With input prices increasing, it is more important than ever to manage fertilizers in the most economically feasible way possible. Here are several things to consider when making fertility decisions.
First, aqua-ammonia is the cheapest and most efficient nitrogen source for California rice growers. When applying N, apply as much as possible in this form. Second, do not plan to top-dress, but apply enough N preplant for an average yield. At panicle initiation, access your crop to see if it needs additional N. Third, in 2021, we analyzed soil data from 83 rice fields (in 28 fields we also analyzed leaf samples) around the Sacramento Valley. Importantly, no soil or leaf samples were deficient for either sulfur or zinc. Many growers apply these nutrients routinely. Skipping these applications (i.e. applying urea instead of ammonium sulfate) may help reduce costs; however, if in question, it would be good to take your own soil samples.
In previous years, aqua-ammonia has not always been available. The best option for replacing it is to use urea. Urea (liquid or granular) works equally well if applied preplant and is put several inches below the soil surface. Liquid urea is hard on the pump, so make sure to clean out your pump very well at the end of the season.
If you are applying the N after planting, you need a split application for best results. Due to reduced N-use efficiency, the total rate may need to be increased by 5% to 10%. I suggest putting on the starter N (plus P and K) about three weeks after planting. This should represent about 15% to 20% of your total N rate. The rest of the N should be applied at four (30%), five (30%) and six (20%) weeks after planting.
In our survey of rice field soils, we found many growers are under applying phosphorus (P). A typical rate was 40 pounds per acre, which is enough P to replace the P in the harvested rice grain of a field that had an 80 cwt/acre yield.
Yields are usually well above this now and straw removal is also more common. If your field is yielding 100 cwt/acre, you need to apply 50 pounds per acre to replace the P removed in the grain. Add an additional 5-10 pounds per acre if you are baling rice straw.
Finally, many fields did not get flooded this winter, due to water limitations. Furthermore, the October rains prevented straw incorporation in many fields. If there is still a lot of straw in the field at the start of land preparation due to poor winter decomposition, early season N may need to be increased. This is one area where I suggest adding a higher N rate in the starter fertilizer (as opposed to increasing aqua-ammonia N rate) and tilling the starter fertilizer in with the straw.
Phosphorus and potassium
With the high cost of fertilizer, don’t forget the importance of phosphorus (P) and potassium (K).
Phosphorus is a constituent of nucleic acids and is needed for several plant-essential processes, including photosynthesis, respiration, energy storage and energy transfer among others. Rice removes P from the soil as an orthophosphate ion, primarily H2PO4- or HPO4-2.
Fertilizer P is always expressed as a percent P2O5 equivalent and on a fertilizer label is located as the second number (example: 0-46-0).
Soil test P and soil test-based fertilizer recommendations can be reported as either P2O5 equivalent or P. Convert P to P2O5 equivalent by dividing by 0.44. Convert P2O5 equivalent to P by multiplying by 0.44. Soil P is most available to plants in soils with a pH around 6.5. Typically, plant available P is decreased as a soil becomes more alkaline or acidic.
When a rice soil is flooded and becomes anaerobic (no oxygen), the pH of the soil migrates towards neutrality over time regardless of the initial pH. Therefore, in most cases P becomes more available to rice after permanent flood establishment. Rice takes up approximately 0.0086 pounds of P2O5 (0.02 pounds of P) per pound of dry rice harvested. Approximately 75% of the P taken up is contained in the harvested grain.
Potassium is needed to regulate normal metabolic processes in the plant including photosynthesis and protein synthesis as well as maintaining plant osmotic pressure. Deficiencies of K in rice have been related to increased disease incidence — typically brown spot, stem rot and blast. Rice takes up K in the ionic (K+) form. Deficiencies of K in rice are more often found on coarse-textured soils with a relatively low cation exchange capacity (CEC).
Fertilizer K is expressed as a percent of K2O equivalent, and on the fertilizer label it is located as the third number (example 0-0-60). Soil test K and soil test K fertilizer recommendations can be reported as either K2O or K. Convert K to K2O by multiplying by 1.2. Convert K2O to K by multiplying by 0.83. Rice takes up approximately 0.024 pounds of K2O (0.02 pounds of K) per pound of dry rice harvested.
Approximately 17% of the K taken up is contained in the grain while the remaining 83% is contained in the straw.
Phosphorus and potassium fertilizers are mobile nutrients within the rice plant. When a plant becomes deficient in one, it is translocated from the older (lower) leaves into the newer (upper) leaves. Phosphorus symptoms include stunted, erect plants with slender stems, and leaves tend to have a very dark green color. Other symptoms are reduced tillering, slowed plant development and necrosis (death) of leaves beginning with the oldest.
Potassium deficiency symptoms include stunted plants, interveinal chlorosis (yellowing) of oldest leaves beginning at leaf tips and moving backward down the leaf. Other symptoms are necrosis (death) of leaf tissue following chlorosis beginning at leaf tips, increased incidence of brown spot and potential for increased disease pressure.
Phosphorus and potassium fertilizer rates should be determined from a recent soil test. Phosphorus and potassium fertilizers are most efficient in rice production when applied just before planting until permanent flood establishment.
Carefully adjust fertilizer rates
Fertilizer price inputs for the 2022 season have been the hot topic for rice. Producers need to avoid emotional decisions and be extremely careful when adjusting their fertilizer programs to manage overall input costs this season. Hopefully, we’ve been proactive with our nutrient management programs following soil-test based recommendations and have adequate soil test levels.
Nitrogen (N) is essential for setting yield potential and is the focal point of most fertilization programs. Unfortunately, urea market volatility will remain quite high throughout 2022. Stay close to the N rates that have made you successful. Slightly trimming N rates likely won’t reduce yield, but savings can add up. As N rates increase, return on investment declines.
Taking N-STaR (Nitrogen Soil Test for Rice) samples can help fine-tune N rates. We can also evaluate rice at midseason using Greenseeker to determine if additional N is needed. Using these tools, we may be able to see cost savings related to N fertility, but with a higher degree of management. Heavy clay soils and rice following rice scenarios typically see the largest benefit (N rate reductions) using N-STaR.
What about phosphorus (P) and potassium (K) fertilization? We prefer to rely on a soil test history (consider the past five to 10 years) rather than a single year. This history indicates whether P and K levels are being maintained, increasing or decreasing. If soil test levels are decreasing, fertilization rates should not be reduced as the soil is being mined of nutrients.
With stable or increasing soil test levels, there is a chance to reduce P and K rates without hurting yield potential. However, it is unwise to completely omit applications. A smaller amount is better than skipping applications entirely. Weak field areas aren’t always captured in soil tests and will suffer from having no fertilizer applied.
For P in particular, the main concerns are soils with high pH (>7.0) and very low or low soil test P levels (<9 ppm). A P deficiency has a limited window to correct in-season and still maximize rice yield. For K, where applications are typically required on lighter-textured soils, rates could be lowered, and deficiencies are often easily corrected in-season.
If you have high soil pH (>6.0) and very low soil test Zn levels (<2.5 ppm) you cannot afford to skip or reduce Zn rates. The cost is similar between applying granular Zn and foliar Zn, but only the granular application builds soil test levels.
Fertilization is a little different with furrow-irrigated rice (FIR), and it appears P and K are not available in the soil to the same degree as flooded rice. These fields likely do not need to have rates reduced or deficiencies may be encountered. The soil test recommended P rate of P2O5 is the minimum that should be applied to FIR. Additional work is needed to improve fertilizer recommendations for P and K in FIR.
Finding ways to stay profitable is the name of the game for 2022.
Let’s be smart about how and where we trim our input costs. Remember that small reductions can add up to big savings but attempting to cut back drastically could lead to losses we can’t recover. Anything we can do to increase nutrient uptake and nutrient use efficiency will pay dividends this season so always focus on the 4 Rs of nutrient management: Placement, time, rate and source. Good luck in 2022 and let us know if we can help.
Don’t starve your rice crop
Fertility is a topic on many minds, especially due to the volatility of input prices over the past five to six months. While the prices of urea and DAP have fallen off their highs at the end of 2021, the input prices remain up compared to last year. To that point also, despite a relatively dry fall, the sharp upswing in prices led to very few phosphorous (P) and potassium (K) fall applications.
Our first message is to make sure you have an up-to-date soil test for your ground. It is impossible to guess the nutrient levels and how much may need to be applied without one. While a common theme across much of the area where I am located is adequate to excessive soil test levels of P and K, many factors can affect those levels. You could be doing yourself a disservice by assuming levels are high.
Once you receive a soil test reading and recommendation, it is especially important to keep up with your fertility if you’re in the low or very low categories. A “fertilize the crop” mentality will usually benefit the bottom line more than a “fertilize the soil” approach in our area, regardless of fertilizer prices. Soil test recommendations will consider the amount of each nutrient needed for your rice yield goal.
One emerging philosophy that could be used in years where fertilizer price is higher is to wait and apply K near mid-season timing if there is deficiency through plant sampling. Recent studies have shown that maximum yield can still be achieved if K deficiency is corrected by mid-season. However, that philosophy is not recommended in furrow-irrigated rice due to less K availability in a non-flooded environment.
With September rough rice trading near $6.50/bushel, the numbers still work for growing rice profitably. The N rate should not be shorted in any situation. But using a single pre-flood application as opposed to a split with a midseason application on varieties can slightly decrease the total N rate and be just as effective where good water management can be practiced. A late boot application is still recommended on hybrids, but if urea hits $1,000 per ton I would definitely want to reconsider.
Of course, this is still a fluid situation that changes nearly every day. If big changes occur, we will certainly keep you in the loop. If you have any questions, feel free to contact me or Justin Calhoun (firstname.lastname@example.org), UM soil and cropping systems specialist. As always, eat Missouri rice.