Introduction
Over the past few decades, I have observed the development of increasingly divergent views of the roles of both calcium as a nutrient, and lime in itself, in New Zealand agriculture. Many of these newer views are lacking in scientific verification, but nevertheless are likely to be reflecting a great number of anecdotal observations, and should therefore not be ignored. In the absence of adequate scientific research in the last 20 years, some agricultural scientists have been guilty of refusing to look beyond what was proven in scientifically-conducted trials decades ago. I believe that there are now different questions, that no scientist had reason to ask in past decades, that must be asked now.
In trying to cover all relevant aspects of the role of lime and calcium (Ca) in New Zealand agriculture in the past and today, under ten or so key headings for easier reading, there will be some repetition. I have tried to keep this to a minimum, but the intertwined benefits of lime and calcium makes it impossible to avoid completely.
This article aims to present an overview of how attitudes to lime and calcium have developed within New Zealand, and very briefly overseas, to try and overcome some of the confusion that exists, and to discuss implications for future management of soil fertility and pasture production in this country. There is far more detailed information on a range of sub-topics to be found elsewhere, for example on lime requirements to maintain soil pH on different soils, Ca in animal nutrition, in broadacre and horticultural crop nutrition, and in crop storage life.
Early recommendations for lime use in New Zealand
The importance of lime in pastoral agriculture is nowhere better recognised than in New Zealand. This is because the development of introduced ryegrass and clover pasture, which quickly came to be the backbone of the New Zealand economy, could not have happened on the previously acidic and nutrient-poor soils without lime, phosphorus (P) and sulphur (S). Clover establishment in particular, even with inoculation with Rhizobia, was very difficult without these vital inputs.
A large number of lime trials conducted throughout the country enabled lime requirements for establishment and maintenance to be fairly well understood by the 1950s. Summaries were made by Peter During in his famous book ‘Fertilisers and Soils in New Zealand Farming’ in the 1960s, and updated and refined by others such as Mike Floate (South Island) and Doug Edmeades in the 70s and 80s. Recommendations were based around a heavy initial application (typically 2-5 tonnes/ha) to get the soil pH on the top 7.5cm to 5.6-6.0, then maintenance in this range by the application of 1-2 tonnes/ha every 3-5 years. This generally meant a saw-tooth effect on soil pH over a range of 0.3 to 0.4 between applications, and in increasing risk of undesirably low pH in the top couple of cm (see later). Several different summaries of trial data all demonstrated that there was little or no benefit to maintaining the soil pH on New Zealand soils above 5.8.
Standards for agricultural lime were set in 1940 that required at least 50% by weight to pass a 0.5mm sieve (30 mesh), and not more than 5% to be greater than 2.0mm (8 mesh). While these standards basically still apply to groundspread lime, as does declaration of the actual CaCO3 content, latitude has been given to aerially-spread lime in the name of flowability and therefore safety, to the extent that much aerial lime is now so coarse, and so lacking in the finer 0-0.25 mm particle size range, that initial responses in soil pH can be very slow. In addition, the fraction over 1 mm in size is essentially useless in the context of predictably maintaining soil pH levels over a reasonable time frame.
It is important to realise that the focus of the use of lime on pasture in New Zealand in the last century was almost solely in the context of its ability – or more specifically the ability of the carbonate anion attached to the calcium cation – to raise and maintain the soil pH (that is, to reduce the concentration of hydrogen and other acidity-producing ions such as aluminium), and so allow pasture seedlings to grow. This, and possibly the fact that single superphosphate (commonly abbreviated to ‘super’ and used throughout New Zealand), contains 20% Ca as well as 9-10% P, lead to the reasonable belief, based on thousands of pasture analyses, that actual deficiency of calcium as a plant (or animal) nutrient simply did not exist in New Zealand pastures. As a result, it is probably the most under-researched nutrient in New Zealand. As one example, During’s book contains 25 pages on the use of lime per se, 37 on soil and fertiliser P, 11 on soil potassium (K), and just one on soil Ca.
Naturally-occurring sources of calcium – liming and non-liming
Calcium is the fifth most abundant element, making up 3% of the earth’s crust. However, it does not occur naturally by itself. Calcium metal, produced artificially in 1808 initially, reacts violently with air or water to form calcium oxide (CaO) or calcium hydroxide or ‘slaked’ lime, Ca(OH)2, respectively. In nature, calcium forms a host of different relatively stable minerals with other elements.
The most abundant forms of calcium are the sedimentary rocks limestone (CaCO3) and gypsum (CaSO4. 2H2O). Gypsum, which New Zealand has remarkably little of, has no liming effect, and is used purely as a source of Ca and S, and to improve infiltration and drainage by de-flocculating soil clay particles. A common (though not in NZ) subset of limestone is dolomite, in which some of the Ca has been replaced by lighter magnesium (Mg). The lighter atomic weight of Mg than Ca, and therefore higher carbonate ratio, gives dolomite a higher liming value per tonne than normal limestone (typically about 10%).
Much limestone is formed by Ca that is continually being washed from the land into the sea in rivers precipitating with soluble carbonate on the ocean floor. This makes Ca an incredibly important element in controlling global warming extremes, as it provides a ‘sink’ for carbon dioxide. The sedimentary phosphate ‘rocks’, many of which are now above ground and mined for their P content, were formed by phosphate in sea water replacing the carbonate in limestone over time to make tricalcium phosphate, or ‘hard’ phosphate ‘rock’, a far more stable compound, which has to be treated with strong acids to make phosphate fertilisers. ( I have put the word rock in inverted commas as it is an unfortunately misleading misnomer – most phosphate deposits actually exist as sands, and only become more ‘rock-like’ if they have become compressed by the weight of other material deposited over them over huge periods of time). The strength and durability of tricalcium phosphate is why it is nature’s building block in skeletons.
Where the conversion of limestone to tricalcium phosphate on the sea floor was not compleed before earthmovements or dropping sea levels left it above sea level, the crystal lattice contains both carbonate and phosphate bonded to Ca, and we have what are called ‘reactive phosphate rocks’ or RPRs, which can be used directly as a fertiliser on acid soils. New Zealand’s own underwater Chatham Rise phosphorite deposit is an RPR in the making.
Many other types of calcium compounds are formed by igneous or volcanic acivity. Many of these are also compounds with phosphate, often incorporating iron and aluminium as well, which reduces their usability for fertiliser, directly or through manufacturing. Many of these compounds have been changed further by weathering and deposition of bird or bat excreta, which in isolated island or cave conditions can result in the natural fertiliser known as guano.
Calcium and phosphate in fertilisers
The earliest recorded use of ground limestone to improve ‘sour’ soil is by the Romans. Gypsum has been used to improve infiltration and drainage for a long time as well. Both ground bones and calcium phosphate minerals were widely used as P fertilisers in the 1800s. Then it became known that treating either with sulphuric acid to make what Lawes in England called ‘superphosphate’ was far more effective in most cases. The sulphuric acid converted the tricalcium phosphate to water soluble (and hence immediately, if only temorarally) available to plants. The Ca that was liberated by this forms gypsum with the sulphate from the sulphuric acid. “Super’ made from pure tricalcium phosphate is by weight 45% monocalcium phosphate and 55% gypsum. The key point is that all the Ca from the phosphate rock remains in the superphosphate. “Super’ became by far the most widely used fertiliser in New Zealand.
With the development in the USA in the 1920s of technology to separate the intermediary phosphoric acid from the gypsum during the superphosphate manufacturing process, everything changed, in most countries. First, triple superphosphate (20.5%P but only 9%Ca) could be made, and phosphoric acid, containing no Ca, could be reacted with ammonia gas to make MAP and DAP. The latter has become increasingly widely used in NZ in the last 10-15 years. We will come back to this.
It is important to note that all phosphate rocks have some liming effect. This is due to the fact that soil acidity slowly – albeit much more slowly than in a factory – dissolves the tricalcium phosphate, and acidity is consumed in the process. Liming effects of phosphate rocks range from about 25% for apatite and other hard phosphate rocks, to over 50% for RPRs, which contain destabilising carbonate in the lattice.
Calcium in New Zealand soils
Calcium is very widely distributed in soils, especially in high-pH alkaline soils (the reason these soils are high pH in the first place is that they have a very high natural limestone content. They are effectively low-grade limestone deposits. However, even above and around naturally-occurring limestone deposits, New Zealand has very few naturally high-pH soils. Generally high rainfall keeps leaching carbonate and other liming agents (and with it the more soluble cations like sodium and Mg) out of the soil. So our native vegetation evolved to cope with acid conditions. European settlers discovered we had a very good climate for growing pasture, but we needed to reduce the acidity, and put on lots of P. Surprisingly, our soil Ca levels themselves were reasonably good, given the climate. This is because the parent sedimentary material was high in the tricalcium phosphate (often called apatite) we talked about. However, the rate of dissolution of this phosphate into plant-available form, at about 1-2 kg P/ha annually, was far too slow to meet the demands of high-producing introduced pastures.
Most New Zealand soils have quite a high cation exchange capacity, so the ground lime applied to increase the soil pH meant that its Ca content was not leached too quickly. The use of ‘single’ superphosphate or ‘super’ as the main fertiliser to supply P and S also helped maintain soil Ca levels, although the gypsum component is present as a very soluble form from which the sulphate is easily leached from most soils, taking cations with it. Nevertheless, the infrequent application of ground limestone (‘aglime’) and annual applications of super generally maintained soil Ca at adequate levels for decades.
What does lime itself actually do?
Lime is stable in alkaline soils. However, when applied to acid soils, the H+ ions in the soil attack it, dissolving it the process shown chemically as
CaCO3(lime) + H+(hydrogen ions) = Ca2+(calcium ions) + CO2 (gas) + H2O (water)
This consumes acidity in the form of hydrogen ions, in the process producing Ca as a nutrient, CO2 for photosynthesis by plants, and water. This process is often referred to by lay people as ‘sweetening’ the soil. As H+ ions are removed, many nutrients become more available for plant uptake. However, if too much lime is applied, the reverse can happen. Excess use of lime can cause deficiencies in manganese, zinc and copper. In many soils around the world, a pH of 6.5-7.0 (ie, very slightly acidic to neutral) is regarded as optimum. New Zealand’s soils do not require this much liming however, partly because pasture and wheat are efficient foragers for nutrients, partly because our soils have evolved with more acid-tolerant organisms, and partly because our soils have high cation exchange capacity due to the dominant types of clay and accumulated soil organic matter. Which is all good, because the higher the soil pH you want to maintain in a particular soil and rainfall, the more lime it will require.
As well as making many desirable nutrients more available for plant uptake, lime also reduces some adverse effects of other elements in the soil. A major, in fact the major limitation to plant growth on New Zealand soils below a pH of 5.5 is toxicity from soluble aluminium (Al) to plant roots. Al also physically impedes both the uptake of P by the plant, and destroys DNA if it gets inside, even in tiny concentrations. Lime, by reducing soil acidity as shown in the equation above, greatly reduces the solubility of Al in the soil. It also reduces toxicity from excess iron (Fe) and manganese (Mn).
Ongoing application of lime is necessary to maintain the soil pH and soil Ca levels against excretion of H+ ions by soil roots, acidity produced by the conversion of N fertilizers into nitrate-N, leaching of carbonate and cations from the soil, consumption of CO2 by plants, and removal of Ca in meat, wool and dairy products.
There is often also a stimulation of soil biological activity – for example increased mineralisation of soil organic carbon, N, S and P – when lime is applied. Scientific opinion is divided over whether this is a liming effect per se, or a response to Ca as a nutrient. I favour the former, because the effect does not seem to happen with non-liming forms of calcium compounds such as gypsum.
The fine lime vs aglime debate
This stimulation of soil microbiological activity seems to be able to be triggered by quite small quantities of fine lime, which suggests to me that some soils which have been topdressed with normal (coarse) aglime periodically in the past may develop undesirably low pHs in the top few cm between applications, disguised by the fact that the average pH in the conventional 0-7.5cm soil core sample is 5.6 or over. This effect is likely to be exacerbated where the aglime applied has very little very fine material, as is the case with most aerial aglime these days. The small fine lime component will be utilized quickly, and the coarser particles will be worked down the soil profile by animal treading and earthworm activity. This is great for the sub-surface soil, but we have to look after the “Top-2” cm as well! If we don’t, we get thatch cover, weeds, Al toxicity, poor response to P, and reduced N fixation by clover.
I believe that the use of coarser aerial lime is the main reason for the increasing debate over fine lime vs aglime, with some misleadingly claiming that as little as 50 kg/ha of very fine lime (under 0.1 mm or 100 microns) applied annually will have the same benefit as the traditional 1-2 tonnes/ha of aglime applied every 3-5 years (equivalent to say 300 kg/ha annually), in other words, six times as effective. It certainly is more effective per kg, but in long-term use I would say twice as effective, not 6 times! People have jumped to the wrong conclusion based on startling short-term responses to fine lime on soils that have developed a low pH in the “Top-2” cm, I believe.
Benefits of Ca as a nutrient
Because soil Ca levels have in the past been adequate for vigorous pasture production, and for the animals grazing them, most of our knowledge on calcium nutrition has come from overseas, particularly research on high-value crops. It has been established from such studies that adequate Ca is vital for such things as activating cell division and cell elongation, tolerance to moisture and heat stress, reducing the incidence of disease and blemishes on fruit and vegetables, and improving storage life.
At a plant uptake level, Ca is linked to improving the uptake of several minerals, including boron (B), which anecdotally at least, is becoming increasingly deficient in New Zealand pastures. Whether this is a specific type of chemical synergism, for example between the Ca++ cation and the B(OH)4- anion, or something more simple such as maintaining higher solubility of soil B, is unknown. Regardless, there is a common quasi-scientific view that Ca more or less physically ‘trucks’ other nutrients to the plant roots.
What is also known from overseas research, where dairy cows are typically housed year-round and are raised entirely on various brought-in feeds, is that Ca levels in the feed is crucially important to milk production and quality. In New Zealand, there are changes occurring that I believe mean that we have to look far more carefully at Ca as a nutrient than we have had to in the past, and in the next section I will explain why.
What is changing in New Zealand?
In the past, lime was treated as a separate issue to fertiliser. This was partly because lime in New Zealand was regarded purely as a soil ameliorant, for reducing acidity and with it aluminium toxicity, and not as a source of Ca as a nutrient. Partly because of this attitude, and partly because it suited the spreading industry to deal with dusty lime in fewer, larger applications, the accepted wisdom of maintaining soil available P and S levels, and K on dairy farms, with annual ( and biannual in the case of dairy farms) was never applied to lime itself. The assumption, if considered at all, was that the input of Ca in super would take care of that.
However, two big changes have been occurring in New Zealand for over a decade, one each in hill country farming and dairy farming, that mean that this assumption is no longer valid, even if it used to be.
On hill country farms, several things are causing the regular application of lime, particularly sufficiently fine lime, to diminish. Until 2010, low meat and wool prices meant that often, when the 5-yearly application of lime was due, cash-flow problems meant it didn’t happen. The fertiliser industry ( and the banks ) pressured farmers to put on P as a priority, when in increasingly many cases lime should have been the priority. Now, with a bit more money to spend, farmers are often finding that (a) applications of the increasingly coarser ‘aerial’ lime aren’t giving the same benefit as before, and (b) because of increasingly common aluminium toxicity, they aren’t getting the response to P they used to. In my personal experience, 70-75% of long-term hill country farmers would be of this view. In desperation, many are being sucked into ‘muck and mystery’ wonder products that promise everything, but deliver just a massive hole in the bank account.
What must be done is to increase the fineness of lime considerably. Where aluminium toxicity is possible, then the first option is to apply a capital application of whatever quantity is required to get the soil pH to at least 5.6 (typically 1-2 tonnes/ha), and then maintain it at this level with annual applications mixed with the maintenance fertiliser. The annual maintenance lime requirement will vary with type of fertiliser used, soil type, stocking rate and rainfall, but will typically be around 100 kg/ha. Doug Edmeades and I agree that the new fineness standard for aglime should be 100% minus 0.5mm and 50% minus 0.2mm. To that I would add 25% minus 0.1mm. The actual weight of lime applied must be adjusted according to the CaCO3 content of the actual product used, as commercial lime can range from 80-98% CaCO3.
Where farm budgets simply do not permit capital application of lime, the alternative is to incorporate Quin Environmentals’ “PORTAL” lime-sparer additive with the annual maintenance fertiliser and lime application. PORTAL precipitates soluble aluminium and manganese present in the soil, reducing the need for capital lime and improving P uptake. Where Al and Mn toxicity is not a problem, ryegrass and clover will perform well at soil pH levels of 5.2 and below. In addition, it takes less lime to maintain a soil pH of 5.2-5.5 than one of 5.6-5.8.
Lime as fine as I have specified may be a problem for some contractors, who will be concerned about the risk of bridging in fixed-wing aircraft hoppers and helicopter buckets. Options include the use of flow agents, attaching vibrators to hoppers and buckets, mini-granulating the lime, or fluidising the lime/fertiliser mix. I have come to view the latter as the way forward.
On dairy farms, and on the more intensive sheep and beef farms, the use of ‘super’ is giving way to DAP, due to increased fertiliser N usage and relative costs of P, especially on an applied basis. I believe that overall, this is a good thing, as efficient elemental S can be added to exactly meet S requirements. Super on the other hand contains a fixed 1.4 to 1 ratio of S (all as sulphate) to P, double the actual agronomically required S to P ratio. On many soils the sulphate is so easily leached (taking valuable cations with it), farmers have to use elemental S-fortified ‘sulphur-super’ instead. On the volcanic ash soils that do retain the sulphate in plant-available form reasonably well, the excess sulphate builds up over years to the point where the soil will not hold any more, and then the excess gets leached, taking cations with it.
But DAP contains no Ca, and so it is becoming increasingly important for dairy farmers to maintain soil and pasture Ca levels by ensuring that adequate quantities, again of a sufficiently fine lime, are applied mixed with the maintenance fertiliser. If P is being applied twice a year, each application should include typically 100kg/ha of lime.
Optimising soil and herbage Ca levels is particularly important on dairy farms, where high applications of potash are practiced to maximise pasture growth, but which increase the risk of mineral imbalances in the animal if other cations are marginal.
Given the massive advantages in fertiliser N efficiency that are achieved by applying urea in fluidised form containing a urease inhibitor, and the increasing frequency with which N is being applied to dairy farms, it is to me a no-brainer that fluidised application of all fertiliser and lime is the way forward on intensive farms. With increasing improvements in fluidisation technology, total costs per unit ‘plant-effective nutrient’ or PEN are now fully competitive with conventional application, and the benefits and convenience (including being able to mix anything with anything, solid or liquid) are immense.
And finally, magnesium
After decades of maintaining (more or less) soil pH and Ca levels, but applying virtually no magnesium (Mg), we are causing imbalances in the ratios of Ca, Mg, K and Na in pastures. Dairy farmers have become aware of this more quickly, through flow-on metabolic disorders in cows, and direct animal supplementation (and/or addition of Mg and/or Na products to fertiliser) is now the norm rather than the exception.
The most cost-efficient way forward longer term is to ensure that there are adequate quantities of all cations in the soil itself, and avoiding any particular one to become excessive relative to requirements. Note I am not saying the various cations need to be in some precise ratio to each other, or in a very narrow range of base saturation percentage. Pasture is a very efficient forager for nutrients – we just need to ensure that the quantities of each cation are adequate, that none are present in excessive amounts that will interfere with uptake of the others, and that concentrations of toxic Al and Mn are minimised.
What we now think of as ‘lime’ – straight ground limestone – needs to become accepted as a mix of reasonably fine lime and a suitable form of Mg. Very finely ground serpentine rock (which contains about 18% Mg) will typically be the most cost-effective where soil Mg levels are currently good and only require maintenance, but increasingly this is not the case, and magnesium oxide (MgO) usually represents the most cost-effecive approach to putting things right quickly. And of course, where transport costs over long distances do not make it too expensive, dolomite is a great option.