Updated July 9, 2018:

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Title: Phosphorus fertilizer management decisions for soybean based on time of planting
Principal investigator: Dr. Jasper M Teboh, NDSU Carrington Research Extension Center
Co-Investigators: Eric Eriksmoen, NDSU North Central Research Extension Center; Szilvia Yuja, Kelly Cooper, Heidi Eslinger, Blaine G Schatz, NDSU Carrington Research Extension Center, Dr. Dave Franzen, Department of Soil Science, NDSU.
How do soybean yields differ in response to phosphorus (P) applied when planted early or later? This study assesses soybean yields as impacted by P fertilizer application when planting is early, normal time, or later, and provides an assessment of the resulting yields on net return.
Planting dates used are shown on table 1. P rates were 0, 20, 40 lbs P/ac as triple super phosphate surface applied and incorporated. At Minot, soybean was no-till and P banded with 11-52-0 at 5, 10, and 20 lbs of P2O5 /A. Both Carrington sites tested medium for P, while other sites were high.
Table 1. Locations and planting dates

Planting
time Locations
-------Carrington ------- Minot Oakes Wishek
Dryland Irrigated Dryland Dryland Dryland
Early 11-May 11-May 10-May 10-May 9-May
Normal 24-May 24-May 19-May 24-May
Late 8-Jun 8-Jun 30-May

Results
Across all sites, the effect of P on soybean yield did not depend on planting date (Table 1). At Carrington, under dryland, early planting and normal planting dates produced significantly greater yields than late planting (Figure 1).
• At the Carrington irrigated site, P rate and planting date impacted yields, significantly. Planting on or before May 24 improved yields by almost 12 bushels on average. Yields from early and normal planting dates were not different. P treatments produced results which could not be explained (e.g., greater yields at 0 and 40 lbs treatments than at 20 lbs) but would be verified in ongoing studies.
• At Oakes, yields were significantly increased by P. Yields were not different between 20 and 40 lbs, but improved by 6.5 bushels compared to the check (0 lbs P). Planting on May 10 (early/normal) had a marginally significant yield increase (about 4.5 bushels) compared to planting two weeks later.
• At Minot, prolonged droughts led to low yields, which were half the normal. Yields were not different among treatments.
• At Wishek, yields were also not different between P rates.
• Economically, only the farmer at Oakes would have made some profit. At $8.5/bu for soybeans, $0.707/lbs P fertilizer, and $6.39/ac application cost, the Oakes farmer who applied 20 lbs compared to 0 lbs, would have gained $13/ac, and at 40 lbs, $5/ac when averaged across the two planting dates. Net loss of $25 was estimated at 40 lbs P, at late planting.
• From these results farmers would be better off planting near the second week of May, to boost yield potentials and net return.