Magnesium soil contents and relationships to livestock health
Magnesium (Mg) deficiency (hypomagnesaemia) in ruminant livestock is a serious issue for the agricultural sector and accounts for a significant number of animal deaths annually. It is caused by a diet deficient in Mg, or due to an imbalance in the supply of Mg in comparison to other mineral cations1. Hypomagnesaemia is likely to be responsible for lower-productivity and diminished well-being in more animals in a herd compared to those displaying acute symptoms, given that herds/flocks generally receive a common diet2,3. If Mg deficiency could be prevented, it would be of benefit to both animal welfare and economic productivity. Recent research has confirmed that whilst hypomagnesaemia is commonly reported by UK farmers, the reported use of preventative measures is low, and the use of pasture interventions is lower still4. Pasture interventions can include the application of Mg-rich fertiliser or lime products, or selection of sward species with a propensity to take-up elevated Mg concentrations4,5.
One aspect of dietary supply is the geographic control of pasture and farm-produced fodder. It is known that total Mg and plant-available Mg concentrations in soil are controlled by geological and geographic factors6,7,8,9 and that there is little evidence for any changes in pasture soil Mg concentrations through time6,10. The magnesium content of soil relates to that of the bedrock, where it is high in the bedrock it is high in soil and vice versa. Thus, the composition of all pasture and farm-grown fodder will be influenced by this natural environmental endowment as well as pasture management decisions.
Grass productivity and soil pH
A key pasture management activity is that of soil pH—which here is reported as measured in water (pHw), consistent with standard agronomic laboratory practice in the UK11. Grassland mineral soil is recommended to be maintained at pHw ≥ 6.0 in Britain12,13. In Ireland, where grass-clover pasture is more widely practiced, a pHw threshold of 6.5 is recommended14. However, multiple lines of evidence exist that indicate pasture soil is frequently below these pHw recommendations. Private sector on-farm sample data summaries from the UK consistently show pH typically below recommendations in pasture soil: the most recent annual data synthesis reports 57% of grassland soil with pHw ≤ 5.99, and 27% with pHw 6.00–6.498. This is consistent with systematically collected public sector data across the north of Ireland, where 84% of pasture samples were below the clover-grass recommended threshold of pHw 6.515.
Grassland production is widespread in Wales and western England (Fig. 1). Two environmental factors jointly contributing to the lower pH in these regions are (1) geological—these areas are most often on soils which are developed over rocks with low concentrations of base cations (Figs. 1 and 2); and, (2) these areas are also often upland areas, associated with typically higher rainfall16 which will further leach base cations. Added to these environmental factors are the application of nitrogen fertilisers which have an acidifying effect17. Thus, many pastoral areas require treatment using agricultural lime in order to optimize soil pH for grass growth18.
The use of liming materials
The opportunity to improve grazing livestock Mg nutrition through use of Mg-rich lime is identified in guidance available to farmers12. This can have the dual benefits of maintaining soil pH for grass growth and ensuring Mg levels in livestock feed is at sufficient levels19,20,21. The combination of soil treatment for pH and Mg would therefore appear to be an efficient solution to solve issues surrounding Mg deficiency22. Conversely, for many soils with existing high Mg levels it may be important to treat with low Mg liming materials to ensure an optimal Ca–Mg balance to preserve the soil structure23.
The use of Mg lime is only one of many methods of controlling Mg levels in livestock feed. Other methods, such as direct additions to feeds, salt licks, pelletised fertilisers products are also effective in reducing incidences of hypomagnesaemia and need to be considered as part of holistic review of a individual farms requirements, this is discussed in Kumssa et al.5.
Maintaining optimal soil pH will directly affect the productivity of grass used for grazing, and will increase fertiliser use efficiency14. However, in some cases, for example upland sheep farming, a low investment—low return approach, with minimum interventions such as liming, may be entirety sensible and appropriate to the farm business and local landscape24,25.
The extent to which agricultural lime is used in Britain is captured through the annual British Survey of Fertiliser Practice (BSFP) and can also be inferred from commodity production statistics. Production can be regarded as a good proxy for consumption since due to its high bulk and low price it is not exported in significant quantities.
The BSFP, is an annual Department for Environment, Food and Rural Affairs (DEFRA) survey26, which representatively samples fertiliser and lime use across the British farming sector. This captures information on lime use in three geological material categories as used in arable and pastoral systems, as well as use of sugar beet lime and ‘other’ options. Sugar beet lime use is very low on grassland (generally unrecorded on ‘permanent’ pasture); ‘other’ categories are generally on a par with Mg-lime, but more detailed liming characteristics are not reported. Figure 3 shows a clear trend in decreasing production of agricultural liming material over the last 40 years. Lime use in the UK peaked in the late 1950s and mid 1960s likely due to a subsidy for agricultural lime in place at the time, this ended in 1978, causing prices to increase and subsequently lime use to decrease27. The use of agricultural lime has continued on a declining, or flat trend, likely due to reluctance to engage in soil treatments that are seen to be costly and a lack of knowledge over its potential benefits.
Figure 4 shows the use of geological lime products to be low at present in respect of the proportion of fields to which lime is reported to be applied, and that this is particularly pervasive on permanent grassland, with a 10-year average to 2019 of 2.9% (range 2.0–4.1%). Of this, a 10-year average of 0.4% of fields had Mg-lime applied, with 1.8% of fields having limestone applied. The limited use of chalk (0.1% of fields) probably reflects the distance between the majority of pasture and the outcrop of the chalk. Recent grassland (< 5 years) has a 10-year average of 6.2% of fields, with an average of 0.8% of fields having Mg-lime applied (Fig. 4).
The low use of lime on farms and the low current supply of agricultural lime suggest that currently pastoral land is being under-limed, and this is entirely consistent with the generally low pH of pasture soil reported8,15. Although this relationship is complicated by factors such as the removal of the lime subsidy in 1970s.
Geological resources for agricultural lime in England and Wales
Agricultural lime is any calcium (Ca) or magnesium (Mg) carbonate rich form of crushed rock (limestone, dolostone or chalk) that is applied to soil in order that the high Ca, or Ca + Mg, concentration raises and neutralises the pH of the soil. Individual lime products have different neutralising values, for example, burnt lime which is ground to a fine powder will release CaO into the soil much more rapidly than coarse crushed limestone rock due to its increased solubility. However, the vast majority of agricultural lime in the UK consists of ground limestone or chalk due to the higher bulk and considerably lower prices of these materials when compared to processed products. UK limestones are principally valued for their use as construction aggregates (around 70% of material quarried is for this purpose)28. Agricultural lime has a much smaller market share, less than 5% of the total sales of limestone28.
Carbonate rocks are distributed in discrete areas across England and Wales (Fig. 2), accounting for ~ 13% of the total area; whilst all alkaline in reaction, these rocks have a wide variety of physical properties and variation in their chemistry, which can affect their efficacy when used as agricultural lime. These properties are intrinsically linked to the sedimentary environments in which these rocks were deposited as well as how they have been altered by post-depositional processes. As such, they need to be treated separately when considering their use as agricultural limestone, which is thus mandated in international agricultural lime product standards29.
Carbonate rocks that constitute raw materials for agricultural lime comprise limestone and dolostone. Limestone is a sedimentary rock composed mainly of calcium carbonate (CaCO3) whereas dolostone is composed mainly of magnesium carbonate in the form of the mineral dolomite (CaMg(CO3)2). These carbonate rocks can contain variable proportions of other carbonate minerals and non-carbonate impurities, mainly in the form of siliciclastic, clay and other accessory minerals30. British limestones can be broadly differentiated in their physical–chemical properties according to the geological era in which they were deposited.
In terms of geographic extent the Cretaceous aged chalk of south eastern and eastern England covers the largest area, followed by Carboniferous limestones and dolostones of the Mendips, North and South Wales, Peak District, North Pennines and Cumbria. Permian aged limestone (which predominately comprises dolomite) is also an important carbonate rock, outcropping between Newcastle and Nottingham. Jurassic limestones also occur extensively across Dorset, Northamptonshire, Lincolnshire and the North York Moors. Older limestones of Ordovician, Silurian and Devonian ages are also present across England and Wales to lesser extents. The broad outcrop of these limestones is shown on Fig. 2.
The Cretaceous Chalk is typically softer than other limestones, with lower levels of non-carbonate minerals and thus high Ca contents: this can be advantageous for liming as it is easy to work, crush and will effectively neutralise acidic soils. Jurassic limestones typically are heterogeneous with high levels of impurities, and can be associated with layers of mudrocks and siltstones, but they can also be thickly bedded and relatively soft. Permian limestones, like Jurassic limestones, are known for being soft and lithologically heterogeneous, but they are principally comprised of dolomite. The Carboniferous Limestone is formed of much more competent (stronger) rocks and these are therefore valued principally for their aggregate properties. They can be chemically very pure, or associated with interbedded muds and silts, and they can also be variably and locally dolomitised. Older Ordovician, Silurian and Devonian limestones occur in much smaller areas and have variable properties but have proved to be important resources on a local scale and are predominantly utilised for aggregate applications rather than agricultural lime9,31.
This study aims to develop simple spatial approaches, for England and Wales, to aid in the decision making process around the application of agricultural limes. Specifically, whether soil pH improvement by liming may be a cost effective solution and whether it may be beneficial to lime using high Mg materials for both pH benefits and to protect livestock against hypomagnesaemia. These approaches are fully documented to ensure they can be replicated elsewhere.
Source: Ecology - nature.com