clay in the south to Rhaetic clay-limestone in the north. Topsoil was sampled 0 to 20-30cm depth and upper subsoil from 25-30cm to 50cm by corer or auger. There are some differences of method discussed in NW report which do not significantly distort the conclusions below. Phosphorus Arable land averaged 22 mg P/l (index 2). 40% of samples were below target index, spread across all geological groups except the Chilterns. 31% of samples were above index 2 with some very high P values. The PAAG 2019 laboratory survey found less (22%) deficient though sampled to 15cm rather than the full depth of identifiable topsoil (23-30cm) here. On grassland median P was 14 mg/l (index 1) with 51% of samples below target, again measured on a deeper sample (20cm+) than is usually taken (7.5 or 15cm). PAAG (2019) reported 34% of grassland as deficient. Topsoil P varied up to twofold within some larger cluster areas (4-5 ha). In small cluster groups (3-5 samples within 1ha) standard deviation from the mean was ±15% on arable and ±25% on grassland. P did not correlate with topsoil pH and there was no clear effect of clay type. On heavier arable soils each ∆1% OM was associated with ∆1.9 mg/l increase in P. Topsoil texture was influential: lighter soils tended to higher P except ferruginous sand. Clay topsoils (median 15 mg/l) were 9 mg/l lower than heavy loams. Woodland median was 13 mg/l but with large range - deviation of ±65% of the mean within one hectare. Some woods had good P fertility suggesting they are not ancient but planted on land that has been farmed at some point in the past. Subsoil P averaged 8.7 mg/l arable and 6.2 mg/l grass. Lime-rich (clay) subsoils were usually <5 mg/l. Subsoil texture influenced medians on arable land: Sandy, light loamy (21 mg/l) > medium (16) > heavy loam (13) >> clay (6 mg/l) Clay textured upper subsoils were most likely index 0. Similar but weaker texture trends occurred on grassland and woodland. There was variable degree of correlation of subsoil and topsoil P (r2 0.3 – 0.8). On heavy subsoils P = topsoil P x 0.25-0.37 plus an intercept 1.4-2.9 or simply 0.45-0.47x topsoil P. Woodland was similar (subsoil P = 0.42 x topsoil P + 2.1). Topsoil and subsoil P converged at 5 mg/l. 5 In some groups, subsoil organic matter was influential, ∆0.9–2.9 mg/l P per ∆1% OM, due to carry-down by earthworms, deeper rooting or shallower start depth of subsoil sampling. In heavy loam or clay subsoils, at topsoil P of 10, 20 and 30 mg/l, subsoil P is likely to be 4.5-6 mg/l, 7-10 mg/l (index 0) and 12-15 mg/l (index 1) respectively. Grassland subsoils will be at the lower end of these ranges (much lower if topsoil sampled 0-7.5cm). There seems no difference between geological clays. In light, medium or stony subsoils, at topsoil P of 10, 20 and 30 mg/l, subsoil P is likely to be 6-10 mg/l, 10-20 mg/l and 15-30 mg/l (index 2-3) respectively. Stony subsoils will be at the upper end of these ranges. At very high index (5) subsoil P is typically 0.46x topsoil P but lower in some clays. Only half the variation in subsoil P was explained by topsoil P and texture. On all research projects and crop trials, subsoil should be tested alongside topsoil and to specified depths (e.g. 0-20/25cm, 20/25-40cm and 40-50cm). RB209 revision: despite over two decades promoting RB209 recommendations, a wide range of P levels persists across all textures in arable and grassland. In section 12 revised builds and run-downs are proposed for light/medium and heavy soils to speed convergence on target index. The current target P of mid index 2 (20 mg/l) for arable and grassland should not be reduced. One reason (even with Precision sampling) is to counter short-range variation and avoid patches of index 1. Up to 35 mg/l (mid index 3) is unlikely to result in major P transmission from under-lying heavy subsoil into drains in most instances. Above 35 mg/l phosphate fertiliser application is usually unwarranted, but there is no need for restrictions on applying reduced amounts at 25-35 mg/l topsoil P, where seedbed conditions are suboptimal. Potassium Arable land median was above target (186 mg K/l, index 2+) with 20% of samples below target (similar to 21% in PAAG, 2019). 31% of samples were index 3 or more. Levels were highest in soils on Charmouth (Lias) Mudstone, Oxfordian Clays and Kimmeridge Clay (~225 mg/l), intermediate on Glacial deposits and Whitby Mudstone (140 mg/l) and least on the Chilterns (122 mg/l). Grassland averaged 163 mg K/l (index