Exchangeable cations – and what they can do for you!

Take home messages

• Clay soils, with poor surface structure because of excessive sodium, respond well to gypsum application. Application of gypsum to these soils is typically an economically attractive proposition. Non-clay soils with poor surface structure require different management approaches, such as variety selection to improve crop establishment
• Poor subsoil structure as a result of excess sodium is difficult to address. Deep ripping with considerable gypsum placed into the rip-lines has been successful, but it is expensive and has not been widely evaluated
• The ratio of exchangeable calcium and magnesium in soil will not influence plant growth, except at extreme values seldom encountered in agricultural soils
• Evaluate “novel” nutritional strategies yourself using strip trials, before adopting them on a broad scale.

Preamble

We have been asked in this paper to address two issues

• Dispersive sodic soils – why are they hard to manage, and what about amelioration.
• Calcium and magnesium – do ratios matter?

These are both topics we have addressed previously in GRDC Updates, so in addition to the material we present here, we refer you to earlier papers.

Our 2015 paper gave a lot of the underlying chemistry that drives the adverse effects of excess exchangeable sodium, and also explains how adding calcium (as gypsum or lime) can help. We also covered some of the impacts sodic soils can have on plant nutrition. You can find the paper here: Soil sodicity chemistry physics and amelioration.

Earlier this year we presented a paper that primarily considered salinity and sodicity impacts on subsoil conditions, and we gave some viewpoints on how we might go about ameliorating subsoil constraints. We will reproduce some of that paper here, with the full coverage available here: Subsoil constraints.

Finally, the question of cation balance, and the idea of an “ideal cation ratio” was something I (Menzies) was asked to speak about in 2004. At the time I thought this was a foolish request, because we have known for a long time that no such ratio exists. However, as Peter and I researched this question we found that it has been raised repeatedly through time – so we are not really surprised that after 15 years we are asked to repeat ourselves. The 2005 paper is not available online, so it is extensively reproduced here. Please excuse this self-plagiarism!

Introduction to salinity and sodicity

The presence of excess sodium in soil presents a range of challenges for plants growing in the soil, and some significant problems for growers managing these soils. Where there is free sodium chloride (NaCl) salt in the soil (a saline soil) the plant is presented with an osmotic challenge – it is difficult for it to take up water.  The high levels of chloride (Cl) can be directly toxic to some plants, and the high levels of sodium (Na) can interfere with uptake of other cations, especially calcium (Ca).  At much lower levels of sodium, where there is no free salt, but where the cation exchange has reasonable levels of sodium (>6% saturation), clay can disperse causing soil structural problems (sodic soils). This is the situation we encounter most frequently and will be the focus of this paper.

Sodic soils is an enormous subject, and it would take a reasonable size book to relate the state of our understanding of the subject. For those of you who are interested, we refer you to the excellent book edited by Malcolm Sumner and Ravi Naidu “Sodic Soils, Distribution, Properties, Management and Environmental Consequences”, 1998 Oxford University Press, New York. We have drawn on it liberally in this paper. Within this paper we address several sodicity constraints, in each instance initially covering the underlying science, then providing some commentary on strategies to ameliorate the problem, including when these are likely to work and when they are likely to fail.

In this paper we are primarily focusing on subsoil constraints because they represent a considerably greater challenge to correct than surface soil problems. Nevertheless, because interventions to address the subsoil problem often focus on the surface – for example surface application of gypsum - surface soil condition will frequently come into our discussion.

We recognise a field as having a subsoil constraint when crops are unable to proliferate roots at depth to exploit water and nutrients that may be present there. We most commonly see this expressed as fields where the crop is particularly susceptible to water stress – and if we go to the effort of assessing the soil profile moisture distribution under these water stressed crops, we find that the surface soil is dry, but considerable water may remain in the subsoil. Things become more complicated when we start to consider why the crop’s roots have not been able to exploit the subsoil, as there are a range of problems that all result in the same endpoint; failure of roots to grow to depth. From a practical standpoint, this is a substantial problem as there is no single treatment that can be applied to address all of the problems – different problems need different interventions.

Potential subsoil constraints include salinity, high bulk density, lack of aeration, calcium deficiency, sodium and chloride toxicity. Discussion of these various constraints is made complex by their inter-related nature. A subsoil that contains a lot of sodium chloride, will impact on plant growth because of the osmotic effect of the salt making water less available.  It may also result in a toxicity effect because of the high sodium and chloride concentrations present in the soil solution, or the high Na concentration may induce calcium deficiency.

Finally, there is little truly new here from a scientific perspective. That sodic soils are a problem, and that gypsum is a part of the solution, has been known for a long time. Here is a view from 120 years ago –

“This evil admits of cure by treatment with gypsum” (Warington 1900)

Our problem is knowing how to apply our scientific knowledge in a practical and economically attractive way – so this paper will step away from science a little in order to focus more on the practical.

Effects of sodium on the physical properties of soil

Sodic soils have extremely poor physical characteristics which, in agricultural soils, lead to problems managing soil water and air regimes. The adverse effects of sodicity on surface soils, and their impact on crop performance are well known. The lack of soil structural stability results in dispersion of the surface during rainfall to form a seal. This seal limits infiltration, partitioning a greater proportion of rainfall to runoff.  This reduces water availability for crops growing in the soil and increases the risk of erosion. On drying, the seal hardens as a crust that can prevent emergence of germinating seeds and result in poor crop establishment. In addition, sodic soils are difficult to cultivate and have poor load-bearing characteristics. In the subsoil, poor structural stability as a result of excess sodium is likely a key contributor to high bulk densities, and consequent low hydraulic conductivity and poor soil aeration.

These behaviours are a result of the influence of sodium on the clay fraction in the soil. When the cation exchange is occupied by calcium or magnesium, the individual clay platelets aggregate as illustrated in Figure 1, and the soil behaves in many respects like a silt or sand because the aggregates of hundreds of clay platelets constitute ‘particles’ of similar physical size. An aggregated clay soil has good structural characteristics. When the exchange is occupied by sodium, the individual clay platelets repel each other, and these aggregates of clay platelets break up. The soil structure is destroyed, the clay disperses in water and is easily eroded. The breakaway gullies we commonly encounter in duplex soils are an excellent illustration of how easily sodium saturated clays are eroded – the soil literally melts away. In a cultivated soil, much lower levels of sodium saturation result in the various adverse agronomic outcomes that occur as a result of only a small portion of the clay dispersing. It is always important to remember that sodicity is a problem that impacts on the clay fraction of the soil. In a sand with little clay fraction, sodicity will not result in adverse physical conditions, though there may still be adverse chemical effects as we will discuss later.

Figure 1. Scanning electron micrograph of aggregated clay platelets

Figure 2. Breakaway gully formation illustrates the ease with which sodium saturated clay can be dispersed and eroded

The composition of the soils cation exchange capacity (CEC) dictates the soils’ physical behaviour, with dispersion being the interaction of multiple factors including the type of cation held on the CEC, clay mineralogy, soil texture, organic matter content, etc. Hence, rigid ‘rules’ and classification systems are not particularly effective at predicting soil structural behaviour. This is where an understanding of the underlying processes (and a great deal of experience) are needed to permit us to manage soil sodicity.

At a mechanistic level, two processes, ‘swelling and dispersion’, are responsible for the behaviour of sodic soils, with these two processes governed by the soil surface charge and how it is balanced by exchangeable cations. You can find our refresher on introductory soil science in the 2015 paper flagged on the first page. We will not repeat this material here, other than to restate the take-home message that the extent of soil swelling in soils with montmorillonite type minerals (Vertosols), and the risk of dispersion of clay particles, increases as the level of exchangeable sodium increases.

The most common ameliorant applied to sodic soils to correct soil structural problems is gypsum. It acts to promote flocculation through two mechanisms; increasing soil solution ionic strength and by supplying divalent calcium ions to displace monovalent sodium from exchange sites. The first of these effects (increased ionic strength) is immediate and can be achieved by relatively low rates of gypsum application, but the effect is short lived (especially if the application rate is low). The effect of replacing sodium with calcium is permanent, unless additional sodium is supplied which will displace the calcium (for example, through the use of poor-quality irrigation water).

Gypsum application can be particularly effective as a means of improving soil surface conditions, providing better soil tilth and reducing crusting. However, the effectiveness of gypsum application in improving subsoil conditions is less well established. If we are considering surface application of gypsum, then the rates of application needed to displace sodium from the exchange throughout the soil profile would be considerable. In Table 1, we have provided data for a grey clay soil and the calculated rate of gypsum required to reduce the sodium saturation of the cation exchange capacity to below 5%. The assumptions here are that replacement is perfect, i.e. that all of the calcium from gypsum replaces sodium and none is leached, the gypsum is pure, and the soil bulk density is 1.2g/cm³. For this soil, 17 t/ha would be required to replace sodium to 60 cm, and 33 t to treat to 90 cm. Clearly these rates of application would not be economically attractive! Even if they were, we would need to consider the time it would take to move the gypsum derived calcium through the soil profile. Gypsum solubility is approximately 2.5 g/L, so it will take 40 mm of rainfall to dissolve 1 t/ha of gypsum. In an environment with 700 mm/year rainfall, it would take 2 years for the gypsum to dissolve from the soil surface. However, it will take a great deal longer for the gypsum to move to depth. If we assume that 10 mm/year leaches beyond 90 cm (this estimate is derived from salinity / leaching work on the western Darling Downs), then it would take 65 years for the gypsum to reach the bottom of the soil profile.

Table 1 provides us with a couple of really important points to keep in mind when thinking about sodic subsoils and their management. If the only option we have available to address subsoil sodicity is the surface application of gypsum, then it will take a considerable investment in gypsum, and a long time to see the benefit. There is also some cause to question if simply leaching calcium into a compacted sodic subsoil would help. The high bulk densities in sodic subsoils reflect the effect of high overburden pressure (the weight of the overlying soil) limiting shrink swell behaviour. In order for aggregation of the subsoil to occur, reducing the bulk density and permitting water and air movement, the overlying soil must be moved upward. As a thinking exercise; if we wanted to reduce the bulk density of a soil from 1.6 to 1.4, from 30 cm to 100 cm depth in the soil profile, the surface level of the soil would need to rise by 10 cm – effectively lifting 15,000 t/ha.  Where would the energy to do this come from?

Deep ripping combined with gypsum application has been reported to result in increased crop yields, but the effect has often been short lived (2-3 years). However, it is important to look at what was done, and if we would expect long-term benefits. For example, McBeath et al. (2010) used a gypsum slurry to place gypsum into the subsoil, but the rate of application they could achieve was limited; in one soil they applied 0.5 t/ha, in another 1.4 t/ha. If we consider this in the light of the preceding discussion of short-term (increased soil solution ionic strength) and long-term (replacement of sodium by calcium) effects, the low application rate used would only provide the short-term benefit. In contrast, Armstrong et al. (2015) used deep ripping with 7.5 t/ha of gypsum placed into the slot with a boot on the ripping tine. This rate of gypsum would be expected to provide both a short-term benefit, and a longer-term benefit by lowering exchangeable sodium levels. Substantial yield benefits were obtained for the four-year duration of the trial, and it is reasonable to expect that they would be sustained beyond the life of the trial.

Table 1.  Exchangeable cation data for a grey clay soil, and the estimated gypsum requirement for different depth increments in the profile, and rainfall required to leach this gypsum into the soil.

Ripping gypsum into the subsoil speeds the rate at which a beneficial effect on the subsoil could be expected. Deep placement would also be expected to reduce the total amount of gypsum needed. This is because we do not need to treat the entire volume of subsoil in order to exploit the water it contains – water can move from untreated soil to roots growing in treated rip-lines. Ripping invariably brings some of the highly sodic subsoil to the surface, so a surface application of gypsum is also needed to ameliorate this subsoil material.

While the preceding discussion of remediation of subsoil sodicity has indicated that we still have much to learn, the situation for surface soils is much more clear-cut. Surface soil amelioration is achieved at much lower rates of gypsum application than is required to displace all of the sodium. The expectation from these smaller additions is that they will help to ameliorate the surface soil, increasing infiltration, and encouraging more uniform crop establishment. Repeat applications are typically needed to sustain the surface soil improvement. Such small applications can be economically attractive. In the GRDC funded Combating Subsoil Constraints project (SIP08), a one-time surface applied gypsum @2.5 t/ha increased cumulative gross margins by $207/ha over 4 crops (wheat 2005, chickpea 2007, wheat 2008 and sorghum 2009-10), removed 115 t sodium chloride from the rooting depth and increased plant available water capacity by 15 mm (Dang et al. 2010). Unfortunately, gypsum application is not always profitable, and more effective prediction of gypsum response is needed.

We need to divert briefly here to discuss alkalinity. Where sodium is the dominant cation in soil solution, the pH can rise to higher values than occurs where calcium or magnesium is the dominant cation. This is a simple reflection of the solubility of the respective carbonates of these cations. Calcium and magnesium carbonates are not very soluble, while sodium and potassium carbonates dissolve readily in water to form solutions with very high pH (once again, more detail in the 2015 GRDC Update paper).  So, we can have soils that contain a great deal of calcium present as CaCO₃ but still have a high level of sodium on the CEC. If we add gypsum to an alkaline subsoil to try to increase the exchangeable calcium, the calcium supplied will precipitate as CaCO₃.   This will lower the soil pH but will not raise the exchangeable calcium concentration as much as desired. If the soil pH is greater than 8.5, precipitation of calcium as CaCO₃ is likely. An alternative approach to this problem would be to add an acidifying agent (like elemental sulfur) to lower the pH, increasing the solubility of the CaCO₃ present, and in this way reducing the exchangeable sodium. The rate of sulfur needed to achieve a pH change is much lower than the rate of gypsum to achieve the same pH change, so deep injection of sulfur with ripping may be an effective strategy for dealing with sodic subsoils containing CaCO₃. While plants growing on alkaline soils express a range of nutritional problems, most of these would only be apparent when the whole soil profile is alkaline and are unlikely to be a problem when it is only the subsoil that has an excessively high pH. Sulfur application as suggested above is intended to address soil physical problems, rather than nutritional ones.

Finally, it is worth considering by what mechanism a sodic subsoil may be limiting root growth, and the likelihood that this can be ameliorated. The most direct effect of a sodic subsoil on root growth would be that the high bulk density, and resultant high soil strength, prevent roots from penetrating into the soil. Amelioration of this effect would require a change in bulk density and soil strength; something that may be achieved rapidly through deep ripping (and hopefully stabilised by deep placement of gypsum) within the cultivated layers, or potentially achieved over a much longer timeframe through gradual leaching of calcium to depth and the effects of repeated wetting and drying to develop soil structure. Closely related is the potential that root growth into sodic subsoils is limited by poor aeration. Given the very low air-filled porosities that are considered to exist in these dense subsoils, this is a very likely limitation to root growth and function. Like the soil strength limitation, this could only be overcome by changing subsoil structure. Another potential limitation to root growth is calcium deficiency. Under conditions of inherently low soil solution calcium, combined with high concentrations of soil solution sodium, plant roots may suffer calcium deficiency. Calcium cannot be translocated through the plant to growing root tips - it must be present in adequate levels in the soil solution at the place the root is growing. Root elongation is profoundly affected by calcium deficiency, without any distinctive symptoms showing on plant tops – other than the crop being susceptible to drought because of its poor root system. If calcium deficiency was limiting root growth, simply supplying more calcium, without the need for soil structural change, should improve root growth. Of course, these various limitations do not exist in isolation from each other, and poor root growth in a sodic subsoil is likely to be the net result of several limitations.

Effects on excess salt on plant growth

Salinity is considered to reduce plant growth and performance by several mechanisms including alterations in water relations within the plant, deficiencies or toxicities, and oxidative stress. Salinity is also considered to reduce the plant availability of soil water, with the osmotic effect on soil water potential reducing plant water uptake. As the salinity of the soil increases, so does the soil water content at which permanent wilting point (PWP) is reached. Saline subsoils may therefore remain wet even when the crop growing above them has wilted.

The amount of salt in a soil profile, and its distribution, reflects the balance between long-term input of salt, and the leaching of this salt from the profile in deep drainage. In dryland systems salt input is predominantly in rainfall, while in irrigated systems salt input in irrigation water is likely to dominate. If we change the hydrology of the soil profile, causing more leaching without adding to the salt input, then the profile salt content will drop. This effect is well demonstrated by the Brigalow Catchment Study, where an area of brigalow scrub was cleared in 1982 for pasture and for cropping, and the salinity in the profile monitored periodically (1983, 1985, 1987, 1990, 1997, 2000). Silburn et al. (2009) provide an excellent analysis of the study and its implications. Before clearing, soil under the brigalow scrub contained about 40 t/ha of sodium chloride (NaCl) in the surface 1.5 m of soil, and the level of deep drainage was low (0.13 to 0.34 mm/yr). Use of the land for cropping, increased deep drainage to 19.8 mm/yr, causing displacement of salt from the soil profile. At the new equilibrium for the cropped soil profile, almost all of the 40 t/ha of salt in the surface 1.5 m will have been displaced. The approach to equilibrium is exponential, with rapid gains initially as salt is displaced from the upper layers of the profile, but progressively slower changes as salt is leached from lower levels. Silburn et al. (2009) calculated time to equilibrium at 50 to 200 years – but considerable reduction in the surface 1m was apparent within the first 6 years.

In dryland agriculture, our only option for increasing the leaching increment (beyond the change already achieved by converting native vegetation to cropping) is by increasing infiltration. If we can improve surface soil structure, resulting in greater infiltration and less runoff, we should be able to displace salt from the subsoil, increasing the depth of water extraction by the crop and the lower-limit soil water content. It is interesting to note that this did not happen in the brigalow catchment study where no change in lower-limit water contents or depths of soil water extraction were recorded. In this instance, the starting salinity levels were only marginally limiting to crop growth, so perhaps a change in crop performance should not have been expected. However, other factors complicate our analysis of the system. Leaching sodium chloride from a saline soil can leave us with a sodic soil, with its attendant poor soil structure and permeability. Of course, improving surface soil structure and infiltration will provide yield benefits in many years – reduction of subsoil salinity would be a bonus!

The preceding discussion has primarily considered the osmotic effect of salinity. Salinity may also limit crop growth through toxicity of sodium and chloride. While the mechanism of limitation to the crop is different, the same solution to the problem works for specific toxicities and the osmotic effect – reduce the levels of sodium and chloride by leaching them out of the root zone.

Of course, it may not be possible to remove the salt, so we need to consider strategies to “live with” the problem. Selection of crops that are more tolerant of salinity/sodium/chloride provides an option to manage the problem, and investment in screening/breeding for tolerance to high sodium and chloride could considerably improve this option for the future.

Introduction to cation balance

Our early understanding of crop nutritional requirements and their response to soil conditions came through observation; the progressive development of hypotheses about soil-plant relationships which could then be rigorously tested.  An example of this is the pH – nutrient availability diagram that we see used everywhere to describe the effect of pH on nutrient availability (you can find this in Helen McMillan and John Small’s update paper in the reference section). This diagram was initially proposed by Pettinger (1935) and later modified by Truog (1946). We now understand that the relationships depicted in Turog’s diagram describe what was common for the soils on which he was working (north-eastern US) but are by no means universally applicable.  Indeed, the diagram is probably wrong as often as it is right, and it is a shame that it is so frequently reproduced.

The concept of ideal cation saturation ratios has a similar history. During the 1940’s and 1950’s there were a series of reports proposing “ideal” proportions of exchangeable cations in soil (Bear et al. 1945; Bear and Toth 1948; Graham 1959). The proposed ranges were 65 to 75% Ca²⁺, about 10% Mg²⁺, 2.5 to 5% K⁺, and 10 to 20% H⁺, or approximate ratios of 7:1 for Ca/Mg, 15:1 for Ca/K, and 3:1 for Mg/K. Without question, soils with this cation make-up would not present any problems for plant growth with respect to these nutrients. However, our question is, “will plants grow better if we adjust the cation ratios of the soil to these values?”  A couple of key points need to be made about this approach

• The method was proposed by scientists working in areas of the USA where there are very good soils with negligible nutrient element deficiencies. At the time this work was performed, fertiliser applications were not required to overcome deficiency of the cationic nutrient elements to achieve profitable production. That is, adding cationic fertiliser to these soils usually had no impact on production. The method requires measuring the cation exchange capacity of soils that is balanced by calcium, magnesium and potassium. Then fertiliser advice is provided to achieve the desired ratio of these elements balancing the surface charge. The method does not involve measuring production responses to the added fertiliser.
• We now understand that most of the exchangeable H⁺ that we measure in soils is an experimental artifact; it does not really exist. The exchangeable H⁺ that was measured resulted from an increase in surface charge capacity (CEC) as a result of using a high ionic strength saturating solution (commonly 1 M) (van Olphen 1977). With the development of more appropriate methods of measuring cation exchange capacity (eg (Gillman and Sumpter 1986) exchangeable H⁺ is not found at measurable concentrations except in the most acid soils (pH<4.5).

During the 1940’s, Bear and co-workers conducted a series of studies at the New Jersey Agricultural Experiment Station investigating the growth of alfalfa (Medicago sativa L.). As part of this research Bear and co-workers proposed the “ideal ratio” of exchangeable cations in the soil. Since the publication of these ratio by Bear, it has been assumed by many that optimum plant growth will only occur when these ‘ideal’ conditions are met. This is despite Bear and co-workers’ acknowledgement that maximum growth will occur over a wide variety of cation ratios. In their work, the purpose of providing a high calcium saturation (65 %) was to allow maximum growth whilst also minimising luxury K uptake. Indeed, Bear and co-workers logic was as follows: (1) good growth occurs across a wide range of Ca:K ratios, (2) a high calcium saturation percentage limits luxury potassium uptake, and (3) “potassium is a much more expensive element than the calcium which it replaces” (Bear and Toth, 1948). Thus, the application of calcium to reduce potassium uptake was cheaper than applying potassium which would be taken up by the plant in luxurious amounts. All good science with pragmatic advice to growers.

At about the same time that Bear was conducting his investigations, Albrecht and co-workers were also conducting a series of experiments at the Missouri Agricultural Experiment Station. Much of their research investigated the growth (and N₂-fixation) of legumes, and examined the effect of soil fertility on plant palatability and the nutrition of grazing animals. In many of these studies conducted by Albrecht, clay minerals were extracted from the soil, subjected to electrodialysis which replaces the exchangeable cations with H⁺, the clay was then saturated with various cations such as calcium, potassium, magnesium, and barium. By mixing these clays at different ratios, Albrecht was able to investigate the effect of cation saturation on plant growth. As we will discuss shortly, this approach was profoundly misguided.  In this following material we have provided more detailed referencing than typical in an update paper – as we are directly critical of individual studies, it’s reasonable to provide the reference so our viewpoint can be checked.

Albrecht concluded that it is important to maintain a high calcium saturation percentage. Indeed, it was this observation which would eventually form the basis for much of Albrecht’s concept of the ‘balanced soil’. However, it would seem that the design and interpretation of the experiments used to demonstrate the need for a high calcium saturation were fundamentally flawed. Based on experiments with soybean (Glycine max L.), Albrecht (1937) concluded that (1) the nodulation of legumes in acidic soils is limited by low calcium concentrations more than by the acidity itself, and (2) plant growth and nodulation increase as calcium saturation increases. In fact, Albrecht later stated that “plants are not sensitive to, or limited by, a particular pH value of the soil” (Albrecht, 1975) and that “nitrogen fixation is related to acidity, or pH, only as this represents a decreasing supply of calcium as a plant nutrient” (Albrecht, 1939). However, examination of the data of Albrecht (1937) reveals that nodulation is indeed inhibited by soil acidity; nodulation only occurred when the pH was ≥ 5.5, and no nodulation occurred at pH 4.0, 4.5, or 5.0 at any calcium concentration. Similarly, although Albrecht concluded that growth and nodulation improve as calcium saturation percentage increases, soil pH values were not reported for any treatment, and due to the methodology used, any increase in calcium saturation would have undoubtedly been confounded by a decrease in acidity. Indeed, the various calcium saturations were achieved by mixing H-saturated clay (pH 3.6) (Albrecht, 1939; Albrecht and McCalla, 1938) with calcium-saturated clay (approximately pH 7.0) (Albrecht, 1939; Hutchings, 1936), thus giving clays of varying acidity (Albrecht, 1939). Using the same experimental system, Hutchings (1936) (who was working with Albrecht in Missouri) reported that a treatment containing 0 % Ca-clay (100 % H-clay) had a pH of 3.5, 25 % Ca-clay a pH of 4.3, 50 % Ca-clay a pH of 5.0, 75 % Ca-clay a pH of 5.7, and 100 % Ca-clay a pH of 6.9. In another experiment conducted by Albrecht (1937), calcium-saturated clay was mixed with barium-saturated clay. Here, the poor growth observed at low calcium-saturation (high barium-saturation) was most likely due to (1) barium toxicity (Barium is phytotoxic at < 500 µM), and/or (2) sulfur deficiency, due to the very low solubility of BaSO₄.  All very poor science indeed!

According to ‘The Albrecht Papers’ (Albrecht, 1975), Albrecht (1939) demonstrated that for a ‘balanced soil’, “65 % of that clay’s capacity (needs to be) loaded with calcium, 15% with magnesium”. However, it is unclear how these ‘balanced’ percentages were derived, as examination reveals that the rate of N₂-fixation (measured as the difference in nitrogen content between the plant and the seed) increased linearly with calcium-saturation – the greatest fixation actually occurring at the highest rate of calcium-saturation, i.e. 88 % (vs. the ‘balanced’ calcium saturation of 65 %) (Figure 2). Similarly, the work of Albrecht (1937) showed that both plant mass and nodulation rate increased linearly with increasing calcium saturation. Later, and notably after the work of Bear and Graham had been published, Albrecht stated that “extensive research projects served up this working code for balanced plant nutrition: H, 10 %; Ca, 60-75 %; Mg, 10-20 %; K, 2-5 %; Na, 0.5-5.0 %; and other cations, 5 %” (Albrecht, 1975). Whilst it is unclear as to the exact origin of Albrecht’s ‘balanced soil’, it appears likely that it relied, at least to some extent, upon the ‘ideal soil’ of Bear and co-workers.

That the “ideal” cation exchange ratio idea received so much attention at the time is surprising, given that, at the same time, other researchers were reporting that it did not work. Hunter and associates in New Jersey (Hunter 1949) could find no ideal Ca/Mg or Ca/K ratios for alfalfa (Medicago sativa L.), nor did Foy and Barber (1958) find yield response of maize (Zea mays L.) to varying K/Mg ratio in Indiana. A comprehensive and elegant demonstration of the failure of the approach is presented by the glasshouse and field studies of McLean and co-workers (Eckert and McLean 1981; McLean et al. 1983), where calcium, magnesium and potassium were varied relative to each other. They concluded that the ratio had essentially no impact on yields except at extremely wide ratios where a deficiency of one element was caused by excesses of others. They emphasised the need for assuring that sufficient levels of each cation were present, rather than attempting adjustment to a non-existent ideal cation saturation ratio.

One of the reasons that the cation saturation ratio idea has persisted, is that, in very general terms, there is just enough “truth” in it to make it seem reasonable. The “truth” here being that if one cation is present at high levels, it can interfere with the uptake of other cations. So, let’s explore cation nutrition of plants a little more deeply - if you can do without a refresher in “Plant Science 101”, feel free to skip the next section.

Plant cation uptake

Some people view the general finding that cation ratios do not matter as difficult to reconcile with their apparent success in predicting nutritional difficulties under extreme conditions. To understand this question, we need to consider the process of cation uptake by plants.

Let’s start with potassium, as it is the cation plants require in the greatest amount, and the cationic nutrient with the most specific uptake pathway. A key feature of potassium nutrition is the high rate and efficient means by which potassium is taken up and distributed throughout the plant. There are various potassium uptake systems, mainly specific (meaning they only work for potassium) channels in the plasmalemma (the outer membrane of plant cells). Two main groups of transporters can be identified; a high affinity group which are very selective for potassium and reach their maximum uptake rate at low soil solution potassium concentrations, and a low affinity group which are less selective and require much higher soil solution potassium concentrations to reach their highest uptake rate.  Both types of uptake system work in conjunction with a plasmalemma proton (H⁺) pump (Figure 3). The proton pump pushes H⁺ out through the plasmalemma, creating an electrochemical gradient (more negative on the inside). This gradient then provides a driving force to push potassium into the cell, but it also provides the same driving force for other cations.  The low-affinity transporters can be viewed as a gate which shows a preference for potassium – the gate will only open to permit a potassium ion to enter. However, this uptake approach will only work when there is a reasonable concentration of K⁺ in the soil solution. At low solution concentrations the uptake process has to work against a strong concentration gradient. The soil solution may contain only 10 µM of potassium, while the cytoplasm (the semi-fluid contents of the cell) will contain around 80,000 µM K⁺. This concentration gradient more than cancels out the electrochemical gradient (the inside of the cell is at -120 to -180 mV) – so potassium uptake is prevented. The plant can get around this problem by using its high affinity transporter, but this costs a lot more energy.  When you consider that 25 to 50% of the energy flow in a root hair cell is used to drive the proton pump, to spend still further energy on nutrient uptake is something the plant would only do when it needs to.

The benefit that the plant derives from having these two mechanisms is that it can obtain its potassium requirement in the most efficient manner. As a result, the capacity of the plant to take up potassium is not directly related to potassium in solution.  The uptake relationship for potassium in Figure 4 is strongly curved – the plant can obtain a lot of potassium despite low soil solution levels.

The presence of high concentrations of other cations in the soil solution (e.g. Na⁺, Ca²⁺, Mg²⁺), can interfere with the potassium transporters. Thus, high concentrations of these other nutrients can reduce potassium uptake, especially by the low-affinity uptake system. However, the high-affinity uptake mechanism is so specific, interference by other cations is never so great as to produce a true deficiency of potassium in the plant.  In very saline situations, uptake of sodium into the plant is sufficient to interfere with the plants use of potassium in the leaves, but it does not prevent potassium uptake from the soil.

Calcium represents the opposite end of the selectivity of uptake spectrum for cations. The principal role of calcium in the plant is to stabilise cell membrane and wall components – this is the outside of the “living” part of the cell. Inside the cell, the concentration of calcium is VERY low, and the plant expends a lot of energy to ensure that this condition is maintained. So, unlike potassium where the plant is actively transporting K⁺ into the cell, for Ca²⁺ the plant has transporters to pump it out of the cell.  Uptake of calcium is almost exclusively through the cell walls of young cells, with transport through the plant by the xylem (the “plumbing” system that moves water from the roots to the leaves). Calcium is required at the growing tips of the plant – without a continuous supply of calcium the growing tip dies (Figure 4). For shoot tips it is moved up from the roots by the xylem, for root tips the calcium must be in the soil solution where the root is growing.  The symptoms of calcium deficiency are very apparent.

Figure 3. Schematic representation of a root hair cell showing the proton pump and potassium transporters

Figure 4. Schematic representation of the relationship between nutrient supply and uptake.  The curved response for potassium reflects the plants specific uptake mechanisms for potassium, while the straight line response for boron indicates passive uptake (not actively controlled)

For calcium, the presence of other cations can result in the displacement of Ca²⁺ from the cell membranes and cell wall. This can be sufficient to cause calcium deficiency – deficiency induced by aluminium and manganese are common in acid soils. Of particular interest to us here is the potential for magnesium to displace calcium – is there any basis for the Ca:Mg ratio. Magnesium, because of its identical charge (2+) is an effective competitor with calcium for sites on cell components.

Calcium deficiency case studies

We present two case studies to demonstrate the failure of the ideal cation ratio to predict plant response. The first is the absolute extreme case where there is so much of one cation present that it does interfere with the uptake of other cations (the little bit of truth that sustains the rest of the fiction). The second case study considers less extreme conditions.

Example 1.  Calcium deficiency induced through the use of magnesium oxide as a liming material

With the development of a magnesium mining and refining industry in Queensland, the opportunity to use by-product MgO as a liming material became possible, and was considered a practical approach to ameliorating acid, magnesium deficient soils. Kylie Hailes undertook research on this issue for her PhD. In her work Kylie investigated amelioration of acidity using MgO, mixtures of MgO and CaSO₄ (gypsum), and compared this to lime. She measured short term root elongation of maize and mungbean as an indication of aluminium toxicity and of calcium deficiency, this is expressed as relative root elongation, the rate of elongation in a treatment as a percentage of the control treatment. We have removed the low pH values, where aluminium toxicity will have limited root growth, so that the primary factor influencing root growth is calcium supply. As you can see from the data in Figures 5 and 6, mung bean root growth reaches a maximum by 10% calcium saturation of the exchange, or a Ca/Mg ratio of 0.1.

Figure 5. The effect of Ca saturation on the rate of root elongation (a measure of Ca deficiency) for maize and mungbean in acid soils limed with MgO and mixtures of MgO and CaSO₄

Figure 6. The effect of Ca to Mg ratio on the rate of root elongation (a measure of Ca deficiency) for maize and mungbean in acid soils limed with MgO and mixtures of MgO and CaSO₄

This data demonstrates the competition effect on cation uptake.  When there is sufficient magnesium present it can interfere with calcium uptake to the extent that the plant was calcium deficient the root growth was impacted. We see this for mung bean when calcium saturation drops below 10% - a long way short of the 65% required by the Albrecht approach.  There is no consistent effect on maize, so the critical values would be even lower.

Example 2.  Calcium deficiency investigations by Albrecht’s co-workers

In reviewing this topic for a 2005 update paper, we (Kopittke and Menzies) considered many papers that directly evaluated the effect of cation ratios on plant growth, as well as other unrelated research that had produced data sets that were relevant to this question. This review was published in the Soil Science Society of America Journal (2007, 27:259-265).  As part of this paper we reviewed the effect of cation ratios on soil chemical fertility, soil biological activity, and soil physical properties, all without finding any evidence to support the idea of an ideal ratio in any of these areas.  The following are a couple of studies from Albrecht’s co-workers to demonstrate the lack of an effect on yield.

Specific cation ratios (i.e. Ca/Mg, Ca/K, and K/Mg ratios) were investigated by McLean, who had worked with Albrecht in Missouri during the 1940s, studied the effect of the soil Ca/Mg ratio on the growth of German millet [Setaria italica (L.) P. Beauv. cv. German] and alfalfa (McLean and Carbonell, 1972). It was concluded that plant yields were not affected by the Ca/Mg ratio within the Ca/Mg range studied (2.2:1–14.3:1; Figure 7).

Similarly, Hunter (1949), who had worked with Bear in New Jersey during the early 1940s, investigated the influence of the Ca/Mg ratio on the yield of alfalfa. Even though the experiment covered a wide range of Ca/Mg ratios (0.25:1–31:1), Hunter concluded that there was no “best” Ca/Mg ratio for optimum growth (Figure 8). Indeed, the Ca/Mg ratios of agricultural soils are seldom found outside of the range investigated by Hunter.

Figure 7. Effect of the exchangeable Ca/Mg ratio (2.2:1–14:1) on the relative dry matter yield of German millet in two soils. Data taken from McLean and Carbonell (1972). The dotted line indicates the “ideal” Ca/Mg ratio of 6.5:1 as stated by Bear et al. (1945)

Figure 8. Effect of the exchangeable Ca/Mg ratio (0.25:1–31:1) on the relative shoot dry weight of alfalfa at two P fertilisation levels. Soils were prepared by mixing calcium and magnesium saturated clays at various ratios, with K-saturated clays accounting for 10% of the total. Data taken from Hunter (1949). The dotted line indicates the “ideal” Ca/Mg ratio of 6.5:1 as stated by Bear et al. (1945)

Finally, and as an aside on the cation saturation ratio issue, advocates of the cation saturation ratios approach present an “ideal” situation as being a soil with a pH of 6.0 to 6.5, and a distribution of cations including 12% of the cation occupancy being by H⁺. To a soil chemist, this really calls into question the credibility of the approach; for the simple reason that it would not be possible for the exchange to have so much exchangeable H⁺ at this pH.  Vietch (Veitch 1904) recognized at the turn of the century that acid soil was aluminium saturated, rather than H⁺ saturated. Indeed, even if you deliberately saturate the exchange of a soil with H⁺, the acidity dissolves the soil minerals releasing aluminium which occupies exchange sites. So we never find H⁺ saturated soils. Furthermore, the higher the charge on a cation, the greater the extent to which it is retained by the soil. For example, the cation exchange of soils is dominated by calcium and magnesium (2+ ions) despite the soil solution being dominated by sodium (a 1+ ion); the divalent cations are typically held at least 10 times more strongly. Given that calcium is typically present at concentrations of 1 mM or higher in near neutral soils, then H⁺ would have to be present at this concentration or higher to give the level of saturation listed as ideal. Here is our problem – 1 mM of H⁺ is a very acid soil, pH 3.

The reason for the high H⁺ levels reported is the use of inappropriate (and very out of date) analytical approaches. Without going into detail, we now recognize that the amount of cation exchange on a soil varies with the pH and the ionic strength (concentration) of the soil solution.  As you increase either the pH or the ionic strength, the soil gets more negatively charged (higher cation exchange capacity), and it does this by losing H⁺ from the surface.  By measuring cation exchange with concentrated solutions at high pH, you get a cation exchange measurement that is too large, and you incorrectly measure a lot of H⁺ as being present.

To conclude this section on cation ratios we restate the take-home message we started with,

The ratio of exchangeable calcium and magnesium in soil will not influence plant growth, except at extreme values seldom encountered in agricultural soils.

And add to it Lipman’s conclusion from 1916 (Lipman 1916) when he reviewed the same topic.

“I have known of measures employed in soil management in this state, based on theory of the lime-magnesia ratio as first enunciated by Loew and later exploited by unscientific men, which to the rational-minded experimenter in soils and plants, appeared to be the veriest folly”

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