Soil surveys carried out along the proposed route of HS2, from London to Crewe and Leeds, have produced valuable information which AHDB believes could help fill knowledge gaps identified in reviews of RB209.
Analysis of the results was conducted by Stephen Heming, of Reading Agricultural Consultants, who examined the relationships between soil nutrients, pH, organic matter, and texture between levels in upper subsoil and topsoil.
All samples were tested at the Berkshire-based NRM laboratory for pH, phosphorus, potassium, magnesium, total organic carbon and nitrogen.
Key findings highlight changes in cultivation practices, the impact of sampling depth and the extent of P and K deficiency.
In the survey, samples were taken at depths between 0-30cm and 25-50cm.
The results show P and K deficiency in topsoils is much greater than indicated by the 2019-2020 Professional Agricultural Analysis Group survey of national laboratories, says Dr Heming (see table).
One reason is arable topsoils were sampled to 22-30cm depth compared to the standard 0-15cm, he says.
“Reduced cultivation depth has lulled us into a sense of having better nutrient levels than are actually present over the whole topsoil depth.” Even so, there were also many cases of excess P and/or K indices with soil texture a big influence.
P tended to be higher in lighter soils than heavier ones, while K showed the reverse trend.
Dr Heming believes texture-adjusted guidance is needed in RB209 to make better use of excess soil P or K and to give faster ‘builds’ where the index is low.
These results may help refine maximum soil P levels above which further phosphate additions could enhance leakage to groundwater.
“35mg/litre in topsoil [mid index 3] is the point above which subsoil P can rise sharply in lighter soils.
However, such soils are not an environmental risk if erosion is controlled and P is unlikely to leach into groundwater except in sand or alluvial subsoils.
Nevertheless, above 35mg/litre soil P, applying phosphate fertiliser seems unjustified in most situations.
“In clay subsoils P tends to be lower, index 0, even when there is 35mg/litre in the topsoil, though it can be higher.
Subsoil P needs measuring below 40cm to determine any possibility of P reaching drains.
“The upper subsoil is an important contributor to crops’ potassium uptake.
The analysis shows it contains less available K than topsoil.
When topsoil was within index 2-, clay subsoil was rarely less than upper index 1, whereas in sandy, light loamy or stony subsoils K could be lower, especially where subsoil had little organic matter.” Dr Heming concludes that for a safety margin, it is better to maintain topsoil above 150mg/ litre (mid index 2-).
Few clay subsoils were below 90mg/litre (mid index 1) with the exception of the Carboniferous clays common in the North East where farmers may need to increase K fertiliser applications, Dr Heming suggests.
RB209 could now clarify which heavier soils are likely to be high K-releasing, non-releasing or in an intermediate category, he adds.
“Only 60% of the variation in subsoil K could be predicted from topsoil K and texture, so in all potassium response experiments, upper subsoil to 50cm should be checked also.” RB209’s soil nitrogen supply adjustment uses soil organic matter as a proxy for total nitrogen to predict N mineralisation for crops.
“This is questionable because the carbon to nitrogen ratio varied from eight to one and 14:1.
It also increased with carbon content and reduced with clay content, so total N is best measured directly.” Initial estimates of average carbon stocks to 50cm depth were 95-125 tonnes per hectare in arable land.
“There’s a huge range and it’s influenced by soil type,” says Dr Heming.