Jones C.E. (2001). Stipa Newsletter No. 15, January 2001 pp. 4-9
Reprinted in The Australian Farm Journal, March 2001, pp. 60-63
The Great Salinity Debate: Part III
Soil organic matter: past lessons for future learning
Getting the basics right
In Parts I and II of this series, the issues of groundcover and its management were examined in a historical context. For Part III, the role of soil organic matter and the types of disturbance regimes required to enhance it, will also be placed in a historical perspective. It is important to understand where we've been in order to find a way forward.
The productivity and health of agricultural land depends on i) inherent landscape capability ii) long and short term seasonal effects iii) soil condition and iv) land management practices. The first two factors are beyond our control, the latter two are of fundamental significance for the future of rural Australia.
Have you noticed that we spend an inordinate amount of time and energy mapping landscapes and trying to predict the weather, all the while just "hoping" that soil health and land management skills will somehow find their own way? Land of even the highest productive potential can deteriorate rapidly under inappropriate management. Conversely, the health of degraded landscapes can improve markedly under regenerative management.
Processes such as dryland salinity, soil structural decline, nutrient decline, erosion, sedimentation and the eutrophication of waterways, all derive from inappropriate disturbance regimes which reduce the quality of perennial groundcover and, as a consequence, the levels of organic matter in and on the soil.
Intermittent disturbance regimes
In ecological terms, a "disturbance" is something which affects the growth or reproduction, or rearranges the order, of the components of an ecosystem. For example, mowing your lawn, spraying herbicide, using fertiliser or pulling out weeds are disturbances. Australian native grasslands cannot tolerate unremitting disturbance regimes, such as continuous grazing, or broadacre cultivation. Nor do they thrive when there is no disturbance at all. For maximum health and productivity, an intermittent disturbance regime is essential. Intermediate levels of disturbance also foster high levels of biodiversity, which give plant and animal communities the necessary resilience to cope with natural disturbances such as drought and fire.
Following the extinction of our megafauna thousands of years ago, the functioning of Australian landscapes became dependent on the interactions between the environment, aboriginal people and small native animals. The dietary requirements of native fauna such as dunnarts, planigales, potoroos, bandicoots, echidnas and bettongs included (depending on the species), grasshoppers, beetles, cockroaches, spiders, termites, ants, larvae, worms, small invertebrates, tubers, seeds, berries, herbs, roots, resin and fungi. The activities of these insectivorous/carnivorous/omnivorous animals were essential to maintaining the health of grassy woodland ecosystems, particularly soils (see Greg Martin's article, next issue).
Of particular importance to an understanding of soil health, is to recognise that at the time of European settlement, there were many thousands, indeed millions, of mouse-sized to rabbit-sized, nocturnal, ground foraging mammals, which turned the soil over while searching for a wide variety of foods including insects and fungi. These animals were not grazers. Aboriginal people also dug the soil in small patches to obtain yam daisy (Microseris lanceolata) and other tubers. Although the patch disturbances created by native animals and aboriginal people accounted for only a small percentage of the landscape in any one year, over longer time scales virtually all of the soil would have been turned over, providing an uneven surface and incorporating organic matter. Our environment was skilfully "managed" for sustainable production in keeping with the overriding influences of long and short term seasonal effects.
The loss of protective grassland habitat which accompanied the introduction of broadscale, unremitting grazing regimes, resulted in the rapid demise of ground foraging mammals. Their removal from the ecosystem, coupled with eradication programs for dingos (an essential predator) and grazers such as wallabies and wombats (considered to compete with livestock), created a void into which the rabbit population exploded. This voracious, rapidly breeding herbivore completely devastated what remained of native groundcover and the associated organic matter in many areas of southern Australia.
Over time, other grazers such as kangaroos (previously present in much lower numbers) and introduced predators such as cats and foxes (which occupied the niche of the displaced dingo), added further pressure to an ecosystem on its knees. In the absence of rejuvenating disturbance regimes, Australian soils, already reduced to a relatively inert, compacted state, suffered their final humiliation - broadacre cultivation. The organic matter content of most soils is now so low, and the previous levels so long forgotten, that the fundamental importance of organic matter to ecosystem function, including water balance, is rarely considered.
Soil organic matter - the missing link
When areas were first explored, or newly settled by Europeans, soils were variously described as mulched, peaty, soft, loose, friable and high in humus, even in relatively low rainfall areas. The spongy nature of the soil was frequently lamented. Horses stumbled in the soft conditions and sometimes broke their legs, drays were difficult to pull overland, and much of the rainfall sank straight into the soil to replenish waterways as basal flow, rather than immediately running off to refill the waterholes and creeks being over-utilised by stock.
However, these conditions changed rapidly. There were highly significant increases in sedimentation rates in lakes and lagoons at the time each area was first settled. During these periods of extremely rapid erosion, huge quantities of friable topsoil were lost. These events coincided with the removal of protective groundcover under the inappropriate grazing regimes used for domestic livestock. Prior to settlement, the groundcover was not heavily grazed over wide areas.
Somehow the details of these catastrophic events have become blurred by time. Perhaps the writings from the early settlement period should be compulsory reading? For example, in 1818, John Oxley described the grasslands of the treeless Liverpool Plains as being "of the richest description". In 1842, Leichhardt recorded the constituents of the soils as chiefly "clay and humus" and noted the obvious indications of many small native animals. Now the plains could best be described as "a mosaic of annual monocultures and bare fallows on poorly structured soils, high in clay, low in organic matter and devoid of most living things". The richness of the vegetation, the diversity of the animal life and the high humus content of the soil are factors long since forgotten.
Many of the wells sunk in the Liverpool Plains in the mid 1800s contained brackish water. The saline watertables in those areas today have not come from somewhere else, they have simply risen through inappropriate groundcover management. They will continue to rise unless the basic principles of land management are addressed.
Although the Liverpool Plains are about to detach themselves from the rest of Australia and float away on their own saline sea, their problems are by no means unique. The only factors which differentiate them from other salinity affected parts of the continent are soil type and the seasonality of rainfall, neither of which we can do anything about. However, we can change land management to increase the organic matter content, and hence the water holding capacity, of our soils. Humic materials have a far greater affinity for water than do clay colloids, which in turn have a greater affinity for water than coarse textured materials such as sand. There is no need to despair if you only have sand. You just need more humus!!
Today, most of our agricultural soils are compacted, hard-setting and lifeless. Many contain 2% or less organic carbon, much of which is the non-labile remnant of thousands of years of aboriginal burning, and as such is of no significance as a soil ameliorant. Yet time and again we hear that the organic matter content of soil is not as important as its "nutrient status" (which can apparently be corrected with the addition of fertiliser) and that soil water holding capacity is not as important as "high water use" (which can apparently be obtained by planting trees and introduced grasses). Our ecosystem is a little more complex than that. Were tonnes of fertiliser being applied prior to settlement? Was the landscape vegetated by high water use plants? No and No. So how can those factors restore the balance now?
Have we fallen into the trap of window-dressing a deteriorating landscape because we don't know what else to do? The addition of either fertilisers or high water use plants will do little to bring about the fundamental changes required. Nor will such simplistic solutions revitalise rural communities. Our current problem is that rain does not properly infiltrate where it falls, nor is it properly held in soil. On sloping land, the water moves across the soil surface, taking nutrients and fine soil particles with it, and becomes a watertable and sediment problem somewhere else. In flatter parts of the landscape, low levels of organic matter in soils result in far more water being lost to evaporation and deep drainage than is put to productive use.
In comparison to an equivalent area of trees, a healthy perennial grassland will have a greater distribution of roots at depth, higher levels of soil organic matter, higher levels of microbial biomass and greater soil water holding capacity. The reason for these facts not being widely known can only be that their importance has not been appreciated. Because humic materials are continually being generated by the decomposition of grass roots, grasslands regenerate soils far more quickly than do forests, especially if the grasses are pulse grazed (see Part I).
The emphasis to date in salinity research has been on de-watering soils rather than on controlling the movement of water in the landscape. Water is an extremely precious commodity and one which we need to use effectively to improve the productivity of our natural resource base. We lose production and we create a problem when water is not held where it falls.
Putting it all together
The Australian landscape is more resilient than we imagine, and finding a way forward may not be as hard as it seems. The two essential ingredients for healthy, porous, high water holding capacity soils are:-
i) as close as possible to permanent soil cover (plants plus associated litter) to provide protected habitat for soil biota, invertebrates and small vertebrates
ii) an intermittent disturbance regime to both stimulate biological activity and provide periods of rest and recovery
Provided these criteria are met, the enterprise choices and the plant species used are secondary considerations, although as outlined in Parts I and II, there are many good reasons for choosing native over introduced, or a combination of the two.
With creative thinking, innovative landholders are finding ways to implement the basic principles of high levels of soil cover and intermittent disturbance regimes into their day to day activities. Some of these are summarised and compared to more conventional approaches in Table 1. There could be limitless combinations and variations on these themes. For example, pulse grazed native groundcover can form a productive base for a variety of cropping, horticultural and silvicultural enterprises. The adoption of these practices does not necessarily require all landholders to be graziers. In the United States, there are full-time businesses based on "renting" livestock for these purposes. Too hard? Take a tour through some salt affected land and think again!
In Part I of this series the tendency for high water use plants to exacerbate dryland salinity by drawing up fresh water and bringing salt-laden water closer to the soil surface was noted. In addition to bringing salt closer to the surface, high water use plants can also over-dry soils. Rain simply runs off dry, poorly mulched soils, or enters deep cracks and passes through to the groundwater without properly re-wetting the topsoil. It is therefore important to recognise that we don't have to increase transpiration rates in order to reduce deep drainage. A better result can be achieved by increasing the water holding capacity of the soil. De-watering soils and restoring water balance are entirely different concepts, but appear to have become confused. Even more worrying is the notion that the apparently "incurable" nature of dryland salinity is justification to live with salt as best we can. However, if we simply try to live with the symptoms of an ecosystem out of balance, we will be de-watering saltier and saltier soils until even those options become untenable.
It is widely acknowledged that our vegetation, soils and water are seriously degraded and that this has happened extraordinarily quickly on a geological time scale. Our ecosystems are at the crossroads and the lights are red. Without the participation of rural communities, the regeneration of Australia's natural resource base will be impossible. But even participation is not enough. Simply planting and/or retaining native perennial plants will not reverse salinisation or any other land degradation process. Much more is required. New attitudes, new ways of looking at the land. New ways of looking at ourselves, and how we interact with the land. We need a deeper understanding of how Australian landscapes functioned prior to European settlement. What were the component parts? How did they fit together? How can we stimulate soil forming processes today?
We don't need any more strategic plans. For most of the landscape, we can make fundamental change without drastically altering traditional enterprises. In some areas, more diversification would be of benefit, and in others more trees and shrubs would have enormous ecological advantages. But there will only be one "solution" to our water balance problems. That will be to support, encourage and reward the landholders who are devising flexible management options incorporating intermittent disturbance regimes to achieve healthy, diverse groundcover, and to increase the organic matter content in and on their soils.
I am deeply indebted to Greg Martin of Earth Sanctuaries Limited, for information on the soil building activities of ground foraging native fauna, and his exceptional insight into the role these animals, and their disturbance regimes, play in ecosystem function (see next issue); to Allan Savory, founder of the Center for Holistic Management, for his teachings on the crucial role of decision making processes in regenerative land management and his visionary approaches to the interactions between grazing animals, vegetation and soils; and to farmers Darryl Cluff, Colin Seis and Bruce Maynard, for their unconventional, courageous and highly successful ventures into the brave new world of integrating, in space and time, permanent groundcover, annual crops and livestock.
Click to see chart showing the extent to which land use addresses the fundamental requirements for a healthy balanced ecosystem. (Thanks to Australian Farm Journal for use of their chart layout)
BACKGROUND INFORMATION (not for main article)
The importance of soil organic matter
Volumes have been written on the multiplicity of benefits attributable to organic matter. It exists in many forms, ranging from the pieces of plant material and animal dung we can see with the naked eye, to the humic substances which can only be observed by the darker appearance and crumbly texture of the soil. Direct physical and chemical benefits of organic matter include the buffering of soil temperatures and regulation of air flow in soils, the supply of plant available macronutrients such as nitrogen, phosphorus and sulphur; micronutrients such as zinc, copper and selenium; increased cation exchange capacity and increased soil water holding capacity.
The indirect benefits of organic matter relate mainly to increased levels of soil biological activity. Soil organisms ranging from microscopic bacteria and fungi to invertebrates such as earthworms, all require a supply of food and a safe environment in which to live. In a healthy grassland soil, the soil biota produce materials which glue soil particles together in lumps, creating a crumb structure which is resistant to erosion. There are large pore spaces between these soil aggregates which hold both air and water. Soil microbes also produce vitamins and plant growth hormones which stimulate root growth and enable plant roots to penetrate dense subsoil material. Furthermore, the presence of a diversity of beneficial soil microbes prevents the multiplication of soil pathogens, through the process of microbial antagonism. The habitat and food sources for soil organisms are partly destroyed by continuous grazing (see Part I) and completely destroyed by broadacre cultivation.