Crops
By Dr. Paul Tracy
It is time to consider your 2008 crop nutrient needs
Last month, I discussed how the ethanol industry has placed increased pressure on crop nutrients. Other factors such as fuel prices, a constricted transportation system, commodity prices and strong global economics are adding to this stress. We are experiencing a situation where plant food values have reached an all-time high. Despite this pressure, plant foods still present one of the best overall returns on investment associated with crop production.
As plant food values increase, we need to be more efficient in how we manage them. Our industry and crop producers must constantly juggle prices, availability and logistics with optimum nutrient use efficiencies and environmental stewardship. One of the most commonly asked questions I receive is: What is the most efficient crop nutrient application timing? Obviously, the answer to that question varies depending upon crops grown, economics, weather conditions, materials used and application capabilities.
Fall/winter fertilization programs can be very efficient. With the early harvest this year, hopefully much of next year’s crop nutrition needs have already been addressed.
I always recommend an aggressive soil testing program to determine accurate plant food needs. After more than 12 years of grid soil sampling, I fully endorse this methodology combined with variable-rate plant food applications. Regardless of soil sampling and fertilization programs used, October through December is an excellent time to get fields sampled, evaluated and fertilized. We all know that waiting until spring to get this done places undue pressure on an already stressed system. My experience is that waiting until spring to soil sample often means waiting until next fall, thus delaying the fertilizer use efficiency benefits of soil testing for another field season.
Late fall/winter is an excellent time to apply non-mobile nutrients or soil amendments like phosphorus, potassium, zinc and aglime. There is little chance for these nutrients to be lost from or tied up within the soil system between now and next field season. When possible, avoid application onto frozen fields. If fields are frozen at night and thaw during the day, then recently applied nutrient loss potential should be low. Flat fields are less susceptible to frozen ground nutrient runoff than are sloped fields.
Non-mobile nutrients don’t move readily through soils. Therefore, once incorporated into the soil via rainfall or tillage, the primary loss pathway is soil erosion. Obviously, avoiding erosion is the best mechanism for avoiding losses of fall applied non-mobile crop nutrients.
Sulfur fertilizers generally come in two basic forms; elemental sulfur and sulfate sulfur. Elemental sulfur is not mobile in soils and it is unavailable to crops. It must be converted to the sulfate (which is soil mobile) before being utilized by plant roots. The conversion process is microbially driven and requires warm soil temperatures combined with adequate soil moisture. Therefore, elemental sulfur will stay in the system until next spring before converting to sulfate, and is the only viable form of sulfur to be applied in the fall/winter.
The toughest nutrient to manage in a fall/winter application program is nitrogen. Like sulfate, most nitrogen fertilizer materials are either leachable or can convert to leachable forms fairly rapidly once they enter the soil environment. Agronomically, I generally recommend waiting until spring to apply most of next year’s nitrogen needs. Obviously, not all decisions are based solely on agronomics. Economics, logistics, soil types and time restraints all dictate the best time to apply nitrogen fertilizer. Based on current conditions, fall applied anhydrous ammonia for next year’s corn crop will be a viable option for many of our growers. The following are some key points to consider for fall anhydrous ammonia application in our region.
Whenever possible, avoid it all together and carefully lay out a program to ensure most of your nitrogen can be supplied effectively next spring with a possible split application between preplant and sidedress timings. Agronomically, this is the still the most efficient corn nitrogen management program.
If you decide to apply nitrogen in the fall for next year’s corn crop, then anhydrous ammonia with the stabilizer N-Serve (nitrapyrin) is the most viable option.
Anhydrous ammonia should be applied after the soil temperature near the injection point (hopefully 6 to 8 inches deep) reaches 45 to 50 degrees for 3 consecutive days.
Unless local conditions dictate, I generally do not recommend fall anhydrous ammonia application south of Interstate 70.
Don’t fret about the small amount of nitrogen in phosphorus fertilizer unless facing multiple-year applications or high soil-building amounts of phosphorus in a single application.
The bottom line for fall fertilization in our region is to use common sense and the best combination of agronomic, economic and environmental management options available. Contact your MFA Certified Crop Adviser with any questions.
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