Fundamentals of organic agriculture

Organic agriculture can be traced back to the early 20th century, initiated by the Austrian spiritual philosopher Rudolf Steiner. It was later diversified by a number of key people, and more recent versions are guided by principles issued by the International Federation of Organic Agricultural Movements (IFOAM), founded in 1972. Organic practices were built upon the life philosophies and convictions of the founders regarding how to perceive nature. Today, those original views and ideas are considered as history. However, to understand the principles and opinions of modern organic agriculture, such as the exclusion of water-soluble inorganic fertilisers, we analysed the original ideas and arguments of the founders, who shared the common principle of relying on natural processes and methods, seen as a prerequisite for human health. For example, the British agriculturalist Sir Albert Howard, who together with Lady Eve Balfour founded the British Soil Association, claimed that healthy soils are the basis for human health on earth. In their view, healthy soils could only be obtained if the organic matter content was increased or at least maintained. Later, the German physician and microbiologist Hans-Peter Rusch together with the Swiss biologists Hans and Maria Müller, focused on applying natural principles in agriculture, driven by the conviction that nature is our master and always superior. Even though these early ideas have been abandoned or modified in modern organic agriculture, the principle of the founders regarding exclusion of synthetic compounds (fertilisers and pesticides) is still the main driver for choosing crops and pest control methods

Mobility of heavy metals in soil and their uptake by sunflowers grown at different contamination levels

A pot trial was carried out to study the effect of heavy metals on sunflowers (Helianthus annuus L.) grown on three different soils at different levels of heavy metal loading (added in 1987 as salts). Heavy metal availability in soil was investigated using ammonium acetate and chelating resin (Chelex 100) as extractants. Total heavy metal contents in the soils of the pot experiment did not show changes in the considered period of time whereas the availability of the metals was sensibly reduced. Therefore, it seems that heavy metals revert with time to forms more strongly bonded in soil. The pH decreased with increasing additions of heavy metals. This may be the result of changes in exudate or proton release by the roots. Cd, V and Zn were generally more concentrated in the leaves than in the seeds; the opposite behaviour was observed for Cu and Ni. V was never measured above the detection limit in the seeds. Toxicity effects were clearly related to the mobility of heavy metals in the soil. The methods used to assess the availability of heavy metals were highly correlated. The correlation between heavy metals extracted with ammonium acetate and Chelex and the concentration in sunflower shoot tissue showed that the latter method was slightly better than the former. (© Inra/Elsevier, Paris.)La mobilité des métaux lourds dans le sol et leur absorption par des tournesols cultivés en présence de différents niveaux de contamination. Un essai en pots a été réalisé pour étudier l'action des métaux lourds sur des tournesols (Helianthus annuus L.) cultivés sur trois sols présentant différents niveaux de contamination en métaux lourds. Ceux-ci ont été ajoutés au sols sous forme de sels en 1987. L'acétate d'ammonium et une résine chélatée (Chelex) ont été utilisés pour étudier extraire les métaux lourds et étudier leur disponibilité dans le sol. La teneur totale des sols en métaux lourds n'a pas varié sur la période considérée, alors que la disponibilité des métaux s'est sensiblement réduite. Il semble donc que les métaux lourds retournent avec le temps sous des formes moins disponibles pour la plante dans le sol des complexes fortement liés. Le pH a diminué lorsque les ajouts de métaux lourds ont augmenté. Ceci peut être dû à des changements dans les exudats ou à la libération de protons par les racines. Le Cd, le V et le Zn sont en général plus concentrés dans les feuilles que dans les graines et c'est l'inverse pour le Cu et le Ni. Dans les graines le V est toujours resté sous la limite de détection. Les effets toxiques sont nettement liés à la mobilité des métaux lourds dans le sol. Les méthodes pour évaluer la disponibilité des métaux lourds présentent une forte corrélation. La corrélation entre les métaux lourds extraits par l'acétate d'ammonium ou par le Chelex d'une part et la concentration dans les tissus des pousses de tournesol d'autre part montre que la méthode au Chelex est légèrement meilleure que celle à l'acétate d'ammonium. (© Inra/Elsevier, Paris

Roots contribute more to refractory soil organic matter than above-ground crop residues, as revealed by a long-term field experiment

We revisited the well documented and ongoing long-term 'Ultuna continuous soil organic matter field experiment' which started in 1956 at the Swedish University of Agricultural Sciences. The objective of the experiment is to quantify effects of six organic amendments and mineral N fertilizers on the crop and soil. We used the ‘equivalent soil mass’ concept for estimating changes in the topsoil C stocks in all 15 treatments. C inputs from amendments were measured and those from crops were calculated using allometric functions and crop yields. Clustering C inputs into seven categories by quality allowed us to calculate a ‘humification’ coefficient for each category. Here, these coefficients simply were based on the fraction of total C input that still remains in the topsoil after about 50 years. As indicated by previous studies, this coefficient was highest for peat, followed by sewage sludge, manure, sawdust and aboveground crop residues. The most interesting result from the current investigation is that the optimized coefficient for root-derived C was about 2.3 times higher than that for aboveground plant residues. The calculated results were found to be robust in a sensitivity analysis. Our findings strongly support the hypothesis that root-derived C contributes more to relatively stable soil C pools than the same amount of aboveground crop residue-derived C