Course:EOSC311/2024/Agricultural Lime
Introduction
Agricultural lime, a fine powder created from either limestone or dolomite, is a frequently used soil input on farms. It is most commonly applied to reduce soil acidity, but can also impact the mineral content and texture of the soil. The application of agricultural lime is particularly important in BC due to the prevalence of acidic podzolic soils throughout the province (Lavkulich, 2021). While the acidity of these podzolic soils is ideal for certain crops, such as blueberries (Longstroth, 2012), the majority of crops produce best in neutral soils. Therefore it is necessary for many BC farmers to turn towards soil management tools, including agricultural lime, to increase the pH of their soils. Agricultural lime is also significant in BC due to the province's substantial limestone and dolomite mining industry.
To completely understand agricultural lime, and its effect on BC, it is necessary to delve into aspects of both geology and soil science. As such the subject was of interest to me, because it combined soil science material I already knew, such as the effect of pH on plant growth, with topics that were hitherto unknown to me. Agricultural lime, in particular, intrigued me because I realized the content I knew about it was fairly one sided. I had background knowledge regarding lime use post-processing, such as its effects on soil, its use in BC, and the potential risks when applying. However, I realized I knew absolutely nothing about lime pre-processing, including how it was processed, whether any piece of limestone could be used as lime, or whether we produced lime here in BC. Therefore, after that realization, the topic of lime piqued my interest, because I felt that the information I already had was missing context, context I could obtain through a better understanding of geology.
Formation:
Processing:
Lime is a term that refers to a variety of different limestone (and occasionally dolomite) products. These products include slaked lime (Ca(OH)2), quick lime (CaO), and agricultural lime (CaCO3 or CaMg(CO3)2) (Kar, 2016). The chemical differences between the forms of lime is determined both by the qualities of the rock of origin, as well as how the lime is processed. The qualities of the rock, such as hardness, purity, and reactivity determine what said rock can be used for. In fact the majority of limestone and dolomite mined is not suitable to turn into any form of lime, and is instead used for other purposes, such as cement production (Oates, 2008). If the rock is of sufficient quality it can then be crushed and milled to produce lime. Then the product is either left alone, or undergoes further processing, such as burning in a kiln to produce quick lime, or burning then hydrating to create slaked lime. The resulting limes have very different properties and use cases, with some suited for improving water quality (Kar, 2016), and others for steel processing (Oates, 2008). However in this paper the focus will be agricultural lime (CaCO3 or CaMg(CO3)2).
Agricultural lime is the least processed form of lime and is typically a co-product of procedures used to produce cement, quicklime, road stone, etc. (Mitchell & Mawanza, 2005). It is a a powder produced through the extraction, crushing, and milling of either limestone or dolomite. However, despite the steps in the production of agricultural lime being relatively straightforward, there are some important nuances. Firstly the chemical and physical composition of the extracted rock must be identified, and the rock must be of sufficient quality. Secondly in most cases the stone should not converted to powder by a single machine or process. This would either result in coarse fragments or a damaged machine. Instead, both in small scale (Mitchell & Mawanza, 2005), and large scale (EPA, n.d.) processing, the rock must first be put in a primary crusher, creating small fragments, and only then can it be put into a secondary crusher, or mill, to create lime. It is worth mentioning that the final lime powder can vary in texture depending on its use case. It has been suggested that the ideal texture is 100% <2mm, 60% <400μm, and up to 50% <150μm (Mitchell et al., 1997). However for longer term applications farmers may desire a coarser powder, and for immediate changes a finer powder may be necessary.
Geology:
Limestone is a sedimentary carbonate rock, with calcium carbonate (CaCO3) content of at least 50% by weight (King, n.d.-a). While it can be produced via inorganic means (Schwab, 2003), limestone is generally is the result of seawater organisms. These organisms, which include mollusks, corals, and red/green algae, are able to convert carbon into calcium carbonates. The calcium carbonate is then used to form an internal skeleton, tests (shells), or other structures. Upon death the calcium carbonate structures do not decompose as easily as the rest of the organism and instead are deposited on the seafloor. In areas of low disturbance (i.e. reefs, tidal flats...) these structures remain on the seafloor, and over time the pressure created by the seawater, as well as new deposits, causes compaction and cementation, leading to eventual lithification into limestone. An additional method of deposition is from the shells of small organisms in the open ocean settling out and accumulating in still deep waters (Schwab, 2003). Following deposition, the calcium carbonate is lithified via a similar process to shallow water deposits.
While limestone is common, it cannot form in all areas of the seafloor. In areas of high disturbance calcium carbonate sediments can be swept away and clastic sediments are introduced. The resulting rock often does not contain enough calcium carbon to be considered limestone, and instead is classified as either a sandstone or mudstone (King, n.d.-a). Additionally, in areas of high acidity, which includes the deepest areas of the sea, the calcium carbonate will react and be destroyed faster than it can accumulate. Lastly, the majority of fine grained limestone only began to form after the beginning of the Jurassic period (201.4 million years ago). Before then species that produced calcium carbonate shells did not exist. The precambrian limestone that exists is likely due to the biochemical activity of bacteria, and deposits of CaCO3 precipitates.[4]
In areas where limestone is able to form, small differences in the environment can lead to dramatically different types of limestone. One example of this is coquina. Coquina is a poorly cemented limestone formed via wave action in areas such as beaches or offshore bars with many sand sized fossil fragments. The weak waves of these regions are able to move silt and clay particles deeper into the body of water, but deposit the larger fossil fragments. Then calcium carbonates cement the fossils together, resulting in a very porous, but poorly compacted and cemented, rock (King, n.d.-b). Other examples of limestone types includes chalk, dolomitic limestone, crystallized limestone (which undergoes metamorphism), fossiliferous limestone, and travertine, among others (King, n.d.-a).
In regards to agricultural lime production one of the most important of these is dolomitic limestone. This type of limestone has undergone a process called dolomitization, which results in re-crystallization, and the minerals calcite and aragonite are converted into dolomite. The reason why this transformation is of particular importance is that it alters the chemical composition of the rock dramatically. Calcite and aragonite are both composed of calcium carbonate, while dolomite is composed of calcium carbonate and magnesium. If only some of the rock undergoes dolomitization then it is called dolomitic limestone, however if a large enough percentage is altered then the rock ceases to be limestone altogether and is instead called dolomite or dolostone. Dolomitization, which likely involves exposure to magnesium rich water, is a slow process, even by geological terms, and its exact causes are poorly understood. Whatever processes previously occurred to create dolomite are no longer occurring, which limits geologists ability to understand how it is created (Schwab, 2003).
Properties of limestone and dolomite:
Limestone composition can vary fairly dramatically, however the primary feature of limestone is at least 50% calcium carbonate. Similarly dolomite must contain at least 50% of the mineral dolomite. However the other components in both rocks can include quartz, feldspar, clay and other mineral particles such as chert (King, n.d.-a). As far as agricultural lime is concerned, limestone and dolomite of high purity is preferred. The active ingredient of lime is calcium carbonate, so any impurities only result in less reactive lime. This also applies to dolomite, as its magnesium content makes it less reactive than lime made solely of non-dolomitic limestone.
Purity is not the only property of limestone and dolomite that impact the quality of lime, the hardness of the rock must be taken into account as well. Both limestone and dolomite are relatively soft rocks, with dolomite being marginally harder than limestone (King, n.d.-c). However even though all limestone is relatively soft some types are much softer than others. For instance coquina is much softer than the average marble (King, n.d.-b). The hardness of the rock impacts the degree which the lime breaks down in the soil. Softer lime will naturally break down, and therefore is able to be left in larger chunks than its harder counterparts.
One of the last major determinants of the lime quality is not the rocks themselves but rather how finely they are ground. Larger particles are much less reactive in the short term due to lower surface area. However, in the long term the larger particles break down in the soil, exposing more surfaces, allowing them to impact the soil for years after application. On the other hand, smaller particles have more dramatic and instant impacts on soil acidity, but must be reapplied much more frequently. Because of these traits, the ideal lime texture is 100% <2mm, 60% <400μm, and up to 50% <150μm (Mitchell et al., 1997). The heterogeneity of the particle sizes allow for substantial impacts both long term and short term. Though it is worth mentioning that farmers and producers may stray from this generally ideal texture and use finer or coarser limes depending on their specific needs.
Agricultural uses of lime:
Increasing pH:
The primary use of agricultural lime is to remedy acidic soils. Plants, like all organisms, have an optimum pH level, and in soils outside of those levels crops may struggle to grow, be less fruitful, or die. Optimum pH can vary between species, most crops prefer a neutral pH of around 6.5-7. That can present challenges to farmers working with soils in acidic regions, such as Southwestern British Colombia. One solution is to grow crops which grow best in acidic soils, such as blueberries which thrive at a pH of 4.5-5.5 (Longstroth, 2012). However, for farmers hoping to not grow these types of plants, they are forced to either grow in un-ideal conditions, or modify their soil pH with lime.
Growing plants outside of their pH range is possible, and compared to liming it reduces the farmer's workload and expenses. However it does run the risk of reducing yield and crop quality. Changes in pH can lead to the death of symbiotic fungi and bacteria, making it difficult for the plants to access nutrients or fix nitrogen. This can cause stunting, yellowing, and reduced efficiency of photosynthesis. Additionally, at low pH plants can be exposed to toxic levels of aluminum.[4] Lastly, farmers run the risk of exposing their crops to the many plant disease organisms which thrive in acidic soils.
A reason why acidic soils is a major concern is that farming typically results in the soil becoming more acidic. There are a wide variety of causes for soil acidity which include the release of CO2 from dead organic matter, excess salts in the soil, absorption of basic cations by crops, and ammonium introduced via nitrogen fertilizer undergoing nitrification (OSU, 2024). While many of these are outside the farmers control, several of the actions necessary for farming, such as fertilizer use, and harvesting (which permanently remove the basic cations absorbed by crops from the soil system), cause agriculture as a whole to be an acidifying process. Therefore fields that were previously able to be left untreated with minimal yield loss, may become less productive over time.
In cases where it is preferable to treat the acidic soil it is typically done with agricultural lime. The calcium carbonate in the agricultural lime is able to react with the acids, neutralizing them, and therefore increasing the pH of the soil. The reactivity of the lime, and therefore its ability to make soils alkaline, is dependent on the factors in the "Properties of limestone and dolomite" section. Use of lime in this way is very effective, though there are some limitations. Lime is a powdered solid and has a fairly limited range of effect, as it must come into direct contact with acids to react with them. As a sand-silt sized solid it also takes at least several weeks for lime to penetrate deeper into the soil. Uneven or overenthusiastic distribution of lime will result in overly alkaline patches of soil, which can be as damaging to plants as acidic soil. To properly apply lime it is best practice to distribute it evenly over a field well before sowing. If ploughing is done then it is best to apply once pre-ploughing, and once post ploughing, allowing the lime to impact all depths of the soil (Oates, 2008). If done correctly lime may only need to be applied once every couple of years, though this can depend on the soil conditions and qualities of the lime used.
Minerals:
A byproduct of the use of lime is that it introduces nutrients into the soil. All types of lime contain calcium, a macro-nutrient that is typically not added by traditional Nitrogen-Phosphorous-Potassium (NPK) fertilizers. So lime can offset the calcium lost from the soil system due to harvesting. This is particularly helpful for soils built on parent material that does not contain high amounts of calcium, or soils that are coarse textured and unable to retain nutrients effectively. Additionally lime produced from dolomite also has high levels of magnesium, another macro-nutrient. In most cases this is good, and is a reason why farmers may choose dolomite lime instead of the more reactive limestone limes. However is some cases if excess magnesium is introduced it can negatively impact plant growth. While in the majority of cases this is not a concern, if the Ca-Mg ratio is too low (typically lower than one) it can lead to deficiency issues even if the plant is able to access the typically required amount of each nutrient. This is the case with serpentine soils, which are toxic to most plants (Oates, 2008).
Texture:
The soil texture, meaning the particle size of the material making up the soil, defines many essential characteristics of the soil. These include drainage, nutrient retention, water retention, root space, and aeration. Lime can be ground to a variety of different sizes, but is typically silt-sized or larger (Mitchell et al., 1997).Therefore while the inclusion of lime would not have a substantial textural impact on sandy, silty, or loam soils, it is much coarser than the majority of the material in clay soils. These coarse particles could help force these clay particles apart, improving drainage and aeration, as well as providing roots with more space to grow. This is particularly impact-full for massive clays, which tend to be very poorly structured. Though there is one exception. In high magnesium clay soils dolomite lime can actually worsen texture, since positively charged magnesium can attract the negatively charged clay colloids, causing the soil to bind more tightly (Sinclair et al., 2013).
Agricultural lime in the Vancouver area:
Use:
Lime use is common in BC. The soil order most often found in the province is podzols (Lavkulich, 2021), which tend to be acidic. This is especially true of the southern part of the province, including over half of the agricultural soils in central BC (BC Ministy of Agriculture, 1991). However, it is worth mentioning that lime is not always used in those acidic regions. Many farmers in BC turn towards alternative crops. It is in part due to soil pH that BC farmers often produce blueberries, cranberries, apples, bell peppers, cabbage, raspberries, and cherries, (BCAFM, 2022) all of which are plants that prefer acidic soils (DeStasi, 2023),(Yara, 2018). Additionally, even some cash crops, such as canola, actually grow quite well in acidic soils (PIRD, n.d.). Nevertheless, despite the opportunities presented by acidic soils, they can limit farmer's ability to grow many major crops that prefer a neutral pH, such as wheat, corn, barley, and oats.
Many farmers who grow such crops are recommended not to lime their soils as the yield lost may not be great enough to justify the expense of liming the soil (BC Ministry of Agriculture, 1991). However for some farmers their production has been hurt enough by soil acidity to make liming cost-efficient. It is expected that as land continues to be farmed in British Colombia it will become more necessary to lime, because agricultural practices generally result in soil acidification.
Production:
BC not only uses lime, but produces it as well. The BC limestone mining industry is a large one, with a long history, dating back to the early 20th century. Currently, 5.2 million tonnes of limestone is mined annually in BC (BCGS, 1992). The majority of this production is from a few quarries on Texada island, however there are smaller quarries throughout the province. There is also a dolomite market in the province, though it is substantially smaller at around 100,000 tonnes, and centered around the Canadian Rockies (BCGS, 1992). These combined mining industries produce lime for both use in province, and also export to areas like Oregon, Washington, and Alberta.
While there is plenty of limestone and dolomite mined in the province, the majority is not used for agricultural lime. For the most part agricultural lime is produced mostly as a byproduct. However, historically this was not always the case. In the past, smaller quarries focused on lime production, such as the Blubber bay quarry on Texada island, and the Parsons bridge quarry in Victoria. Even as recently as the 1980's, there were several mines in the northeast part of the province producing lime for use in Alberta (Fischel, 1992). Despite the significant limestone mining operations in the province, both past and present, it is not one of Canada's main producers. That title instead belongs to both Ontario and Quebec, which account for 70% of the countries lime market (Vagt, 2006).
The reason why limestone is so prevalent throughout the province is partially due to its position near the coasts. Limestone forms from sediment on the seafloor, therefore coastlines and islands are likely areas to find deposits. It is for that reason that the largest quarries are on the island. Similarly, dolomite appears to be most common on the coast, with some also occurring in the Rockies. While the majority of modern limestone is found on the coastal areas, there are still many older deposits in the interior of the province. These vary dramatically in cause. For instance some are due to oceanic plates colliding with the ancestral North American plate as it moved (Fischel, 1992). Others are due to bodies of water which no longer exist. The combined effects of this has led to British Colombia having a wide variety of limestone, with different ages and qualities, making it an excellent site for limestone mining.
Conclusion
Agricultural lime is often mentioned as a one size fits all remedy for soil acidity. However, this is clearly not the case. The different rocks used and processing methods lead to different types of agricultural lime, with varying impacts on the soil. The traits of the limestone and dolomite, are caused by the environment they are created in, such as the biota in the region, the levels of disturbances, and acidity. These traits then determine if the rock can be processed into lime, and if so they impact the reactivity, magnesium content, and ability to break down of the resulting product. The processing of the rocks then can determine how reactive the powder is, as well as how often it needs to be reapplied. As such farmers need to take into account the properties of their soil, such as degree of acidification, mineral content, and texture when choosing an agricultural lime.
While this may seem unimportant, in a low margin, environmentally impactful field, such as agriculture, these small changes can have a significant effect. Incorrect lime choice or application can force farmers to reapply lime sooner than otherwise necessary, could make the lime ineffective, or in extreme cases could damage the soil. Especially in a province such as British Colombia, with acidic soils and a growing population, it is imperative that farmers are able to use the tools at their disposal efficiently.
Looking to the future, it is probably that more and more farmers in British Colombia will begin to use lime. Luckily the province is more than able to supply enough to meet demands. The combination of being on the coast, an area where limestone forms, and having a wealth of deposits from past eras, means that it is unlikely that there will be a lime shortage any time soon.
References
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