ISSUE 3    May 18, 2006

FERTILIZING SOYBEANS

Changes in recommendations have been made within the last year that have not appeared in our NDSU Extension circulars, but have been published in the latest Crop Production Guide. Due to new research that indicates a role of soil N availability in the severity of iron deficiency chlorosis, NDSU has dropped the N recommendation for soybeans in certain situations to zero. The recommendation for N for all conceivable preplant situations for soybeans is now zero.

Changes have not been made regarding other nutrients. Phosphate is still recommended if soil P levels are medium (Olsen test of 11) or lower. Broadcast application will provide the greatest return in low and very low testing soils (Olsen test below 8), however, yield increases if some P is banded will also be achieved in low and very low testing soils. Banded P should only be considered if row widths are less than 15 inches. The P fertilizer MAP is safer to use in a band compared with DAP, but DAP can be used if N rates are less than 10 lb/acre. Expect some stand loss with any with-seed banded applications, although soybean is very resilient to stand loss.

 

FERTILIZING DRY BEANS

Nitrogen fertilizer is used on many acres of dry beans due to the poor relationship in some soils between the nitrogen-fixing bacteria and dry beans. Some growers have very good dry bean yields without any additional inoculation or N fertilizer. Other growers are happy with inoculation only. Still other growers use N only. Most growers have been in dry bean production long enough to know what works for them. A review of about thirty site-years of N trials from NDSU and Univ. of MN trials in the northwest part of MN show that there is a different response from N depending on whether the trials were inoculated or not. Inoculated trials maxed out in N response at about 40-50 lb N/acre. Non-inoculated trials maxed out at about 100 lb N/acre. In both populations of studies, the response curve was very flat, meaning that added N resulted in small increases in yield. In wheat and corn N research, check plots (plots received no supplemental treatment) might only reach 30% of the maximum yield for the plots. In the dry bean work, the lowest check plot yield was at 70% of maximum. In the non-inoculated trial, there was very little difference whether the N rate was 70 lb/acre or 100 lb/acre.

Therefore, with N prices what they are, it would make sense in dryland production in North Dakota to limit N rates to about 70 lb N/acre, including residual soil N, if the field is not inoculated, and if the fields are inoculated, no more than 50 lb N/acre, including residual soil N, should be used.

 

INOCULATION

Nitrogen- For most legumes, if the rotation is short, meaning the legume has been grown within 3 years, and the crop was well nodulated the last time it was grown, research on soybeans in North Dakota and field peas north of our border suggest that no supplemental inoculation is needed. Inoculation will not hurt the crop, but it might not help it either.

Many growers inoculate with a less expensive inoculant anyway, just for insurance, but the data show that it is probably not necessary. For legumes not grown for four years or more between plantings, inoculation might be worth the time and effort. In fields that were previously flooded, we recommend inoculation just to be on the safe side.

Phosphorus- These inoculants are more a soil inoculant than those that affect plants directly. The P inoculants marketed today are a fungus that acidifies its immediate environment. The acidity dissolves carbonate minerals that surround some P in high pH (over 7) soils with carbonates present. This may release as much as 10 lb P2O5 in those high carbonate soils. If there are soils within the field that have no carbonates or the pH is less than 7, the product will have no benefit in terms of P nutrition. Many fields, although they test high in pH on a composite soil test basis, have pockets and sometimes major areas that are lower in pH where the product will not benefit. For example, in the Valley, the product might help the Hegne and Bearden soils, but not the Fargo. In the till plain, they might benefit the Buse and Hamerly, but not the Barnes and Svea. Compare the cost of these materials and the probability of having favorable soils with 10 lb P2O5 from commercial fertilizer to see if it would help out in a particular field.

Dr. Dave Franzen
Extension Soil Specialist
701-231-8884
david.franzen@ndsu.edu


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