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Soil Nutrient Concentrations (Table 4)
•59% of fields tested had very low soil
test P (≤4 ppm)
•Most fields had moderate to high soil
K and S levels
Plant Tissue Nutrient Conc. (Table 4)
•35% of GMs contained <2.0% N
•65% of GMs contained <0.20% P
Relationship between Soil P and Plant
Tissue P (Figure 2)
•Soil P <5 ppm → low plant P
•Soil P 5-10 ppm → variable plant P
•Soil P >10 ppm → high plant P
Assessing Nutrient Status of Organic Grain Farms on the Canadian Prairie
Joanne R. Thiessen Martens1, Martin H. Entz1 and Derek H. Lynch2
1Department of Plant Science, University of Manitoba, Winnipeg MB
2Department of Plant, Food and Environmental Science, Dalhousie University, Truro NS
Introduction
In this study, we developed and
piloted two tools for assessing
nutrient status on organic farms:
1. Green manure (GM) bioassay:
to assess the soil’s nutrient-
supplying power, or
“functional fertility” (esp. P, K
and S).
2. Whole-farm nutrient budget:
to identify and quantify
nutrient surpluses and deficits
at the farm level
Methodology
Results: Green Manure Bioassay 2015
•Green manure productivity was sub-optimal
on many organic farms in the study.
•Where soil test P was very low, GMs were
unable to access sufficient quantities of P.
•Where soil test P was moderately low, many
GMs were still able to access sufficient P.
•Whole-farm nutrient budgets calculated over
several years can expose trends in nutrient
accumulation or depletion.
Conclusions
On many organic grain farms, soil fertility is
not being optimized (low crop yields; low
soil test levels of certain nutrients).
Green Manure Species and Biomass Production
•GM species: annual, biennial and perennial legumes, often in
mixtures with a cereal or grass (Table 3).
•Many GMs did not meet target values for adequate N supply to
following crops:
•76% of GMs produced <5000 kg/ha of biomass
•41% of GMs contained <45% legume by weight
•41% of GMs contained >25% weeds by weight
Standard soil tests do not always accurately
reflect the nutrient-supplying power of soils
under organic nutrient management.
Organic farmers need better tools
to assess the nutrient status of their fields and farms
and allow them to make improved nutrient management decisions.
Figure 1. Farm system co-design process and timeline for one
growing season, outlining the activities carried out for the two
tools developed in this study.
These tools are part of a “farm system co-design” process that brings together farmers and
agronomists or researchers to support sound, farm-specific decision-making (Figure 1).
Table 1. Summary of data collected for the Green Manure Bioassay pilot.
Year Farms and fields Data collected
2014 2 farms; 3 GM fields Plant biomass; Plant tissue analysis
2015 11 farms; 17 GM and hay fields Plant biomass; Soil and plant tissue analysis
2016 9 farms; 12 GM and hay fields Plant biomass; Soil and plant tissue analysis
8 farms; 12 crop fields (GMs in ‘15) Plant biomass; Plant tissue analysis
Table 2. Summary of data collected for the Nutrient Budget pilot.
Year Farms Data and information collected
2014 4 farms Quantities, nutrient concentrations of:
-Nutrient imports:
-Seed, manure, other crop inputs
-Nitrogen fixation by legumes
-Animals, feed, bedding, etc.
-Nutrient exports:
-Crops: Grain, hay, etc.
-Animals, other livestock
products, manure
2015 8 farms
2016 9 farms
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0 10 20 30 40 50 60 70
Plant P concentration (%)
Soil P concentration - Olsen (ppm)
Figure 2. Plant tissue phosphorus (P) concentration
(% by weight) as a function of soil P concentration
(ppm; Olsen) in green manures sampled on 17 farm
fields and 3 research station fields in 2015.
Table 4. Characteristics of green manures sampled on 17 organic
farm fields in 2015.
Parameter Mean Median Min. Max.
Crop biomass (kg/ha) 3830 3760 1740 6900
Proportion legume (% by weight) 47 47 14 98
Proportion weeds (% by weight) 23 20 0 60
Soil analysis, 0-15 cm depth
Nitrate-N (kg/ha) 12 11 7 28
P (Olsen; ppm) 11 4 3 61
K (ppm) 262 248 58 520
S (kg/ha) 43 22 7 135
Plant tissue analysis (% by weight)
N concentration 2.2 2.1 1.4 3.7
P concentration 0.17 0.14 0.08 0.32
K concentration 1.8 1.7 1.0 3.1
S concentration 0.18 0.18 0.08 0.31
Table 3. Green manure crop
types sampled in 2015.
Green manure type n
Pea-cereal mixture 5
Red clover and/or sweetclover 4
Alfalfa-grass mixture 3
Fababean-cereal mixture 2
Multi-species annual mixture 2
Hairy vetch-cereal mixture 1
TOTAL 17
•All farms had a N surplus (7-54 kg/ha).
•Most farms importing manure and/or
feed had surpluses of P, K and S (one
exception). Most farms do not import
manure every year.
•Farms not importing manure or feed had
small deficits of P, K and S (usually <1
kg/ha).
•Nutrient exports were often low due to
low crop yields.
GREEN MANURE BIOASSAY:
Crop and soil samples were collected from farm fields in Manitoba (MB) and eastern
Saskatchewan (SK) and were analyzed for nutrient concentration (Table 1). Samples from green
manure (GM) crops are used to assess the sufficiency of P, K and S. Samples from the crop
following the GM are used to assess the N contribution of the GM in the previous year.
WHOLE-FARM NUTRIENT BUDGET:
Budgets for N, P, K and S were calculated for farms in MB and
eastern SK, based on information provided by farmers, analyzed
grain and hay samples and appropriate estimates (Table 2).
Collecting soil samples in a
sweetclover green manure, July 2016.
Left: A poor pea-barley green manure, dominated by weeds.
Right: A healthy pea-barley green manure, dominated by peas.
Acknowledgements:
Thank you to the Organic Science Cluster II (Agriculture and Agri-Food Canada) and Organic Valley for financial support.
Many thanks to the farmers participating in this project for their time, hospitality and willingness to share their knowledge.
Results: Nutrient Budgets