The basics of elementary education typically center around the three Rs: reading, writing and ’rithmetic. In the field, farmers need to focus on the four Rs of fertility management: the right product, right rate, right time and right place.
"Choosing the right product and then applying it at the right time, at the right rate and in the right place makes you money by using fertilizer more efficiently," says Farm Journal Field Agronomist Ken Ferrie. "It also makes you a better environmental steward by preventing unused nutrients from escaping and getting into water sources."
The four Rs, developed by the fertilizer industry, are so important that Farm Journal is launching a new series called Nutrient Navigator to help put the concept into motion on your own farm.
The key to the four Rs of fertility is that all of the components are applied together. "If just one—the product, the rate, the timing or the placement—is off, you miss out on the benefits of the whole process," Ferrie says.
To set the stage for fertilizer success, let’s look at the role of rate, timing, product and placement.
Rate and timing. Rate and timing are related. To understand rates, you must understand when each crop, as well as soil microorganisms, needs nutrients. For example, both corn plants and
microorganisms need nitrogen early in the growing season.
"When we talk rates, we’re talking about the rate for a certain window of time," Ferrie says. "Your total nitrogen application for a corn crop might be 200 lb. per acre. However, you might
need 50 lb. of that nitrogen applied in late fall or early spring to pay the carbon penalty."
The carbon penalty results from microorganisms decomposing old crop residue. The penalty is greater when residue is incorporated with tillage.
"The corn needs nitrogen for early growth and plant health—a crucial time when yield is being set," Ferrie says. "Microorganisms need nitrogen to promote the breakdown of old crop residue. This triggers the mineralization of nutrients and makes them available to the growing crop.
"You might need to apply 10 lb. of your total nitrogen application as starter fertilizer to make sure the roots of your young corn plants can reach it," Ferrie continues. "You might also need to put some phosphorus in your starter to make sure it is available to the young plants early in the season, when cold, wet conditions might cause phosphorus in the soil to be tied up."
Learn when each crop needs each nutrient. "For example, phosphate, sulfur and zinc need to be taken up by plants early in their lives," Ferrie says. "Those nutrients are not mobile, and they must be present in the plant to satisfy its vegetative growth needs early in the season."
Product. There are various products on the market to meet the demands of the plant. "With phosphorus, potassium or micronutrients, you need a product that becomes available in time to meet the plant’s needs," Ferrie says. "With phosphorus, for example, some forms are water soluble, and some are citrus soluble. The water soluble forms become available faster.
"Phosphate applied as monoammonium phosphate [MAP] or diammonium phosphate [DAP] forms must be mineralized and converted to orthophosphate," he explains. "Orthophosphate becomes available faster than diammonium phosphate; and diammonium phosphate becomes available faster than triple superphosphate [monocalcium phosphate]."
With lime, depending on where you farm, you might have a choice of powdered, pelleted or coarse material. "Coarse limestone mineralizes slowly, so it could miss the window of need for your plants," Ferrie says. "In some situations, pelleted lime could become available too quickly."
Plants use sulfur to form proteins and convert nitrogen to a usable form, so the nutrient must be taken up early. But elemental sulfur is slow to be mineralized by microbes into sulfate, the form plants can use.
"You must apply elemental sulfur in the fall," Ferrie says. "If you apply it in the spring, you need a sulfate source, such as ammonium sulfate or thiosulfate, that microbes can oxidize rapidly."
Your choice of products might be limited by availability. "In central Illinois, there are situations where I would love to apply ammonium nitrate or potassium nitrate," Ferrie says, "but they aren’t widely available in this area.
"Occasionally, I find a situation where rainfall has flushed nitrate out of the soil and it needs calcium. Calcium nitrate makes corn green up quickly. It seems to only be available in areas where tomatoes are grown, though."
Potassium nitrate is a form of nitrogen that is handy on soils that are low in potassium. Ferrie’s area is blessed with lime quarries, so limestone is readily available. Elsewhere, farmers might have to lime with byproducts. "We can deal with availability issues by modifying our timing and
placement," he says.
Place. Placement can help you build efficiencies into your fertilizer program and ensure that availability matches timing. "Banding any nutrient, whether in strip-till or by sidedressing, is more efficient than broadcasting, and it makes the nutrient available for a longer period of time," Ferrie says.
"With phosphorus, banding fertilizer next to the plant helps roots take up the nutrient in cold soil. Placing nitrogen in a band next to young corn plants also helps them avoid problems with the carbon penalty and low phosphorus availability."
Placement can be critical with one form of early season nitrogen. "If you apply anhydrous ammonia in the spring in strip-till and plant too soon after application, the ammonia could kill the seed," Ferrie says. "Typically, you need at least 2" of rain after a spring ammonia application to avoid burning the roots and killing the seed.
"If your tractor is equipped with GPS and auto-steer, it’s possible to plant the same day you apply ammonia if you can apply it between the rows, rather than under the seed."
In future Nutrient Navigator stories, we’ll study the major and minor nutrients to learn more about how rate, timing and placement work together.
The Roles of Nutrients
When planning the timing of your fertilizer applications, it might help you to visualize the purpose of each nutrient.
"As all corn growers know, nitrogen fertilizer has an immediate effect," explains Farm Journal Field Agronomist Ken Ferrie. "You apply nitrogen to supply the growing crop and sometimes to feed microorganisms that tie up soil nitrogen."
With phosphorus, you make periodic applications based on soil tests and crop removal to maintain the level in the soil and ensure it’s available to plants. However, soil phosphorus can become unavailable during cold, wet spring weather, so you might need to apply some phosphorus as starter fertilizer.
Soil pH and calcium are used to maintain long-term balance for the microbial system and soil health. "Rates are influenced by how heavy the soil is and what type of tillage you do," Ferrie notes.