Despite the challenge of defining 'sustainable agriculture' there is a general consensus amongst farmers and other interest groups that sustainability in agriculture is an essential and worthwhile objective [Reeve, 1990]. A positive aspect to the concept of sustainable agriculture is that it represents and gives a vision or goal to farmers and other interested parties to individually or jointly work together. The Land Care movement, for example, demonstrates this aspect as its goal is the sustainable use and management of land [Campbell, 1994]. There is still however, an ongoing debate about what in reality is a sustainable agricultural system. In hindsight it is relatively easy to identify sustainable and non-sustainable systems for time has proven the obvious success or failure of previous systems. Traditional systems such as rice paddy cultures in South-East Asia [Lampkin, 1992], are often hundreds, if not thousands of years old [King, 1926]. These traditional systems which are currently operational today are clearly identifiable as sustainable farming systems, although they are often relatively small in area. In contrast non-sustainable agricultural systems are also well documented [Mitchell, 1946; Carter and Dale, 1974 and Reeve, 1990]. For example, in North Africa the expansion of the Roman Empire caused increasing areas to be used for agriculture systems. Those systems proved to be unsustainable with the result that once plentiful savanna and forest areas were transformed into desert [Carter, Dale, 1974].

The two extremes of sustainable and non sustainable systems represent a continuum whose middle ground is very ambiguous and difficult to interpret, as it deals with the parameters of space and the uncertainties of absolute time frames and levels of permanence [eds Edwards et al. 1990; Campbell, 1994; Roberts, 1995; Reeve, 1990]. These areas will be discussed later.

According to Kiley-Worthington [1993] the fashionable and politically correct philosophy of sustainable agricultural systems is incomplete, for what are the costs and ethics of such a concept? Should foodstuffs be imported into a country or on to a farm, so that animals can be kept as part of an integrated system? Is it right to be reliant on chemical fertilisers that can have a detrimental effect on soil organisms? Indeed, is it ethical to destroy one ecosystem in order to sustain another such as with phosphate rock? These are all open questions that make the issue of sustainability very complex and challenging.

There are many differences of opinion as to the application of the term 'sustainable agriculture'. Sustainability has become a catch phrase in marketing as the term is being increasingly used as a positive way of promoting the sale of agro-products, for example, the association of the herbicide Roundup* with sustainability. This labour saving product can greatly assist in weed control by reducing the potential need for costly soil cultivation and reducing the risk of accelerated soil erosion. This allows soil resources to be theoretically more sustainable. In contrast, this markedly differs from the promotion of N.A.S.S.A. In their concept of sustainable agriculture, no synthetic herbicides should be used.

The above illustrations show a general agreement as to the importance and merit of sustainable agriculture, but also the differences in their interpretation of its practical application. An agronomist may view ongoing increased production as a goal in itself and providing this is achieved the agricultural system is sustainable. Richardson [1978] suggests that productivity alone can no longer be treated as just a single isolated dimension, as to continually increase individual yields and reduce the productivity of the land is a self defeating goal. In addition sustainable systems must not degrade the land that it uses and recognise that exotic weeds and pests need to be controlled [Campbell ,1994].