From the Field-Agronomy Notes: Vol. 2, Num. 9
April 26th, 2017-
After having an additional 24 hours to assess the damage from recent weather events, it appears that saturation and flooding are worse than originally thought in Eastern North Carolina. Excessive water is likely an issue in the Piedmont as well; however, the good news is that very little of the crop has been transplanted in that region.
Yesterday we released an article outlining fertilizer leaching adjustments for conventional growers, but failed to include information relevant to organic production. Information in this post will attempt to provide insight. One point that should be mentioned is the fact that determining fertilizer losses in organic systems is even more difficult than determining those in conventional systems.
The first discussion point relates to understanding the forms of nitrogen (N) that dominate the organic fertilizer sources used in tobacco production. General observation has shown that pelletized feather meal and poultry litter are probably the most commonly used fertilizer sources. Both sources primarily contain N in the various forms of proteins, amino acids, and DNA/RNA, none of which are plant available. This means that these forms of N must be mineralized by soil microbes which convert them into ammonium (NH4+) and then nitrate (NO3–), both of which can then be used by plants. This also means that those forms of N are not very water soluble and are not typically subject to leaching. The single exception to this overview is Sodium Nitrate (16-0-0/15-0-2). Sodium nitrate is mined from South America and processed as nitrate (NO3–) nitrogen, so it is immediately plant available. Of course, it’s also very leachable.
The next point for discussion becomes how much of the applied N has mineralized into a plant available form? This is probably the most difficult question to answer and will essentially set the stage for how large or small a leaching adjustment will be. Research from laboratory studies where soil conditions are maintained constant for an extended period of time have indicated that 45-55% of applied N can be mineralized as quickly as two weeks after application (1,2). I’m a bit skeptical that we would see this much mineralization in a field environment given the fluctuations we typically observe with soil temperature and moisture. As a general rule of thumb, mineralization increases as soil temperature and soil moisture increase and slows as both factors are reduced. Soil temperature measurements as recent as two weeks ago were between 65 and 75° at a depth of about four inches. However, I’m confident that the cooler temperatures we’ve intermittently experienced might have slowed microbial activity to some degree. In addition, soil moisture in most areas was fairly low prior to this event, which might have further decreased the rate of N mineralization. Of course, all of this is speculation at this point as I have no good way to accurately estimate what fraction of fertilizer N was released.
Another point for consideration is the length of time that organic fertilizer had been applied. One rule of thumb I can provide is this: The longer the materials is in the soil, the more N there should be available to plants. If producers applied all fertilizer prior to transplanting and then transplanted two weeks ago, it could be probable that as much as 50% of their N was removed during the storm. Alternatively, if the same practice was employed one week ago, then we might be considering a 20 to 25% loss. Again, there’s not a great way to estimate what was lost but it’s probably safe to say that recent organic N applications will have more N left in the soil profile.
Lastly, and probably most importantly, is the question “do organic tobacco producers need to consider a leaching adjustment?”. I would venture to guess that yes, they may very well need to consider it, but after a period of observation. The largest concern I have regarding N application in an organic system is how much is too much. I worry that a leaching adjustment in excess of 50% N replacement could result in late season greening, which would be caused by late season N uptake by plants. To hedge against this issue the following scenarios can be considered:
- Apply 100-135 pounds of sodium nitrate as soon as fields are dry enough. This application will provide 15-20 pounds N/acre and should give plants a kickstart. Fields should be monitored for signs of N deficiency, with additional applications of 15-0-2 made IF needed.
- Take the most conservative approach by allowing the plants to tell you what is needed. Once plants begin visually appear pale, apply 100-135 pounds of 15-0-2 per acre for a quick fix. This situation might be the most preferable since it could potentially prevent a producer from over-supplying N.
Other points for consideration are as follows:
- When considering a leaching adjustment in organic systems, spoon-feeding the crop is probably the safest option. Again, this comes back to the prevention of late-season N uptake and late-season greening. There’s just too much at stake with organic tobacco and so few options when it comes to management.
- Selecting a fertilizer source is up to a grower, but I have serious concern with applying poultry litter or feather meal later in the growing season. Given the variability in material breakdown and mineralization there is potential for slowed N release from these products. My primary concern relates to a layby application that is followed by a hot, dry June and July. Those two environmental factors are likely to reduce N availability.
- When considering a leaching adjustment, a conversation should be had with your buyer(s) and your third party organic certifier. You should verify that you choices will be acceptable to the industry.
- What about potassium (K)? We have recommended that conventional producers replace K at a 1:1 ratio with N, especially in fields with >12 inches to clay. This means that for every one pound of N applied, one pound of K is also applied. This will not be the case for organic systems because we don’t expect N losses to be as severe when compared to conventional systems. We do, however, expect K losses to mirror those in conventional systems. To account for this, it might be useful to apply K at a 2:1 ratio with N. Of course, there’s always the option to wait and see how the crop looks at layby.
As I said before, determining N losses in an organic system are going to be very tricky and will require significant thought. The key is to not get heavy handed with N and to strongly consider using a material that is plant available at the time of application.
As always, please let us know if you have any questions.
- Hadas, A. and Larissa Kautsky. 1994. Feather Meal, a Semi-Slow-Release Nitrogen Fertilizer for Organic Farming. Fertilizer Research. 38: 165-170.
- Hartz, T.K. and P.R. Johnstone. 2006. Nitrogen Availability from High-Nitrogen-Containing Organic Fertilizers. HortTechnology. 16(1): 39-42.