When it comes to the 4Rs of crop nutrition (right source, right rate, right time, right place), many farmers focus most of their attention on right rate. However, as Dr. T. Scott Murrell explains, the 4Rs should be thought of as a system that intertwines and works together to create a well-managed crop nutrition program. The R that coordinates them all is right time.
"It's hard to compartmentalize all of the 4Rs," Dr. Murrell says. "To be performed properly, nutrient management must include more than just rate. Timing, placement and source are just as important. Right time, in particular, is important, because it is the best way to synchronize the application of nutrients for the time that the crop needs them."
Murrell is a director at the International Plant Nutrition Institute (IPNI) and has spent much of his 20-year career studying corn-and-soybean systems in the Midwest.
Serving as a synchronizer, optimized timing helps maximize fertilizer effectiveness, improves nutrient use efficiency, and lessens nutrient losses to the environment.
"Fall applications are appropriate for phosphorus, potassium, zinc and elemental sulfur," says Murrell. "Elemental sulfur requires time to convert to sulfate sulfur - a couple of months or more, according to some university guidelines. Phosphorus, potassium and zinc form chemical bonds with minerals in the soil. As a result, their movement becomes limited in the soil to just a few inches. In the spring, they will still basically be where they were placed the previous fall."
Phosphorus (P), potassium (K), zinc (Zn) and elemental sulfur (S0) applied in the fall should be broadcast on the soil surface and mixed into the soil with tillage or banded below the soil surface. This helps avoid potential surface runoff during more intense or longer-duration rainfall that could take place in the days following application.
According to Murrell, fall typically has a greater number of suitable days for fieldwork than spring. Days good for this are measured as "days suitable for fieldwork," a metric that is defined and tracked by the USDA National Agricultural Statistics Service. A suitable day is defined as one during which "weather and field conditions allow producers to work in the field most of the day." Allocating some of the nutrient applications to the fall helps ease the workload in the spring, when fewer suitable days are available.
On sandy, coarse-textured soils, spring applications are best. These soils have fewer minerals present to hold these nutrients in place over the winter. Spring applications are also best for K on organic soils.
"Spring or in-season applications are best for nitrogen and sulfate sulfur," Murrell points out. "Nitrogen fertilizers, if they don't already contain nitrate, will eventually form nitrate, which is mobile in soils and moves with water in soil. Sulfate sulfur moves in soils the same way. Spring applications increase the effectiveness of these nutrients, reducing the chances for unwanted losses to the environment. This results in a healthier crop, higher yield and, ultimately, a greater return on investment."
Applying a portion of the nitrogen preplant and another portion side-dress is known as a split application. A split application is commonly used as a risk management strategy.
"The time between preplant and side-dress applications can also be used to assess the growing season," states Murrell. "Once a preplant nitrogen application is made, farmers can monitor the crop and assess its nutritional status. If more nitrogen is needed, more can be applied as a side-dress application. There are a growing number of tools that help farmers make this assessment.
"Although lower rates must be used, a very efficient way to apply all of these nutrients is at planting," Murrell adds. "Applying them in a band near the seed puts them in the right position to be accessed by young root systems early in the season."
Unlike P, K and Zn, the right time to apply S is always the springtime, Murrell says. This is because, like nitrogen, sulfur is mobile and acts as a multi-nutrient in the soil. When applying S in the spring, farmers should utilize a starter application - applying nutrients at the same time as seeding. It is particularly effective to apply S in early springtime for certain soil profiles.
Some soils contain sulfate sulfur (SO42-) lower in the soil profile than a young root system can reach. Because of this, early-season S deficiencies can occur, but starter applications of SO42- can prevent them. Later in the season, roots grow farther down into the soil, where they can finally reach and absorb the S in the soil.
"Early-season nutritional deficiencies can have repercussions later on," notes Murrell. "If you don't fertilize and the soil is deficient, nutritional problems will lower the yield and the quality of the crop. That's why it's important to ensure that plants are getting all they need - starting early in the season and continuing throughout the entire period of plant nutrient uptake."
Sulfur deficiency can be particularly problematic for oil crops, such as soybeans and canola, which rely upon S for oil development in the seeds.
"Soybeans and canola can be more sensitive to sulfur deficiencies in the soil than other crops," Murrell says. "When those crops have sulfur deficiencies, they don't have as high of an oil content, so a sulfur deficiency can affect their yields substantially."
Although decisions about nutrient timing are dependent on nutrient availability in the soil and many other management factors, according to Murrell, farmers have a greater opportunity than ever before to understand the optimum fertilizer timing for their crops. One way this can be done is by participating in on-farm research.
"It's easier than ever before to participate in on-farm research, thanks to farmer networks that provide support and precision agriculture tools, which have reduced the time and labor required to put out trials and collect data," Murrell says. "If farmers want to know if nutrient timing can be improved under their specific set of conditions, I highly recommend they put out a trial and find out for themselves."