The discussion above leads to the following considerations: (1) today's agriculture has achieved the scientific and technical ability to provide food for a steadily increasing world population, but the price paid to achieve this success, in terms of environmental decay and quality of life, cannot be accepted and there is ample reason to fear an irreversible decay of agro-ecosystems in the future; (2) strategies for a sustainable agriculture are urgently needed and an arsenal of sometimes contrasting ways to achieve sustainability is available, but sustainability is an elusive concept widely varying with the various farms and agricultural systems; (3) progress towards sustainability can be achieved provided that prejudice-free, flexible system approaches are adopted, apt to the diverse circumstances and objectively supported by appropriate indicators.
Although thoroughly validated, really holistic system approaches are not immediately available; it is possible today to significantly improve present agricultural systems by enhancing the knowledge of the multifarious aspects of agricultural reality and their implications rather than passively accepting pre-concocted, all-purpose solutions.
Sustainability is a moving target wrote Hoag and Skold (1996) and as such it requires flexibility in selecting the practices to be adopted.
The coordinated combination of practices and techniques selectively picked from those tested and suggested by the 'alternative agriculture' groups, so defining all those groups exploring ways to alleviate the high burden imposed by the high-input agriculture, can offer sound, although not formally optimized, solutions, provided that the necessary holistic and synergic approach be maintained by selecting and combining the best from the various proposing groups rather than embracing any of them as a religion, rancorously rejecting the others.
The costs and benefits of various agricultural practices must be based on local values and local constraints, causing sustainable practices to be region and culture specific (Tilman et al. 2002): no universal recipe exists.
In the following part a necessarily incomplete review will be exposed of the possible impacts of the principal farming practices that must be simultaneously evaluated in order to avoid neglecting some important aspects while giving too much emphasis to others. Only for the sake of clarity, although admittedly the close interrelationships linking them all should not be overlooked, the management practices to be examined will be grouped under four headings:
Practices in every single group will be analysed for their impact on the three 'pillars' of sustainability:
Again, since the economic, environmental and social impacts are closely interlocked, a separate analysis is in principle incorrect; however, it is deemed necessary for sorting out the outcomes of the various possible actions. Furthermore it must be considered that conflicting indications may result for every single impact, such as, for instance, the need to associate no-tillage positive effects for protecting soil fertility, sequestering CO2 and minimizing off-site damages with the negative effects of spraying herbicides, depending on the risk of local and downstream pollution. Similarly, social aspects to be privileged can include increasing labour, which conflicts with farm net profit and above all environmental pollution, human energy being notoriously by far the most polluting of all.
A win-win solution can be found rather easily when only a couple of aspects are considered, but finding the 'best compromise' solution can become a difficult task when three or more conflicting aspects are simultaneously considered and a weight must be assigned more or less arbitrarily to each of them. The scope of the present review is to present a down-to-earth framework at farm level and evidence some rather diffuse misconceptions, with the aim of assisting farm operators in selecting sustainable management strategies and rejecting charlatanisms.
Various forms of soil cultivation, or non-cultivation, exist ranging from mouldboard ploughing to no-tillage, as listed below (from CTIC and Conservation Technology Information 1998):
• Conventional tillage: mouldboard ploughing is followed by disking or harrowing, implying soil inversion
• Mulch tillage or mulch ripping: the soil is tilled prior to planting with chisels, disks, sweeps or blades; weed control is obtained with herbicides and/or cultivation
• Ridge tillage: the soil is left undisturbed from harvest to planting except for nutrient injection; planting is completed in seedbeds prepared on ridges with sweeps, disk openers, coulters or row cleaners. Residue is left on the surface between ridges. Weed control is accomplished with herbicides and/or cultivation. Ridges are rebuilt during cultivation
• No tillage or zero tillage: the soil is left undisturbed from harvest to planting except for nutrient injection. Planting or drilling is accomplished in a narrow seedbed or slot created by coulters, row cleaners, disk openers, in-row chisels. Weed control is accomplished primarily with herbicides. Cultivation may be used for emergency weed control
It is worth to report preliminarily the conclusions of a research conducted in Canada by Clements et al. (1995), who found no significant relationship between the yields for a corn-soybean-winter wheat rotation and the energy expended for the frequency and depth of cultivations, which implies that with intensive cultivations there is ample room for energy saving and input reduction.
Conservation tillage, defined as any tillage and planting system that maintains at least 30% of the soil surface covered by residue after planting (CTIC and Conservation Technology Information 1998), encompasses a variety of solutions, basically those defined above as mulch tillage, ridge tillage and no tillage. Although attractive under several points of views (in the USA about 40% of corn is conservation tilled according to Uri 1998; similar advantages can be expected in vast European areas, Tebrügge and Düring 1999), conservation tillage finds limitations in soils -heavy, clay soils as well as soils prone to crusting are not apt to be conservation tilled, Adeoye 1986; climate - conservation tillage cannot be adopted in humid climate areas, due to excessive water intake rates consequent to macropores from large earthworm burrows and root holes, Dunham 1979, as reported in Fowler and Rockstrom 2001; and crops - vegetables, potatoes, beets, tobacco, peanuts cannot be conservation tilled (Peet 2001; Uri 1998). As a consequence, a careful assessment
of local conditions is required before embarking in a conservation tillage program. Obtaining clear-cut and definitive information on the comparative efficiency of the various solutions is not easy, not only because of the impact of local conditions -soil, climate, crops - but also because real differences in results can be appreciated only after a long-term experimentation; however, there is general consensus supported by some experimental evidence that reduced tillage and even more so no-tillage are more advantageous than conventional systems not only in terms of environmental protection and energy savings but also in terms of farm profit. The results of a long-term experience conducted in Spain, for instance, demonstrated that zero-tillage with only 0.72 kg/ha of glyphosate outperformed both conventional and minimum tillage (Hernanz et al. 2002); opposite to that, a long-term trial in Argentina could detect no significant difference in yield between conventional and no-till management (Diaz-Zorita et al. 2002).
In plantations on a sloping land, environmental damages from erosion due to mechanical cultivation, namely in-site and off-site effects, are certainly higher than those from one or two yearly sprayings with glyphosate at the dose of less than 1 L/ha, which demonstrates that pollution from physical origin can be more harmful than that from chemical origin. Mulching with polyethylene sheets, permitted in organic farming (Haas 2006) is certainly much more polluting than spraying gly-phosate. Similarly the flame weeders permitted in organic farming are not only more costly than glyphosate (Kang 2001), but also much less efficient in the control of perennial weeds and more demanding in terms of energy; therefore, they are ultimately much more polluting.
Anderson (2007) reports encouraging results obtained with field crops in the semi-arid steppe of the USA through the adoption of no-till in a dualistic approach of prevention and control which permitted to reduce to about 50% the amount of herbicides.
Cover crops, often suggested as a means for weed control, are certainly attractive but unfortunately can be applied only under certain conditions, since they compete with the main crop for water, disturb water distribution patterns with some irrigation systems, increase frost risk in some areas and are unable to compete with some perennial weeds such as Cynodon dactylon (bermudagrass) or Sorghum halepense (Johnsongrass). Cover crops are usually, but not necessarily, associated to conservation tillage, concurring to enhance the system sustainability, thanks also to their potential in enriching soils in organic matter and nitrogen and their action in combating weeds. Their acceptance is limited by their opportunity cost adding to the explicit costs and in some regions by their competition with limited water resources. The potential of plant cover in reducing water erosion is well acknowledged: for instance Rizzo et al. (1994) demonstrated that increasing plant cover from 15-40% to 50-90% reduced run-off from about 25 to about 3 mm after 1 h simulated precipitation with the intensity of 48.7 mm/h on 9% sloping plots.
Buffering strips, otherwise called filtering strips, are one further method suggested to protect the agro-ecosystems (e.g. Parsons et al. 1995; Vought et al. 1995). They are based on the plantation of vegetated strips at some interval (varying with land slope, soil intake rate and precipitation intensity), which check overland flow and diminish water speed; this in turn entails the deposition of transported solids with their load of pollutants, thus avoiding their accumulation. Additional benefits of filtering strips include the encouragement of water infiltration into the soil and the uptake of some chemical pollutants by protecting plants. The suggested width of strips usually ranges from 5 to 50 m, particularly when they are used as riparian buffers along a watercourse (e.g. Lal et al. 1999); however, examples can be found of narrow grass hedgerows not wider than 50 cm (Huang et al. 2008), very effective in reducing run-off and soil erosion. More forms of non-conventional agriculture exist, aiming at reducing inputs and protecting the agro-ecosystem, including precision agriculture. Although attractive, promising and sound in its principles since it is not rational to manage entire fields uniformly, ignoring soil variability, it presently does not enjoy a vast acceptance, requires a high-tech equipment, for instance linking GIS to GPS, and a skilled management and can only be applied under special conditions (e.g. Verhagen et al. 1995; Power et al. 2001; Precision Agriculture and University of Minnesota 2002).
In those cases that conservation tillage, particularly no tillage, can be adopted, advantages can be appreciable under diverse aspects:
• Economic: reduced tillage operations automatically reduce costs; particularly with no-tillage, when feasible, root system in tree plants is not disturbed and yield is often increased; grain yield is enhanced through the encouraged rainwater infiltration; costs for irrigation are reduced; in-site and off-site damages depending on erosion and downstream pollution are mitigated;
• Environmental: reduced cultivation implies reduced energy inputs (e.g. Swanton et al. 1996), therefore determining less pollution; soil is less disturbed and its structure is protected; accumulation of organic matter, a fundamental component of fertility, is encouraged; microbial biomass and soil fauna are increased; CO2 releases to atmosphere are much reduced (e.g. Halvorson et al. 2002), with a potentially appreciable alleviation of greenhouse effect; soil erosion and downstream pollution are mitigated; wildlife habitat is remarkably improved; chemical contamination is lessened, in spite of the herbicide use required by conservation tillage, in particular by zero-tillage;
• Social: workers conditions are improved due to the reduced/eliminated tractor trips; a wide-ranging alleviation of pollution is achieved, from local fertility decay consequent to erosion to off-site damages such as reservoirs siltation, recreational areas impairments, rivers eutrophication, gas emission, water body quality impairment, etc. It is worth to mention here that there is a general consensus that off-site damages consequent to erosion far exceed in-site damages: consequently, the advantages to the society of adopting a large-scale soil conservation program implying the adoption of herbicides when necessary exceed those to single farmers.
Synthesis of Subsection 3.2.1 - The possible cultivation modes range from mouldboard ploughing to mulch tillage, ridge tillage, zero tillage and each solution has pros and cons. Generally the trend is to reduce cultivation adopting some form of weed control. Well-managed herbicides are less polluting than plastic mulches and flame weeders. Cover crops and buffering strips can be very useful solutions. Factors to be considered in the choice include fertility maintenance, CO2 sequestration, aquifer protection, erosion control and gas emissions.
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