Down to Earth
Getting to know your soil
A soil is made up of:
Mineral particles – sands, silts and clays
Minerals that are attached to the soil particles and organic matter and are either available or unavailable for plants to absorb through their roots
Air and water
Micro-organisms - from bacteria to earthworms
The proportions of these give the soil its physical characteristics and productivity.
Soil texture is based on the relative proportions of sand, silt and clay. We can assess this absolutely in a laboratory, or we can make a good assessment in the field.
Sand is pretty inert. It takes in rainwater easily but lets it through easily. So, a very sandy soil will tend to hold little water. Plants take up water during the hours of daylight. So, the best time to water plants in a sandy soil is first thing in the morning. Because sands are largely tiny inert pieces of quartz they hold little in the way of plant nutrients. Nutrient holding capacity can be increased by adding compost, which is organic and has myriads of sites where plant nutrients can be held by chemical bonds.
At the other end of the scale are the clays, which are made up of microscopic particles that are chemically changed and have an enormous surface area compared to sands. Clays take in water more slowly than sands, but once it is absorbed it can be held for a much longer time for plants to use. So, it is less important when you irrigate plants in clay soils. Clays can hold on to chemicals that make up the plant food. However, clays have their downside too. They can be so dense that they don’t let water through very easily, and plant roots can have trouble penetrating them. Such clays may be excellent for building dams in, but they may be less useful for growing plants.
I don’t know of any Southwest soils that are dominated by silt. Silt has a soapy feel, and is a minor component of local soils.
Then we have the mixtures of sand, silt and clay that produce soils of varying value. An ideal top soil for many purposes is a sandy clay loam, which combines the good properties of sands and clays. SCLs let in water more easily than a clay, but hold it better than a sand. They allow water, air, roots and microbes to move in the soil more easily than in a clay, but hold more plant nutrients than a sand.
Soil profile describes how the soil texture and other characteristics change as you dig deeper into the soil. Generally, the amount of clay in the soil increases with soil depth. When this happens gradually we say the soil has a gradational profile; when there is a distinct and sudden increase in the amount of clay in the soil (subsoil) we call this a duplex soil. We are starting to find in the Southwest that duplex soils, particularly on sloping sites, can develop erosion problems some years after the sites are cleared of trees. This affects the land’s ‘Capability’ to sustain certain land uses. More on this elsewhere on the website.
Porosity is the term used to describe the amount of pores and passageways through the soil. It is in and through these pores that water and air move, roots extend and it is in the water-lined pores that the soil microbes live. Porosity is very important, and relies on the stable structure of the soil, which in turn is maintained by soil carbon. A healthy soil can have up to 50% pore spaces. An unhealthy, compacted soil has been likened to a demolished building. All the materials are still there but the rooms and the corridors are now filled with rubble.
What could possibly cause such compaction, such destruction? Machinery, quad bikes, the ute, concentrations of livestock and your gumboots are obvious causes. Raindrop impacts on bare soil or digging the soil to ‘aerate it’ may seem less obvious causes, but are just as important.
Keep the soil covered
Disturb it as little as possible
Improving soil porosity for soil health
The paper examines the key role of soil pore space in mitigating runoff and erosion, improving drought resistance, and facilitating the activities of the myriad organisms that contribute to soil health. The pore spaces are where the action is! The paper discusses how soil pore space is lost, regained and retained. In this context, one of the most ignored causes of loss of soil pore space, decreased infiltration, and initiation of soil erosion is the action of rain drops on bare soil.
Improved knowledge of soil biology and soil chemistry, termed ‘Soil Health’ comes at a critical time of uncertainty due to climate change. A significant proportion of the soils in the Southwest are degraded in some form or other, as are agricultural soils around the world. Yet it is these soils that must continue to be productive if they are to supply the needs of the growing world population. Along with the biology and chemistry of soil health must go soil physical conditions and soil hydrology. This paper provides some fresh interpretations of how we see these important factors.
Like crop husbandry and livestock husbandry, I commend to you the term ‘land husbandry’ to help create the right thinking that will lead to appropriate actions for the productive and ecological management of soil, land and water.
Within this land husbandry context, the paper concentrates on aspects of soil management that are required to facilitate improvement in soil health. In so doing the paper re-interprets causes of soil degradation to help focus on management essential to establish, improve and maintain the relationship between production and soil ecology.
The trouble with soil
Soils have become degraded and less productive. At times soils can set like concrete and at others they have the consistency of porridge, making it difficult to find a time when the soil can be ‘worked’. Water does not penetrate the crust on the surface, soil moisture is reduced while a larger portion of the rainfall runs away, taking the soil with it. In dry times wind erosion and dust are problems. There’s a need to apply more and more fertiliser to maintain crop yields. Does this sound familiar?
Runge and van Gool (1999) produced a comprehensive assessment of land degradation and land capability in the South West of WA, extending from Albany to Gingin and east to Merredin. In the document they highlight eight causes of land degradation. Five causes will be considered here.
Soil Structural Decline
Table 1 gives the percentage of the area surveyed within selected Shires that is ‘susceptible to moderate and high soil structure decline’ after Runge and van Gool (op cit). Of a total surveyed area of 3,747,090 ha, 34.4% showed susceptibility to moderate soil structural decline and 6.5% showed susceptibility to high soil structural decline. Runge and van Gool quote Needham et al, (1998) to explain soil structural decline as being caused by excessive tillage, with the extent of the problem relying on various soil factors. They identify that a common symptom of soils suffering structural decline is a crusting or hard setting of the soil surface, but do not explain its creation.
Table 1. Percentage areas of moderate and high susceptibility to soil structure decline in selected Shires based on the actual area surveyed (after Runge and van Gool, 1999)
In the section on Subsurface Compaction Runge and van Gool (op cit) again turn to Needham et al, (1998) for the causes, which are heavy vehicular traffic on tilled soils (‘traffic pan’) and repeated cultivation at the same depth over a number of years (‘plough pan’). Stock trampling can also lead to shallow soil compaction. Wilson (1986) noted that subsurface compaction increased the harmful effects of pathogens in the topsoil. Table 2 highlights the susceptibility to subsurface compaction in certain Shires after Runge and van Gool. Not surprisingly there are similar results between Tables 1 and 2. Soil structural decline and soil compaction go hand in hand.
Table 2. Percentage areas of moderate and high susceptibility to subsurface compaction in selected Shires based on the actual area surveyed (after Runge and van Gool, 1999)
Water Erosion, Wind Erosion and Phosphorus Export
In their treatment of Water Erosion, Runge and van Gool (op cit) assert that susceptibility to water erosion refers to the likelihood of soil being detached and transported as a result of rainfall, run-off and seepage. They go on to say that water erosion (as defined in their document) is caused by the surface or shallow subsurface flow of water, and is affected by factors which affect the amount of run-off (slope angle and length, soil permeability, cohesion and organic matter content) and the soil’s inherent erodibility. Wind erosion is caused by strong winds, a lack of soil cover and loose dry soil (Runge and van Gool, op cit). They do not explain the process by which surface soil becomes loose, but make the point that the main causes of wind erosion are over-stocking and inappropriate cropping systems. The main causes cited for Phosphorus Export are moving water and water erosion. Figures for susceptibility to water erosion and Phosphorus export have been combined in Table 3. A simple correlation between water erosion and Phosphorus export is not obvious from the figures. It would appear that other factors are involved, most likely among them soil type, fertiliser application and land and soil management (land husbandry).
Table 3. Percentage areas of moderate, high and extreme susceptibility to water erosion and moderate and high susceptibility to Phosphorus export in selected Shires based on the actual area surveyed (after Runge and van Gool, 1999)
Sixteen years on, we have better understandings of some of these processes.
Advances in understanding – the problem with rain drops
What has not been made clear in the above descriptions is the process of water erosion. Leaving aside stream bank erosion and tunnel erosion, the vast majority of water erosion is initiated when raindrops impact bare soil. Rainfall of 1mm falling on one hectare of land has a volume of 10m3, and a mass of 10 tonnes. So in a common rainfall event of 20mm the ground in one hectare is struck by 200 tonnes of raindrops hitting the ground at around 9.8m/s (35 kph), depending on the intensity of the rainfall . The energy in such rainfall is far higher than the energy in the resulting run-off water, as demonstrated by Hudson (1971). He compared the kinetic energy (energy imparted as a result of motion; KE = 1/2mass x velocity2) of rain and the runoff it produced. Assuming that 25% of the rain became runoff, he calculated that the rain has 256 times more kinetic energy than the surface run-off it generates. The result indicates clearly that rainfall has more energy to disrupt the soil surface than run-off flowing over the soil. Geeves (1997) found that the rate and degree of surface sealing were influenced primarily by raindrop kinetic energy with highly energetic impacts leading to significant surface sealing.
The kinetic energy at raindrop impact has to be dissipated. On bare soil this results in (1) soil compaction, (2) structural damage and detachment of soil particles from soil aggregates, breaking them apart, and (3) splashing soil particles into the air. Particles splashed into the air have one of three fates. If they land in runoff water they are entrained and carried downslope; if they land back on the soil surface they are small loose soil particles ready for water erosion by the next raindrop direct hit in this or the next rainstorm, or they are available to be eroded by wind in dry times. On sloping sites soil particles are splashed differentially further downslope than upslope. So, even without runoff, there is a movement of soil down slope. The third fate for splashed soil particles is to land back on the soil surface and be washed or sucked into a soil pore, thereby reducing its size and capacity to infiltrate water. The vast majority of soil crusts that Runge and van Gool noted are developed in this way. Any plant nutrients attached to these soil particles share their fate. Soil erosion is also nutrient erosion.
Reduction in surface soil pores and development of soil crusts reduce water infiltration. With nowhere else to go, the water runs off, and soil moisture storage is reduced. Run-off, flooding, drought conditions and soil productivity are all made worse when raindrops hit bare soil.
Soil cover is imperative to reduce raindrop impact on the soil surface. Soil cover is important for other reasons, amongst them minimising water evaporation, which causes loss of soil moisture and leads to capillary rise of salts, and reducing the harmful effects of the sun’s UV rays on soil microbes in the top 1-2cm of soil.
Advances in understanding – the problem with soil compaction and its link to Soil Organic Carbon (SOC)
Shaxson et al (2014) likened soil compaction to the demolition of a building. In this analogue, all the solid, mineral parts are still present, but the spaces in which people lived and worked are dramatically reduced and all the services to the building (power, water, ventilation) are destroyed. Soil compaction reduces pore spaces, particularly water-lined pore spaces in which the soil microbes exist and where the myriad chemical and biological activities of the soil occur. A compacted soil has reduced ability to store and transmit water, and reduced capacity for gaseous exchange between the root zone and the atmosphere above the soil. So, it is little wonder that compacted soils commonly produce poor crops or pastures. Clearly a well structured soil with a good arrangement of pore spaces is essential for microbial activity, root elongation, water holding capacity and movement of water and minerals – and earthworms – within the soil.
Paradoxically, cultivation to aerate the soil commonly causes loss of soil pore space in the long term. Aeration causes increased oxygenation within the soil, which stimulates the respiration of soil microbes, encouraging increased utilisation of their energy source, which is soil organic carbon (SOC). The increased microbial respiration also generates increased emissions of CO2 from the soil. Reicosky (2001) reported on experiments that found CO2 emissions of 229 g/m2/24 hours from soil ploughed to a depth of 280mm compared to an emission rate of 10 g/m2/24 hours from un-cultivated soil. The culmination of this is large scale reduction in SOC and the ‘soil services’ it supports. Reporting on a longitudinal study of 100 years in Kenya, Marenya and Barrett (2009) showed that SOC declined from 12% to around 2% following the conversion of woodland to agriculture, and the decline in SOC was paralleled by declines in plant size and cob weights of the staple maize crops. In Australia, Yin Chan (2008) suggests that there was a general halving of soil carbon in the thirty years from woodland clearing for agriculture, but a slow increase in soil carbon since reduced tillage techniques were introduced.
Soil microbes rely on SOC as their source of energy and nutrients. Loss of SOC, therefore, adversely affects the ‘soil services’ fulfilled by soil microbes. Amongst those ‘services’, it has been discovered that soil fungi exude a sticky material named ‘Glomalin’ (USDA, 2014). It is thought to contain up to 30% of the soil’s carbon, and importantly glues soil mineral particles together to promote or re-build soil structure. Similarly, mucilage and ‘glues’ from soil organic matter and other sources within the soil biota are likely to be contributing to the stabilising and reconstruction of soil structure and pore space. However, these processes rely on an on-going supply of dead organic matter as their substrate. Here is an underlying reason why minimum tillage or no-till farming methods with stubble retention are improving both soil structure and SOC, although the process is generally a slow one.
Advances in understanding – the plant root zone
It has been established that there is a symbiotic relationship between plant roots and the microbes in the root zone (rhizosphere) (e.g. Stapper, 2006). Plants have been found to exude carbohydrates, on which the microbes feed, and in return the suite of microbes attached to a particular plant help ward off pathogens, facilitate the flow of water and nutrients to and into the plant roots, and moderate the soil pH in the rhizosphere, allowing vigorous plant growth even in an acid soil (Stapper, op cit). The diversity of microbes associated with plant roots is in the order of tens of thousands of species. This complex plant - microbial community is crucial for plant health. Berendsen et al (2012) show that upon pathogen or insect attack, plants are able to recruit protective microorganisms, and enhance microbial activity to suppress pathogens in the rhizosphere. The rhizosphere is a lot more complex than previously thought. Yet to promote or maintain the beneficial activity of the soil microbes and allow the transmission of water and nutrients within the soil, the soil structure and, in particular the soil pore space has to be in good condition. Now we can better understand Wilson’s (1986) observation that subsurface compaction increased the harmful effects of pathogens in the topsoil. Compaction reduces the ability of the ‘good’ soil microbes to protect plants against pathogens. Soil management to increase pore space will have a direct benefit on crop health.
Discussion and conclusions
Within a land husbandry context, this paper emphasises the importance of soil pore space in soil and water conservation and in promoting the beneficial activities of soil microbes, which in turn promote productivity of the soil and the plants that grow in it. Maintaining surface pore space is essential for rainwater infiltration, and minimising runoff and soil erosion. Emphasis has been laid on the destructive effects of raindrops on a bare soil surface, and the need to keep the soil surface covered. In cropping systems, stubble retention is a primary method for achieving this, and ‘keeping the soil covered’ is enshrined as one of the three principles of Conservation Agriculture.
Mechanical structures such as contour banks and grade banks have been viewed for too long as the first line of defence against soil erosion. They are an important back-up and second line of defence for those very intense storms when rainfall intensity outstrips soil infiltration rate. However, to rely on them solely misses the point that erosion starts at the soil surface when it is allowed to be bombarded by high-energy raindrops.
‘Keep the soil covered’ should be equally applied to livestock farming. Overgrazing, particularly in summer and autumn can leave the soil exposed to wind erosion in dry weather and exposed to raindrop impacts, soil and nutrient erosion, runoff and reduced infiltration during summer and autumn rainfall (which tends to be intense), and exposed to the opening rains of the next autumn/winter. Three grazing management techniques are likely to reduce the incidence of bare areas in paddocks. The first is rotational grazing. By resting paddocks from grazing, the plants have increased opportunity to recover from grazing, re-cover the soil surface, and grow larger, thereby increasing annual pasture production compared to systems of set stocking. The second technique is reducing livestock numbers going into summer. However the possibility of doing this is largely controlled by marketing opportunities and arranging the whole livestock production system to suit. The third technique is known as deferred grazing, under which the livestock are removed from paddocks over summer and autumn, held in a well shaded holding paddock and maintained on conserved fodder and supplements. Such holding paddocks should be on level, stable sites, and should be rested from livestock for the remainder of the year.
Improving soil pore space starts with management of the soil surface; regaining and retaining pore space throughout the soil relies on the soil biota, and keeping it fed.
Soil cultivation increases microbial respiration, depletes SOC and leads to increased CO2 emissions from the soil and reduced soil structure and porosity. As shown above, soil microbes, their exudates and the mucilage from organic matter produce the ‘glues’ that can repair, maintain and improve soil physical conditions. No wonder the second principle of Conservation Agriculture is ‘minimise soil disturbance’. An exception would be deep ripping massive subsoils and plough pans to expand soil moisture holding capacity, promote increased root depth and root and microbial foraging for mineral nutrients, and alleviate seasonal waterlogging. However, the implement chosen should minimise soil aeration for the reasons provided earlier. (For completeness, the third principle of Conservation Agriculture is ‘use crop rotations’, however, this third principle is not discussed in this paper.)
Minimum or no-till farming remove the opportunity to use cultivation as a means of controlling weeds; other options are needed. Weed control is important for optimum crop growth and yield, purity and price. Herbicides are widely used to control weeds, particularly around the time of seedling emergence. Results from both field and laboratory research have shown symptoms of microbial stress (reduced new microbial growth) even at recommended application rates for up to six to eight weeks following the application of some herbicides (Gupta, n.d.). These effects were found to be soil type dependent. For example, in sandy soils with low organic matter and low microbial activity the negative effects of these herbicides were found to be greater. However, this six to eight week period is when the new crop seedlings should be developing symbiotic relationships with soil microbes and starting to build the rhizosphere to help ward off pathogens and improve availability of nutrients and water to the establishing crop. If used, herbicides need to be carefully chosen on the basis of their effect on soil biota, or alternative methods to cope with weeds need to be considered.
In this time of climate change and continued growth of human population we face an uncertain future, with some dire predictions if comprehensive mitigation of greenhouse gas emissions is not undertaken soon. Much of the land used for agriculture here and around the world is degraded and less productive than previously. Inputs to agriculture that are derived from fossil fuel are becoming more expensive and cause emissions of greenhouse gases. It is timely that more is being discovered about the remarkable properties and abilities of the ‘soil organism’, and ways are being discovered to naturally rejuvenate the soil and return it to higher productivity. In the quest to feed the world’s growing population the biological and chemical concepts and practices relating to soil health are sure to become more widely adopted. Within such land husbandry it will be equally important to adopt concepts of physical soil management for improved surface conditions and soil pore space so that maximum benefit can be gained from soil hydrology.
This paper draws heavily for its inspiration on the paper by Shaxson et al (2014). I am grateful to Mikkel Christensen for promoting my participation in the conference, and to the South West Catchments Council for its support.
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Geeves, G.W. (1997) Aggregate breakdown and soil surface sealing under rainfall. PhD thesis, Centre for Resource and Environmental Studies, ANU
Gupta, V.V.S.R (nd) Herbicides and life in the soil, CSIRO Land & Water Resources R&D Corp. Retrieved 10 August 2015 from:
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Shaxson, T.F., Williams, A.R. and Kassam, A.H. (2014) Land husbandry: an agro-ecological approach to land use and management Part 2: Consideration of soil conditions. International Soil and Water Conservation Research, Vol. 2, No. 4, pp. 64-80
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Wilson, J.M. (1986) Soil compaction, deep tillage and root disease, In: Perry, M.W. (Ed.) A review of deep tillage research in Western Australia, Western Australian Department of Agriculture, Div. Of Plant Research, Perth, pp.131-136
USDA (2014) Glomalin: hiding place for a third of the world’s stored soil carbon. Retrieved 10 August 2015 from: http://ars.usda.gov/is/AR/archive/sep02/soil0902.htm
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