Er der krummer i din jord – eller er det bare knolde

Jordens krummer er vigtige for vækst. Både vækst af afgrøden og de svampe og bakterier, der laver bindemidler til nye krummer. Vi har undersøgt, hvordan praktisk økologisk og konventionel drift påvirker dette samspil. Et varieret sædskifte med meget græs og husdyrgødning gav gode stabile krummer med højt indhold af biologiske bindemidler. Et ensidigt kornsædskifte med kunstgødning gav i stedet stabile knolde, hvor bindemidlet var ler. Sådan nogle knolde kan give et dårligt såbed, fordi våd jord bliver blød som mudder og tør jord hård som beton

Soil aggregation – a matter of proper management

Soil crumbs are important to soil functions. These include plant growth, and the growth of fungi and bacteria forming agents for binding and bonding of new crumbs. We have studied how commercial organic and conventional farming affect this interaction. A diverse crop rotation, including grass, and animal manure resulted in stable crumbs with a high content of biological binding and bonding agents. A cash crop rotation with the addition of only synthetic fertilizers resulted in small, stable aggregates – more like clods - with clay as binding agent. Such a soil will provide poor conditions for preparing seedbeds, because prolonged rain makes it soft and muddy while drought makes it hard as brick

The epidemiology and natural history of Crohn’s disease in population-based patient cohorts from North America: a systematic review

To quantify, through systematic review, the epidemiology and natural history of Crohn’s disease in North America. Methods:  The selected articles contained: (i) population-based samples of patients followed from the time of diagnosis; and (ii) objective diagnostic criteria for disease. Studies on the natural history of Crohn’s disease also contained sufficient follow-up. Data collection and analysis:  For prevalence studies, data on the incidence, prevalence, gender and age at diagnosis were extracted. For natural history studies, data on the disease activity, use of medications and surgery were extracted. Main results:  The prevalence of Crohn’s disease in North America ranges from 26.0 to 198.5 cases per 100 000 persons. The incidence rates range from 3.1 to 14.6 cases per 100 000 person-years. Most patients have a chronic intermittent disease course, while 13% have an unremitting disease course and 10% have a prolonged remission. Less than half require corticosteroids at any point. During any given year, approximately 10% are treated with corticosteroids and 30% are treated with 5-aminosalicylates. Up to 57% of patients require at least one surgical resection. Conclusions:  Between 400 000 and 600 000 patients in North America have Crohn’s disease, and the natural history is marked by frequent exacerbations requiring treatment with corticosteroids, 5-aminosalicylate products and surgery.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72031/1/j.1365-2036.2002.01140.x.pd

Den danske dyrkningsjords tilstand og kvalitet - konsekvenser af trafik og jordbearbejdning

Et alsidigt sædskifte og/eller tilførsel af organisk stof til jorden giver en god jordkvalitet (her defineret som en gunstig jordstruktur). Ensidige driftsformer kan i sig selv give anledning til en dårlig jordkvalitet. Jordpakning via trafik på jorden kan eliminere de gode effekter af et godt sædskifte og tilførsel af organisk stof. Trafik på jorden og i endnu højere grad intensiv jordbearbejdning øger risikoen for, at jordens lermineraler dispergeres til vandfasen. Dette kan give tilslemning og skorpedannelse på jorden og indebærer endvidere en risiko for nedvaskning af de vigtige lermineraler fra dyrkningslaget. Der bør foretages undersøgelser af omfanget af denne proces med de i dag anvendte bearbejdningsteknikker. Komprimering af jorden under normal bearbejdningsdybde via pløjning med hjulet i furebunden og via kørsel med meget tunge maskiner er i dag en realitet for stort set hele den danske landbrugsjord. Det anbefales stærkt, at landbruget overgår til pløjesystemer med alle fire hjul ’på land’ (’on-land’ pløjning). Det anbefales ligeledes, at belastningen (vægten) ved kørsel på forårs- og efterårsvåd jord ikke overstiger 6 tons på enkelt-aksel og 8-10 tons på boogi-aksel. Anvendelse af bæltekøretøjer er muligvis en løsning på pakningsproblemet. Det forudsætter dog, at de anvendte maskiner giver anledning til reelt meget lave marktryk (<50 kPa) over hele trædefladen. Der bør gennemføres flere undersøgelser af, hvorvidt de i dag tilgængelige bæltekøretøjer opfylder disse kriterier ved alle arbejdsoperationer

Short-term changes in soil pore size distribution : Impact of land use

Changes in land use affect the pore size distribution (PSD) of the soil, and hence important soil functions such as gas exchange, water availability and plant growth. The objective of this study was to investigate potentially damaging and restorative soil management practices on soil pore structure. We quantified the rate of change in PSD six years after changes in land use taking advantage of the Highfield land-use change experiment at Rothamsted Research. This experiment includes short-term soil degradation and restoration scenarios established simultaneously within long-term contrasting treatments that had reached steady-state equilibrium. The land-use change scenarios comprised conversion to grassland of previously arable or bare fallow soil, and conversion of grassland to arable and bare fallow soils. In the laboratory, we exposed intact soil cores (100 cm3) to matric potentials ranging from −10 hPa to -1.5 MPa. Based on equivalent soil mass, the plant available water capacity decreased after conversion from grassland, whereas no change was observed after conversion to grassland. Structural void ratio decreased after termination of grassland and introduction of grassland in bare fallow soil, while no change was seen when changing arable to grassland. Consequently, it was faster to degrade than to restore a complex soil structure. The study illustrates that introducing grassland in degraded soil may result in short term increase in soil density

Soil degradation and recovery – changes in organic matter fractions and structural stability

The combination of concurrent soil degradation and restoration scenarios in a long-term experiment with contrasting treatments under steady-state conditions, similar soil texture and climate make the Highfield land-use change experiment at Rothamsted Research unique. We used soil from this experiment to quantify rates of change in organic matter (OM) fractions and soil structural stability (SSS) six years after the management changed. Soil degradation included the conversion of grassland to arable and bare fallow management, while soil restoration comprised introduction of grassland in arable and bare fallow soil. Soils were tested for clay dispersibility measured on two macro-aggregate sizes (DispClay 1-2 mm and DispClay 8-16 mm) and clay-SOM disintegration (DI, the ratio between clay particles retrieved without and with SOM removal). The SSS tests were related to soil organic carbon (SOC), permanganate oxidizable C (POXC) and hot water-extractable C (HWC). The decrease in SOC after termination of grassland was greater than the increase in SOC when introducing grassland. In contrast, it was faster to restore degraded soil than to degrade grassland soil with respect to SSS at macro-aggregate scale. The effect of management changes was more pronounced for 8-16 mm than 1-2 mm aggregates indicating a larger sensitivity towards tillage-induced breakdown of binding agents in larger aggregates. At microscale, SSS depended on SOC content regardless of management. Soil management affected macroscale structural stability beyond what is revealed from measuring changes in OM fractions, underlining the need to include both bonding and binding mechanisms in the interpretation of changes in SSS induced by management

Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films

X-ray photoelectron diffraction is used to directly probe the intra-cell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO3 films. Our measurements, combined with ab-initio calculations, unambiguously demonstrate non-centro-symmetry in films a few unit cells thick, imply that films as thin as 3 unit cells still preserve a ferroelectric polar distortion, and also show that there is no thick paraelectric dead layer at the surface

Pløjefri dyrkning: effekter på jordens frugtbarhed

Interessen for pløjefri dyrkning er steget voldsomt gennem de seneste år. Fokus har primært været på de kortsigtede økonomiske gevinster som følge af mindsket tidsforbrug og lavere maskinomkostninger og i langt mindre grad på de langsigtede gavnlige effekter af pløjefri dyrkning på jordens frugtbarhed. Forskellen er slående i forhold til Nordamerika, hvor den udbredte brug af pløjefri dyrkning i høj grad er båret af et ønske om en langsigtet sikring af jordens frugtbarhed (jfr. betegnelsen ”conservation tillage” anvendt for pløjefri jordbearbejdning. I Nordamerika har man især været optaget af pløjefri dyrkning, som et middel til bekæmpelse af vind- og vanderosion. Pløjefri dyrkning er også et anvendeligt redskab til at begrænse pakningen i dybden og den omfattende jordflytning ved jordbearbejdning – såkaldt jordbearbejdsningserosion. De danske erfaringer med pløjefri dyrkning i 1970 og 80’erne var brogede. Der var problemer med håndtering af planterester og i mange tilfælde fandtes skadelig pakning af overjorden. Nyt effektivt udstyr til snitning og fordeling af halm og avner kan afhjælpe problemerne med planterester, mens problemet med skadelig pakning af overjorden fortsat eksisterer. Til løsning af pakning i overjorden anbefales ikke-vendende jordløsning til den behovsbestemte dybde, der sikrer at jorden ikke opblandes og en skånsom påvirkning af regnorme m.v. Genpakning af jorden er dog et problem, der skal tages højde for ligegyldigt hvilken strategi og hvilket redskab, der vælges til ikke-vendende løsning. Forebyggelse er det bedste middel imod pakning af overjorden, hvilket kan ske ved kontrolleret trafik i spor, der anvendes år efter år. Mindsket afhængighed af ukrudtsmidler og optimeret gødningsudnyttelse bør også være centrale elementer i fremtidige pløjefri dyrkningssystemer