Soil fertility

Summary
The project addressed two aspects of importance to organic farming. One was concerned with the development and evaluation of tillage strategies of special relevance to organic farmers. The other aspect covered key soil quality parameters and the effects of long-term management practices.

Four tillage strategies were set up and tested in a five-course rotation at the organically managed Rugballegård Experimental Station. Two systems included traditional ploughing to depths of 10 and 20 cm, respectively. Two other systems introduced a non-inverting tillage strategy by using subsoil loosening to a depth of approximately 35 cm. One of these treatments involved a simultaneous seedbed preparation and sowing by implements attached to the subsoiler. The results showed that the new non-inverting systems were realistic alternatives to traditional ploughing. There was a tendency of a higher number of weeds in these systems. However, yields were comparable to those obtained by the ploughing systems. Soil studies revealed the existence of a dense plough pan, which was effectively broken by the subsoiler. The studies should be continued for a longer period in order to evaluate the new systems properly.

The investigations of soil quality were performed as multidisciplinary case-studies on farms, which had been managed organically for more than forty years. This offered us a unique opportunity to investigate the soil ecosystem and to identify soil problems in commercial organic farming. In an attempt to increase our understanding of the agricultural soil as an ecosystem, closely situated conventionally cultivated systems with different fertiliser practice and crop rotation were included in the study. This gave us the opportunity to elucidate which management characteristics have the most pronounced influence on the soil system. The main conclusions can be listed as follows:

• All organically and all conventionally managed soils had a dense pan below ploughing depth. This im-peded root growth.
  
  
• The negative effects of a high traffic intensity on structure stability as expressed by clay dispersibility overshadowed the positive effect of a diversified crop rotation and organic manuring.
  
  
• Generally, our results showed that soil physical characteristics in differently managed soils must be evaluated with due reference to both long- and short-term effects as illustrated by the following:
  
  
• For an organically managed soil, the densification caused by traffic during the time prior to sampling caused a lower ease of fragmentation and a higher wet aggregate stability as compared with its conven-tionally managed reference soil.
  
  
• For another organically managed soil, a lower ease of fragmentation and a higher wet aggregate stability as compared with its conventionally managed reference soil were ascribed to differences in biological bonding and binding mechanisms.
  
  
• For a third organically managed soil, a higher ease of fragmentation and a lower wet aggregate stability as compared with the conventionally reference soil reflected a pronounced difference in density and structure due to either crop rotation or organic manuring.
  
  
• For five soils receiving animal manure and grown with a diversified crop rotation, a significant linear cor-relation was observed between microbial biomass C and the volume of protective pore space. Two soils dressed only with synthetic fertilisers had a lower microbial biomass C than would be expected from the relation obtained for the other soils.
  
  
• A high level of ergosterol was observed for soils with a high frequency of grass leys in the crop rotation.
  
  
• The ratio between ergosterol and microbial biomass C indicated that the high stability / low ease of frag-mentation for the organically managed soil was due to bacterial activity rather than to hyphal enmesh-ment.
  
  
• Some of the biological indicators showed differences between organic and conventional farming systems:
  
  
• Microbial biomass C was higher in the organically managed soils than in their conventionally managed counterparts, especially in those receiving only synthetic fertilisers.
  
  
• A high abundance of fungal species with reported antagonistic properties was observed for all organi-cally managed soils.
  
  

The quantitative scientific laboratory methods were supplemented with descriptive methods in the field in order to evaluate the conclusions drawn from the classical methods. It can be concluded that the visual descriptions by the spade analyses were in accordance with quantifying field tests, which further correlated to laboratory tests. With regard to the laboratory methods, clay dispersibility appeared to be a better index for structural stability than wet sieving of macro-aggregates but our results show that a high energy input in the shaking procedure prior to determination of clay dispersibility was beneficial for evaluation of long-term effects.

The mycorrhiza potential (MP) was negatively correlated to the level of plant available soil P, while the farming system as such had no effect. A pot experiment with subterranean clover and buckwheat grown in Askov soil, which had received no fertilisers for 100 years, showed that plant growth was increased by inoculation with an exotic mycorrhizal fungus. No clear relationships were observed between soil aggregate stability and the occurrence of mycorrhizas.

Considering the improvement and development of sustainable farming systems, our results strongly indicate that farmers should pay more attention to tillage and traffic practices in order to avoid structural damages to the soil. Furthermore, farmers and extension services may benefit from using descriptive methods in the field to evaluate the physical and biological status of the soil as these observations seem to correlate well with results obtained with classical laboratory methods.