in-season nutrient management considerations

DR. KURT STEINKE

MSU ASSOCIATE PROFESSOR, SOIL FERTILITY AND NUTRIENT MANAGEMENT

Now that the 2023 planting season and the hot, cold, wet and dry conditions of yet another typical Michigan spring are behind us, our attention turns to mid- to latesummer soybean growth and development. While I often use the term “frustrating” to describe soybean yield in response to nutrient applications at above-critical soil test values, the same terminology may also apply to inseason nutrient management. There are however some considerations to think about heading into the latter half of the growing season.

Is a nutrient deficiency showing from actual lack of a nutrient or related to weather conditions? Remember plants do not grow to water, they grow through water. Whether a nutrient moves via mass flow or diffusion, reductions in soil moisture decrease water transport and diffusion pathways, reducing nutrient uptake. Due to this, nutrient deficiency symptoms may be visible even when soil test levels are above the critical level under dry soil conditions.

Soybeans remove more potassium per bushel than corn. A 60 bu/A soybean crop removes about 70 lb K2O/A as compared to 40 lb K2O/A in a 200 bu/A corn crop. The key point to remember with soybeans however is that K uptake occurs much earlier in the season than other nutrients with about 25% of K uptake complete by R1 and 70% by R4 (seed-fill). Unlike some of the other macronutrients, much of the K uptake after seed-fill is remobilized from the plant and not taken up from the soil, further demonstrating the importance of pre-plant soil test K concentrations (> 100 ppm for CEC < 5; > 120 ppm for CEC >5) for maintaining tissue K concentrations later in the season. Due to the large quantity of K that soybeans uptake, foliar applications struggle to provide sufficient K. Soil-applied K applications (i.e., drop-nozzles, Y-drops, etc.) have been investigated on many occasions with rarely significant results and are not recommended or suggested. If a producer wants to make an in-season K fertilizer application to the soil, be sure to use a soluble K source, focus on known high-yielding areas and apply near rainfall or irrigation prior to the R1 growth stage.

Manganese (Mn) remains the most common soybean micronutrient deficiency in Michigan with both dry soil conditions and high soil pH (> 6.5) contributing to visible deficiency symptoms. Soybeans will respond to Mn applications when deficient. Typically foliar applications of 1-2 lbs. Mn/A will correct the deficiency, but growers should note which areas of the field develop Mn deficiency as symptoms are likely to reoccur when Mn-sensitive crops are grown.

Despite the ability to fix a substantial percentage (44-72%) of N from the atmosphere and access the remainder via soil N mineralization, residual soil N, or atmospheric deposition, questions persist about soybean N applications. Unlike potassium, soybean N uptake is only about 20% and 50% complete at R1 and R4, respectively. Hence almost 50% of N uptake occurs late in the season with a sizable percentage of this coming from the soil. The time-frame for late-season N uptake, however, also coincides with peak N availability via soil mineralization, which when combined with biological fixation often provides sufficient N for soybean yield. Have soybean responses to N application occurred? Yes. Should a soybean response to N application be expected? Not likely. Thus N applications to soybean are not recommended unless nodulation failure has been documented. If a grower wishes to attempt an in-season N application, growers should focus on season-long N availability, shy away from soluble pre-R1 N applications, and remain in the 50-80 lbs. N/A range. High-yielding (> 70 bu/A) field areas with sandy soils and minimal organic matter accumulation may be the most responsive to N application but again should not be expected. Starter fertilizers at rates < 25 lb N/A have shown consistent biomass response with negligible impact on N fixation but inconsistent yield responses have often followed. Be careful not to assume that a larger soybean canopy will result in greater yield. Excessive soybean canopy development can result in too much water usage, white mold occurrence, and plant lodging creating difficulties with pod set and grain fill.