Subsoil constraints: the many challenges of sodium, chloride, and salinity and what you can do about them

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.

Subsoil salinity can be ameliorated by increasing the leaching increment to move the salt deeper into the soil profile.  In dryland agriculture, the only means of increasing the leaching increment is to increase infiltration (and reduce runoff). Gypsum application may help to achieve this.

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.

Introduction

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 (NaCl), 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 (Na) and chloride (Cl) concentrations present in the soil solution, or the high Na concentration may induce calcium (Ca) deficiency.

‘Subsoil constraints’ 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 subsoil 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.

While we are primarily focusing on subsoil constraints here, our interventions to address the subsoil problem often focus on the surface – for example surface application of gypsum. So, surface soil condition will frequently come into our discussion.

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 the 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. Last time I spoke to the GRDC on this topic, I provided a refresher on introductory soil science. We will not repeat this material here, but you can find the earlier talk at Soil sodicity chemistry physics and amelioration

The take home message is 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 CEC 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.

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.

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.

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 earlier GRDC paper  Soil sodicity chemistry physics and amelioration.  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, preventing 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, is a reflection of 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/y). Use of the land for cropping, increased deep drainage to 19.8 mm/y, 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.

References

Armstrong RD, Eagle C, and Flood R (2015) Improving grain yields on a sodic soil in a temperate, medium-rainfall cropping environment. Crop and Pasture Science 66, 492-505.

Dang Y, Dalal RC, Buck SR, Harms BR, Kelly R, Hochman Z, Schwenke GD, Biggs AJW, Ferguson NJ, Norrish S, Routley R, McDonald M, Hall C, Singh DK, Daniells IG, Farquharson R, Manning W, Speirs S, Grewal HS, Cornish P, Bodapati N, and Orange D (2010) Diagnosis, extent, impacts, and management of subsoil constraints in the northern grains cropping region of Australia.  Australian Journal of Soil Research 48:105-119.

McBeath TM, Grant CD, Murray RS, and Chittleborough DJ (2010) Effects of subsoil amendments on soil physical properties, crop response, and soil water quality in a dry year. Australian Journal of Soil Research 48:140-149.

Silburn DM, Cowie BA, and Thornton CM (2009) The Brigalow Catchment Study revisited: Effects of land development on deep drainage determined from non-steady chloride profiles. Journal of Hydrology 372, 487-498 .

Warrington R (1900) Lectures on some of the physical properties of soil.  Clarendon Press, Oxford, UK.