Bob Henson, North Dakota State University Carrington Research Extension Center

Jay Goos, North Dakota State University Soil Science Department, Fargo

Dave McAndrew, Agriculture and AgriFood Canada, Morden, Manitoba

Soybean production in the Northern Plains is increasing tremendously. In North Dakota, acreage has jumped from 500,173 acres in 1990 to 2,108,129 in 2001 (North Dakota State Farm Service Agency, 1990 and 2001). Reasons for this increase include better varieties in the early maturity groups, fewer disease problems than with some crops, relative market prices, increased familiarity with the crop, and the advantages a legume offers in a crop rotation.

Among the advantages a legume offers are benefits in soil fertility, particularly nitrogen (N). In association with the soil-borne bacteria Bradyrhizobium japonicum, the soybean plant forms nodules on the root system. By a process termed N₂-fixation, the bacteria living in the nodules capture N₂ from the atmosphere and convert it to a form useable by the host plant. This is a symbiotic relationship, since both organisms benefit from the association. The bacteria receive carbohydrates and minerals from the plant and the plant receives N from the bacteria. However, available N (residual and fertilizer) in the soil reduces N₂-fixation. How to best manage these two N sources, alone and in combination, is a recurring question among growers.

The basic rules of soybean inoculation are:

• Use soybean inoculant (note expiration date on package),
• Avoid excessive temperature, and
• Keep the inoculant out of direct sunlight.

Since the bacteria which nodulate soybeans are specific to that crop, the proper inoculant must be used. The bacteria are living organisms and will be killed by high temperature or UV light. As a general rule, plant seed the same day it is inoculated, the population of bacteria decreases with time after inoculation.

Inoculants are manufactured in various formulations: peat-based, liquid, and granular. The granular form may be broadcast or applied in-furrow. Peat-based and liquid inoculants are normally mixed with the seed at planting, but liquid also may be applied in the furrow. The liquid form is easy to use and may be applied to seed as it is augered into the truck or planter, but bridging may occur if the seed gets too wet. Both peat and granular formulations provide some protection against desiccation, but the liquid form is quite susceptible. Data comparing the performance of the various formulations are somewhat limited. As a general recommendation, use the formulation which is most convenient for your operation and follow recommendations for rate and handling.

Logically, the Bradyrhizobium bacteria which nodulate soybean are, like soybean, native to China and are not naturally-occurring in the soils of the Northern Plains. As such, inoculation is an essential step in establishing a good population of the proper bacteria for first-year soybean. However, the bacteria move very little in the soil and nodules only form on roots which come into contact with the bacteria. Thus, inoculating the seed will result in a cluster of nodules around the seed and very few on the rest of the root system. Some growers double the inoculation rate on a first-year crop, but the problem is not the number, but rather the distribution. One option to improve the distribution of bacteria in the soil would be a broadcast application and incorporation of granular inoculant. Another technique is to inoculate the previous crop, such as wheat. This will allow the bacteria to establish prior to planting soybean and any tillage operations will benefit distribution in the soil. Some growers say that the maximum benefits from N₂-fixation will only be realized in the second and subsequent cycles of soybean.

Many growers ask if it is necessary to continue inoculating soybean every year, once a population of Bradyrhizobium has been established in a field. Although there may be little or no response, the cost of inoculation is relatively low compared to other inputs, and many feel that the soybean crop should be inoculated every year.

It is important to remember that the process of N₂-fixation is very sensitive to environmental stresses and growing conditions. A response to inoculation may not be observed every year or in all fields. Results in Manitoba have shown that year, location, cultivar, inoculant, and fungicide seed treatment may significantly affect nodulation and yield.

Current NDSU recommendations state that if the soil tests less than 50 lbs NO₃^--N / acre (0-24"), apply up to 50 lbs N. This may be especially critical in fields that do not have an established population of Bradyrhizobium. However, the soybean seed is very sensitive to fertilizer. In 15" or wider rows, do not apply fertilizer with the seed. In 6-7" rows, do not apply more than 10 lbs N / acre with the seed.

When soybean is a recurrent crop in the rotation, opinions vary on the use of N fertilizer. There is frequently no yield response to N fertilization. Some suggest that applying a low rate (starter or pop-up) of N results in faster crop establishment and initial growth, without reducing N₂-fixation. There is no clear-cut answer to this question.

Literature Cited

North Dakota State Farm Service Agency. 1990. 1990 Reported Acreage.

North Dakota State Farm Service Agency. 2001. 2001 Reported Acreage. October 15.