Thoughts on Managing Nitrogen Fertilizer

Thoughts on Managing Nitrogen Fertilizer

For the past three years, we have been doing research at the Rice Experiment Station on how to manage N fertilizer in a rice field when the previous year, the rice was in fallow. Here is what we have found from that research. First, the yield potential in rice after fallow is similar to rice after rice. While this may be a bit surprising, it is similar to what we found in an analysis of our Yield Contest data.

While the yield potential is similar, more N fertilizer is required in rice after rice to achieve those yields. Most plant-available N comes from applied-fertilizer N and from the soil. Using labeled N fertilizer, we found that the fertilizer N was used similarly between the two systems. However, what was available from the soil differed. In rice after fallow, there was more soil N available from PI to harvest than for rice after rice. Thus, comparing these two systems, rice after rice needs more N to reach its yield potential compared to rice after fallow.

I mentioned above the analysis of our Yield Contest data. We have run the Yield Contest from 2015 to 2023 and have recently done a complete analysis of that data. A lot of interesting things have come out. Related to fertility, we found two important things that I would like to mention.

In considering when to apply the starter fertilizer application to rice, growers have two options: apply preflood or 20 to 30 days after seeding. The later option helps to avoid scum and allows a grower to move an operation to later in the season when it is less busy. From the Yield Contest, we saw high and winning yields for both practices.

Secondly, I have talked a lot about top-dress N and if it is needed. Our data have shown that you can apply all of the required N up front (as aqua and starter N) and do not need to plan for a top-dress. However, we do recommend monitoring the crop at PI to see if a top-dress is necessary. The Yield Contest data support this. We saw high and winning yields for both growers who apply a top-dress and for those that apply all of their N up front. These data are helpful in that it demonstrates flexibility on how and when to apply fertilizers.

The Yield Contest data also showed that M-211 has higher yield potential than many of the other medium grain varieties being grown. In our variety trials, we typically see yields being 200 pounds to 400 pounds per acre more than other varieties. A higher yield potential suggests higher N rates are needed to achieve that potential. Assuming there is a 300 pounds per acre yield advantage, the crop will take up an extra 5 pounds per acre of N. Assuming a fertilizer N recovery of 50%, growers would need to add an extra 10 pounds of N per acre to achieve those yields.

Fertility is a Balance

The 2023 season was one for near-record yields across crops in Arkansas. That level of production will most certainly have an impact on nutrient availability in the 2024 season. Last year was riddled with reports of nutrient-deficient rice, especially potash, which was most likely the result of record-high fertilizer prices in the preceding years.

With some moderate improvement in fertilizer prices and the amount of nutrients removed from fields in the harvested grain, we need to make sure we are putting back adequate nutrients for the upcoming crop. Let’s take a close look at our fertility programs going into the upcoming season.

Phosphorus (P) fertilization in flooded rice is straightforward, and soil test recommendations should be followed. If growing furrow-irrigated rice, there is a definitive need to apply the minimum recommended rate, and it is likely beneficial to increase that rate to some degree. Our current P rate recommendations are based on delayed, continuous flooded rice. Flooding makes P more available in the soil, so when we don’t flood, we have to fertilize to manage that shortfall.

Potassium (K) has a few new wrinkles for recommendations. Check out the Potash Rate Calculator (https://agribusiness.uark.edu/decision-support-software.php) for assistance. Basically, using historical K rate trials, our K fertilizer rate can be better tailored to the individual field using crop price, fertilizer price, and yield expectation. Using the Potash Rate Calculator gives us the most economical K fertilization program as a starting point. With this, we can more accurately spend our money.

Additionally, we now recognize the ability to take Y-leaf samples during reproductive growth to determine our K levels and whether we would benefit from additional K fertilizer. The window of opportunity to effectively manage K in-season doesn’t close until late reproductive stages, so we have time to make adjustments post flood if warranted and economical.

Zinc (Zn) has not appeared to be as great a problem in recent years. However, this can be a continual problem with hidden hunger implications. Seed treatments can help manage or sometimes mask moderate deficiencies, but lower soil test levels need to be met with real corrections through proper fertilization.

Zinc sulfate is not cheap but serves to build Zn soil test levels and should be considered in high pH, low Zn soil test fields. Following the current soil test guidelines for Zn will ensure that it is not limiting and will help avoid potential yield-limiting disasters in season.

Balance is always a key for rice production. If we’re going to push nitrogen (N) rates to attempt to push yield, we need to push our base fertility to keep up and balance out the plant. A balanced nutritional program will provide the rice plant the greatest opportunity for consistently high yields. Good luck and let us know if we can help.

Early Fertilization for Rice

Rice seed is mostly carbohydrates stored in tissue called endosperm. The embryo makes up most of the rest of the seed. Germination begins with imbibition of water. The seed swells and gains weight. The conversion of carbohydrates to sugars begins, and the embryo is activated.

Nutrition from the endosperm can supply the growing embryo for about three weeks. Fertilization timing and water management are similar for both drill-seeded and dry-broadcast-seeded rice. Phosphorus (P), potassium (K), and micronutrient fertilizers should be applied preplant or pre-flush based on soil texture and soil test results.

The addition of 15 to 20 pounds of preplant or pre-flush nitrogen (N) is generally recommended to ensure against nitrogen deficiency in seedling rice. Early nitrogen deficiency can reduce tillering and yield potential. Application of large amounts of preplant nitrogen should also be avoided in a dry-seeded system since wetting and drying cycles before the permanent flood is established can lead to the loss of much of this nitrogen.

The majority of the nitrogen fertilizer should be applied to a dry soil surface within 3 days prior to permanently flooding the field. The remainder of the nitrogen requirement should be applied mid-season. In some cases, all of the nitrogen fertilizer can be applied ahead of the permanent flood if the precise nitrogen requirement for a field is known and if the permanent flood can be maintained throughout the season. If a field must be drained, however, for any unforeseen reason such as water weevil larva control or straighthead, appreciable amounts of nitrogen can be lost requiring reapplication of nitrogen.

When the required nitrogen fertilizer rate is not known or the field will be drained before harvest for any reason, apply 60% to 70% of the estimated nitrogen fertilizer requirement prior to flood establishment. Additional nitrogen fertilizer should be applied at mid-season at the beginning of internode elongation (IE) green ring.

Native soil fertility, soil type, and other factors affect nitrogen fertilizer efficiency. Rice growers should determine the nitrogen rate that provides optimal grain yield on their soil and production system.

Fertility Management

When looking at 2024 fertility plans, it is important to remember what happened in 2023. First, we ran into some situations with nitrogen (N) in which we had to be creative last year. One of the few rains and probably the biggest came at a time when we were just preparing to apply our pre-flood N. This left fields that had not had their biggest N application flooded before we were ready.

The first option here was to go with a spoon-fed approach like has commonly been used in furrow-irrigated rice. The second option was to drain the field and wait for the ground to get nearly dry and apply a N source treated with a N-stabilizing product. It is important to note the second approach provides the best yield response due to achieving greater nitrogen efficiency, but growers have to consider many factors and make the best decision for their operation.

The next issue that arose often in 2023 was potassium (K) deficiency. Rice needs K to perform many important roles within the plant. Two of these roles include the ability of rice to resist disease and grain quality. With the K deficiencies observed last year, late-season disease became an issue. Often, this disease was low in the canopy and not problematic, but there were cases where foliar K and fungicides were applied as a “band-aid” to finish the crop. Additionally, the K deficiencies were likely a contributor to the milling issues seen with the 2023 rice crop.

The final issue we ran into in Mississippi was artificially high soil pH due to groundwater quality. In areas where our groundwater is high in bicarbonates, the soil pH is raised by irrigation. This leads to phosphorus (P) in the soil becoming unavailable for plant uptake. To combat this, we often make in-season applications of a readily available P source.

Going into 2024, I like to remind people to plan on two N applications in flood-irrigated rice. The first of these is the most important. The first application should be about 2/3 of the total N, treated with a N stabilizer, and applied to dry ground. Immediately following this application, establish and maintain the flood. Also, with what we saw last year, check soil-available K and, if necessary, plan to at least apply maintenance rates of K. Finally, test your water source so you know what you are dealing with. With acres on the rise in Mississippi the past couple of years, I am excited to see what 2024 holds.

Fertility Adjustments for Rice Following Rice

With the way markets and prices look in the beginning of 2024, combined with a reasonably high rice acreage in 2023, it makes sense that additional ground will be rice following rice for the 2024 season. Typically, we see 10% to 20% of the Missouri rice crop as rice following rice, so this may be a rare practice for some farmers. We should be cognizant of the potential need for additional fertilizer when following rice.

First thing first is the obvious and usually most yield-influencing nutrient, nitrogen (N). We typically recommend increasing N rate by 20 pounds per acre (43 pounds of urea per acre) for flood-irrigated rice following rice. Regardless of whether the field was burnt, we expect to have less N availability following rice compared to soybean. As a conservative estimate, burning rice straw can release well over 120 pounds of N per acre. If rice straw is left in the field, the very high C:N ratio of rice residue causes the tie up of a lot of N in order to break down the residue. Bottom line, less N for the short-term regardless of management practice.

Most other nutrients do not require more input for rice following rice, but we should pay attention to notes from the previous year and whether we had any deficiency issues. If P, K, or Zn deficiency was an issue in 2023, then applying the proper amount of the limiting nutrient near planting could increase our likelihood for maximizing yield potential. One exception could be with potassium (K). A 200-bushel rice crop will remove an average of 32 pounds of K2O per acre, while a 70-bushel soybean crop can remove over 80 pounds K2O per acre. Theoretically, that leaves 50 additional pounds of K2O after rice compared to after soybean, BUT it’s best to run another soil test before gambling on whether a nutrient may or may not be available.

It’s always worth noting that in-season tissue testing is a great tool to help diagnose any possible nutrient deficiencies, especially if using reduced fertilizer rates or if you’re lacking a current soil test report.

If you have fertility questions now or in-season, you can always contact us via e-mail at jchlapecka@missouri.edu or catch us at some of the upcoming meetings and conferences through the first of March, including the Missouri Rice extension meeting Feb. 13 and the Missouri Rice Council annual meeting Feb. 27. As always, God bless and eat MO rice!