Crop Rotation and
Wheat Nutrient Content: Glenlea

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 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 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.

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 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:

dilution effects due to differences in crop yield
nutrient supply and availability in the soil
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 concentration of these minerals in the grain; however, soil micronutrient levels have not been measured in the 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 (see The Effect of Crop Rotation and Chemical Inputs on Mycorrhizal Colonization), 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

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.

Crop Rotation and Wheat Nutrient Content: Glenlea