ST. CLOUD, Minn. – With any input applied to a field, yield is always the goal, but yield is not the only goal. Growers place as much focus on optimizing inputs as they do yield. At this year’s Conservation Tillage Conference, focus was put on optimizing those inputs, particularly phosphorus and potassium applications.
“Both potassium and phosphorus share some commonalities in how they move in the soil, and when I say they move in the soil, I really mean they don't move that much in the soil,” said Lindsay Pease, assistant professor with the University of Minnesota Northwest Research and Outreach Center, during her presentation at the 2019 Conservation Tillage Conference.
Potassium carries a positive charge and will bind very tightly to the negatively charged surface of clay minerals. The potassium gets sandwiched between the clay particles and it is very slow to release back into the soil.
“If you don't have naturally high levels of potassium in the soil, you're going to want to add a little bit each year to increase the amount that's in the available pool for the plants to uptake,” Pease said.
Phosphorus on the other hand, while it carries a negative charge and cannot bind to the clay particles like potassium, still does not move readily in the soil. It will bind to other positive minerals in the soil, calcium being the most common.
When making a fertilizer application, growers should consider the four Rs of nutrient stewardship: right source, right rate, right time and right placement.
“You can be using manure, you can be using commercial fertilizer, you can be using different sources of commercial fertilizer, but you really want to be thinking about what else is in that fertilizer,” she said.
Some fertilizers will contain nitrogen or salts, both of which could be damaging to a seed and affect emergence if not used properly.
The second question with phosphorus and potassium application is rate.
Research Pease conducted in Ohio compared manure to commercial fertilizer and the amount of phosphorus runoff in the tile lines and surface water. The goal was to determine if manure was more subject to leaching.
“When you're applying manure and commercial fertilizer at the same rate, you have the same environmental losses,” she said.
The difference is that traditionally manured fields tend to have received more phosphorus application over time and have more phosphorus built up in the soil.
Phosphorus applied to the soil tends to stay in the soil, until there are no more binding sites or places for that phosphorus to go. Once the soil has reached its phosphorus or potassium capacity, it does not hold anymore, and additional applications are subject to leaching.
Long-term research done by the Northwest Research and Outreach Center has shown that soils in the medium or high category for phosphorus soil test levels do not benefit from additional phosphorus applications.
“We stopped seeing a yield benefit to applying additional phosphorus,” Pease said. “Once you get to that point, just applying that little bit of starter phosphorus with the seed is going to be enough.”
For those fields that do require applications of phosphorus and potassium, the next question is where to place it. The goal is to optimize that fertilizer and it is important to keep in mind, both phosphorus and potassium do not readily move in the soil.
“Whether it was the University of Minnesota, Manitoba, NDSU, pick your favorite Extension service, everybody found that banding tends to be more efficient than broadcasting when it comes to phosphorus and potassium,” she said. “The reason for that goes back to the chemistry – these two elements don't like to move in the soil.”
When placing phosphorus and potassium on the soil surface, it tends to stay on the soil surface.
Farmers during Pease’s presentation that have been in a no-till and cover crop management system for several years did comment that they have excellent results with broadcast on the surface. This could be due to increased microbial activity in the soil and improved infiltration.
Not only do the organisms provide channels in the soil for the nutrients to fall into, they also cycle and move those nutrients through the soil profile.
In most situations, surface applied phosphorus and potassium, without incorporation, is riskier.
The last part of fertilizer application is timing. For the last few seasons, timing has been a real issue in the Midwest. Fall versus spring, both have been wet and the windows for application have been short.
“This really is more of a question about what time do you have as a producer,” she said. “Once it comes to that nutrient application question, you guys are going to do what you have time to do.”
Research has indicated that spring applications can be more efficient, but there is not always enough time in the spring to make that work. Timing of application is going to focus more on what is optimum for a given producer’s operation.