Dry fall and early spring soils have led to questions about starter fertilizer application this spring. While that planting with starter in a dry seedbed can significantly increase the risks, the overall effect will not be known until after planting. Assessing the situation after emergence will be the best way to determine if damage has occurred due to "pop-up" fertilizer application. With some corn already planted and fertilizer decisions made there are a few key points to remember when dealing with starter fertilizers.
Identifying starter damage
All fertilizers contain salts. Salts contain ions which are charged molecules that attract water wince water is a polar molecule. The higher the concentration of salt in one area the stronger water can be held. If this is in the root zone it can make it difficult for roots to take up water and ultimately damage the living root tissue by drawing water out of the roots. In severe cases of starter damage the radical, or the first emerging root, will be killed off when it reaches the starter band. For "pop-up" application direction on the seed this would be soon after it first emerges. Fields exhibiting damage will have very uneven emergence and the plants that do emerge may be spindly in appearance. If you dig the plant the radical will typically be short and the tip will be brown or black. Plants may recover by growing new roots but typically they have been set back significantly enough that yield reductions are highly likely. At this time fields will have to be assessed for possible replant if the damage is severe enough.
Impact of nitrogen
While salts are of major concern work within the last 10 years in Minnesota has demonstrated that the amount of nitrogen in the starter may have a larger impact than salts. In particular, fertilizer sources that liberate high amounts of ammonia-N are of major concern. Urea poses the highest potential risk since ammonia is liberated as the urea molecule is being converted to a plant available form. While most liquid N-P-K blends do not contain 100% urea, it is added to some in order to bring up the amount of nitrogen. It can be important to check to see what the percentage of urea N is in starter mixes since this can significantly affect when damage occurs when you compare sources. In the case of 10-34-0, nitrogen is in the ammonium form. Ammonium differs from ammonia in that is has one more hydrogen atom and is charged and can be held by clays vial cation exchange. While there are differences, high rates of 10-34-0 can still present a significant risk even in dry years.
Thiosulfate ionAnother source to be aware of are fertilizers containing the thiosulfate ion. The thiosulfate ion can severely damage plants. While some growers have been using small amounts of ammonium thiosulfate in starter to supply sulfur to corn, there is significant risk associated with this practice especially if the seedbed is dry. Current research has found that while low rates of ATS may not seriously reduce stand in loamy soils, plant growth is reduced when soil moisture is near field capacity. When a sandy soil with half the available water holding capacity is compared, the potential for damage was two times or more greater. If soils remain dry into planting then a different application method should be considered to get the fertilizer away from the seed. If soils are warm then the benefit from P applications will likely be less. Surface banding ATS to the side of the row can be effective and greatly reduce the risk for stand damage.
The recent rains may alleviate some of the risks but monitor fields especially when high rates were applied on the seed with early plantings. In our experience damage from starter will occur at the initial onset of emergence therefore by the time you can see the damage there is no way to correct it. We currently are researching different starter options for corn. While starter can be beneficial there are risks associated with the practice and there may be better options than others in specific situations. For instance in corn on corn situations, surface banded ammonium thiosulfate seems to be a safe option if the band is placed to the side of the row and may have a greater impact on yield.
Remember, adequate moisture will alleviate many of the problems so the more rainfall we get during the early growing season the lower the risk and more flexibility to what can be applied. In addition, any placement where the band is placed away from the seed will lessen the risk. For growers still using 2x2 or any other placement with at least 1" of soil between the seed and the fertilizer band there is far less of a risk because the first roots that emerge do not need to growth through the band. That does not mean that there necessarily will not be any risk for damage but the risks is far less. There is not a 100% safe source of fertilizer for "pop-up" placement so knowing the risks is important to ensure the best possible outcome and even emergence after planting.