A Nelson

C Welsh

J Froese

M H Entz

M Tenuta

Y Y Chen

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3rd  QLIF Congress, Hohenheim, Germany, March 20-23, 2007 
Archived at http://orgprints.org/view/projects/int_conf_qlif2007.html 
 
 
Assessment of selected soil parameters in a long-term  
Western Canadian organic field experiment 
M.H. Entz, C. Welsh, M Tenuta, Y. Y. Chen, A. Nelson and J. Froese 
Keywords: nitrogen, phosphorous, mycorrhiza, aggregate stability 
Abstract  
A long-term field study was used to compare soil nitrogen and phosphorous status, 
and soil aggregate stability in organic and conventional cropping systems.  Two 
rotations were tested: a grain only and a grain-alfalfa hay rotation.  The organic 
systems had a lower nitrate leaching potential than the same rotations under 
conventional management.  After 13 years, one organic system (the grain-alfalfa; no 
manure return) is suffering serious soil P depletion. However, the grain only and the 
grain-alfalfa with manure return to land systems had soil P levels similar to the prairie 
grass control treatment and showed no signs of P deficiency. Despite having lower 
levels of organic carbon, the organic soils had higher levels of wet aggregate stability 
than conventionally managed soils. 
Introduction  
Soil fertility and erosion risk are major concerns in Canadian organic grain production.  
Surveys of organic fields in western Canada indicate that while soil nitrogen (N) supply 
appears adequate and sometime excessive, soil available phosphorous (P) is often 
lacking (Entz et al., 2001).  Our first question regards N leaching potential in organic 
compared to conventional systems under the variable weather conditions experienced 
in western Canada. We were interested in whether the 2006 drought, which resulted in 
low crop N uptake that year, increased the risk of future N leaching in organic and 
conventional systems.  Our second objective was to compare soil P in different 
organic farming systems and determine whether long-term organic management is 
depleting soil P.   Western Canada is dominated by large fields and soil erosion poses 
a major threat to soil sustainability.  Hence, our third objective was to determine the 
effect of long-term organic management on soil erodibility through measures of wet 
aggregate stability. 
 
Materials and methods  
The Glenlea study is located near Winnipeg, Canada on a Black Chernozemic soil 
(9% sand; 26% silt; 66% clay).  Annual precipitation is 500 mm. Two, four-year 
rotations have been investigated at Glenlea since 1992 (grain only and grain-alfalfa).  
The grain only rotation consists of flax-oat-fababean-wheat, while the grain-alfalfa 
system consists of flax-alfalfa-alfalfa-wheat.  Each rotation is conducted under 
conventional and organic management. The experiment also includes a restored 
prairie grass plot that serves as an ecological benchmark treatment. The experimental 
design is a randomized complete block with three replications. All grain crops are 
harvested for seed. Fababean is green-manured in the organic system and harvested 
for seed in the conventional system.  Alfalfa is harvested for hay.  In 2000, all alfalfa-
grain system plots were divided and one-half received a one-time application of 10 
t/ha composted manure, while the other half did not.    
  Archived at http://orgprints.org/9887/   
3rd  QLIF Congress, Hohenheim, Germany, March 20-23, 2007 
Archived at http://orgprints.org/view/projects/int_conf_qlif2007.html 
Measurements relevant to this study include crop yield and quality, soil carbon, soil 
nutrient status (N and P), mycorrhizal colonization of flax roots and wet mean weight 
diameter of soil (Angers and Mehuys 1988).  A P balance was conducted using 
differences in P removed by crops, added through fertilizer (or manure) between 1993 
and 2004. 
Results  
N content of soils to 120 cm was determined on samples taken in September 2006. 
The 2006 growing season was very dry and crop N uptake averaged <60 kg/ha (data 
not shown). Both conventional rotations had very high residual N levels (Table 1), and 
N leaching risk was considered high. Much lower N levels were observed in both 
organic rotations (Table 1); N leaching potential in organic was considered very low. 
Results suggest that organic systems, even those containing alfalfa, were at lower risk 
for N leaching than conventionally-managed systems. 
A thorough examination of soil P was conducted from soil sampled in 2003.  Results 
for total soil P indicate relatively high levels for all treatments, reflecting the natural 
fertility of these soils (Table 2).  However, the grain-alfalfa system with no manure 
returned to land had lower (p<0.05) levels of P than other systems. Sodium 
bicarbonate extractable P was lowest (p<0.05) in the grain-alfalfa system (<10 kg/ha; 
data not shown).  These observations suggest that a farming system that exports 
alfalfa hay but does not receive animal manure is at risk of a P deficiency.  The results 
from our expected P balance analysis (Table 2) confirm that the alfalfa-grain system 
with no manure added had the highest level of P removal.  It was interesting to 
observe that the grain-alfalfa with no manure treatment is showing physical signs of P 
deficiency in the alfalfa.  For example, in 2006 alfalfa yields were reduced by 75% in 
the organic compared with the conventional plots (data not shown).   
Mean wet aggregate stability was measured to assess soil erosion potential.  The 
organic systems had significantly higher soil stability than the conventional systems in 
both the grain only and the grain-alfalfa systems (Table 3). This observation was 
surprising, especially given that soil C levels were actually lower (p<0.05) in the 
organic system (4.8 vs. 5.1% for conventional).  Higher soil stability under organic 
compared with conventional management may be due to greater mycorrhizal 
colonization in the organic system (Entz et al., 2004) in 2003 (Table 3). 
Tab. 1: Soil nitrate-N (kg/ha) in three soil depths for two farming systems (grain only and 
grain-alfalfa) under organic and conventional management at Glenlea, Manitoba in 
September, 2006.  Numbers followed by different letters are significantly different at P<0.05. 
Soil depth 
(cm) 
Grain only 
conventional 
Grain 
only 
organic 
Alfalfa-grain 
conventional 
Alfalfa-
grain 
organic 
0-15 91 23 100 12 
15-60 62 20 67 31 
60-120 31 15 43 15 
0-120 184a 57b 210a 57b 
 
 
 
3rd  QLIF Congress, Hohenheim, Germany, March 20-23, 2007 
Archived at http://orgprints.org/view/projects/int_conf_qlif2007.html 
 
 
Tab. 2: Total soil P (kg/ha) measured in autumn of 2003 and expected P balance (1993-2004) 
for different crop rotations under organic and conventional management at Glenlea. 
Crop rotation  System Total soil P Expected P 
balance 
Grain Conventional 305a* +18 
Grain Organic 301a -56 
Grain-alfalfa Conventional 285a +41 
Grain-alfalfa Organic 219b -120 
Grain- alfalfa Conventional 
with manure 
315a +65 
Grain-alfalfa Organic with 
manure 
275a -100 
Prairie Organic 310a  
     * numbers followed by a different letter are significant for P<0.05  
 
Tab. 3: Soil mean weight diameter and percent root colonization by mycorrhiza for two 
rotations conducted under conventional and organic management at Glenlea in 2003.  
Crop rotation  System Aggregate 
Mean Weight 
Diameter 
(Wet) 
Mycorrhizal 
colonization 
(% root infected) 
Grain Conventional 0.786 b 30 b 
 Organic 0.946 a 50 a 
Grain-alfalfa Conventional 0.885 ab 27 b 
Grain-alfalfa Organic 0.986 a 45 a 
Prairie Organic - 82 a 
* numbers followed by a different letter are significant for P<0.05  
Discussion 
Dry seasons often cause available N to accumulate in prairie soils. This N poses a 
leaching risk.  Both organic rotations in the present study showed less N buildup than 
the conventional rotations, which means the organic system was at lower risk for N 
leaching.  This observation contradicts some previous studies where organic rotations, 
especially those containing perennial legumes, were found to have a high leaching 
potential.  P depletion was discovered in the organic grain-alfalfa hay system.   
Therefore, despite having adequate levels of soil N, the alfalfa hay export system is 
not sustainable. The problem of P depletion in the grain-alfalfa rotation appears to 
have been overcome by returning some composted manure to the soil (Table 2).  
Organic farmers who produce alfalfa for off-farm export need to monitor the soil P 
status of their soils and be prepared to take action to reverse P depletion.   
Conventional soil science wisdom suggests that a decrease in soil C will result in less 
aggregated soil.  In the present study, higher levels of wet aggregate stability could 
not be attributed to soil C since soil C was lower in the organic system regardless of 
rotation type.  Perhaps it is the improvement in soil biology, namely glomalin 
production by mycorrhiza, that is responsible for more stable organic soils. 
3rd  QLIF Congress, Hohenheim, Germany, March 20-23, 2007 
Archived at http://orgprints.org/view/projects/int_conf_qlif2007.html 
 
 
Conclusions  
After 13 years, soil P levels in organic grain and grain-alfalfa (with manure) systems 
were similar to the natural prairie and showed no signs of depletion. Soil P depletion 
was observed in the grain-alfalfa rotation (no manure).   Crop-livestock integration 
appears important on organic farms. 
After a drought year, organic systems were found to be less prone to nitrate leaching 
than conventional systems. 
Organic systems were found to have greater soil wet aggregate stability than 
conventional systems, despite having lower soil C concentrations.  Additional soil 
ecology research is required to better understand these results. 
Acknowledgments 
We gratefully acknowledge the Government of Manitoba for long-term funding of this 
study.  Special thanks also to Mr. Keith Bamford for technical assistance. 
References  
Angers, D.A. and G.R. Mehuys. 1988.  Effects of cropping on macro-aggregation of a marine clay 
soil. Can. J Soil Sci. 68:723-732. 
Entz, M.H., R. Guilford, and R. Gulden. Crop yield and nutrient status on 14 organic farms in the 
eastern Northern Great Plains. Can. J. Plant Sci. 81:351-354. 
Entz, M.H., K.R. Penner, J.K. Vessey, C.D. Zelmar and J.R. Thiessen Martens. 2004.  Mycorrhial 
colonization of flax under long-term organic and conventional management. Can. J Plant Sci. 
84:1097-1099.  


Assessment of selected soil parameters in a long-term Western Canadian organic field experiment

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Assessment of selected soil parameters in a long-term Western Canadian organic field experiment

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