English: The aggregated results of Clark & Tilman’s study is shown in the chart below. This comparison measures the relative impact ratio of organic to conventional agriculture, whereby a value of 1.0 means the impact of both systems are the same; values greater than 1.0 mean the impacts of organic systems are higher (worse) (for example, a value of 2.0 would mean organic impacts were twice as high as conventional); and values less than 1.0 mean conventional systems are worse (a value of 0.5 means conventional impacts are twice as high). We see these relative impacts measured by food type across our range of environmental impacts with averages and standard error ranges shown.

We see large differences in impact patterns across environmental categories and food types. For some impacts, one system is consistently better than the alternative; whilst for others, results are mixed depending on crop type and the local agricultural context. The clearest results are for land and energy use. Organic systems consistently perform worse in terms of land use, regardless of food type. As we explore in detail in our entry on Yield and Land Use in Agriculture, the world has achieved large gains in productivity and gains in yield over the past half-century in particular, largely as a result of the availability and intensification of inputs such as fertilizer and pesticides. As a result, the majority of conventional systems achieve a significantly higher yield as compared to organic systems. Therefore, to produce the same quantity of food, organic systems require a larger land area.

This produces the inverse result for energy use. The industrial production of chemical inputs such as fertilizers and pesticides is an energy-intensive process. The absence of synthetic chemical inputs in organic systems therefore means that their energy use is predominantly lower than in intensive conventional agriculture. The exception to this result is vegetables, for which energy use in organic systems tends to be higher. Some of this additional energy use is explained by the use of alternative methods of weed and pest control in organic vegetable farming; a technique widely applied as an alternative to synthetic pesticide application is the use of ‘propane-fueled flame weeding’. The process of propane production and machinery used in its application can add energy costs – especially for vegetable crops.

Acidification and eutrophication potential are more mixed, but tend to be higher in organic systems; average values across all food types are higher for organic, although there are likely to be some exceptions in particular contexts. Why are organic systems typically worse in these measures? The supply of nutrients in conventional and organic systems are very different; nitrogen supply in conventional agriculture is supplied with the application of synthetic fertilizers, whereas organic farms source their nitrogen from manure application. The timing of nutrient release in these systems is different: fertilizers release nutrients in response to crop demands, meaning nitrogen is released when required by the crops, whereas nitrogen released from manure is more dependent on environmental conditions, such as weather conditions, soil moisture and temperature.

Nutrient-release from manure is therefore not always matched with crop requirements – excess nutrients which are released but not taken up by crops can run off farmland into waterways such as rivers and lakes. As a consequence, the pollution of ecosystems with nutrients from organic farms are often higher than conventional farms, leading to higher eutrophication and acidification potential.

Across all food types, there is no clear winner when it comes to greenhouse gas emissions. Results vary strongly depending on food type, although most lie close to a ratio of one (where differences in impact between the systems are relatively small). Based on average values, we might conclude that to reduce greenhouse gas emissions, we should buy organic pulses and fruits, and conventional cereals, vegetables, and animal products. In general, the greenhouse gas emission sources of organic and conventional systems tend to cancel each other out. Conventional systems produce greenhouse gases through synthetic fertilizer production and application, which is largely balanced by the higher emissions of nitrous oxide (a strong greenhouse gas) from manure application.