Dr. Dan Talks Agronomy

By Daniel Davidson
DTN Contributing Agronomist

We like to give a crop a fast start, but it is possible for starter or pop-up fertilizer placed directly in the furrow to damage corn seedlings. Too much fertilizer volume, too high of salt index or dry soil conditions can lead to in-furrow fertilizer damage, especially in lighter-textured or sandy soil or soil low in organic matter content.

Placing starter fertilizer directly on the seed or immediately below the seed is when germination or emergence problems due to salt damage typically occur. Fertilizers containing nitrogen, potash or sulfate act as salts in the soil. Phosphate is not a salt and isn’t a concern.

The salt index is a measure of the salt concentration in the soil solution when a fertilizer dissolves. Knowing the salt index of the fertilizer is important so you can prevent potential damage. Low volumes of high-salt fertilizer or high volumes of low-salt fertilizer can burn seedling roots, set back seedling growth or kill seedlings.

So why is salt a risk? Soils create osmotic pressure that pulls water away from seedlings, causing cell desiccation, tissue damage and possible death. Salt burn appears as necrotic-like lesions on seeds and seedling roots. Seedlings die or emergence is slow; the plant can’t compete and yield is lost.

Just how much starter can be safely applied depends on the product, gallonage and where it is placed. The greater the distance from the seed, the more that can be applied safely. Soil is the primary buffer against fertilizer burn. Fertilizer applied directly in the furrow is generally restricted to only 10 pounds per acre of nutrients. Extend the application zone to one inch (1- by 1-inch band) and you can apply up to 20 pounds (10 pounds in sandy soils and 20 pounds per acre in silts and clays).

Fertilizer placed in a 2- by 2-inch band provides a degree of safety by placing a wider soil barrier between the seed and the fertilizer. The rule of thumb for this placement is up to 20 pounds per acre in sandy and 40 pounds per acre in silts and clays.

There are tables available that list the salt content of fertilizers that are commonly used as starters (Table 4; http://www.spectrumanalytic.com/…). However, in starters, I like to stick with the 10-pound limit and then calculate how much is in a gallon of product and use that information to determine how many gallons to apply. You can easily do this if you know the analysis and weight of the product per gallon. For example, applying 5 gallons per acre of 10-34-0 on the seed provides only 5.5 pounds per acre (5 gallons x 11 lbs. per gallon x 10% N).

Any low-analysis fertilizer — such as 3-18-18, 6-24-6 and 10-34-0 — usually makes a good starter.

Avoid high-analysis fertilizers such as UAN 28-0-0 and 32-0-0 or 7-21-7 don’t make good in-furrow starters. Ammonia thiosulfate (ATS), diammonium phosphate (DAP) and urea produce free ammonia that will damage germinating seeds and seedlings.

The amount of starter fertilizer you can apply safely depends on the fertilizer’s salt index value, the distance between the fertilizer and the seed, the soil texture and soil moisture at planting. It is important to remember that no fertilizer application placed close to the seed is without risk.

Understanding the variables involved can help to minimize the risks associated with in-furrow or pop-up placement of starter fertilizer in corn. If in doubt, talk to your fertilizer dealer about the salt content of the starter and the rate they recommend for in-furrow application.

Dan Davidson can be reached at AskDrDan@dtn.com

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