Starter fertilizers have not been widely used in soybean production. However, new research results and recent shifts in planting equipment and planting dates may lead to an increase in starter fertilizer use.
There is tremendous interest in increasing soybean yields and producers are wondering if starter fertilizers can contribute to this goal. Historically, starter fertilizers have not been recommended for soybean production. There are numerous reasons for this:
Soybeans are twice as sensitive as corn to salt injury.
Yield responses to near-seed placement of phosphorus and potassium have not performed better than broadcast applications in university trials. Soybeans have typically been planted later than corn and into warmer soils. Phosphorus soil test levels are high in most of our soils.
Two factors that determine the likelihood and size of a yield response from starter fertilizer are: 1) phosphorus and potassium soil test levels, and 2) soil temperature. The phosphorus and potassium soil test levels are the most important factors determining soybean response to starter fertilizer. Economic returns to starter fertilizer are not likely to occur when phosphorus levels exceed 30 ppm and potassium levels exceed 130 ppm.
Cool soil temperatures are common during early planting, when planting into heavy crop residues, or when planting into poorly drained soils. The probability of an economic yield response to starter fertilizer is higher in any of these situations.
Recent university research has shown that early planting is critical to maximizing soybean yields. This information is motivating producers to plant soybeans earlier.
Soybeans can be safely planted into soils at 50°F, however germination, root growth and root uptake of phosphorus and potassium will be reduced in cool soils due to a decreased rate of nutrient mineralization.
A starter fertilizer containing phosphorus and potassium may be beneficial in these conditions as both nutrients promote early root development. Earlier root growth will also encourage earlier nodule development and nitrogen fixation.
Horst Bohner and Keith Reid conducted fertilizer trials comparing five different fertilizer treatments in Ontario in 2009 and 2010. They found that two of the treatments (6-24-6 at 3 gallons per acre applied in furrow and 50 lbs. of 11-52-0 applied in furrow) were profitable. Please refer to Table 1.
*Four of the five test sites had low to medium soil test levels for phosphorus and potassium and one site had high soil test levels.
The two most common ways to apply starter fertilizer are in the seed furrow (pop-up) and in a band, two inches beside and two inches below the seed (2 x 2). When applying phosphorus and potassium in a 2 x 2 band, the starter can contain up to 100 lbs. of P2O5 and 60 lbs. of K2O per acre. This is an advantage of the 2 x 2 placement as maintenance application rates of phosphorus and potassium can be applied to replace the nutrients removed by the crop.
Placing fertilizer in contact with the seed increases the risk for salt injury. Salt injury is also more likely to occur if the fertilizer has a high salt index, if the soils are coarse-textured and if the soil is dry. Consider these factors when selecting fertilizers and application rates for pop-up fertilizers.
Dr. Ronald Gelderman at South Dakota State University developed an Excel spreadsheet to help producers identify the maximum amount of fertilizer that can be placed in contact with the seed.
The spreadsheet is available online. Producers planning to apply pop-up fertilizer to soybeans this spring should use this spreadsheet. Producers planning to apply pop-up fertilizer to soybeans this spring should use this spreadsheet.
Due to the level of producer interest and the positive research results recently generated in Ontario, we plan to conduct soybean pop-up starter fertilizer trials in Michigan in 2011 and 2012. The 2011 results will be shared with producers at winter meetings and summarized in future articles.