NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665
March 30, 2000
Plants make use of phosphorous (P) in several ways, but a key way is for transferring energy. In other words, P does for a plant what a drive shaft does for an automobile.
Most North Dakota soils were originally high in P under native prairie, but that changed with tillage, which loosened topsoil and enabled a great deal of soil to blow away or wash away. Due to that erosion, along with the removal of P through the harvesting of hay and grain, North Dakota soils became relatively depleted in P, explains a soil scientist at North Dakota State University.
Now P is one of the most-used fertilizers in North Dakota, and it is restored to the soil through the addition of P-containing fertilizers and manure. And as with other management decisions that producers make, timing the application of P and determining how much to apply are driven by concerns about efficiency and overall economics.
"In small grains and other grassy crops, an efficient and effective way to apply P is at seeding, either with the seed or slightly away from the seed," says Dave Franzen, extension soil specialist at NDSU. "This banded application concentrates the P fertilizer and makes it harder for soil minerals to react with the P and transform it into materials that are less available to plants. It also places the P near emerging and growing roots, so that P uptake by the plants is at a maximum early in growth. In many cases, recommended rates of P fertilizer are reduced because of placement and concentration."
Soil Test Categories and Rates of Return
Soil test levels of P are categorized into Very Low, Low, Medium, High and Very High. Franzen says an inverse relationship exists between those categories and the rates of return producers will realize from their investment in P fertilizer. For example, the chances are excellent that a producer will realize a profitable return on P application to soils testing Very Low and Low, especially for crops such as wheat and corn. Likewise, producers will realize high rates of returns to applications on Medium testing soils.
"There is a low rate of return on High and Very High testing soils, especially with broadcast P applications," Franzen says. "Most P applications on soils testing High are designed for soil-test maintenance and have little effect on yield or quality of crops in that year."
Applications of P should not be made to soils testing Very High because the chance of economic return is nearly zero, Franzen says. But there is often an economic return for banded applications of P on crops such as wheat and corn when soil test levels are High due to reduced uptake in cool spring temperatures.
"Broadleaf crops behave differently with respect to P placement, and soybean is a good example," Franzen says.
In research studies, soybeans consistently performed better when P was broadcast, compared to banded application, even when the bands were separated from the seed. Some soybean-fertility researchers have speculated that a soybean plant performs better when its roots are bathed in nutrients rather than having the fertilizers concentrated. Franzen says another possibility is that the nutrients, when concentrated in a band, are too salty for soybean roots to take up efficiently.
Many broadleaf crops, including soybeans as well as dry beans and sunflowers, are very good scavengers of P, and recommendations suggest either no P or minimal P rates when soil test levels are Medium or higher. Franzen says there are exceptions to that recommendation for potatoes and sugarbeets, which have greater P requirements due to the importance of tuber and root development.
Placement Restrictions with the Seed
The amount of P to place with the seed is limited because of the salt content of the fertilizer, the free ammonia produced by the fertilizer, the sensitivity of the crop, the moisture content of the soil and the soil's texture (sand, silt and clay content). Franzen says research conducted at NDSU has established the amounts of P that can be placed with the seed before significant stand loss and yield decreases occur. This information is available through county offices of the NDSU Extension Service or via the Internet at the Web site for NDSU Extension Service crop production publications (http://www.ag.ndsu.nodak.edu/cropprod.htm).
"When it comes to placement of fertilizer with the seed, it's important for producers to remember that the question should not be, `How much fertilizer can I put with the seed and not get yield losses?'" Franzen says. "Instead, producers should be asking themselves, `How much fertilizer can I put with the seed and still get a yield increase?'"
The answers to those two questions are different numbers, with the second answer being the lower number. Franzen says growers would be wise to approach seed-placed fertilizer conservatively.
As producers upgrade equipment, they should invest in boots that separate seed and fertilizer. Seed and fertilizer separation results in the same convenience of seed-placed fertilizer but without risk of injury, Franzen says. In addition, seed and fertilizer separation provides growers with much more flexibility; for example, they can apply materials such as ammonium sulfate to canola in a single trip.
When Broadcast is Better
Franzen says broadleaf crops tend to be more sensitive to seed-placed fertilizer than small grains. Although it is possible to apply some fertilizer with certain broadleaf crops, the rates are quite restrictive, and broadcasting fertilizers often results in greater yield.
"So why bother with seed-placed fertilizer?" Franzen asks.
A strategy producers in the Corn Belt take advantage of is fertilizing their small grain or corn crop more heavily with P and letting the broadleaf crop feed on what is left. Franzen says this strategy should work equally well for many North Dakota farms.
When soil test levels are Very Low or Low, some broadcast P should generate a favorable economic return in broadleaf crops, but in soils testing Medium or higher, there is generally little chance of achieving an economic return, Franzen says. Unlike wheat, corn and other grasses, most broadleaf crops receive little starter boost from a banded application of P.
When Times are Tough
When economic conditions are tough and loans are difficult to obtain, cutting P out of the program when soil test levels are Very Low or Low is a mistake, Franzen says. For that reason, he encourages lenders to view soil testing as an important economic planning tool and to understand that a producer's crop-consultant budget is an investment in profitability. If a farmer's operation is stretched thin financially, the full recommended rate of P need not be applied, Franzen says, but some P should always be applied to soils testing Very Low or Low; otherwise, achieving profitable yields is a long shot.
"Growers cannot afford not to apply P when they are in economic trouble," Franzen concludes. "Lenders should realize its value and encourage its application."
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Source: Dave Franzen (701) 231-8884
Editor: Dean Hulse (701) 231-6136