Holger Kirchmann

Soil compaction (SC) is a major threat for agriculture in Europe that affects many ecosystem functions, such as water and air circulation in soils, root growth, and crop production. Our objective was to present the results from five short-term (−1) and subsoiling at the Swedish SS improved root penetration into the subsoil by about 10 cm—but the effects of SICSs on yields were generally small. These case studies also reflected difficulties in implementing SICSs, some of which are under development, and we discuss methodological issues for measuring their effectiveness. There is a need for refining these SICSs and for evaluating their longer-term effect under a wider range of pedoclimatic conditions.

This paper reviews the original reasons of the organic farming movement for excluding mineral (inorganic) fertilizers. In this paper, their theories and decision criteria for excluding use of inorganic fertilizers in crop production were revisited. Original reasons for banning inorganic fertilizers were subjected to scientific scrutiny, which was not possible when they were formulated 50–100 years ago due to limited knowledge of the soil-crop system. The original reasons were as follows: Rudolf Steiner, the founder of biodynamic farming, played down the physical role of plant nutrients and pointed out “flow of forces” as being most important for soils and crops. Eve Balfour and Albert Howard, founders of the Soil Association in England, claimed that inorganic fertilizer increases the breakdown of humus in soil, leading to a decline in soil fertility. Hans-Peter Rusch, the founder of biological organic farming, considered inorganic fertilizers to be imbalanced products not matching crop composition and not in synchrony with crop demand. When testing these historical statements as scientific hypotheses, older and modern scientific literature was used for validation. Steiner’s belief about the “flow of forces” has not be verified using current methodologies. The claim by Balfour and Howard that inorganic fertilizers accelerate soil organic matter decomposition is not substantiated by data from long-term field experiments on carbon and nitrogen cycling in soil-plant systems. The statement by Rusch that inorganic fertilizers supply crops inappropriately is difficult to uphold, as the composition, time, and rate of application and the placement of fertilizer in soil or on foliage can be fully adapted to crop requirements. In light of accumulated scientific evidence, the original arguments lack validity. The decision to ban inorganic fertilizers in organic farming is inconsistent with our current scientific understanding. Scientific stringency requires principles found to be erroneous to be abandoned.

The aim of this article is to provide information about crop production data based on large-scale organic farming and to point toward major consequences. National statistics show lower organic yields than compiled in meta-analyses from farm- and plot-scale. Yields of organically cropped legumes were 20% and nonlegumes 40% lower than those of conventionally grown crops. Area estimates showed that almost two of three crops were legumes or legume mixtures in organic farming, whereas one of three crops was a legume in conventional cropping. Doubling land use for legumes in organic farming affected the type of food produced, being dominated by milk products and red meat. Over all crops, the organic yield gap was 35%. Since yields are lower under organic than conventional practices, more land is required to produce the same amount of agricultural crops. A 35% yield gap means that 50% more arable land is required. A demand for 50% more farmland imposes huge land use changes and makes one realize the wide-ranging environmental consequences that follow when converting to organic farming. In a relevant comparison between organic and conventional cropping systems, environmental consequences caused by land use change such as lost products (timber, fiber, energy, etc.) and lost ecosystem services (sequestered carbon in soil, wildlife, biodiversity, etc.) must be included. The concept of organic farming was founded on philosophical views about nature, not biological science. Natural means and methods were assumed to be superior. Verification of the reasoning and statements of the founders on why to abandon mineral fertilizers cannot be corroborated by science and is incorrect. Scientific evidence for the concept to abandon synthetic mineral fertilizers as nutrients for crops is lacking. The scientific community is obliged to follow rigorous scientific criteria—not biased views, prejudices, or beliefs.

An incubation study with fresh, frozen, and dried clover and rye-grass shoots gathered in autumn was performed in the laboratory (43 days). The aim was to study inorganic and organic phosphorus (P) release from plant material during decomposition without soil. Plant materials (2 cm size) were mixed with small glass beads and placed in large syringes for consecutive water extractions on 7 occasions. Leachates were analysed for inorganic and total P. At the first leaching event, 8% of total crop P was released from fresh, 24% from frozen and 27% from dried plant material. During decomposition, both inorganic and organic P was released following first order kinetics, with grass releasing mostly inorganic P and clover mostly organic P. After 43 days, 42%–50% of total crop P from fresh, 49%–51% from frozen and 57%–69% from dried material was released, with significant differences between treatments. Using the results in calculations on field scale showed that P released from overwintering crops under cold climate conditions can amount to several kg P ha⁻¹. Thus, senescence of overwintering aboveground biomass can be a significant source for P leaching from soils.

The composition of municipal wastewater and sewage sludge reflects the use and proliferation of elements and contaminants within society. In Sweden, official statistics show that concentrations of toxic metals in municipal sewage sludge have steadily decreased, by up to 90 %, since the 1970s, due to environmental programmes and statutory limits on metals in sludge and soil. Results from long-term field experiments show that reduced metal pollution during repeated sewage sludge application has reversed negative trends in soil biology. Despite this Swedish success story, organic waste recycling from Swedish towns and cities to arable land is still limited to only about 20 % of the total amount produced. Resistance among industries and consumers to products grown on land treated with sewage sludge may not always be scientifically grounded; however, there are rational obstacles to application of sewage sludge to land based on its inherent properties rather than its content of pollutants. We argue that application of urban organic wastes to soil is an efficient form of recycling for small municipalities, but that organic waste treatment from large cities requires other solutions. The large volumes of sewage sludge collected in towns and cities are not equitably distributed back to arable land because of the following: (i) The high water and low nutrient content in sewage sludge make long-distance transportation too expensive; and (ii) the low plant availability of nutrients in sewage sludge results in small yield increases even after many years of repeated sludge addition. Therefore, nutrient extraction from urban wastes instead of direct organic waste recycling is a possible way forward. The trend for increased combustion of urban wastes will make ash a key waste type in future. Combustion not only concentrates the nutrients in the ash but also leads to metal enrichment; hence, direct application of the ash to land is most often not possible. However, inorganic fertiliser (e.g. mono-ammonium phosphate fertiliser, MAP) can be produced from metal-contaminated sewage sludge ash in a process whereby the metals are removed. We argue that the view on organic waste recycling needs to be diversified in order to improve the urban–rural nutrient cycle, since only recycling urban organic wastes directly is not a viable option to close the urban–rural nutrient cycle. Recovery and recycling of nutrients from organic wastes are a possible solution. When organic waste recycling is complemented by nutrient extraction, some nutrient loops within society can be closed, enabling more sustainable agricultural production in future.

We tested the hypothesis whether fertilization has an impact on subsoil development using data from two long-term soil fertility experiments. We chose treatments without nitrogen (N) and high N fertilizer addition in two crop rotations with and without ley and farmyard manure application. The soil properties measured included soil organic matter, microbial composition, aggregate stability, readily dispersible clay and micronutrient content in topsoil (0–30 cm) and upper subsoil (30–40 cm). Studies of two adjacent soil profiles revealed that the upper subsoil of an N-fertilized treatment had more soil organic matter, darker colour, more aggregation, more fine roots and was 10 cm thicker than that of the no-N treatment. This pronounced subsoil development in treatments with higher yields (high N input) was corroborated by carbon mass determinations based on systematic sampling including bulk density. Soil organic matter contents were higher in N-fertilized than unfertilized treatments, either with or without manure. Nitrogen fertilization caused distinct differences in microbial composition at one study site. Phospholipid fatty acid analysis revealed: (1) that mineral N fertilizer promoted nitrifiers; and (2) that bacteria were favoured over fungi in treatments with N supply probably due to nitrogen-rich crop residues. Soil aggregate stability (water dispersion) was correlated to soil organic matter and clay content. We also tested the hypothesis whether trace element contents in the top- and subsoil were affected by N fertilization through removal by large yields. Despite long-term depletion, crop removal of micronutrients was too small to cause significant differences in soil between treatments.

The aim of this paper was to synthesize a series of investigations from a long‐term field experiment addressing the turnover of organic matter in soil. This paper is based on four organic matter inputs (soil treatments): none (fallow), small amounts of roots+stubble cropped without N fertilizer (no‐N), moderate additions of animal manure, and moderate additions of peat. After 42 years, soil carbon stocks declined in the fallow and no‐N treated soil but increased in the animal manure and peat‐amended soil. Gentle fractionation of soil particles and aggregates revealed that the silt‐sized fraction contained most of the soil C and was most responsive to changes in input of organic matter. The clay‐sized fraction ( clay > fine sand > coarse sand. The highest natural abundance of ¹³C and ¹⁵N was found in the clay‐sized fraction, the fallow being most enriched in ¹³C and the animal manure‐treated soil in ¹⁵N, indicating that the organic matter of the clay‐sized fraction had been turned over most intensively. The ¹³C inventory showed that the transfer from silt‐ to clay‐sized carbon was most intensive in the soil treated with animal manure and least intensive in the peat‐treated soil. Bacterial diversity increased from sand‐ to clay‐sized fractions revealed by 16S rRNA genes. Fungal activity was highest in coarse‐sized fractions as indicated by enzyme measurements. The quality and amount of organic matter input had no significant effect on the community structure of soil bacteria.