Charles Sturt University-Riverina. Wagga Wagga. NSW 2650
Two-thirds of Australia’s croplands are in a degraded state through soil structure breakdown. Thirty-two million hectares are salt-affected and more than 17 million hectares are sufficiently acid to impair production (Pratley, 1987). These areas are increasing in size annually.
Not all the problems are the result of farming practices - many have been occurring over millions of years. Nevertheless farming practices over the past 200 years have certainly not helped. Clearly current farming practices are not sustainable. It thus raises the question “Can farming systems be sustainable?”
What Is Sustainable Agriculture?
Sustainability can be defined as the ability of a system to maintain its productivity when subject to stress (Conway, 1986). This stress can be a regular, sometimes continuous, small disturbance such as encroaching salinity or acidity or an infrequent, large and unpredictable disturbance such as drought, flood or a new pest.
Sustainable agriculture infers that the level of productivity of a particular farming system can, at least, be maintained over a defined period of time. Such a definition requires both the period of time and the level of productivity to be defined.
Most farmers are in the farming business for a lifetime and generally wish to leave their farm to their heirs in productive condition. The period of time is therefore long term, though economic pressures may reduce this consideration to much shorter periods - i.e. survival from year to year.
The level of productivity is a much harder criterion to determine. Pressures from bankers, family, government, environmental lobby groups and even weather conditions strongly influence what productivity is achieved.
Sustainability is all about BALANCE. If the level of inputs balances the level of outputs then productivity can be sustained. As the productivity level is increased so must the level of inputs be raised if the resources are not to be mined and deterioration of the system take place at some time in the future. We ought not to delude ourselves that low level productivity per se can be sustained - salinisation, for example, could be the end result. We should not delude ourselves that organic farming per se or chemical-free farming per se are sustainable. They can be, but only if some basic principles are followed.
Sustainability has at least three dimensions:
- the paddock or farm level
- the regional or catchment level
- the global level
The Farm Level
Previous Outlook Conferences have described in great detail the importance of maximising the value of the root zone of our soils. We have encouraged you to:
- reduce tillage in order to improve soil structure (Osborne, 1983; Pratley, 1983), to reduce compaction (Cockroft, 1984; McGarry, 1984), to improve infiltration and storage of water (Cornish, 1984), to reduce erosion and provide better utilisation of soil nitrogen and phosphorus (Taylor, 1980; Batten, 1989);
- to maximise the utilisation of root zone water through the crop plants by optimum sowing, appropriate plant density, good crop and pasture vigour, better nutrition, fewer weeds (Reeves, 1982). This is important because of the close relationship in our environment between crop water use and yield (Cornish, 1984);
- to increase the level of phosphorus applied to raise the level of productivity. Each tonne of wheat for example removes about 2.6 kg P - a relationship which requires higher P inputs to sustain higher grain outputs (Batten, 1980).
We have encouraged you to do these things as a means of raising productivity. In the past they have tended to be considered in isolation but clearly each component interacts with other components of the system. What we should do is evaluate them in terms of sustainability of our farming systems - how important is it to aim for high productivity in order to sustain the productive capacity of the land? This theme is developed by consideration of three cycles - moisture, nutrient and soil biology cycles.
In our environment, crop and pasture yields are limited by the availability and utilisation of soil water. Balance is achieved by matching moisture input with moisture output but yield is determined by how much of the output is used by the productive plants. The components of this hydrological or moisture cycle are shown in Figure 1.
Figure 1. The Moisture Balance In Cropping Soils
To control yields we therefore need to exert control over the various components of the system.
(a) Increase infiltration - this is achieved by improvement of surface soil structure and can be enhanced by the presence of crop residues or other vegetation on the surface. Increasing infiltration reduces the amount of runoff which otherwise is lost to the system.
(b) Increase soil water storage - the capacity of the root zone to hold water deteriorates as structure declines, as soil compaction increases, and as organic matter levels are reduced. These occur usually through excessive cultivation, and hence reduction in tillage generally has a positive effect on soil water storage capacity.
(c) Reducing runoff and drainage losses - increasing infiltration has a significant effect on reducing runoff. Thus the potential for erosion is reduced. The increased utilisation of moisture by high yielding deep-rooted plants reduces the likelihood of deep drainage of water and provides capacity in the root zone for storage of further precipitation. Downward movement of water results in the leaching of soluble nutrients.
(d) Reducing non-productive water loss through evapotranspiration - unnecessary losses through soil surface evaporation (the rate of which can be reduced by surface residues or by reducing cultivation) and through weed transpiration are non-productive and reduce the amount available for the crop or pasture.
(e) Increasing Moisture utilisation - the Moisture stored in the soil is there for use by the plants. Deeper rooted species increase the potential utilisation by increasing the size of the root zone. Most grasses, including cereals, are reasonably deep-rooted as are lupins, rapeseed and lucerne. It is questionable whether the shallow rooted annual pasture legumes make much contribution in this regard and whether they should be the sole component of the pasture phase. Optimum sowing times and other factors improving plant vigour also contribute to this utilisation.
(f) Increasing the efficiency of utilisation of soil moisture - water is used by the plant in metabolism, as a nutrient carrier and in plant structure. It is also a significant part of the heat-regulating mechanism of the plant through transpiration. In this role the size of the stomatal openings are controlled, closing during moisture stress periods. As carbon dioxide entry into the plant is through the stomates, yield is determined by whether the stomates are open or closed.
Thus if the conditions for transpiration were made less demanding, water would be used more efficiently, for yield rather than cooling. Trees and shelter belts perform this useful function by reducing wind speeds, and hence evaporation and transpiration rates. Figure 2 demonstrates the effects on yield. Animal production benefits are also obtained from these shelter belts (Lynch and Donnelly, 1980; Lynch et al., 1980)
Figure 2. The Plant Yield Response Associated With Windbreaks
Savings in non-productive water loss during the growing season can therefore contribute to increased moisture availability at the end of the season where effects on yield can be pronounced.
The nutrient cycle is another important component of the sustainable farm equation. Nutrient level and balance largely determine how close yields get to water-limited potential yields. Phosphorus is clearly the most important and controls to a large extent how much nitrogen is available via nitrogen fixation by legumes. Thus high crop water use to achieve high productivity requires a non-limiting supply of essential plant nutrients -the higher the productivity level the higher the requirement for plant nutrients. In order to sustain productivity, therefore, account must be taken of the amount of each nutrient removed in plant and animal products and steps taken to replace them - i.e. a balance must be achieved between nutrient removal and nutrient input (Figure 3). This principle applies equally to hay and pasture as to crops. Except for nitrogen which is considered separately, nutrient input involves fertiliser application or, in the case of organic farmers, the provision of organic matter. To not do this means exploitation of the system which is therefore not sustainable. For organic matter to be effective it must at least replace phosphorus and other nutrients to the extent that they have been removed. The level of nutrients in the organic matter thus needs to be known.
Phosphorus, unlike some other nutrients, does not move very much in the soil and, after a period of years, a phosphorus-rich layer accumulates near the soil surface where the fertiliser was drilled. Its role is very important in that it is instrumental in the development of a strong and elongating root system which assists in the determination of the size of the soil’s root zone and thus the utilisation of moisture.
In soils which are susceptible to erosion at the surface, the phosphorus-rich layer thus becomes susceptible to being removed from the system in the erosion process. This is yet another reason for developing and maintaining well-structured surface soils through reduced tillage operations.
A refinement of the nutrient cycle which hitherto has been neglected is the maintenance of soil p11. Removal of crop and pasture products constitutes removal of alkali from the paddock (Cregan and Helyar . 1986), i.e. it is an acidifying process. This may have been masked to some extent by the burning of crop residues which leaves behind an alkaline ash although this usually gets blown or washed away. The principle being expounded here is the replacement on a regular basis of the alkali being removed. Thus, part of the planning activity should be to use lime, say once a rotation phase, in amounts equivalent to that removed over the phase (see Table 1).
Table 1. The Lime Required To Balance Product Removal From Paddocks (Cregan And Helyar, 1986)
(kg CaCO3/t product)
An important part of the soil pH story relates to nitrogen management. Soil nitrogen fertility largely depends on a vigorous legume/rhizobium symbiosis - a process which is inexpensive and important in the maintenance of high crop yields. This process, together with mineralisation of organic nitrogen to nitrate-nitrogen, is acidifying. However, if there is good utilisation of this nitrate by plants, such a process is alkaline - hence high yielding crops utilising a high proportion of formed nitrate help stabilise the pH level. Leaching of nitrate, however, contributes to soil acidity and reinforces the principles of maximising water utilisation through plants and minimising non-productive water loss through drainage. Figure 4 shows the pH relationships of nitrogen management and notes that ammonia-type fertilisers are also acidifying.
Figure 4. The Acidity Balance Of Croplands
A much neglected part of the ecosystem is the soil biology. We tend to think of soil microbes mainly in their negative or disease-promoting activities. Rather we should be thinking of soil microbes as the principal agents of soil health rather than agents of disease. The cliche “a healthy soil is full of life” should be foremost in our thinking.
The level of microbial activity is directly related to the level of soil organic matter. Our intensively cultivated farming systems in the past have tended to reduce organic matter and thus microbial populations have declined. Farming systems were relatively simple, being confined largely to cereal crops. As systems become more simple they often become more unstable and cereal monoculture allows the buildup of specialised pathogens which infect and debilitate such cereal crops (Kohlmorgen et al., 1983; Reeves et al., 1984; King, 1984). Diversity of forms tend to be self-regulating - hence the desirability of rotations.
Thus, high organic natter levels and rotations become part of the sustainable agriculture scenario. Direct drilling, for example, has been shown at Wagga to reduce the incidence of eye-spot lodging (Pratley, 1986) and, elsewhere, other diseases. Whilst the reasons have not been clearly identified, the maintenance of organic matter levels and soil microbe populations under this method of farming must make some contribution.
The value of rotations must be emphasised. Evidence is clear that crops yield better after non-disease host crops. Work by S. Sutherland (unpublished) at Wagga showed a spectacular difference in wheat yields through the control of grasses in the last year of the pasture phase. Paddock monitoring work by N. Clark (unpublished) showed that the top 25% of wheat crops followed a legume pasture or non-cereal crop whereas the bottom 25% of wheat crops usually followed wheat or a grassy pasture.
Rotations also offer the prospect of more options for weed management. An understanding of allelopathic relationships between crops and weeds may well allow reduced chemical input for weed control which will reduce the cost and please the environmentalists, which hopefully we are all becoming.
A major limitation to the implementation of a sustainable farming system is the way in which decisions are made as to what crop to grow. Farmers respond quickly to market price and generally make their decisions according to the relative gross margins of the alternatives. Thus if wheat offers the best price this year, farmers will grow wheat. If wheat prices are down, farmers may choose better economic options.
Such an approach takes no account of the interaction between successive crops, or between crops and pastures over time - a crucial aspect of sustainability. To consider a crop in isolation takes account largely of the price received and ignores potential productivity. High prices for low yields may well be less profitable than lower prices for higher yields.
One option for improving this problem is suggested by Finlayson (1988) and involves comparison of rotation gross margins. Thus a pasture:wheat (1:1) rotation could be compared with a pasture (3 year):canola:wheat:barley rotation by:
This then takes account of the positive effect of a pasture on wheat production or the negative effect of wheat on barley production and provides an average yearly gross margin for the rotation.
Achieving Sustainable High Productivity
On the farm therefore, sustainability is achieved by a healthy soil producing high yielding crops and pastures under reduced cultivation. Water utilisation and its efficiency must be maximised. A balance is achieved by replacing nutrients and alkali removed from the system. Strategic planting of trees increases production and assists in the management of soil moisture.
It must be emphasised that the achievement of high yields cannot be sustained on a piecemeal basis. All the components must be brought together. This is reinforced by the findings of the 10 kg Dryland Wheat Club in the Finley district of NSW (J. Lacy, 1989, personal communication). This Farmer Club has as its objective to produce 10 kg grain per mm of growing season rainfall and all their activities are monitored. Results show that profitability and yield are directly related to the number of key factors which have been successfully implemented. Lacy has identified a minimum of eight key factors involved:
- previous break crop or pasture
- soil structure
- recommended sowing date
- good phosphorus nutrition
- good nitrogen nutrition
- minimum plant population
- weed control
Figure 5 shows the importance of getting all of it right. Those farmers who only achieved five of the eight key factors only achieved half the yield of those achieving eight key factors.
Figure 5. Effect On Profitability And Yield Of The Number Of Key Factors Adopted By Finley Farmers (J. Lacy 1989, Personal Communication)
Number Of Key Factors Achieved By Farmers
The Catchment View
Individual farmers only have control of their own tarn;. However, it has to be recognised by everybody that the actions of one farmer can have consequences for neighbouring farmers and beyond. Two particular effects are worth noting:
(1) The clearing of land removes deep-rooting, high water-using plants from the ecosystem. This allows a greater volume of water to enter the soil, raising the water table and resulting in salinity at lower places in the valley (Figure 6). Clearly then the removal of trees in the recharge area has consequences some distance away and often on other farmers. The solution to this problem is not often under the control of the farmer suffering the productivity loss and it requires closer cooperation between neighbours if these issues are to be sorted out. Choice of tree is an important consideration. Eucalypts are noted for their high water usage and high growth rate. Recent recommendations encourage indigenous varieties,because their control agents are present and prevent uncontrolled proliferation of the species.
Figure 6 - We Effect Of Weed Removal On The Level Of The Water Table And Subsequent Salinisation
(2) The second effect is called eutrophication. This is the effect of rising nutrient levels in water courses, endangering the ecology and choking the waterways with vegetation. This becomes a danger with increasing productivity and is a major problem in European countries. In the Australian context this basically means phosphorus and nitrogen.
Phosphorus enters the water course system; through surface runoff associated with erosion. This loss is clearly an expensive one for farmers as it increases the subsequent need for P replacement through fertilisers. The solution is clear, however. Erosion must be reduced to a minimum and the technology for that is available through reduced tillage and maintenance of soil cover by plants or plant residues. Prevention of erosion also reduces the silting up of water courses.
Nitrogen pollution of waterways is minimised by holding the soil N reservoir for plant usage in organic form - i.e. by N fixation by legumes and minimising tillage to reduce its mineralisation in large amounts. Previous management strategies outlined where the leaching process is reduced and drainage away from the paddock is avoided thus prevents this N from joining up with water courses.
The Global Problem
Briefly, the global problem relates particularly to the greenhouse effect described by Gifford (1989). This seems to be largely a problem associated with the carbon cycle. Again it is a question of balance - or rather imbalance - that carbon is being released into the atmosphere (e.g. as C02, methane, and hydrofluorocarbons) faster than carbon can be fixed by plants, particularly trees, by photosynthesis (Figure 7). Thus forests must be regenerated at a faster rate than they are removed and deserts must be reclaimed again by selective tree planting. Long-term carbon fixation, i.e. by trees and other perennials, would contribute more significantly than the short-term annual carbon fixation of crops.
Figure 7 - The Carbon Balance Of Croplands
As individuals we can have little effect. Collectively we can have a slightly larger but yet. rather insignificant effect. The fact that it is insignificant should not. mean that we ignore it. It is our moral duty to mankind to make a contribution and keep our house in order so that others may follow.
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