Can Plants Access P Pools not Accounted for in Traditional Soil Tests? A Study of Organically Managed Soils Across Saskatchewan

Tandra Fraser¹, J. Diane Knight¹ and Brenda Frick²

Abstract
Saskatchewan is the Canadian leader in organic farming. A previous study of organically farmed fields across Saskatchewan revealed a widespread deficiency of available P using standard soil test protocols. Despite the low available P test results, producers reported crop yields exceeding what was expected.

In 2009, a study of organic farms across Saskatchewan was initiated to determine if plants grown under organic management are able to access pools of P that are not measured using traditional soil tests. Three organic farms were recruited from each of the Brown, Dark Brown and Black soil zones of Saskatchewan and one from the Gray soil zone. One conventional farm from each zone was sampled for comparison. Soil and plant N also were investigated to expand the scope of the study and obtain a more comprehensive understanding of nutrient cycling in organic systems.

The objectives of the study were to examine P and N mineralization at each site, determine plant P and N uptake over the growing season, quantify arbuscular mycorrhizal fungi (AMF) associations, and assess the relationship between the analysed variables. A comparison of one organically managed site and one conventionally managed site in each soil zone was included to assess if the traditional soil tests performed better on conventionally managed soils. One field seeded to a pulse and one field seeded to a cereal was sampled at each location. Spring soil samples were collected for initial soil characterization and for use in N and P mineralization studies. Plant Simulator Probes (PRS™) were used to measure field mineralization of P and N over the growing season. Plant samples were collected in June, July, August and just before harvest for nutrient analysis. Roots samples were collected mid-season for quantifying AMF colonization.

Results from this province-wide study will provide an understanding of the severity of the perceived P deficiency for organically managed soils.


Source
Proceedings of the Joint Conference of the Canadian Society of Soil Science and the Canadian Society of Agronomy, Beyond Organics session. Saskatoon, Saskatchewan. 20-24 June 2010.


Author Locations and Affiliations
(1) Department of Soil Science, College of Agriculture and Bioresources, University of Saskatchewan
(2) Department of Plant Science, College of Agriculture and Bioresources, University of Saskatchewan


Posted July 2010