Seedrow starter blend recommendations for lentil and soybean

A rate of no more than 9 lbs. N + 9 lbs. P2O5/ac as a starter seed-row 28-26-0 fertilizer blend was found to be the optimum rate that maximized yield in soybean and lentil. Higher rates reduced emergence and decreased the proportion of nitrogen derived from fixation.

Legume crops obtain the majority of their nitrogen requirements through biological nitrogen (N) fixation, which reduce N input costs. However, biological nitrogen fixation with Rhizobium bacteria can take 1 to 2 weeks for nodules to become established. Seed-row placed N and phosphorus (P) was thought to possibly be of benefit in early crop stages, but safe seed-row placed fertilizer rates for N and P fertilizer combinations had not been established for soybean and lentil.

This research was conducted to assess the effect of a seed-placed fertilizer blend of 50% Urea (46-0-00) + 50% mono-ammonium phosphate (MAP; 11-52-0) on soybean and lentil emergence, grain and straw production, and nutrient uptake. Additionally, the research looked at the influence of starter N on nitrogen derived from biological fixation in soybean.

The study was conducted on a farm field located at the boundary of the Brown and Dark Brown soil zones at Central Butte in south-central Saskatchewan. Prior to the field trial in 2018, wheat was grown on the site in 2017, and canola in 2016. Soil test nitrate-N was 20 lbs./ac in the top 12-inch soil depth, and Modified Kelowna extractable PO4-P was 10 lbs./ac in the top six inches.

A starter blend of 50% urea and 50% MAP with a blend analysis of 28-26-0 was applied at four rates of starter N at 0, 9, 18, and 27 lbs. N/ac  (0, 10, 20 and 30 kg N/ha). This blend also supplied 0, 9, 18, and 27 lbs. P2O5/ac (0, 10, 20 and 30 kg P2O5/ha). The fertilizer was placed in the seed row using a narrow seed-row opener with an approximately 15% seedbed utilization.

The proportion of nitrogen derived from fixation was determined in soybean using N-15 dilution technique, and compared to wheat as a reference crop.

Low rate of starter fertilizer recommended

Rates greater than 9 lbs. N/ac of the fertilizer blend reduced average soybean and lentil emergence by 20 to 25% as determined by plant emergence counts at 19 days after seeding. The low moisture conditions at spring seeding may have increased the amount of fertilizer burn in the year of the study.

In both soybean and lentil, slightly higher grain yields were observed at the 9  lbs. N/ac starter rate compared to the control, with yields trending lower at higher seed-row starter rates. Lentil appeared to respond less to starter N than soybean. At the 9 lbs. N/ac starter rate, lentil yielded approximately 31 bu/ac, and soybean yielded around 38 bu/ac at that rate.

In soybean, the N uptake in grain and straw was not significantly increased by N and P fertilization. In lentil, similar results were observed, with no significant difference in N and P uptake in the grain. Lentil straw uptake of N and P was significantly higher with starter fertilizer compared to the control, although only slightly higher.

Analysis of the proportion of plant N derived from the atmosphere (%ndfa) through biological N fixation by the soybeans showed that N fixation was reduced with starter N rates of 18 and 28 lbs. N/ac. A starter rate of 9 lbs. N/ac resulted in % ndfa that was not significantly different than the unfertilized control. A reduction of about 10% ndfa was observed at the 28 lbs. N/ac starter rate. This was expected because high levels of inorganic N in soil are known to reduce biological fixation in legume crops.

Overall, the researchers concluded that a rate of 9 lbs. N /ac + 9 lbs. P2O5/ac as a starter seed-row placed urea-MAP blend was the rate that did not significantly reduce emergence, stand count or biological nitrogen fixation, and was sufficient to maximize yield, N and P uptake. Higher rates would reduce the benefits of biological N fixation and increase input costs even though yields would be similar.

This research was supported by the Agriculture Development Fund-Government of Saskatchewan and Saskatchewan Pulse Growers.

Dona, W.H.G., Schoenau, J.J. and King, T. (2019) Response of Soybean and Lentil to a Seed-Row Placed Starter Nitrogen-Phosphorus Fertilizer Blend in a Brown Chernozem in South-Central Saskatchewan. American Journal of Plant Sciences, 10, 1813-1829.

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