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Soils - Part 6: Phosphorus and Potassium in the Soil

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Application Methods

There is little producers can do to change the basic soil and climatic characteristics that affect crop response to applied fertilizer. However, one can control phosphorus availability by managing the soil pH (acid soils), increasing organic matter, and by proper placement of phosphorus fertilizer. Research has shown that band application of phosphorus is much more efficient than broadcasting. Wheat studies in Nebraska have shown that profits from application with the seed are double those of broadcasting. This is because each pound of applied phosphorus with the seed increased yield much more than a pound broadcast. There is little economic justification for broadcasting phosphorus on wheat in Nebraska.

Another banding method (dual placement) applies liquid phosphorus (10-34-0) at the same time as anhydrous ammonia with a separate tube delivery for each fertilizer. Dual placement has been found to be equal to seed application on wheat and equal to or better than row application for corn and soybeans. While band applications of phosphorus require special application equipment and require extra time at planting, these methods are generally economically superior to broadcast phosphorus. The primary exception being broadcast phosphorus applied to growing alfalfa, grass, or in no-till farming systems. When residues remain on the soil surface, research studies indicate broadcasting phosphorus can be nearly as effective as dual placement. This is attributed to increased root activity in the residue-soil interface where soil moisture and mineralizing nutrients from the residues stimulates root development. This is believed to give a broadcast application the advantages of a band application. This is sometimes referred to as a “horizontal band.” The horizontal band, which is unincorporated, has limited soil-fertilizer contact and is in a position of increased root activity.

Seed placement is another method of banding that can be very effective. The problem with seed application is that starter fertilizer contains salts from the nitrogen and potassium sources; and, when applied in excessive amounts, reduces seed germination. Phosphorus fertilizer without nitrogen has little effect on germination, but mixed fertilizers containing potassium, sulfur, and nitrogen are very damaging, unless water moves the fertilizer from the seed. A major factor affecting salt concentration in the seed row is row spacing. Since wheat is planted in 7- to 12-inch rows, the concentration of 18-46-0 fertilizer is only one-third of the concentration in a 30- or 36-inch corn row. Phosphorus fertilizers, even with nitrogen, can be safely used on wheat at normal phosphorus application rates. For row crops, such as corn, sorghum and soybeans, rates must be limited, because germination will be decreased about one percent for each pound of salt applied (pounds of nitrogen + potassium + sulfur) for corn. Soybeans are more susceptible to germination damage, and so any fertilizer should be kept from contacting the soybean seed.

Row application to the side and below the seed is favored over seed application for row crops, even though this method requires more expensive application equipment than seed applications. This method is also referred to as a “starter” method for row crops and is more effective than broadcast incorporation methods on soils low in available phosphorus. It is, however, important to remember that increased early growth from starter fertilizer application does not always indicate increased yields at harvest.

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