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CropTalk: The Nutrient Cycling Puzzle

January 2022

Published on Saturday, January 1, 2022

Like many farmers, you may be wondering if you can “rob” the soil nutrient bank in 2022 to avoid paying record, or near-record, fertilizer prices. While this is usually a poor strategy, there may be some fields that can forgo a year of applied fertility if the soil nutrient bank is full and cycling properly.

 

MICROBES THAT MATTER

There are four main groups of microbes that assist and drive nutrient cycling in our soils: bacteria, fungi, algae, and protozoa. In many cases, nutrient cycling results from these microbes simply consuming and excreting one another. Nutrients are also released as these microbe groups consume residue from previous crops and then help release them in a plant-available form.

 

SHOW ME THE MONEY

There are three primary reserves of fertility that exist in every field: organic matter (OM), residues, and parent material. The question becomes, how do we put a value to these nutrients. A study by The Ohio State University found that every 1% of OM equates to a value of roughly $700/A. in nitrogen, phosphorous, potassium, sulfur, and carbon. But what about all the residue from last year’s crop? Figure 1. shows approximately how many pounds of nutrients are left behind after a 250 Bu./A. corn crop is harvested. Think about how much it would cost to apply these nutrients at current fertilizer prices. What are some factors that can enhance the soils’ ability to cycle these nutrients?  

 

 

SOIL TEMPERATURE

On average, the microbial population in the soil will double with every 10-degree Fahrenheit increase in temperature. There is a limit, though. Once the soil reaches 86°F, the microbe populations will begin to decline. Interestingly, most agricultural soil microbes prefer the same temperature range as a corn plant, between 78 to 86°F. In many cases, we tend to think of warm soils in spring as the most important for mineralization, but cool soils in the middle of summer are just as vital towards efficient nutrient cycling for the current crop.

 

SOIL STRUCTURE

Providing the microbes with a good home is the first step to ensuring they do their job. Ideally, the soil is chemically composed of 25% air, 25% water, 45% parent material, and 5% organic matter. Soil compaction and poor drainage lead to the air and water percentages becoming out of balance, leading to the destruction of the microbe’s ideal home.

 

CARBON: NITROGEN “C:N” RATIO

More than 20 years of Practical Farm Research (PFR)® have taught us that cornafter- corn rotations require approximately 30 additional pounds of nitrogen to achieve the best return on investment. Why is this? It’s all about the carbon (C) to nitrogen (N) ratio. Corn residue has a C:N ratio of roughly 57:1. Soil microbes need the C:N ratio to be roughly 24:1 to begin breaking down the residue. By applying additional nitrogen in a corn-after-corn rotation, we are simply lowering the C:N ratio so that the microbes can do their job of cycling the residue. This will then begin the process of unlocking nutrients contained in last year’s crop residue.

 

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Author: Ben Puestow

Categories: CropTalk, 2022

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