Constraints to the sustainability of a stockless arable rotation

The sustainability of an organic stockless arable rotation on a fertile soil in eastern England was assessed from 1990 to 2005. The good water and nutrient holding characteristics of the silty clay loam soil were well suited to a stockless organic rotation. Fertility-building clover crops were the most difficult to establish, and failed completely in some years despite one or two re-sowings. Crop yields were good, particularly for cereals, with an average for winter wheat of 7 t ha-1. Crop yield did not show any particular trend with time; there was no evidence of either a post conversion adjustment period, or a fall in yield due to declining fertility. High organic crop prices in the 1990s, resulted in significantly higher gross margins than from comparable non-organic farms. However, falling organic crop prices from 2000 resulted in profitability only similar to non-organic. Supply of N, P and K was probably not a major limitation to crop growth and yield. However, in the longer term, additions of sustainable sources of plant-available phosphorus and potassium would be necessary, even on the nutrient retentive and potassium rich soil. Effective mechanical weeding was difficult on the silty soil. The rotation favoured perennial weeds, particularly creeping thistle which increased progressively despite efforts at control with mechanical and hand weeding

Testing the sustainability of stockless arable organic farming on a fertile soil (OF0145)

This is the final report of Defra project OF0145. If organic farming is to expand in the arable east of England, where the knowledge, infrastructure and capital for livestock are not available, viable stockless systems will be needed. The aim is to maximise economic performance and in turn encourage conversion. Project OF0112 showed that a stockless arable rotation was consistently more profitable that a comparable conventional rotation on the fertile silty clay loam at ADAS Terrington. Project OF0145 researched challenges to sustaining that level of performance into the second crop rotation. The project was a combination of systems comparison, replicated experiments and monitoring of commercial farms. The core of the project was an unreplicated systems comparison with field-scale plots to allow meaningful study of patchy problems such as perennial weeds and give confidence to farmers that the system could work on a farm scale. Conversion was completed in 1995 and the rotation has since been clover, potatoes, winter wheat, spring beans, undersown spring barley. The greatest agronomic challenges continued to be with the establishment of fertility-building legumes. Despite these problems, crop yields have been maintained with good rolling average yields of 25 t/ha for potatoes, 7.5 t/ha for winter wheat, 3.5 t/ha for spring beans and 4.1 t/ha for spring cereals. Disease levels in cereals have remained low and posed minimum threat to yields. Slugs and blight have affected potatoes in wet years; control of these is particularly difficult in an organic system leading to greater yield variability than would be expected in a non-organic rotation. Calabrese has grown and yielded well with few problems but weed control in onions proved both difficult and expensive and they have been dropped in the successor project OF0301. Weighted rolling-average gross margins show a consistent and large advantage to organic (£912/ha conventional, £1757/ha stockless with potatoes and £1148/ha stockless with vegetables). The advantage to organic has increased with time as yields and prices have been maintained whilst conventional crop prices have fallen. Soil fertility as measured by carbon and nitrogen contents has shown little change since the start of conversion in 1990. Soil available P and K have remained at ADAS Index 1 to 2 despite continued crop offtakes. However both are showing a slow progressive decline, less so with P, perhaps partly due to the rotational applications of Aluminium Calcium Phosphate. Annual weeds are proving relatively easy to control, being worst where crop growth is poor for reasons such as compaction on headlands. However, the perennial weeds couch grass, creeping thistles and docks are an increasing problem. Hand pulling of thistles and docks is containing the problem but the cost of this has risen dramatically in the last two years. The distribution of potato cyst nematodes (PCN) was mapped within all five plots in January 1998, 1999 and 2000. Sampling was in 25 m x 25 m sub-plots. In January 2000 viable cysts were found in 7.6 % of sub-plots, all at fewer than 10 eggs per g of soil. There was no evidence of a significant multiplication of PCN following potatoes. Growing a variety other than the resistant Sante may have allowed multiplication. A manure utilisation booklet was compiled in association with Elm Farm Research Centre. The text was agreed with MAFF in May 2001 and it should be published by July 2001. Specific challenges deserving further study include: • The ecology of perennial weeds and agronomic strategies for their control. • Quantification of net nitrogen fixation by legumes and subsequent release to crops. • The impact of potato cultivar on PCN multiplication. • Better and more reliable grain quality. • Control of slugs and potato blight. The development of companion and bi-cropping systems for arable rotations (linked to results from OF0181 and OF0173)

Organic Arable Systems at ADAS Terrington OF0112

Project OF0112 contributes to MAFF's main policy focus of encouraging conversion to organic farming methods. It is part of a long term rotational study that began in 1990 as OF0102 and has recently been extended to 2001 as OF0145. The overall objective of these three projects is to evaluate the cost of conversion to organic arable production on a fertile soil, to assess the physical and financial performance of the organic rotation, to identify and overcome limitations to sustainabilty and to compare the results with conventional arable production. The project comprises a field-scale unreplicated systems comparison and associated replicated experiments at ADAS Terrington, and a financial analysis of ten commercial 'linked' organic farms. The silty clay loam soil at Terrington has proved ideal for organic production, primarily because it has very good water and nutrient and retention. As expected, organic crop yields have been less than conventional, averaging 70% for winter wheat; yields have been, on average, double that of the linked farms, and reached a peak of 10 t/ha in 1996. Variable costs have been lower and organic prices have been twice or more that for conventionally grown potatoes and wheat. Crop grossmargins (i.e. the value of the crop harvested minus the drect variable costs of growing it) have been consistently higher from organic than conventional. Even allowing for the lower value of the other three crops in the rotation, i.e. beans, spring cereal and clover (Set aside), overall gross margin from organic was higher than from conventional (average from 1993 to 1997 was 1,878 v 1,290 #/ha). Crop yields and gross margins were generally lower on the linked farms, probably mainly because they were on lighter soils more prone to leaching losses. However, all were viable businesses and had similar profitabilities to conventional farms of their size. The most profitable rotations in cluded potatoes and/or vegetables. In the absence of animal manures and synthetic fertilisers, the main driver of crop yield and key to sucess, will be the fixation of sufficient atmospheric nitrogen by the Rhizobium bacteria in the root nodules of legumes. Replicated experiments comparing a range of species have shown that in terms of gross accumulation of nitrogen in the cut foliage, and in the yield of a following wheat crop, red clover, lucerne and white clover are all very effective fertility builders, with red clover on average just the best. A second experiment has compared wheat, barley and oats as the cover crops for the undersowing of red clover. In 1997 clover dry matter at harvest uder oats was only 4.5kg/ha compared with 88 under wheat and 74 under barley. Baley was also the most profitable crop, however this was affected by relative grain prices which vary between years. A third experiment tested timing of manure application across the rotation. The modest quantity applied (30 t/ha per rotation) was chosen as what could have been produced from animals fed on crops grown within the rotation. There was only one isolated response in crop yield over four different crops. This was probably a reflection of the high inherent fertility and nitrogen retention capacity of the silty clay loam soil at Terrington. There are real current business opportunities for conversion to arable production. The linked farms, mostly with mixed arable livestock rotations, show profitability comparable with conventional; stockless arable production was consistently more profitable than conventional on the fertile nutrient retentive soil at Terrington. However there remain questions about the longer term sustainability of a stockless arable rotation, even on such a well suited soil. The next phase of the project (OF0145), which has just started, will focus on sustainability studying potential threats from perennial weeds, nutrient supply and soil-borne pests and diseases. The use of manure will be discontinued and, in that part of the study area, vegetables will be introduced in place of potatoes to test an alternative rotation

Companion cropping for organic field vegetables (OF0181)

Typical organic crop rotations are extensive with at least one year in four as a fertility building crop. However, the economic viability of organic systems may be compromised by having 75% or less of the farm productive at one time, limited further by the absence of the Arable Area Payments Scheme, particularly Set-aside, for vegetable crops. In addition, the system gives rise to a high fertility/low fertility sequence which is inefficient in terms of nutrient management (particularly nitrogen). To try to address this, the use of permanent beds of companion crop grown alongside the vegetable crops has been developed under various conditions around the world and is perceived as a possible alternative in organic husbandry. Companion crops also have the potential to reduce the impact of pests and weeds. A potential disadvantage of companion crops is competition with crop plants for space, light, water and nutrients. The companion crop, therefore, is likely to have to be mown or grazed to control competition and encourage nutrient transfer. On the positive side, companion crops have the potential to reduce the impact of pests, and weeds. The challenge is, therefore, to develop appropriate crop layouts and machinery to balance these interactions and result in profitable crop production. Project OF0181 was delivered with Elm Farm Research Centre and was guided by a Steering Group. The core of the project was the further development and evaluation of a seven-crop companion crop system initially developed by Professor Martin Wolfe at Wakelyns Agroforesty, Fressingfield, Suffolk, a Soil Association registered organic farm. The system was based on 1.5 m beds, with three 20 cm vegetable rows alternating with 30 cm leguminous companion strips. Within each bed, there was a seven-course crop rotation: potatoes, alliums, Umbellifers, spring oats, legumes, brassicas and spring wheat. To establish and manage this system, Martin Wolfe and his co-workers (P. J. & M. J. Wards) had by spring 1999 developed a range of purpose-built machinery including a strip rotavator, 3 row precision seed drill, straight tine or L-blade strip cultivator with/without discs, rotary strip mower, strip irrigator and a strip compost spreader. Two large experiments were established at Wakelyns in spring 1999; it was planned that these be continued for the full three years of the project. One experiment compared a factorial combination of a) three companion crops: white clover, vetch and nil, b) companion crop mowings left to fall, or deflected onto the vegetable rows, and c) the presence or absence of added composted manure. A second experiment compared factorial combinations of winter cover crops of rye and vetch grown in the vegetable rows with additional approved inputs of phosphorus and potassium. All seven crops were grown but assessments were made only on brassicas, alliums and carrots. Conclusions Companion cropping has the potential to improve economic viability, and pest, disease and annual weed control in organic cropping systems, particularly in field vegetables which are not supported by the Arable Area Payment Scheme. However, in practice, in project OF0181 these benefits were not realised: • Grass weeds were favoured and were difficult to control once established. • There were problems with seedbed preparation and crop establishment; these may be less on lighter soils. • Some crop species were better suited to companion cropping. In 2000 and 2001, there was: - a high yield in both years from beetroot, spinach, chard and kale; - a high yield in one year from Brassicas (some cabbage, swede, turnip), endive, lettuce, parsley and parsnip; - a low yield in both years from Allium crops (leeks, onions), Brassica (sprouts, some cabbage, calabrese), carrots, celeriac, broad and dwarf beans. • Clover used soil available nitrogen in preference to fixed nitrogen, starving less competitive crops such as alliums of the nutrient and resulting in very low yields. • Even with reliable yields, companion cropping in the form tested may only be suited to small-scale labour-intensive production. A system with greater spatial separation of companion and vegetable crops, with vegetables and companion crops grown alone in separate beds or strips, may give the reported benefits of companion cropping with less competition and be practical for large scale production

Limits of the energy-momentum tensor in general relativity

A limiting diagram for the Segre classification of the energy-momentum tensor is obtained and discussed in connection with a Penrose specialization diagram for the Segre types. A generalization of the coordinate-free approach to limits of Paiva et al. to include non-vacuum space-times is made. Geroch's work on limits of space-times is also extended. The same argument also justifies part of the procedure for classification of a given spacetime using Cartan scalars.Comment: LaTeX, 21 page

Thermodynamic Structure of the Solar Corona: Tomographic Reconstructions and MHD Modeling

We carry out a study of the global three-dimensional (3D) structure of the electron density and temperature of the quiescent inner solar corona ($r<1.25 R_\odot$) by means of tomographic reconstructions and magnetohydrodynamic simulations. We use differential emission measure tomography (DEMT) and the Alfv\'en Wave Solar Model (AWSoM), in their latest versions. Two target rotations were selected from the solar minimum between solar cycles (SCs) 23 and 24 and the declining phase of SC 24. We report in quantitative detail on the 3D thermodynamic structure of the core and outer layers of the streamer belt, and of the high latitude coronal holes (CH), as revealed by the DEMT analysis. We report on the presence of two types of structures within the streamer belt, loops with temperature decreasing/increasing with height (dubbed down/up loops), as reported first in previous DEMT studies. We also estimate the heating energy flux required at the coronal base to keep these structures stable, found to be or order $10^5 erg\, cm^{-2} s^{-1}$, consistently with previous DEMT and spectroscopic studies. We discuss how these findings are consistent with coronal dissipation of Alfv\'en waves. We compare the 3D results of DEMT and AWSoM in distinct magnetic structures. We show that the agreement between the products of both techniques is the best so far, with an overall agreement $\lesssim 20\%$, depending on the target rotation and the specific coronal region. In its current implementation the ASWsoM model can not reproduce down loops though. Also, in the source region of the fast and slow components of the solar wind, the electron density of the AWSoM model increases with latitude, opposite to the trend observed in DEMT reconstructions

Relative energetics and structural properties of zirconia using a self-consistent tight-binding model

We describe an empirical, self-consistent, orthogonal tight-binding model for zirconia, which allows for the polarizability of the anions at dipole and quadrupole levels and for crystal field splitting of the cation d orbitals. This is achieved by mixing the orbitals of different symmetry on a site with coupling coefficients driven by the Coulomb potentials up to octapole level. The additional forces on atoms due to the self-consistency and polarizabilities are exactly obtained by straightforward electrostatics, by analogy with the Hellmann-Feynman theorem as applied in first-principles calculations. The model correctly orders the zero temperature energies of all zirconia polymorphs. The Zr-O matrix elements of the Hamiltonian, which measure covalency, make a greater contribution than the polarizability to the energy differences between phases. Results for elastic constants of the cubic and tetragonal phases and phonon frequencies of the cubic phase are also presented and compared with some experimental data and first-principles calculations. We suggest that the model will be useful for studying finite temperature effects by means of molecular dynamics.Comment: to be published in Physical Review B (1 march 2000

Temporal lobe surgery in childhood and neuroanatomical predictors of longterm declarative memory outcome

The temporal lobes play a prominent role in declarative memory function, including episodic memory (memory for events) and semantic memory (memory for facts and concepts). Surgical resection for medication-resistant and well-localized temporal lobe epilepsy has good prognosis for seizure freedom, but is linked to memory difficulties in adults, especially when the removal is on the left side. Children may benefit most from surgery, since brain plasticity may facilitate post-surgical reorganization, and seizure cessation may promote cognitive development. However, the long-term impact of this intervention in children is not known. We examined memory function in 53 children (25 boys, 28 girls) who were evaluated for epilepsy surgery: 42 underwent unilateral temporal lobe resections (25 left, 17 right, mean age at surgery 13.8 years), 11 were treated only pharmacologically. Average follow-up was 9 years (range 5-15). Postsurgical change in visual and verbal episodic memory, and semantic memory at follow-up were examined. Pre- and post-surgical T1-weighted MRI brain scans were analysed to extract hippocampal and resection volumes, and evaluate post-surgical temporal lobe integrity. Language lateralization indices were derived from functional magnetic resonance imaging. There were no significant pre-to post-operative decrements in memory associated with surgery. In contrast, gains in verbal episodic memory were seen after right temporal lobe surgery, and visual episodic memory improved after left temporal lobe surgery, indicating a functional release in the unoperated temporal lobe after seizure reduction or cessation. Pre-to post-surgical change in memory function was not associated with any indices of brain structure derived from MRI. However, better verbal memory at follow-up was linked to greater post-surgical residual hippocampal volumes, most robustly in left surgical participants. Better semantic memory at follow-up was associated with smaller resection volumes and greater temporal pole integrity after left temporal surgery. Results were independent of post-surgical intellectual function and language lateralization. Our findings indicate post-surgical, hemisphere-dependent materialspecific improvement in memory functions in the intact temporal lobe. However, outcome was linked to the anatomical integrity of the temporal lobe memory system, indicating that compensatory mechanisms are constrained by the amount of tissue which remains in the operated temporal lobe. Careful tailoring of resections for children undergoing epilepsy surgery may enhance long-term memory outcome

Segre Types of Symmetric Two-tensors in n-Dimensional Spacetimes

Three propositions about Jordan matrices are proved and applied to algebraically classify the Ricci tensor in n-dimensional Kaluza-Klein-type spacetimes. We show that the possible Segre types are [1,1...1], [21...1], [31\ldots 1], [z\bar{z}1...1] and degeneracies thereof. A set of canonical forms for the Segre types is obtained in terms of semi-null bases of vectors.Comment: 14 pages, LaTeX, replaced due to a LaTex erro