Alfalfa Mulch as a Nitrogen Source for Organic Wheat Production

Matthew Wiens, Department of Plant Science, University of Manitoba
Winnipeg, Manitoba, Canada
Email: umwiensm@cc.umanitoba.ca
Ph: (204) 474-6089

Perennial alfalfa cropping is well known as an alternative to using chemical inputs. Alfalfa fixes nitrogen and chokes out certain weeds. These benefits may be difficult for "straight-grain" organic farmers to capture since growing alfalfa as a cash crop means exporting large amounts of hard-to-replace nutrients. One solution is to add livestock to the farm operation and return the nutrients in the form of manure. Another option is to use alfalfa hay directly as an organic nitrogen fertilizer, thereby extracting value from the hay, without exporting nutrients off the farm.

Strip-farming: a Novel Alternative
A strip farming system may be the best way to deliver alfalfa mulch to organic grain crops. Alfalfa could be grown in strips across a field for three years to maximize its soil-building benefits. Annual crops would be grown between the alfalfa strips, to receive mulch harvested from the alfalfa strips. After three years, the alfalfa strips would be rotated with the cropped strips. Whether this system is feasible depends, in part, on how the mulch affects the annual crops.

Over the past two summers I have studied the effects of broadcasting direct-cut alfalfa mulch on a growing crop of spring wheat. The goal of this study was to determine if alfalfa can be successfully used as an organic nitrogen fertilizer on spring wheat. I measured wheat yield, wheat N uptake, weed density, and soil moisture.

Barrie HRS wheat was seeded in late May at 2 bu/ac. Alfalfa mulch was then harvested with a walk-behind flail mower and applied at 3 rates (low, medium and high) corresponding to the amount of alfalfa harvested from an area 0.5, 1 and 2 times the wheat plot area (see Fig. 1). To refine the timing of application, mulch was applied either before emergence or at the 3-leaf stage. Response to mulch was compared to 4 levels of N as ammonium nitrate - 0, 18, 36, and 54 lb N/ac - broadcast before wheat emergence. Field trials were located at Winnipeg, Carman, Clearwater and Kenton.

Photo 1. This picture was taken 9 days after early mulch application (before emergence) at a rate of 1.3 tons/acre (3000 kg/ha). Wheat has successfully grown through mulch layer.

Photo 2. Centre plot received 1.9 tons/acre of alfalfa (dry weight) (4300 kg/ha), bearing 105 lb N/ac (118 kg/ha). Plot is noticeably darker green than pathways, indicating N response from mulch. Mulch was applied June 13, 2003 at three-leaf stage of wheat and photo was taken on July 9, 2003. The dark green plot behind and to the left received 54 lbs N/ac (60 kg/ha) as broadcast ammonium nitrate.

Weed density decreased as mulch rate increased (Fig. 3). However, the lowest mulch rate had significantly higher weed density than the control. It appears that low levels of mulch improved conditions for weed establishment, while high mulch rates suppressed weeds. Significantly higher soil moisture was observed on several dates in the top 10 cm under the heaviest rate of mulch compared to the control plots.

Figure 2. The effect of mulch rate and application Figure 3. The effect of mulch rate and application timing on wheat yield at Winnipeg in 2003; timing on weed density at Winnipeg in 2003.

† Bars topped with the same letter are not significantly different from each other (P=0.05).
‡ The 18lbsN, 36lbsN and 54lbsN treatments received 18, 36, and 54 lbs of inorganic N per acre, respectively, in the form of broadcast ammonium nitrate applied before wheat emergence. Early (before emergence) mulch rates were 0.44, 0.88, and 1.76 tons/acre (dry weight) for the 0.5x, 1x, and 2x rates respectively. Late (3-leaf stage) rates were 0.48, 0.96, and 1.93 tons/acre for the 0.5x, 1x, and 2x rates, respectively.

Conclusions
This study demonstrated that using alfalfa as mulch on spring wheat is a successful way to extract value from alfalfa hay without feeding it to cattle. Regardless of early or late application timing, wheat yield increased as alfalfa mulch rate increased, mainly as a result of mulch-supplied nitrogen. If practical methods of field-scale application can be developed, using alfalfa as mulch will be an avenue for "straight-grain" organic farmers to increase alfalfa acreage in order to capture its soil building benefits.

This article first appeared in Farmer's Independent Weekly

NEW!! Abstract from published article (Can. J. Plant Sci. 86: 121-131): Agronomic benefits of alfalfa mulch applied to organically managed spring wheat