Episode 4 - Fall versus Spring Applications with Trent Roberts Arkansas Row Crops Radio, providing up to date information and timely recommendations on row crop production in Arkansas. Trent Roberts: Hello and welcome to Arkansas Row Crops Radio. This is Trent Roberts, Soil Fertility Specialist with the University of Arkansas System Division of Agriculture. I will be your host for this episode. Today's topic will focus on fall versus spring-applied phosphorus and potassium. First off, I think it is important to remind everyone of the four Rs of nutrient management: right source, right place, right rate, and right time. Notice that I listed time as the last of the four Rs because that will be the focus of our discussion today. In order to maximize nutrient uptake, we want to try and apply and incorporate nutrients as close to plant uptake and demand as possible. Having said this, we also have to understand how nutrients react in the soil and their relative mobility and loss potential to truly address this question. Although the ideal time to apply and incorporate phosphorus and potassium fertilizers is immediately prior to planting, we all know that this is not a feasible practice due to weather patterns such as rain and the amount of ground that has to be covered in a short amount of time. Previous research in Arkansas has shown that when soil test recommendations are followed that fall-applied phosphorus and potassium yield and perform just as well as spring-applied phosphorus and potassium. There's several factors that need to be considered to help you better manage your pre-plant applied phosphorus and potassium and potentially help you identify which fields are better suited for fall application versus spring application. The first question you should ask yourself is how the field will be managed in the winter or is it prone to flooding? If you plan to flood the field for waterfowl or it is prone to prolonged flooding conditions in the winter, then these are situations where you want to delay your phosphorus and potassium application until the spring when the water has been dropped or the flood water recedes. Although we tend to think of phosphorus and potassium as being immobile in the soil, they can dissolve into the floodwater and move with the water. Also when we have prolonged flooding this can lead to increased leaching within the soil profile. Another consideration specific to fall-applied phosphorus is how the alternating reduced and aerobic conditions influence the phosphorus chemistry. This may lead to lower PA availability the following spring. The next question you should consider is whether or not the field has a history of phosphorus and potassium deficiency. In fields with low soil test values or fields that have exhibited phosphorus and potassium deficiencies in the past, you want to try and wait until the spring to apply phosphorus and potassium to those fields. Areas that have low soil test values or have a history of nutrient deficiencies often require the largest fertilizer application rates. But these fields also result in the largest field gains from the successful management of those nutrients. Spring application of phosphorus and potassium to fields with low soil test values will often result in greater return on investment due to sound nutrient management practices. Fields that tend to have higher soil test values or areas where fertilizer applications are based on a maintenance approach are ideal candidates for fall application, as these soils tend to have lower phosphorus fixation capacity and potential yield penalties for under application of fertilizer and nutrients are much less. Less focus on soil PH effects, which will mainly influence phosphorus considerations and soil PH has little effect on potassium cycling and availability within the typical Arkansas soil PH ranges. For rice soils in particular that are acidic in the low to mid 5s or alkaline with a PH 7.5 or above, we want to try and move those application timings as close to planting as possible. In these soils where the PH is beyond the 6.5 to 7.5 PH range, the potential for phosphorus fixation is increased. And we want to reduce the amount of time between applications and plant demand to help increase phosphorus availability and ultimately plant uptake. With any pre-plant application of phosphorus or potassium, it is important to incorporate those nutrients into the soil, especially fall applications. Unincorporated phosphorus or potassium fertilizers applied in the fall are more prone to nutrient runoff and potential loss, and therefore every effort should be made to incorporate these fertilizers following application. A specific question came in on this topic of incorporating phosphorus and potassium into the bed in the fall or surface broadcasting across the tops of the beds in the spring. In an ideal world, we want nutrients to be incorporated into the soil regardless of whether they are applied in the spring or in the fall. I would encourage you to consider the topics that were discussed earlier to make this decision. What are the soil test levels? Are they low or medium? Is there a recent history of deficiency in the field? In fields that are better suited to spring fertilizer applications, the nutrients can be broadcast across the top of the beds. But making a past to re-pull or incorporate those nutrients is always a good practice. One issue with applying pre-plant fertilizer to stale or hardened beds in the spring is that much of the fertilizer falls to the center of the furrow and it can take the crop much longer to access and take up these nutrients. So let's say that my field has low soil test values and I'm only going to pull the beds once in the fall. What is my approach? I would rather you put out your pre-plant nutrients in the fall and make sure that they are incorporated into the beds, than wait until the spring and not incorporate them. So in this case, I think that placement of those nutrients take some precedence over timing of application. My closing thought for you is to remember that there are advantages and disadvantages to both fall and spring applications of phosphorus and potassium fertilizers. But both can be effective fertilizer management application times. The best approach is to use soil test levels, planned management practices, and previous field history to identify which fields are ideal candidates for fall applications of phosphorus and potassium. I would like to thank the support of the rice, soybean, corn, and grain sorghum checkoff programs, as well as the fertilizer tonnage fee for supporting the research to address fall versus spring-applied fertilizer applications. As always, let us know if there is any way that we can be of assistance. Thank you for joining us on Arkansas Row Crops Radio. Arkansas Row Crops Radio is a production of the University of Arkansas System Division of Agriculture. For more information, please contact your local county extension agent or visit uaex.edu.