Glenlea long-term rotation

Carkner, M., Bamford, K., Thiessen-Martens, J.R., Wilcott, S., Stainsby, A., Stanley, K., Dick, C., and Entz, M.H. 2020. Building capacity from Glenlea, Canada's oldest organic rotation study. In Long-Term Farming Systems Research. p. 103-122. Academic Press.

Fraser, T.D., Lynch, D.H., O’Halloran, I.P., Voroney, R.P., Entz, M.H., and Dunfield, K.E. 2019. Soil phosphorus bioavailability as influenced by long-term management and applied phosphorus source. Canadian Journal of Soil Science 99(3): 292–304.

Westphal, M., Tenuta, M., and Entz, M.H. 2018. Nitrous oxide emissions with organic crop production depends on fall soil moisture. Agriculture, Ecosystems and Environment 254: 41–49.

Braman, S., Tenuta, M., Entz, M.H. 2016. Selected soil biological parameters measured in the 19th year of a long-term organic-conventional comparison study in Canada. Agriculture, Ecosystems, and Environment. 233: 343–351.

Fraser, T.D., Lynch, D.H., Bent, E., Entz, M.H., Dunfield, K.E. 2015. Soil bacterial phoD gene abundance and expression in response to applied phosphorus and long-term management. Soil Biology and Biochemistry. 88:137-147.

Entz, M.H., Welsh, C., Mellish, S., Shen, Y., Braman, S., Tenuta, M., Turmel, M.S., Buckley, K., Bamford, K.C., and Holliday, N., 2014. Glenlea organic rotation: a long-term systems analysis. In Managing Energy, Nutrients, and Pests in Organic Field Crops. p. 236-259. CRC Press.

Xu, N., Wilson, H.F., Saiers, J.E., and Entz, M.H. 2013. Effects of crop rotation and management system on water-extractable organic matter concentration, structure, and bioavailability in a chernozemic agricultural soil. Journal of Environmental Quality 42(1): 179-190.

Bell, L.W., Sparling, B., Tenuta, M., and Entz, M.H. 2012. Soil profile carbon and nutrient stocks under long-term conventional and organic crop and alfalfa-crop rotations and re-established grassland. Agriculture, Ecosystems & Environment.158:156-163.

Briar, Shabeg S., Corinne Barker, Mario Tenuta, and Martin H. Entz. 2012. Soil nematode responses to crop management and conversion to native grasses. Journal of Nematology 44(3): 245–54.

Li, R., Khafipour, E., Krause, D.O., Entz, M.H., de Kievit, T.R., and W.G.D. Fernando. 2012. Pyrosequencing reveals the influence of organic and conventional farming systems on bacterial communities. PLoS ONE 7(12).

Kirk, A.P., Entz, M.H., Fox, S.L., and Tenuta, M. 2011. Mycorrhizal colonization, P uptake and yield of older and modern wheats under organic management. Canadian Journal of Plant Science 91(4): 663-667.

Humble, S.M., Entz, M.H., Holliday, N.J. and VanAcker, R. 1999. Weed and ground beetles (Coleoptera: Carabidae) as influenced by crop rotation type and crop input management: The Glenlea crop rotation study. Proceedings of the Expert Committee on Weeds Comité d’experts en malherbologie, p.45.

Turmel, M.S., Entz, M.H., Bamford, K. and Thiessen Martens, J.R. 2009. The influence of crop rotation on the mineral nutrient content of organic vs. conventionally produced wheat grain: Preliminary results from a long-term field study. Canadian Journal of Plant Science. 89(5): 915-919.

Welsh, C., Tenuta, M., Flaten, D.N., Thiessen-Martens, J.R., and Entz, M.H. 2009. High-yielding organic crop management decreases plant-available but not recalcitrant soil phosphorus. Agronomy Journal. 101:1027–1035.

Hoeppner, J. W., Entz, M.H., McConkey, B.G., Zentner, R.P., and Nagy, C.N. 2006. Energy use and efficiency in two Canadian organic and conventional crop production systems. Renewable Agriculture and Food Systems 21(1): 60–67.

Entz, M.H., Penner, K.R., Vessey, J.K., Zelmer, C.D., and Thiessen Martens, J.R. 2004. Mycorrhizal colonization of flax under long-term organic and conventional management. Canadian Journal of Plant Science. 84: 1097-1099.

The Glenlea long-term rotation study was established in 1992 to determine the interaction of crop rotation and crop inputs (fertilizer and herbicide). Crop rotation ranged from simple annual systems to more complex forage-based systems. The present study reports on results from the first 12 years of this study.

Test crop yield

Flax was grown as a test crop at the end of all rotation systems. Flax is an important organic crop in the Northern Great Plains currently receiving high prices. Flax was chosen as the test crop in this experiment due to its poor competitive ability with all weed types. The influence of crop rotation and crop inputs on yield should, therefore, be evident. The yield of this flax test crop is discussed below.

Flax is used as a "test" crop at the end of each 4-year rotation cycle. Flax yields in bu ac^-1 for 1995, 1999 and 2003 in the three main rotations are shown below. 

Rotation	Inputs	1995	1999	2003
Annual (Wheat-pea-wheat-flax)	F+H+	30	22	27
	F+H-	16	10	1.3
	F-H+	21	17	15
	F-H-	15	10	4
Green Manure (Wheat-clover-wheat-flax)	F+H+	29	29	10
	F+H-	20	18	1.2
	F-H+	18	25	8
	F-H-	16	16	3
Alfalfa (Wheat-alfalfa-alfalfa-flax)	F+H+	27	23	21
	F+H-	21	16	1.5
	F-H+	25	24	20
	F-H-	22	22	8

Crop Rotation Effect

• The beneficial effects of alfalfa and clover green manure appeared to be stronger in the first and second rotation cycle (1995 and 1999) than in the third rotation cycle (2003).
• After two or three cycles of the crop rotation, when weed pressure was much higher, green manure and forage did not offer weed suppression benefits to the following flax crop.
• A two-year stand of alfalfa had a greater effect on subsequent grain yield than did a clover green manure.
• By 2003, organic flax yields were low in all three rotations, however. Alfalfa yields began to suffer possibly due to low soil P and S. This may have reduced alfalfa's ability to suppress weeds.

Input Effect

• There was a greater yield loss when herbicide was removed (F+H-) from the system than when fertilizer was removed (F-H+) suggesting that weeds have a greater limiting impact on crop production than nitrogen fertility.
• Flax yield was very low in systems that used fertilizer but no herbicide (F+H-) because weeds took better advantage of the added N fertilizer than the crop.

Interaction of Crop Rotation and Inputs

• In both low input systems (F+H- and F-H+), flax yields were lowest in the annual crop rotation vs the sweet clover and alfalfa-containing rotations.
• Under organic production conditions (F-H-), flax in the alfalfa-containing rotation had the highest grain yield, followed by flax in the sweet clover rotation, followed by flax in the annual crop rotation.

Why is this important?

Crop rotation is important to farming with fewer chemicals. In this study, when alfalfa or green manure were included in the rotation organic flax yield was better than when only annual crops were in the rotation. Even when these legumes were included in the rotation, by the end of the third cycle of the rotation, organic yields were low. Soil P and S may be a big limiting factor to the success of alfalfa as a weed clean-up crop. In the fourth rotation cycle, treatments will be further subdivided to include manured treatments and early and late seeding dates.

Weed Dynamics

Weed dry matter (DM) was measured at harvest time in each of the flax test crop years to determine the effect of crop rotation and input management on weed production.

Crop Rotation Effect

• Crop rotation only had a significant effect on weed production in 1995 when weed DM was significantly less in the rotations that included green manure or forage (Figure 1). The biggest increase in weed DM production between 1995 and 2003 occurred in the forage rotation. This most likely can be attributed to the additional nitrogen that the alfalfa in that rotation provides to the weeds.
• Weed DM for the annual rotation was comprised of predominantly green foxtail (millet), wild oat, lady's thumb (green smartweed) and Canada thistle (data not shown). Stinkweed (field pennycress), a winter annual weed, wild buckwheat and Canada thistle were associated with the green manure rotation. Volunteer alfalfa, dandelion and wild mustard were associated with the forage rotation.

Figure 1. The effect of crop rotation on weed dry matter (kg ha^-1) at flax harvest at Glenlea.

Input Effect

• Crop input management had a strong influence on weed DM production in all three test crop years (Figure 2, below). As expected, weed DM production was consistently lower where herbicide was used (F+H+ and F-H+). In all years of the study, weed DM was greater for the fertilizer-only treatment (F+H-) than for the organic treatment (F-H-). In this situation, the weeds were taking better advantage of the added fertilizer than the crop.
• The management of crop inputs influenced the weed spectrum (data not shown). Briefly, hemp nettle was most abundant when fertilizer and herbicide were used (F+H+). This may be due to the fact that herbicides effectively controlled all other weeds thus allowing this species to occupy available niches. Wild mustard, wild buckwheat, wild oat, and stinkweed were in the greatest abundance in the fertilizer-only system (F+H-). When no inputs were used (F-H-) Canada thistle was more abundant than when fertilizer was used but no herbicide (F+H-).

Figure 2. The effect of crop input management on weed dry matter (kg ha^-1) at flax harvest at Glenlea.

Interaction of Crop Rotation and Inputs

• During the first rotation cycle, there was an interaction between crop rotation and input management for weed DM. In 1995, when herbicide was removed but fertilizer remained (F+H-), weed DM was greater in the annual crop and green manure rotations than in the forage rotation (Figure 3). Therefore, the presence of alfalfa in the rotation suppressed weeds during the first rotation cycle.
• After three rotation cycles, the decrease in yield when herbicide (F+H-) was removed from the system was similar for all rotations (2003). Thus, by the third cycle of the rotation, herbicide was necessary to decrease weed DM regardless of the crop rotation. This suggests that even when a crop known to be competitive with weeds is consistently in the crop rotation (i.e. alfalfa hay), herbicide may be necessary to control weeds.

Figure 3. The interaction of crop rotation and crop input management on weed dry matter production (kg ha^-1) at flax harvest in 1995 at Glenlea.

Ground Beetles

The maintenance of maximum biological diversity within a cropping system is important as biodiversity is an index of the health of the cropping system. Bioindicators are organisms within an ecosystem that are sensitive to changes in the environment, and their populations are affected by disturbance. Ground beetles are excellent bioindicators. Ground beetles have often been considered a beneficial insect as some species eat weed seeds and other eat pest insects. Therefore, the ability to maintain or enhance ground beetle populations could lead to less reliance on therapeutic pest control measures.

Methods

In this study, pitfall traps were used to trap insects from 1992 to 1999. Traps were placed in each sub-plot (i.e. F+H+, F+H-, F-H+, F-H-). Insects caught in the traps were identified, separated and counted.

Results Indicate: 

• Ground beetle populations were higher in the annual rotation (wheat-pea-wheat-flax) than in the forage (wheat-alfalfa-alfalfa-flax) or green manure (wheat-clover-wheat-flax) rotations, because weed populations were highest in the annual rotation.
• Beetle populations were more consistent or stable between input treatments in the forage and green manure rotations. This suggests that the forage and green manure rotations are more "robust" or less fragile than the annual rotation.
• Beetle populations were highest in the F+H- treatment because these weedier plots provided a better habitat due to increased humidity and abundant food supply.
• Ground beetle populations were least in the F-H+ system because of a lack of potential food sources and poor habitat.
• Four consistent associations were observed between beetle and weed species, however. These were Harpalus pensylvanicus with redroot pigweed; Amara carinata with stinkweed; Agonum placidum and Calosoma calidum with wild mustard. Harpalus and Amara are weed seed eaters.

Harpalus pensylvanicus

+

Redroot Pigweed

 

Agonum placidum

Calosoma calidum

+

Wild Mustard

Why is this important?

The association of Harpalus and Amara the weediest subplots (F+H-) suggests that these ground beetles may be used to sustainably manage weed populations through seed predation. Other studies have indicated that 50 to 80% of weed seeds in the soil may be consumed by insect seed predators. In this study, however, the abundance of weeds in the no herbicide plots (F+H- and F-H-) was too high for the beetles to reduce weed seeds significantly. Ground beetles may be more effective in an Integrated Pest Management (IPM) system where some herbicide is used to keep weed densities at levels lower than in the present study. In a system that depends entirely on herbicide for weed control, populations of ground beetles remain small and never have the opportunity to build to beneficial levels.

Energy Use

Reducing non-renewable energy use and carbon dioxide (CO₂) production, and increasing energy efficiency can make cropping systems more sustainable. Nitrogen benefits of legumes to succeeding non-leguminous crops are well documented, while organic cropping systems have reduced levels of crop inputs. The long-term effects of perennial forage legumes and organic cropping practices on energy use, CO₂ production, and energy efficiency of crop production were examined at Glenlea.

• The first objective of the current study was to examine the effectiveness of perennial forage legume crops in reducing energy use and CO₂ production, and improving the energy efficiency of cropping systems, by replacing a portion of the nitrogen fertilizer required for crop production with nitrogen fixed by the alfalfa.
• The second objective of this study was to examine the effectiveness of organic crop production in reducing energy use and CO₂ production, and improving the energy efficiency of cropping systems, by eliminating fertilizer and pesticide inputs.

Methods

Data collected included levels of all crop inputs, such as seed, fertilizers, pesticides, and fuel and machinery needed for all field operations, and crop yields from each treatment. This information was then converted to energy values (MJ/ha) using energy coefficients assigned for each crop input, field operation, and crop harvested.

Rotational ENERGY USE was calculated by adding the energy coefficients of all the field operations and crop inputs. Rotational ENERGY PRODUCTION was calculated using energy output coefficients assigned to the different crops through laboratory bomb calorimeter tests. ENERGY EFFICIENCY was calculated for each treatment by dividing rotational energy production by rotational energy use. Rotational CO₂ production was calculated by multiplying total rotational energy use by CO₂ coefficients used by Gulden and Entz (1996).

Total rotational energy consumption (MJ/ha) at the Glenlea long-term cropping systems study, 1992-2003
Rotation	Inputs	Total Energy Consumption	Seed Energy	Fuel and Lube Energy	Machinery Energy	Pesticide Energy	Fertilizer Energy
WPWF	F+H+	68498	7902	16133	2367	7116	34980
WPWF	F-H-	24233	7902	14229	2102	0	0
WAAF	F+H+	49255	3657	18184	2515	3499	21400
WAAF	F-H-	22181	3657	16213	2311	0	0
WPWF = wheat-pea-wheat-flax; WAAF = wheat-alfalfa-alfalfa-flax

Results Indicate:

Energy Use and CO₂ Production

• When comparing the same treatments between rotations, the organic and conventional wheat-pea-wheat-flax (annual) systems used 9% and 39% more energy than the organic and conventional wheat-alfalfa-alfalfa-flax (forage) systems, respectively.
• When comparing the conventional and organic systems within rotations, the conventional forage system produced approximately 2 times as much CO₂ as the organic forage system, while the conventional annual system produced approximately 2.5 times as much CO₂ as the organic annual system.
• When comparing the conventional and organic systems within rotations, the conventional forage system consumed approximately 2.2 times as much non-renewable energy as the organic forage system, while the conventional annual system consumed approximately 2.8 times as much energy as the organic annual system
• When comparing the same treatments between rotation, the organic and conventional annual systems produced 6% and 34% more CO₂ than the organic and conventional forage systems, respectively.
• The reductions in energy use and CO₂ production for the forage system were primarily through reductions in nitrogen fertilizer requirements.

Energy Production

• Within the annual rotation, rotational energy production was 85% higher in the conventional system, while rotation energy production in the forage rotation was approximately 26% higher in the conventional system.
• The forage systems had levels of rotational energy production that were approximately two- and three times higher than the organic and conventional systems in the annual rotation, respectively.
• The apparent benefits of including alfalfa in the rotation become more and more evident, the more years that the rotation was in existence, as the systems in the annual rotation appear to have energy production that is dropping more than in the forage rotation.

Energy Efficiency

• It was found that energy efficiency in cropping systems containing perennial forage legumes was up to 222% greater than in the conventional cropping systems, as a result of decreases in energy use and increases in energy production.
• Within each rotation, the organic system was more efficient than the conventional system. This was primarily due to substantially higher levels of input energy in the conventional systems, where energy use was more than twice as great as in the organic systems. While output energy for both of the conventional systems was higher than in the organic systems, it was not enough to negate the higher amounts of input energy consumed, in terms of rotational energy efficiency.

Why is this important?

The findings of this study point to the importance of including legumes in cropping systems in order to improve energy efficiency. The vast majority of energy used in crop production is in the form of fossil fuels. When burned, these fossil fuels release carbon dioxide, a greenhouse gas, into the atmosphere. With ever-increasing concern over global warming as a result of greenhouse gases, any steps taken to improve the energy efficiency of agriculture should be welcome steps indeed.

Soil Nutrient Status

Soil samples were collected from the three main rotations in the fall of 2003. Only the full input (F+H+) and no-input (F-H-) systems were tested. Samples were taken in six locations per subplot in all three replicates. The replicate samples were bulked prior to lab analysis.

Results show both a strong crop input and a strong crop rotation effect for many of the nutrients (see Table below). Nitrogen levels were affected by both fertilization and crop rotation. The alfalfa-containing rotation had the highest level of N, followed by the sweet clover containing system.

Soil nutrient status in three different crop rotations after 12 years of cropping. Rotation 1, wheat-pea-wheat-flax; rotation 2, wheat-sweet clover green manure-wheat-flax; rotation 3, wheat-alfalfa-alfalfa-flax. Values in kg available nutrients per hectare.
Rotation	Input Level	N	P	K	S
wheat-pea-wheat-flax	F+H+	32	46	1316	141
wheat-pea-wheat-flax	F-H-	22	33	1312	86
wheat-clover-wheat-flax	F+H+	29	24	1169	87
wheat-clover-wheat-flax	F-H-	31	37	1116	76
wheat-alfalfa-alfalfa-flax	F+H+	81	24	1140	63
wheat-alfalfa-alfalfa-flax	F-H-	37	11	1073	26

Phosphorous levels were adequate for all systems except the unfertilized alfalfa rotation (see Table). The phosphorous mining effect of alfalfa forage crops is well documented. It is important to note that these values only reflect the available P levels, yet about 50% of soil P is in the organic form (which these soil tests do not measure). Soil potassium and sulphur levels were also lowest in the unfertilized alfalfa-containing rotation (see Table).

Why is this important?

Results of the present study show no evidence of serious nutrient deficiencies in organic systems for the annual or sweet clover containing rotations (see Table). Only in the organic alfalfa-containing rotation did signs of nutrient deficiencies occur (see Table). In a survey of commercial organic farms in the eastern prairies, Entz et al. (2001) reported that long-term organically farmed soils had extremely low levels of inorganic phosphorous.

 

Economic Analysis

Economic analysis of 8 years of the Glenlea Rotation Study The following table shows the cost of production and net income for the 8 years (1992-1999) of rotations 1 and 3 at Glenlea. Costs are in "1996 Canadian dollars" while commodity prices are in 1996 dollars (minus 20%). Values are averaged over the entire 8 year rotation and are shown in mean annual dollars/acre.
Crop Rotation	F+H+ Full inputs	F+H- Low input	F-H+ Low input	F-H- Organic system
wheat-pea- wheat-flax	Input cost 104.14 Net return 27.87	Input cost 77.17 Net return 30.87	Input cost 71.36 Net return 26.67	Input cost 43.44 Net return 40.23
wheat-alfalfa- alfalfa-flax	Input cost 71.68 Net return 77.83	Input cost 51.92 Net return 93.42	Input cost 55.92 Net return 73.73	Input cost 36.08 Net return 93.77

Results Indicate:

• With full inputs, input costs were lower and net returns were higher in the alfalfa-containing vs the annual crop rotation.
• In the annual rotation, removing either fertilizers or herbicides did not seriously reduce the net return, but did reduce input costs (equal reduction in input costs).
• In the alfalfa-containing rotation, removing inputs increased net return in a number of cases compared with the full inputs treatment.
• The organic systems had the lowest cost of production and the highest net returns over the 8 year period in both the annual grain and alfalfa-based rotations.
• Organic flax production appears to be very economically attractive when included in an alfalfa-based crop rotation.

Background

Arbuscular Mycorrhiza (AM) is a soil fungus that has a symbiotic relationship with plant roots. AM enhances plant nutrient uptake by increasing root surface area, provides protection against harmful fungi and nematodes, and improves soil structure. Most crop plants are at least partly dependant on mycorrhizae for uptake of immobile nutrients, especially phosphorous. Improved access to phosphorus is particularly important in low-input or organic farming as these nutrients are often limiting. One important plant group which does NOT associate with AM is the Brassica species, which includes plants such as canola and wild mustard.

There are many factors that affect mycorrhizal colonization rates. Long-term phosphorous fertilization and tillage have been shown to decrease mycorrhizal colonization. Alternatively, winter cover crops have been shown to increase mycorrhizal colonization, by providing a continuous host plant for AM colonization. Lots of non-mycorrhizal crops (such as canola) in rotation may inhibit AM colonization.

Photo of root tip infected with AM. The blue-stained areas are the fungus. Photo courtesy of Dr. Carla Zelmer, University of Manitoba.

Study Objective

The aim of this study was to measure the long-term combined effects of crop rotation and chemical inputs on the level of mycorrhizal colonization in flax at the Glenlea Long-Term Crop Rotation Study.

Experiment Description

Flax plants were sampled from organic and conventional (full input) plots in Wheat-Alfalfa-Alfalfa-Flax (W-A-A-F) and Wheat-Pea-Wheat-Flax (W-P-W-F) rotations. The conventional plots received fertilizer based on soil tests and pesticides based on economic thresholds. The organic plots received no fertilizers or pesticides. The plots were in their twelfth year of this treatment, the fourth year of the third rotation.

Soil cores were taken randomly from each plot forty days after seeding when flax was 21 cm high on average. These soil cores were then washed to obtain the root systems of individual flax plants. The roots were cleared in 10% KOH and stained with Trypan Blue. Length of root colonized and total root length was determined. Arbuscles, the structure between the fungus and host plant cell, were used as identification for colonization.

Results

In this experiment, the level of flax root colonization with AM was significantly higher under organic management. This is most likely due to the fact that soil P levels were lower under organic management. Other research has shown that mycorrhizal colonization is higher when P levels are low.

AM colonization was somewhat higher in the wheat-pea-wheat-flax rotation, although this difference was not statistically significant. Lower AM colonization in the wheat-alfalfa-alfalfa-flax rotation was most likely due to the greater amount of tillage used to terminate the alfalfa. Other research has shown that tillage negatively affects AM colonization.

Management of inputs (full inputs vs. organic) had a smaller effect on AM colonization in the forage rotation than the annual rotation. Although the exact cause of this is not clear, we can speculate that weeds may have caused this difference. The forage rotation had high numbers of wild mustard, while the annual rotation had very few wild mustard plants. Wild mustard is not associated with AM, and may even inhibit AM colonization. Therefore, AM populations may be lower than expected in the forage rotation because of the abundance of this non-host weed. Alternatively, green foxtail is known to have an association with AM. The wheat-pea-wheat-flax rotation had very high numbers of green foxtail. More green foxtail in this rotation may have encouraged AM populations. More testing needs to be done in order to determine if this is, in fact, what is occurring.

Table 1. Mycorrhizal colonization, soil P, and weed populations in two crop rotations under organic and conventional management.
Rotation	Management	Length of root colonized	Soil P 0-15 cm	Green foxtail	Wild mustard
		%	mg/kg	plants/m2
W-A-A-F	Organic	59	<5	50	1201
	Full Inputs	44	10	21	185
W-P-W-F	Organic	81	15	1731	126
	Full Inputs	44	21	1212	5

 

Recommendations

• Organically managed crops have higher levels of AM.
• Tillage negatively affects AM levels.

The effect of crop rotation, use of synthetic inputs, and soil management on mycorrhiza populations need to be considered when making management decisions.

Background

Demand for organic foods is growing, due largely to the perceived human health benefits of eating organic. The benefits most often associated with organic food include lower levels of chemical residues and higher nutritional value.

In previous research studies comparing the nutrient content of organic and conventional produce, results have been inconsistent. Some studies report higher levels of micronutrients, vitamins, and high-quality nitrogen in organic products, while others report little or no difference.

Crop rotation is known to affect soil nutrient status and nutrient uptake by crops, and it follows that grain nutrient content will also be affected by crop rotation. Inconsistent results in previous studies may be due to the wide range of crop rotations practiced in both organic and conventional agriculture. It is important to compare the nutritional quality of organic and conventional produce in a well-controlled study where the role of crop rotation can be determined.

Study Objectives

The aims of this study were:

• to determine if the mineral nutrient content of organic wheat differs from conventional wheat, and;
• to determine the influence of crop rotation (grain-only vs. forage-grain) on the differences in the mineral nutrient content of organic vs. conventional wheat.

Experiment Description

Figure 1. Rotation and management system treatments in the Glenlea Long-Term Rotation study, 1992-2003.

The grain nutrient content study was part of a larger ongoing trial, the Glenlea Long-Term Rotation Study. The Glenlea Study was established in 1992, 20 km south of Winnipeg, Manitoba, and compares the productivity and sustainability of annual (wheat-pea-wheat-flax) and perennial (wheat-alfalfa-alfalfa-flax) crop rotations under both organic and conventional (full input) management systems (see Figure 1). The organic plots received no fertilizers or pesticides. The conventional plots received fertilizer based on soil test recommendations and pesticides based on economic thresholds.

The wheat grain used in this study was taken from stored samples from the years where wheat was grown in both the annual and forage-based crop rotations. Samples were analyzed for concentration of ten different mineral nutrients: nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu). Wheat yields were also compared.

Results

Wheat grain yields averaged over the years of the study were lower in the organic production system than the conventional system. Organic yields were 41% and 47% of conventional yields in the annual and perennial rotations, respectively. Yields in organic systems were reduced due to lower soil nutrient availability and higher weed pressure. There was no significant difference in yields between rotations.

Both crop rotation and organic vs. conventional management affected the concentrations of several nutrients in wheat. In many cases, these differences can be traced back to the effects of crop rotation and inputs on soil nutrient levels. Soil nutrient status in spring 2003 is displayed in Table 1.

Table 1. Soil nutrient status in two crop rotations under two management systems in 2003, after 12 years of cropping.
Rotation	System	N	P	K	S
		kg available nutrients ha-1
Annual	Conventional	32	46	1316	141
Annual	Organic	22	33	1312	86
Perennial	Conventional	81	24	1140	63
Perennial	Organic	37	11	1073	26

 

Nitrogen and sulfur

Wheat produced in the organic annual rotation had the lowest levels of N and S compared to the other systems (Figure 2). However, N and S levels in wheat grown in the organic perennial rotation were no different than levels of these nutrients in wheat grown in the conventional systems.

Figure 2. Concentrations of N, P, and S in wheat grain produced in two crop rotations (R) under conventional and organic management systems (S). *, ** and *** indicate significance at P < 0.05, 0.01 and 0.001, respectively.

In the organic annual rotation, where the only source of N was one grain legume crop (peas) in a four-year rotation, soil N levels were very low, resulting in lower plant uptake and a low N concentration in the grain. In the organic perennial rotation, on the other hand, the two-year stand of alfalfa provided an adequate supply of N to the annual crops in the rotation.

It was interesting to note that wheat S concentration in the perennial rotation was similar under organic and conventional management, even though soil S levels were very different between these systems (Table 1).

Phosphorus

Phosphorus (P) concentration was lower in wheat grown in the perennial rotations (both organic and conventional) and was lowest in the organic perennial rotation (Figure 2). Available soil P was also very low in this system (Table 1) because of high rates of P removal in alfalfa hay crops with no return of nutrients to the system. Low soil P levels likely caused lower plant uptake and thus lower P concentration in the grain. In the conventional perennial rotation, exported nutrients were replaced using phosphate fertilizers and so P supply to the wheat crop was adequate.

P concentration in wheat grown in the organic annual rotation was comparable to conventional wheat. The annual rotation had lower P removal rates than the perennial rotation, and therefore soil P levels were still adequate in the organic annual rotation.

Manganese, Zinc, and Copper

Mn and Cu concentrations in wheat were affected by crop rotation but not by the management system (Figure 3). Mn concentration was higher in wheat grain grown in annual rotations than in perennial rotations, while the reverse was true of Cu concentration.

Figure 3. Concentrations of manganese, zinc and copper in wheat grain produced in two crop rotations (R) under conventional and organic management systems (S). * and ** indicate significance at P < 0.05 and 0.01, respectively.

Production system and crop rotation had an interactive effect on Zn concentration in wheat (Figure 3). Wheat produced organically in the perennial rotation had higher Zn content than all other treatments while there was no difference in Zn concentration between wheat produced in the organic annual treatment and wheat produced conventionally.

Micronutrient concentration in grain may be affected by a variety of factors including:

1. dilution effects due to differences in crop yield
2. nutrient supply and availability in the soil
3. the ability of the plant to take up nutrients

While a dilution effect could be expected to cause lower concentrations of minerals in higher-yielding conventional crops, the lack of significant system effects suggests that micronutrient dilution was not a major factor in the present study.

The availability of soil micronutrients at the Glenlea study may have affected the concentration of these minerals in the grain; however, soil micronutrient levels have not been measured in this study. Doing so would provide important insight into the dynamics of micronutrient availability and plant uptake.

Another possibility is that the plants' ability to take up nutrients is different in the two crop rotations, regardless of availability. In previous work on the Glenlea study, we have observed increased mycorrhizal colonization of plant roots in the perennial crop rotation, due in part to low levels of available soil P in this rotation. Other researchers have observed increased Zn and Cu content and reduced Mn content under conditions of low soil P availability and increased mycorrhizal activity (Liu et al, 2000).

Potassium, Calcium, Magnesium, and Iron

Concentrations of potassium, calcium, magnesium, and iron were not affected by crop rotation or input management system.

Conclusions and Recommendations

• Including green manures and forage legumes in organic rotations can provide an adequate supply of nitrogen to maintain grain nitrogen content. Where soil N is very low, grain N content will also likely be low.
• Low soil phosphorus levels in organic forage-based rotations can contribute to low P content in wheat grain.
• Low soil P levels can increase mycorrhizal colonization, which in turn can increase plant uptake of zinc and copper and reduce plant uptake of manganese.
• Crop rotation and its effects of soil nutrient status and soil biology are major contributors to nutrient uptake by plants and should be considered when comparing the quality of organic and conventional produce.

References

Liu, A., Hamel, C., Hamilton, R. I., Ma, B. L., and Smith, D. L. 2000. Acquisition of Cu, Zn, Mn, and Fe by mycorrhizal maize (Zea mays L.) grown in soil at different P and micronutrient levels. Mycorrhiza 9: 331-336.

Background

Phosphorus (P) management is a major challenge facing organic farmers. While P is continually exported from the cropping system in grain and forage crops, returning P to the system is more difficult. Studies from across western Canada and the world have shown that in many cases, available soil P is deficient on organic farms.

Available soil P is only a portion of the total P in the soil, however. Phosphorus exists in the soil in a variety of forms that range from highly available to highly unavailable to plants. It is important to understand how the different pools of P respond to depletion of the most plant-available fraction, as these less-available pools may provide P to the plant-available pool over time. It is also important to determine the role of crop rotation and crop management practices in depleting, replenishing, or facilitating the use of soil P reserves in order to develop cropping systems that use this resource in a sustainable manner.

Study Objectives

• to determine if depletion of P observed on organic farms is a general depletion or only a reduction in the plant-available fraction.
• to determine the role of crop rotation (grain-only vs. forage-grain), and application of composted beef manure in depleting, replenishing, and facilitating the use of soil P reserves.

Experiment Description

The soil phosphorus study was part of a larger ongoing trial, the Glenlea Long-Term Rotation Study. The Glenlea Study was established in 1992, 20 km south of Winnipeg, Manitoba, and compares the productivity and sustainability of annual (wheat - pea - wheat - flax) and perennial (wheat - alfalfa - alfalfa - flax) crop rotations under both organic and conventional (full input) management systems.

In the fall of 2002, after the eleventh crop year, composted beef manure was applied to one-half of each plot in the perennial rotation at a rate of 10 t ha^-1 (4.5 ton ac^-1), creating perennial rotations with and without manure addition. Other than the manure compost in the perennial rotation, the organic plots received no fertilizers or pesticides. The conventional plots received fertilizer based on soil test recommendations and pesticides based on economic thresholds.

Soil samples were collected from all plots to a depth of 15 cm in October 2004, after 13 years of the rotation study. Soil P in these samples was measured using two methods:

• the modified Kelowna technique was used to measure "soil test P". This is the same technique used by many commercial soil test laboratories. Kelowna-P was later converted to Olsen-P using a mathematical equation. Olsen-P is the measurement used in the Manitoba Soil Fertility Guide.
• a sequential extraction procedure, described in Table 1, was used to measure the size of 4 pools of soil P, separated based on availability to plants.

Table 1. Description of sequential phosphorus (P) extraction procedure and forms of P extracted.
Order of Extraction	Extractant	P form	Availability to plants
1	water	orthophosphate	very high
2	sodium bicarbonate	inorganic P weakly bound to aluminum and iron, and organic P weakly bound to soil organic matter	high
3	sodium hydroxide	inorganic P tightly bound to aluminum and iron, and organic P tightly bound to soil organic matter	moderate
4	hydrochloric acid	apatite-type inorganic P (rock phosphate)	very low

The sizes of the different pools of P, as well as total extractable P, were compared between crop rotations, between management systems, and between perennial rotations with and without manure. Grain yields for years where crops were in common between rotations (flax and wheat) were also compared, as well as other soil parameters.

An estimated cumulative P budget was calculated for the 13 years of the study (1992-2004) to help clarify the roles of P removal and P addition in different crop rotations and input management systems. P removal was calculated from actual yield data from the Glenlea Study and estimated P content of harvested crops, based on reference values in the Manitoba Soil Fertility Guide. P additions were based on the known nutrient content of fertilizers and manure added to the system.

Cathy Welsh collects a soil sample from the Glenlea Long-Term Rotation Study for her Master's work on soil phosphorus.

Results

Soil Test P

Plant-available P, or "soil test P" was affected by the management system and by crop rotation. Available P was lower in organic systems than conventional systems and was lower in perennial rotations than annual rotations. Available P was especially low in the organic perennial system. P export from this system was highest of all systems, for two reasons:

• alfalfa crops have very high P requirements and when alfalfa is removed from the system as hay, large amounts of P are exported from the system.
• grain yields were higher in the organic perennial system than in the organic annual system, resulting in greater P export from the perennial rotation.

Adding manure to the perennial rotations increased available P somewhat, although P levels were still much lower in the organic perennial rotation with manure than the conventional perennial rotation. Since manure was applied only once during the 13 years of the rotation study, the increase in available soil P as a result of manure application was quite small. More frequent application of manure would help to replenish soil P reserves.

Sequentially Extracted Fractions of P

Figure 1. Soil P extracted by water, sodium bicarbonate, sodium hydroxide, and hydrochloric acid in three crop rotations after 14 years of organic and conventional management.

Crop rotation and management system had a significant effect on the three more available fractions of soil P (water, sodium bicarbonate, and sodium hydroxide extractable). Trends in P concentrations in the first three fractions were similar to trends observed in soil test P — P levels were lower in organic systems than in conventional systems and were lower in perennial rotations than in annual rotations (Figure 1).

The reduction in the moderately available (sodium hydroxide extractable) fraction of P in organic systems suggests that as highly available P became scarce, plants began to access the less available P. Some of this P may have partially replenished the more available fractions, and some may have been used by plants more directly.

The unavailable fourth fraction (hydrochloric acid extractable) was not affected significantly by crop rotation or management. Since the fourth fraction of P was not reduced, even when the other fractions were, it appears that this highly unavailable fraction of P was not being depleted — at least not yet. Further study is required to determine whether unavailable P reserves will be affected by crop rotation or management system in the future.

When all four P fractions were added, the total extractable P ranged from 259 to 345 parts per million (Figure 2). In contrast, soil test P for the same systems ranged from 9 to 38 ppm. While total extractable P was still lower in the organic perennial system than the other systems, the differences between systems were not as extreme as in soil test P.

Adding manure raised soil P levels in available and unavailable fractions, but this increase was not statistically significant. Again, more frequent application of manure would likely increase soil P levels over time.

Estimated P Budget

Calculating a cumulative P budget for the duration of the study emphasized the trends observed in measurements of soil P levels. P removal was greatest in perennial rotations and was higher in conventional systems than organic systems (see Table 2 below). The cumulative P balances were negative for organic systems and positive for conventional systems.

Table 2. Estimated cumulative (1992-2004) phosphorus balance at the Glenlea study.
Rotation	Management System	Cumulative P Removal	Cumulative P Addition	Cumulative P Balance
		---------------------------- kg P / ha ----------------------------
Annual	Conventional	107.26	139.88	32.61
Annual	Organic	52.05	0.00	-52.05
Perennial	Conventional	160.07	230.56	70.49
Perennial	Organic	117.92	0.00	-117.92
Perennial with compost	Conventional	160.56	255.02	94.46
Perennial with compost	Organic	118.83	24.46	-94.37

Differences in the cumulative P balance between organic and conventional systems were much larger than observed differences in soil P levels. While the reasons for this discrepancy are not clear, using estimated crop P concentrations to calculate P removal may have been a contributing factor. Measuring the P content of harvested crops and using those values for calculating the P budget would have given more accurate results.

P Limitation in Organic Systems

The difficulty of replacing P is one of the major concerns in organic systems. Without added P from external sources, P reserves will eventually be depleted. However, the onset of P limitation in organic systems depends on three major factors:

1. the rate of P removal through crop harvests
2. the ability of plants to access unavailable soil P reserves, if present, and
3. the rate of replacement of P removed from the system.

Rate of P Removal

Alfalfa hay crops remove much more P from the system than annual grain crops, due to alfalfa's high P requirements. As a result, the perennial organic system became P-limited after approximately 12 years of production. The annual organic system, on the other hand, was not yet P-limited after 13 years of production and based on annual P removal rates, could be expected to continue another six years before becoming P limited.

Accessing Unavailable P Reserves

When large amounts of unavailable, or recalcitrant, P are present in the soil, P limitation in organic systems may be delayed by promoting the cycling of unavailable P into plant-available forms. Processes that make recalcitrant P more available to plants include mobilization of unavailable forms by plants such as legumes and buckwheat or microbes such as Penicillium bilaiae, the active ingredient in JumpstartTM.

Colonization of plant roots with endomycorrhizal fungi can also help to delay the onset of P limitation since endomycorrhizae can help plants absorb P from the soil.

Replacement of P

While mobilization of phosphorus reserves may delay P limitation in organic systems, replacement of nutrients taken up by crops is required to make these systems sustainable in the long term. In this study, the one-time application of manure-compost increased available P slightly in the perennial rotations. Applying manure compost more frequently would help replace the P removed by the crops.

Conclusions and Recommendations

• Organic crop production can deplete available soil phosphorus (P) over time if P is not replaced.
• Perennial hay systems remove large amounts of P from the soil, largely due to alfalfa's high P requirements.
• Annual grain systems do not remove as much P from the soil but may be lower yielding if N supply through green manures is not adequate.
• Adding composted livestock manure increased available soil P levels somewhat. More frequent application of compost could prevent P depletion in organic systems.
• Reserves of unavailable P may gradually replenish reserves of available P, but at a rate that is too slow to keep pace with P uptake by plants.
• Helping crops access reserves of unavailable P with practices such as promoting mycorrhizal fungi may delay the onset of P limitation in organic systems.
• For long-term sustainability, replacement of P removed by crops is necessary.

Further Reading:

Thiessen Martens, J. 2008. Where has all the phosphorus gone? Organic Agriculture Centre of Canada.

Welsh, C.M., 2007. Organic crop management can decrease labile soil P and promote mycorrhizal association of crops. M.Sc. Thesis. University of Manitoba, Winnipeg, Manitoba.

Welsh, C., M. Tenuta, D. Flaten, C. Grant, and M. Entz. 2006. Organic Crop Management and Soil Phosphorus. Better Crops, vol. 90, no. 4, pp. 6-7.