A long-term study at the University of Manitoba is adding more weight to the theory that crop rotations in organic agriculture can affect nutrient content in grain.
Martin Entz, Soleil Turmel and Keith Bamford of the university’s plant science department compared the concentration of 10 mineral nutrients in wheat grown organically and conventionally in two different crop rotations over the first 15 years of the Glenlea long-term rotation study south of Winnipeg.
Preliminary results indicate that including a perennial alfalfa stand in a grain crop rotation boosted the concentrations of certain nutrients in wheat.
Read Also
VIDEO: Green Lightning and Nytro Ag win sustainability innovation award
Nytro Ag Corp and Green Lightning recieved an innovation award at Ag in Motion 2025 for the Green Lightning Nitrogen Machine, which converts atmospheric nitrogen into a plant-usable form.
The two crop rotations were a perennial-based rotation of wheat, alfalfa, alfalfa and flax and an annual rotation of wheat, pea, wheat and flax.
Each rotation was grown under conventional and organic management. Crop rotation was the only source of fertility. In the organic system, no animal manure or other products were applied. In the conventional system, crops were fertilized to soil test recommendations.
Zinc and copper concentrations in wheat were higher and phosphorus concentrations were lower in organic rotations that included alfalfa. Nitrogen and sulfur concentrations in wheat were lower in an annual organic rotation than in a perennial organic or conventional rotation.
Concentrations of other nutrients, including potassium, calcium, magnesium, iron and manganese, were not affected by crop rotation or organic-conventional selection.
While the reasons for these differences in nutrient content are not always clear, the researchers suggest that the effect of crop rotations on soil nutrient levels is likely a major factor.
In the annual organic system, where the only source of nitrogen was one legume crop in a four-year rotation, soil nitrogen levels were low, resulting in lower plant uptake and a low nitrogen concentration in the wheat. In the perennial organic system, the nitrogen supplied by a two-year stand of alfalfa provided an adequate supply to the annual crops in the rotation.
This does not mean all organic annual crop rotations are nitrogen deficient.
Entz said an annual rotation that includes legume green manure on a regular basis can supply enough nitrogen for the other crops in the rotation.
The frequency of the green manure crop will depend on the region, the amount of biomass produced by the green manure and the nitrogen requirements of the other crops. The annual rotation in the Glenlea study was modified in 2004 and now includes a fababean green manure.
The alfalfa hay crop in the perennial rotation also caused other differences in nutrient content. Harvesting alfalfa hay removed large quantities of phosphorus from the field, and because this phosphorus was never replaced, soil phosphorus levels became low in the perennial organic system.
On the other hand, less phosphorus was removed from the annual organic system. A low phosphorus concentration in wheat is not considered to be a problem from a nutritional perspective because it is readily available in many foods and unlikely to be deficient.
Low levels of available soil phosphorus in the organic perennial rotation may have also caused higher zinc and copper concentrations in wheat.
When available phosphorus levels are low, plants such as flax, legumes and cereals associate more closely with mycorrhizal fungi, a naturally occurring soil micro-organism that forms mutually beneficial relationships with plant roots.
These fungi increase the uptake of certain nutrients, including zinc and copper.
While it is possible that these trace minerals were simply diluted in the higher yielding conventional crops, the differences between the two organically managed rotations seem to indicate that crop rotation was at least partially responsible for this phenomenon.
As consumers continue to seek healthier food, the nutrient content of organic products will likely gain more attention. Knowing how crop rotation and soil nutrient levels affect the nutritional value of crops is a key component in comparing the quality of organic and conventional food.
Joanne Thiessen Martens is a research and extension associate with the Organic Agriculture Centre of Canada working in collaboration with Martin Entz at the University of Manitoba.
