As with most grains, N management is very important in obtaining high yields. Nitrogen deficiency is a common problem.
Wild rice paddies are very different from upland fields. Nitrate-N build up in groundwater caused by over fertilization is not a problem. Most N losses occur by the process of denitrification that produces N2, which makes up 78% of the atmosphere. Dry urea, liquid ammonium fertilizers, or anhydrous ammonia are appropriate sources of N. Nitrate fertilizers are ineffective because of denitrification upon flooding.
Basal N is often applied in the fall. To maximize carryover of N to the following spring, application of basal N should be followed by flooding within 2 or 3 days. Fall flooding prevents oxidation of the ammonium forms of N to nitrate and the subsequent losses caused by denitrification when flooding occurs. When fall application is desired, but flooding is not possible, application should be made when soil temperatures are well below 50°F, preferably as close to freezing as possible.
Basal N can also be applied in the spring immediately before flooding. Both spring and fall N should be incorporated to minimize losses by the nitrification/denitrification process in the surface soil that can result in N losses. See Table 1 for N rates.
A minimum of one topdress application of 30 – 40 lb/ac of N as urea, applied by aircraft, is necessary for high yields. A second topdress is typically needed to maximize yields. A third application may be needed if no basal N was applied or basal N was lost by nitrification followed by denitrification. Drainage of paddy water should not occur for about 5 days after N application. Field testing in mid June for soil ammonium N can be used to guide topdressing decisions. The SPAD chlorophyll meter or a color chart are also a useful aids in determining the timing and number
Phosphorus is an important nutrient for plant growth. In well-fertilized paddies, P accumulates over many seasons and P deficiency is now uncommon. Phosphorus is the nutrient most limiting for nutrient for algal production in fresh waters, and hence, is of great concern in surface water quality. Phosphorus in paddies can move into surface waters both in the soluble form and as part of soil particles (by soil erosion). When phosphorus is surface applied, it can dissolve in paddy waters causing algal blooms. Incorporation of P fertilizer is very highly recommended and erosion from ditches, etc. should be minimized to prevent phosphorus from entering drainage waters.
Phosphorus can be applied in the fall or spring. It must be incorporated by plow down or injection. The rate of application should be determined by soil testing. When the Bray P 1 test exceeds 16 ppm, do not apply P. See Table 1. If it is not possible to incorporate P it is best not to add any phosphorus fertilizer. The reserve P in the soil will normally be sufficient for good plant growth.
Potassium is required by wild rice both for high yield potential as well as helping in protection against some diseases. Potassium at the levels applied to wild rice paddies is not an environmental concern. Potassium can be applied in the fall or spring. Usually it is applied with the phosphorus, but, unlike phosphorus, incorporation of K is a necessity. See Table 1 for rates. Application of potassium with the topdress N is possible. This increases the late season uptake of K and might help prevent some diseases.
Table 1. Guidelines for fertilization of wild rice.
|Nutrient||Criteria||Organic Soils||Mineral Soils|
|Amount to apply (lb./acre-N)|
|Soil Test, Bray P-1 (ppm)||Amount to apply (lb./acre-P2O5)|
|Amount to apply (lb./acre-K 2O)|
Water Drainage Before Harvest
Nitrogen in the water can be detected in wild rice fields for 3 to 5 days after fertilization but at drainage time most nutrients in the water have been consumed the plants. However, to avoid erosion of drainage ditches, the water should be released slowly during a one to two week period before harvest. The soil particles from erosion can carry phosphorus, in addition to the any soluble P, into surface waters. Drainage ditches should be stabilized with grasses if possible.