Phosphorus (P) also impacts water quality and is a common cause of hypoxia in surface water bodies. Phosphorus can be lost as soluble P in a water leachate or move off the landscape with soil particles and enter watersheds.
Phosphorus (P) Management:
Right Rate: Phosphorus rates for Illinois are based upon the “Soil Buildup and Maintenance” fertilization scheme. The Buildup is based on the inherent P-supplying power (low, medium and high) of the various soil regions in the state and has slightly different associated soil test values (visit the Illinois Agronomy Handbooks to view the state chart). The main points to consider for all regions are that with:
- Buildup and Maintenance fertilization, crop yields are dependent on this amount of P fertilizer.
- Maintenance fertilization only, subsequent crop yields are dependent on P fertilizer.
- No fertilization due to high P soil tests, yields are not dependent on P fertilizer.
As a side note, when you do have high P testing soils it is also important to know your soil test levels for zinc and to monitor zinc levels in the plant tissue. In these high P testing soils the zinc can be tied up by the P and create a phenomenon called “phosphorus induced zinc deficiency.”
Soil testing and plant tissue testing are key components here. Utilize soil testing to determine your soil P levels and use that information so you apply at the correct, non-excessive rates. Utilize the plant tissue testing to monitor crop health progress.
Finally, the use of variable rate fertilizer application technology (VR or VRT) can be of great economic and environmental benefit. While the technology fee might be a little more expensive than you budget for, you are putting on only the amount of fertilizer needed and only where it is needed.
Right Source: Just as with nitrogen sources P sources all have their strengths and weaknesses, but should provide relatively the same yield when used correctly and with the equivalent amount applied. These sources can be diammonium phosphate (DAP), triple super phosphate (TSP), ammoniated phosphates (APP/MAP), or organic N sources (composts/manures).
All sources are good P sources and will provide the necessary, needed nutrients. The sources that need the highest degree of management are livestock manures. When applying manures at the N requirement needs of a crop, P will be over-applied. For every 6 units of N required by a crop, approximately 2 – 2.5 times the P requirement will be applied. Instead, apply manure at a reduced N utilization rate to help keep the soil test P values lower and then supplement the required nitrogen with commercial fertilizer.
It is important that you collect a representative manure sample and have it analyzed for its nutrient content. Then you can plan subsequent manure and/or fertilizer application rates accordingly. An additional key component when applying manure is to do a physical calibration of the application equipment. Do not use book values for manure nutrient content or application rates from the equipment manual. You will be surprised at the variability observed in the application and in the crop response
Right Time and Place: Here is one of the first points where we may have to “agree to disagree.” In the interests of water quality and nutrient management I would like to see P fertilizer applications just ahead of crop planting in the spring for corn and soybeans, with wheat again being the exception. I would also like to see a light or shallow incorporation of P fertilizer if using dry P fertilizer sources in a broadcast situation, with No-Till being the exception. If you are applying a P fertilizer that also contains N, take advantage of this N by putting it just ahead of crop planting.
I know logistics come in to play here with growers trying to cover large acres in a short window, especially if applied by retailers/custom applicators. A fall surface application of a product like DAP has the potential for P loss all winter. It also contains N that can be potentially lost to the environment. The ammonium portion in DAP can either be adsorbed to the soil CEC, taken up by winter annual weeds, or be converted to nitrate and potentially leached deeper into the soil profile or to drainage tile lines. If the latter occurs, this will be counterproductive to the Illinois NLRS.
There has been a lot of discussion on the best placement of P nutrient sources. There are several different P placement methods such as: broadcast, starter/row, seed placed/pop-up, strip/strip till, or deep banding fertilization. Of these, broadcast fertilization is still the most common and the most efficient time- and equipment-wise. The starter, seed placed, and strip fertilization techniques are designed to apply a small portion of the total P (and N) needed to promote early season plant health in cool, wet soils. Deep banding of P is not a common practice in Illinois, but the reasoning is to apply the P in concentrated bands in soils that may have a high P-fixing potential. This is a common practice in the calcareous till soils of the Western States, where soils with very high pH can fix/complex the P and render it plant unavailable. Deep-banding generally is done when strip-tilling.
My Phosphorus Best Management Practices include:
- Conduct routine soil testing to know your soil test P levels.
- Validate fertility with routine tissue testing.
- Know the sensitive areas on your farm—high soil test and high surface runoff potential areas.
- Inject manures or band phosphorus where applicable.
- Lightly incorporate surface P applications.
- Abide by any “no application setbacks” if applicable.
- Apply P at the recommended rates and possibly use variable rate application.
- Consider reduced or no-till to maintain more soil cover to reduce erosion losses.
- Consider using cover crops to scavenge P (and N) so it is not lost to the environment.
Terry Wyciskalla is an independent crop and soils consultant based out of Nashville, Illinois. He specializes in soil sampling, fertility recommendations, precision ag services and crop problem diagnoses. He serves a 12-county area throughout Illinois. He earned his 4R Nutrient Stewardship certification in 2015 and was part of the 2016 Soy Envoy team.