Organic Crop Production Overview

This publication provides an overview of the key concepts and practices of certified organic crop production. It also presents perspectives on many of the notions, myths, and issues that have become associated with organic agriculture over time. A guide to useful ATTRA resources and to several non-ATTRA publications is provided

Radiative forces on macroscopic porous bodies in protoplanetary disks: laboratory experiments

In optically thin parts of protoplanetary disks photophoresis is a significant force not just for dust grains, but also for macroscopic bodies. The absolute strength on the supposedly highly porous objects is not known in detail as yet. We set up a low pressure torsion balance and studied photophoretic forces. We investigated the dependence on plate dimensions and on ambient pressure and considered the influence of channels through the plates. As samples for full (no channel) plates we used tissue with 2mm thickness and circular shape with diameters of 10mm, 30mm and 50mm. The influence of channels was probed on rectangular-shaped circuit boards of 35mm x 35mm area and 1.5mm thickness. The number of channels was 169 and 352. At low pressure, the absolute photophoretic force is proportional to the cross section of the plates. At high pressure, gas flow through the channels enhances the photophoretic force. The pressure dependence of the radiative force can (formally) be calculated by photophoresis on particles with a characteristic length. We derived two characteristic length scales l depending on the plate radius r_1, the channel radius r_2, and the thickness of the plate which equals the length of the channel d as l=r^{0.35} x d^{0.65}. The highest force is found at a pressure p_max = 15 x l^{-1}Pa mm. In total, the photophoretic force on a plate with channels can be well described by a superposition of the two components: photophoresis due to the overall size and cross section of the plate and photophoresis due to the channels, both with their characteristic pressure dependencies. We applied these results to the transport of large solids in protoplanetary disks and found that the influence of porosity on the photophoretic force can reverse the inward drift of large solids, for instance meter-sized bodies, and push them outward within the optically thin parts of the disk.Comment: Accepted by A&

Robert Thayer Wilce, pioneer of Arctic marine botany (9 December 1924 – 26 February 2022)

Acknowledgements: Special thanks are due to Bob Wilce, Donna Parker and Alexander Wilce for their help with the interviews for this article, as well as to Ambassador Mark G. and Patricia Hambley and Leslie and Lyman Wood (Springfield, Massachusetts) for their hospitality and logistical support which made the author’s visits possible (the last one during the difficult circumstances of the COVID19 pandemic). Also, the author would like to thank Michael Wynne (University of Michigan, Ann Arbor), for a critical read and fact-checking details of this article.Peer reviewedPublisher PD