Nov. 15, 2022 10:50 AM

Crop nutrition experts offer insights for soil testing and 2023 planning

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Corn growers who harvested their crop for silage rather than grain in 2022 have unique challenges to consider as they develop nutrient plans for 2023 and subsequent crops. Harvesting corn as silage removes the entire plant from the field and with it the crop material that would normally contribute nutrients to the soil profile after decomposition. Drought conditions can skew soil test results, offering a different set of variables that must be considered in developing a sound crop nutrition plan.

Crop nutrition experts at Mosaic have insights to offer growers so they can navigate fertility planning following silage harvest with efficiency and ease. The most common nutrient additions to soil in the fall include phosphorus, potassium and lime, but fall nitrogen applications are also common and may be needed depending on the crop rotation.

Soil nutrients after silage harvest

After corn is harvested for grain, the remaining plant residue is left to decompose and return important nutrients into the soil. Since the early 2000s, more producers have been harvesting their corn as silage each year. Regardless of whether it is being done because of drought, prices or feed requirements, soil nutrients are removed from the field and must be replenished.

“A corn crop harvested as silage rather than grain removes approximately twice as much nitrogen, 40% more phosphorus, over four times more potassium and twice as much sulfur from the soil each year," explains Keith Byerly, agronomist near Bloomfield, Neb., and commercial sustainability lead for The Mosaic Company.

“When corn silage is harvested from the same fields for consecutive years, this nutrient depletion adds up, and additional fertilizer applications will be necessary. Harvesting corn for silage due to drought also removes nutrients that would normally be cycled for the following years' crops. It is important to soil test and take this into consideration for 2023 and 2024 crop years. It is also key to remember that baling dry residue after harvest can remove almost as many nutrients as silage in some cases,” Byerly says.

Dry conditions affect soil nutrient test results

In October 2022, over 62% of the contiguous United States was experiencing drought conditions. While the drought affected yields for producers it continues to create challenges by skewing fall soil sample results. Mosaic experts recommend producers work closely with their local ag retail or co-op agronomist to evaluate soil nutrient results, adjust nutrient plans accordingly and apply fertilizer most efficiently.

One result of a soil test taken during a dry period may be a pH reading which is lower than the soil’s true pH. Drought conditions make soil seem more acidic due to increased amounts of soluble salts that have not cycled out of the soil with rainfall.

“Producers might overcompensate by adding lime to increase soil pH, but they should be cautious and wait for normal rainfall to resume to determine accurate soil acidity and most effectively use their fertilizer budget,” says Byerly.

When calculating the amount of fertilizer required to maintain critical levels of NPK, it is also important to remember that during drought plants take up less nutrients from the soil. Microorganisms and nutrient compounds rely on water as the medium to move into plants, so nutrient solutes will often remain in the soil during a drought. Nitrates are prone to remain in the soil and this should be accounted for when determining N fertilization rates for 2023. Additionally, if dry conditions and below-normal rainfall persist year after year the needed fertilization rate for N decreases.

“Ideally, soil sampling should be delayed until meaningful rainfall occurs, when possible. This will increase the probability of obtaining reliable and accurate test results. When rainfall is not likely prior to fall nutrient application, growers should turn to their local retailer, certified crop advisor or certified professional agronomist for their expertise in soil fertility testing, interpretation of the results and fall application tips,” Byerly stresses.

Fall fertilizer application best practices

Regardless of soil moisture, producers should follow best practices to responsibly apply fertilizer in the fall and maximize its effectiveness. Using cover crops helps prevent erosion, helps increase organic matter, preserves soil moisture and discourages weed growth.

Anhydrous ammonia can be effectively applied in dry soils but remember to wait until temperatures are consistently below 50 degrees. When dry soils are cloddy and do not seal properly, the ammonia can be lost during application or seep through large pores between clods after application. Therefore, proper depth of injection and good soil coverage are a must for application into dry soils. If soil is dry and in good physical condition, it holds more ammonia than soil that is moist.

“Cover crops or light tilling are excellent ways for growers to lock in nutrients for the upcoming season after applying dry fertilizers in the fall, especially after a silage harvest that depleted NPK levels more than a grain harvest,” emphasizes Byerly.

Growers should also take manure applications, frequency of nutrient applications and soil type into account when determining fall nutrient decisions and use fertilizer as efficiently as possible.

For more information on fertilizer application after corn silage harvest visit