Soil structure, biological activity and management
Chair: Mark Measures (IOTA)
Christine Watson (SAC): Improving phosphate availability: green manures and better timing.
Christine Watson holds the position of Reader in Organic Farming Systems at SAC. She has a degree in soil science and a PhD in farming systems. She leads a research team working on resource use in farming systems. Current research focuses on relationships between management practices and nitrogen and phosphorus cycling. She leads the Scottish Government RERAD research Work Package 1.7 (2006-2011) on sustainable cropping systems. Christine is on the editorial board of Journal of Agricultural Science, Cambridge and the Journal of Organic Agriculture. Current research is funded by EU, Defra, Natural England, FORMAS (Swedish Research Council on Environment, Agricultural Sciences and Spatial Planning), SNIFFER and Scottish Government
Presentation - Improving phosphate availability: green manures and better timing.
Julia Cooper (Nafferton Ecological Farming Group): Soil structure, management and effect on nutrient availability and crop production.
Julia Cooper is a Lecturer in Soil Science in the School of Agriculture at Newcastle University. She has worked closely with the organic industry in Canada where she was an organic farm inspector and a small-holder. Since entering the academic arena she has focussed on the study of soil organic matter dynamics and the effects of manure and crop management on the supply of nutrients to the crop. She is a member of the Nafferton Ecological Farming Group and works closely with the N-TOOLBOX project and the NUE-CROPS project, both of which are focussed on strategies to improve the efficiency of N use at the farm level.
Presentation - Soil structure, management and effect on nutrient availability and crop production
Paul Gosling (University of Warwick): Managing soil life for crop production.
Presentation - Managing soil life for crop production
Heather McCalman (IBERS, Aberystwyth University): The Pro Soil project
Presentation - The Pro Soil project
It was clear from all three presentations that tillage, rotations, and nutrient additions must all be thought of as part of the soil management strategy, whether for structure, nutrient profile or microbe activity.
There are multiple ways to affect phosphate availability in organic farming. One of these is through plant breeding, for characteristics which enhance nutrient uptake efficiency, for example through more abundant and longer root hairs. Phosphate rock added to soils does not automatically increase the amount of phosphorous available to plants, since it is not in a bio-available form. Buckwheat, however, is able to liberate and utilise soil phosphate (possibly through an acidic root exudate), and is attracting interest as a green manure. In experiments, it showed strong responses to phosphate additions where other crops showed little or no response. This effect was observed both in acidic Scottish soils and in southern English soils with a higher pH of 7-8. The P-enriching effect of buckwheat as a green manure can be achieved in a short time: the crop can be in the ground for as little as 8-10 weeks before cutting. Brassicas are also able to efficiently utilise soil phosphate. Meanwhile, experiments have been testing whether phosphate enrichment of soils can be better achieved through the composting of phosphate rock. This is a challenge, because science does not yet fully understand the phosphorous and other mineral dynamics of the soil. Results do not show composted phosphate to be much more effective than rock phosphate immediately following application, but it could be that more effects emerge later in the rotation.
Good soil structure is defined by low bulk density (mass / volume) and high porosity (volume of pores / volume of soil). The key management strategy for good soil structure is the addition of organic matter, which increases water-stable aggregates. Fresh (as opposed to composted) organic matter is more active, contains more bio-available carbon sources and has the best effect on soil structure. Biochar is very stable, therefore inactive. Meanwhile, root exudate is organic matter, and definitely promotes good soil structure.
In terms of managing tillage for best soil structure, good practices include: 1. Reducing the number of passes of machinery; 2. Limiting stock density and moving feed terminals around the field.
Dr. Cooper stressed the importance of “getting off the tractor and getting to know your soil”! She recommended a practical website with advice on how to examine and evaluate soils:
The Environment Agency has also published a more detailed manual on soil evaluation and management, ‘thinksoils’:
There is a profusion of different microorganisms in the soil, which interact with the plants growing there. Understanding soil organisms will be of great use in organic farming. Research into effects of biopesticides (naturally occurring substances or microorganisms that control pests) designed to suppress soil microorganisms damaging to crops found that application of a product to control the root fungus Rhizoctonia increased sugar beet yields, but biopesticides perform erratically and an alternative approach to biopesticides is to support general enhancement of soil life by adding feedstuffs for soil life, and reducing tillage. Adapting rotations to benefit mycorrhizae is also beneficial: leys, rotations, crop selection and green manures can all be used.
Soil mycorrhizae are still not well understood. This may explain why mycorrhizal products on the market generally perform quite poorly. Another puzzling phenomenon is that phosphorous additions can be detrimental to mycorrhizae, but less so in organic soils.
Although there is no clear relationship between the percentage of Soil Organic Matter and microbial activity levels in the soil, it is generally thought that adding ‘too much’ organic matter is not possible.
The Welsh PROSOIL project will pin down any effects on the nutrient profile of the crops and livestock of the soil on which they are grown and grazed. It will also develop best practice for soil management among Welsh farmers.
- Compost versus green manures for microorganisms: compost adds phosphorous and potassium but its nitrogen sources are largely stable (therefore unavailable); fresh organic matter and green manures provide a big injection of nitrogen but are low in P and K.
- Some weeds are an excellent residue of mycorrhizal fungi, so farmers should not be afraid to leave a few in their fields, year to year.
- Tillage does not permanently destroy mycorrhizal populations, but it will delay their colonisation of subsequent crops. As long as bare fallow is not left, mycorrhizae will arrive in due course. A ‘no rotation’ / ‘natural farming’ approach may be good for mycorrhizal populations, but can lead to the build-up of parasitic microorganisms detrimental to the crop. Intensive horticulture operations (both organic and non-organic) tend to have low presence of mycorrhizae because they grow a lot of brassicas (which are non-mycorrhizal), eliminate weeds completely, and apply high levels of nutrients.
- Additions to enrich soil microbes: except for rhizobia, it is probably not worth using microbial additives to improve crop performance; compost teas have been found effective in controlled lab experiments but remain unproven in field conditions.
- Soil management for flood tolerance: good soil structure aids drainage; in particular, high soil organic matter content increases flood resistance and livestock carrying capacity.
- How deep should farmers plough their soils? As shallow as they can get away with for some mineralisation, aeration and weed control. The less the better.