Crop rotation key to increasing nutrient content of grains

By Joanne Thiessen Martens

Crop rotation is known to have many benefits, especially in organic production, and researchers at the University of Manitoba are adding another one to the list – crop rotation can affect nutrient content in grains such as wheat.

Martin Entz, Soleil Turmel and Keith Bamford of the Department of Plant Science at the University of Manitoba compared the concentration of ten mineral nutrients in wheat grown organically and conventionally in two different crop rotations over first 15 years of the Glenlea Long-Term Rotation Study, located just 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 grain.

The two crop rotations used during the nutrient study were a perennial-based rotation of wheat, alfalfa, alfalfa, flax; and an annual rotation of wheat, pea, wheat, flax. Each of these rotations was grown under both conventional and organic management. In the organic systems, crop rotation was the only source of fertility; no animal manures or other products were applied. In the conventional systems, crops were fertilized to soil test recommendations.

Zinc and copper concentrations were higher and phosphorus concentrations were lower in wheat grown in the organic rotation that included alfalfa than in any of the other rotations. Nitrogen and sulfur concentrations were lower in wheat grown in an annual organic rotation than in the perennial organic rotation or in either conventional rotation.

Concentrations of other nutrients, including potassium, calcium, magnesium, iron, and manganese, were not affected by crop rotation or organic vs. conventional management.

While the reasons for these differences in nutrient content are not always clear, the researchers point to crop rotation and the effect of different crop rotations on soil nutrient levels as likely being major factors.

In the annual organic system, where the only source of nitrogen 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 perennial organic system, on the other hand, 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 that all organic annual crop rotations are nitrogen deficient. According to Entz, an annual rotation that includes legume green manures 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 caused other differences in nutrient content as well. Harvesting alfalfa hay removed large quantities of phosphorus from the field, and since this P was never replaced, soil P levels became very low in the perennial organic system, limiting plant uptake of P. Phosphorus removal from the annual organic system, on the other hand, was much lower and therefore P was not limiting in this system. A low phosphorus concentration in wheat is not considered to be problematic from a nutritional perspective, as it is readily available in many foods and unlikely to be deficient.

Low levels of available soil P in the organic perennial rotation may have been the cause of higher zinc and copper concentrations in wheat grown in this system as well. When available P 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 the trace minerals 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 out healthier foods, the nutrient content of organic products will likely enter the spotlight more prominently. 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 foods.


Joanne Thiessen Martens is a Research and Extension Associate with the Organic Agriculture Centre of Canada working in collaboration with Dr. Martin Entz at the University of Manitoba.


“We gratefully acknowledge funding support from Agriculture and Agri-Food Canada for for production of this publication. Agriculture and Agri-Food Canada (AAFC) is pleased to participate in this project. AAFC is committed to working with industry partners to increase public awareness of the importance of the agriculture and agri-food industry to Canada. Opinions expressed in this document not necessarily those of the OACC or AAFC.”


en français


Posted June 2008