With the variation in conditions we have seen this spring there are a few issues that may show up in fields related to cool and wet soils. Purpling of corn leaves due to phosphorus (P) deficiency and early season interveinal striping due to sulfur (S) may occur if temperatures remain cool and we continue to have frequent rains. I want to take some time and outline these issues and some of the related research that has been conducted in the past five years.
Starter Fertilizer Use And Purple Corn
I have had a few questions related to starter fertilizer use this spring. With planting being delayed there may be some questions as to what can be eliminated from the planting operation to speed things up. If we continue to see the temperature changes that have occurred thus far I would except to see some purpling on corn that has already been planted as it is coming out of the ground. Whether this purpling actually affects yields is debatable.
We typically say that starter helps get corn out of the ground in cool and wet soils we typically see in the spring. Survey work on research data from Wisconsin by Dr. Larry Bundy indicated that the potential for a significant return to starter was greater with later planting dates with full season hybrids. So, the questions on forgoing the starter fertilizer are valid as we progress into what should be warmer planting days. However, the Wisconsin data gives an alternate prospective that may indicate that some starter still should be used.
In a joint project between Jeff Coulter and Myself we studied the effects of starter fertilizer use on medium to high P testing soils for three differing hybrid relative maturities (94, 99, and 104 day) at three different planting dates over three years at the Southern and Southwest Research and Outreach Centers at Waseca and Lamberton. The main conclusions that can be drawn from the data is that 5 gallons of 10-34-0 applied "pop-up" consistently increased plant height early in the growing season and sped up the time to silking by 1-2 days across hybrids and planting dates. However, we found very inconstant yield responses. In most of our studies we have been seeing that if broadcast P is applied the starter P has less of an impact on yield. The exception to this may be high pH soils, but many our responses have been similar to more acid soils with high P availabilities.
Certain years seem to favor responses to starter regardless of broadcast P applied or high testing soils. One option is to reduce the rate applied for those whom wish to still apply some starter. In most of our studies we have seen little difference between 2.5 and 5 gallons of 10-34-0 per acre. Reduced rates would allow coverage of more areas per fill of the planter and is a viable solution for those not wanting to eliminate "pop-up" in their fertilizer program. An application of 2.5 to 3.0 gallons per acre still can give a satisfactory growth response and will likely give a similar yield increase if we occur a year where there is a high probability of a response to starter.
Striping On Corn Leaves Due To Sulfur
As we have become more aware of sulfur deficiencies the instances of early season striping on the upper corn leaves have seemingly increased. With the spring we have had at this point I would expect some lower mineralization rates and potentially some striping to occur. I have seen this happen in some of our studies even when sulfur is applied so the overall effect on yield should be small unless the issue is present through the fifth leaf stage (about 12" tall). We have seen little difference when corn yields respond to sulfur when the fertilizer is applied at plating or by the fifth leaf stage. As long as there is a source of sulfur at that point yields should not be limited. If mineralization begins by that point sulfur may not need to be applied.
Watch fields applied with elemental sulfur in the fall closely. With the cool temperatures there may be some fields that may show yellowing because of poor conversion of elemental sulfur to sulfate. Some recent data we have collected shows that our soils have a capacity to mineralize elemental sulfur fairly rapidly at high soil temperatures. But the process still takes time and with the cool conditions we have seen the conversion will be very limited.
If a deficiency is detected sulfur can be applied early in the season. Remember that broadcast AMS does pose some risk for damaging corn leaves if the fertilizer gets into the whorl. Normally at low rates of S this damage should not result in a large reduction in yields. Gypsum and potassium sulfate could be used as substitutes. Ammonium thiosulfate liquid could be used, but not as a sprayed on application over the top of the plants as significant leaf damage may occur. ATS is best suited for surface dribbled or knifed in applications for corn to avoid tissue damage.