Environmental impacts of organic farming

Summary

Many farming practices used in organic farming reduce the risk of nutrient losses to environment, especially the absence of inorganic fertilizer inputs, which decreases the leaching risk of nitrogen (N) and phosphorus (P). However, frequent use of animal manure and leguminous crops may induce excessive release of nitrate N in soil during periods when the uptake of N by crop is low. The lower yield level obtained in organic farming compared with conventional farming may further reduce the beneficial environmental impacts, if they are calculated per yield instead of per hectare. In Finland, the area under organic farming increased rapidly in the 1990's from 1.1% (1994) to 7.1% (2002). Despite the large support given to organic farming there has been no research to study directly the nutrient losses from organic farming in Finland. The aim of this study was to measure N leaching and P losses in surface runoff and subsurface drainage water from plots under organic farming with grass or cereal crop rotation. The results were evaluated against those obtained on conventionally cultivated plots on the same sites, situated in western and southern Finland (Toholampi, sandy soil and Jokioinen, clayey soil, respectively). The amount of nutrient inputs in manure and commercial fertilizers was the main difference between the organic and conventional cropping systems, while the tillage intensity and the cultivated plant species were kept as similar as possible.
The amount of N leaching was less from organic cropping systems at both sites and through the whole study period (1997-2004). On the sandy soil in Toholampi (1997-2004), 10-35% less N was leached, while on clayey soil in Jokioinen (2001-2004) the respective reduction was 46 %. The yield was decreased by 21-45% (Toholampi) and 56% (Jokioinen), respectively. When N leaching losses were calculated per yield, the values were smaller for organic farming if the yield reduction was less than 30%. During seven years of the study, the higher N balances due to biological N fixation and organic N inputs in manure did not cause larger losses of N from the organic crop rotation compared with the conventional rotation. Moreover, the amount of mineral N in the soil profile in autumn was a poor indicator of N leaching during the following winter and spring. The cropping system affected the P losses through different P balances. The soil P status decreased in the organic crop systems due to smaller or negative P balances, while the larger balances in conventional rotations increased the values of easily dissolved P. However, the differences in soil P status were still so small that the P losses were mostly affected by the technical aspects of fertilizer management, like the presence or absence of surface application. The soil P status was a good indicator of dissolved P concentration in surface runoff, but the actual losses of P also depended on the proportions of surface runoff and subsurface drainage.