Soil phosphorus turnover in long‑term organic and conventional management systems

T. Fraser1, D. Lynch2, M. Entz3 and K. Dunfield1


Low plant available phosphorus levels have been reported in organically managed soils across Canada. Manure can be a valuable source of phosphorus but large distances may prohibit regular application in the Great Plains region. Replacement of phosphorus in long-term organic systems is challenging since limited options are available under organic certification, making the turnover of organic phosphorus in these soils important.

Soil samples were taken from the wheat phase of a long-term field experiment near Winnipeg, Canada, established in 1992. Soil phosphorus pools and turnover of organic phosphorus were measured in the wheat phase of a forage-grain rotation (flax-alfalfa-alfalfa-wheat) managed as organic no input, manure-amended organic (fall applied, 2002 and 2011 only), conventional, and restored native prairie systems.

In spring 2011, Hedley phosphorus fractionation revealed significantly lower concentrations of both soil labile and moderately labile phosphorus fractions in the organically-managed treatments compared to the prairie and conventional systems, although plant phosphorus concentrations showed no difference (after one-time manure application). Alkaline phosphatase activity in July was highest in the no input organic and organic manure amended treatments, with correspondingly low levels of plant available phosphorus. Samples collected in July 2012 from the same plots now in flax, following the second fall manure application, resulted in organic manure-amended alkaline phosphatase activity to be similar to the conventional and native prairie systems.

The influence of long-term organic systems and periodic manure application on the abundance and diversity of bacteria harboring an alkaline phosphatase gene is currently being investigated to examine the relationship between alkaline phosphatase activity, plant phosphorus availability, and phosphorus uptake.


Proceedings of Organic Phosphorus 2013 Integration across Ecosystems. Smithsonian Tropical Research Institute, Republic of Panama. 4-7 February 2013.

Author Locations and Affiliations

(1) School of Environmental Science, University of Guelph, Ontario, Canada
(2) Faculty of Agriculture, Dalhousie University, Truro, Nova Scotia, Canada
(3) Department of Plant Science, University of Manitoba, Winnipeg, Manitoba, Canada

Posted May 2013