I have been fielding more questions on seed placed fertilizer in areas where rainfall has been sparse this spring and soils are dry. In my previous post I discussed the use of in-furrow starter fertilizer. Placing fertilizer on the seed can help speed up early plant growth but also can substantially reduce stand if a fertilizer is over-applied or soils are dry. How dry is too dry? That is a good question and the answer depends on the soil corn is being planted in. For medium and fine textured soils, the risk of damage typically is lessened when the soil moisture content is 25% or greater.
Is there an easy way to determine the general moisture content of your soil? Soil particles will stick together as soil moisture increases in fine textured soils. Soils with adequate moisture should form a ball in your hand. As these soils approach 50% of water holding capacity the soil should form a ribbon when pinched between your fingers. Checking soil in the seed row where the seed is placed can help determine whether in-furrow starter should be avoided. If the soil crumbles easily and will not form a ball where all the soil will adhere to then the water content is likely less than 25% of the water holding capacity.
For sandy soils it is difficult to tell when soils are too dry as a small change in soil moisture can result in major changes in appearance. If soils are sandy and the field is irrigated, supplemental water can help reduce risk for stand damage. Water should be applied immediately following application if a sandy soil is dry. Starter rates have to be low in general for sandy soils but a low rate of liquid fertilizer can still result in damage as the potential. Broadcast fertilization alone may be best in sandy soils.
Seed safe rates are presented assuming soil moisture concentration greater than the 25% threshold. Most tools for assessing damage potential should be viewed more as qualitative as it is hard to quickly quantify soil moisture and the impact that it may have on stand damage. Two things are important to consider when assessing the potential damage from starter.
First, if rates are high the effect may be an inhibition of the seed to germinate. We have observed this effect in the greenhouse. An attempt was made to wash seeds that did not germinate then try to re-plant the seeds to see if they would germinate. None of the seeds that did not emerge under high salt concentrations emerged when the salts were removed. At this point the damage was already done.
Second, the major impact of starter on roots occurs immediately as the seed is germinating. The first root to emerge will encounter the starter band and if the rate is too high the root will typically turn brown and discolored and cease to grow. Secondary roots may grow as the plant attempts to recover but the plants growing in this situation typically are stunted and yield will likely be impacted.
Keeping fertilizer application rate low is the best option even in years with adequate moisture. In-furrow starter fertilizer should not be viewed as a way to primarily supply nutrients to crops. I would not suggest any starter for soybean, particularly in Western Minnesota. Soybean is more sensitive to damage. Low salt fertilizer sources will not help if soils stay dry into soybean planting. We do not suggest on-seed placement in moist years due to the sensitivity of soybean seed to on-seed placement.
For those with 2x2 or more traditional starter placement away from the seed row there is far less risk even in dry soils. At this time I would not suggest a rate reduction in situations where in-furrow placement is not being used. Allowing for at least 1-2” of soil between the fertilizer band and seed row should be enough to lessen the risk to the seed.
I would strongly suggest monitoring fields this spring as replanting may be an option. This is not a time to panic as we do not know how stand will be impacted. Recent rain over the weekend may lessen the risk for corn yet to be planted if the soil moisture was significantly impacted. Since damage typically occurs right at the emergence of the first roots, recent rains will not help if any damage has occurred. Once damaged, nothing can be done other than assessing whether the damage is severe enough to warrant further action. Being vigilant and reducing rates is warranted especially when soil conditions are not favorable for the use of in-furrow starter fertilizer.
For more information go to http://z.umn.edu/nutrientmgmt