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N-P-K: Essentials for Farm Fall Nutrition

When fall begins to approach and harvest is near completion, farmers should evaluate field fertility conditions. Utilizing down time to collect soil samples, provides data needed to guide management decisions for fertilizer demands. High soil and nutrient variability in soils demands the ability to accurately measure this variability. Phosphorus (P) and potassium (K) applications can be applied during the fall. Applications of lime and nitrogen (N), in certain regions, are also routinely made. Tillage, in some areas, provides proper placement of nutrients below soil levels, while preparing the ground for the next planting season. Preparation of fall fertility programs can help spread out fertility work and streamline farmers spring planting operations.

Potassium and phosphorus are two major nutrients often sampled and tested for demand needs. Potassium increases photosynthesis, water-use efficiency, fruit formation, and increases disease resistance in plants. It’s a limiting nutrient in soils due to its low mobility. Availability of K for a plant’s uptake may depend on its location in the soil profile. Potassium applied this year, may not necessarily be available for a crop next year. Tillage incorporates K in the soil and prevents it from being stratified near the surface where plants’ roots may not reach. An 80-bushel soybean crop can remove over 100 lbs. of K and a 200-bushel corn crop can remove over 50 lbs. of K from the soil per acre.

Phosphorus aids in photosynthesis, promotes early growth, vigor, root formation, and respiration in crops. Of the three major macronutrients, (N-P-K), phosphorus is highly immobile and often moves less than an eighth of an inch in the soil as granules breakdown. A Bray and Olsen soil test can determine aluminum fluoride, Ca and Al in soils that tie up and decrease phosphorus availability. Also, fall applications of P are recommended because drier fall weather provides a lower risk for P runoff. An 80-bushel soybean crop can remove over 60 lbs. of P and a 200-bushel corn crop can remove over 70 lbs. of P from the soil per acre.  

Fall nitrogen applications, although most effective in the spring, are sometimes applied in drier regions. Regions in the Northern U.S., with low moisture and consistent cooler winter temperatures, often benefit from fall applied N. Low moisture and cooler soil temperatures allow for microbial life to slow down and decreases the breakdown of N applied in the Fall. Fall applied N is often not recommended in the Southern U.S., due to the variation of soil temperatures. Nitrogen is essential for the formation of amino acids, the building blocks of protein; and needs to be confined in the cation form of NH4+ ammonium, which bonds to negatively charged organic and clay particles. Utilizing Innvictis products such as Preserve N™ will aid in stabilizing applied nitrogen in the soil and limit nitrogen loss from rain. Denitrification can also occur, which is the bacterial conversion of nitrogen into a gaseous form. A nitrogen stabilizer like N-Veil®, treats nitrogen and inhibits urease from breaking down the urea molecule on the soil surface and releasing ammonia gas into the atmosphere. Protecting nitrogen applied to the soil is critical to protecting the farmer’s investment and improving return on investment.

A neutral soil pH of 6.0-7.0 is important for P & K, as well as other nutrients, to be more available to plants. Applications of lime are often applied in the fall, a month before fertilizers, to prevent interference with nutrients. Lime takes time to break down and neutralize soil acidity, so applying in the fall allows ample time before the next planting season. Typically, every ton of lime applied per acre, will increase the soil pH by 0.3-0.5.

Optimum yields are directly influenced by the quantities of nutrients present in the soil. Yield potential can drastically decrease due to low nutrients in the soil. A proactive soil sampling approach will help guide management decisions for fall fertility programs to help increase each acre of crop production.  

Bryan Olivi

Bryan has worked in the agricultural industry for the past 10 years and has published research. Bryan currently works as a Proprietary Product Agronomist for Pinnacle Agriculture. In this role, Bryan conducts research trials of experimental and commercially released products for Innvictis Crop Care and Mission Seed Solutions to evaluate agronomic characteristics and ensure product performance in the field.