Pop-up Fertilizer Considerations

Chapter 6 – Fertility, No. 5, Fall 1986

Roger Veseth

Banding fertilizer in the crop root zone near the seed improves fertilizer-use efficiency and may increase yield potential. Although fertilizer placement has been shown to be beneficial in conventional tillage, it is particularly important for successful minimum tillage and no-till farming systems.

In recent years, innovations in equipment design have increased growers’ options for fertilizer placement. Besides banding fertilizer in a separate operation before seeding in minimum tillage, there are now over 14 different commercial minimum tillage and no-till drills that have one-pass fertilizer placement capabilities at seeding. Countless grower-modifications have also been developed for deep-banding fertilizer.

In addition to decisions on deep banding fertilizer, growers must also choose whether to apply starter fertilizer with the seed at planting, and if so, how much and what type. To address these questions, Robert Mahler, University of Idaho soil scientist and STEEP researcher, has been conducting research on management of starter fertilizer which he calls “pop-up” fertilizer. Mahler feels that pop-up fertilizer has not been adequately evaluated as a fertilizer placement option, in addition to or in place of deep banding or broadcasting of fertilizer.

He emphasizes several potential advantages of pop-up fertilizer:

1. Fertilizer is placed where limited root systems can readily utilize the nutrients if seed-zone moisture is adequate.
2. Weeds are suppressed because fertilizer access is reduced for weeds compared to broadcasting.
3. “One operation” fertilizer placement with seeding saves time and energy.
4. Application through the seed opener reduces power requirements and the expense of separate deep-band fertilizer openers or seed opener modifications.
5. Erosion loss of nutrients is less compared to broadcasting.
6. Rapid early plant growth results compared to broadcasting or widely spaced deep fertilizer bands placed some distance from the seed rows.

However, he also points out some potential disadvantages which include:

1. Danger of salt or ammonia injury to seeds or seedlings from high concentrations of nitrogen (N), sulfur (S) and potassium (K) near the seed, especially with low soil water content.
2. Reduced mineralization and crop utilization of nutrients from organic residues of previous crops when compared to broadcast fertilizer.
3. Greater potential for leaching losses of N overwinter with high rates of pop-up fertilizer in the fall compared to spring broadcasting.

Mahler’s research has involved several aspects of popup fertilizer management. These include: field studies on N rate effects on stand and yield of winter wheat; winter wheat varietal responses; and greenhouse studies on the impacts of soil water, temperature and N source.

Winter Wheat Field Study

Mahler evaluated the effects of different pop-up N fertilizer rates on winter wheat seeded in fall 1983. The experiment was on a Palouse silt loam soil at the University of Idaho Plant Science Farm near Moscow in a 22-inch annual precipitation area. Eight pop-up N fertilizer treatments were evaluated on five varieties — Lewjain, Hill 81, Stephens, Daws and Nugaines. The research trial was seeded after spring peas on September 18 with an experimental drill which has l-inch wide hoe-type seed openers.

Each field received a total N application of 120 pounds N/acre as ammonium nitrate. Of the total, 25 pounds N/acre were broadcast and incorporated preplant on all the plots. The remainder was applied as pop-up with the seed at planting and/or broadcast as a topdress in the spring. Pop-up N rates were O, 15, 25, 35, 45, 55, 70 and 90 pounds/acre. The remainder was top dressed in the spring:

120 pounds/acre -25 pounds/acre preplant incorporated and pop-up rate = spring topdress rate

With the exception of late fall seedings, winter wheat is often seeded into relatively dry soil in the fall in northern Idaho and much of the Northwest dryland cropping region. Conversely, spring wheat is usually seeded under much more favorable soil water conditions. Consequently, Mahler points out that there is often a higher potential for seed damage from pop-up fertilizer with winter wheat. When this field study was seeded, the soil water content was approximately 17 percent by weight (matrix soil water potential of about — 3.0 bars) with soil water content at field capacity being 21 percent.

Figs. 1 through 4 show the effects of pop-up fertilizer rate on stand and yield on four of the varieties in the trial. Treatments which do not have any of the same letters are statistically different at the 95 percent probability level. The highest stand counts (no percent stand reduction) usually occurred where no pop-up fertilizer was applied. However, the highest yields (no percent yield reduction) were with low to moderate amounts of pop-up fertilizer, most frequently about 35 pounds/acre N.

Each winter wheat variety reacted somewhat differently to the pop-up fertilizer. When considering stand count as the basis of tolerance to high rates of pop-up N, Mahler ranked the varieties from most to least tolerant as follows: Stephens > Hill 81 > Daws > Nugaines > Lewjain. Tolerance from a yield reduction comparison gave a ranking of Stephens > Hill 81 > Nugaines > Daws > Lewjain.

Mahler combined the stand and yield reduction data into a table showing the percent stand reduction required for a statistically significant yield decline (Table 1). Lewjain was most sensitive with 35 percent. Nugaines was least sensitive with 53 percent. For Stephens, stand reductions were not high enough to significantly affect yield.

Greenhouse Study

In a 1985 greenhouse study, Mahler evaluated the effects of pop-up N fertilizer rate and N source on germination of Stephens winter wheat under various controlled soil water and temperature conditions. Three N fertilizer sources were compared — ammonium nitrate, ammonium sulfate and urea. Palouse silt loam soil from the University of Idaho Plant Science Farm was used for the research so results could be compared with the 1983-t34 winter wheat field study on pop-up fertilizer.

The pop-up fertilizer was placed in direct contact with the seeds in the soil trays. Mahler explains that direct seed fertilizer contact normally only occurs about 33 percent of the time in the field using standard double disk and hoe type openers. He estimates that the rates used in the greenhouse study should be multiplied by three to make the results on germination rate relevant to field conditions. For example, the reduction in germination with 10 pounds N/acre would be equal to about 30 pounds N/acre in the field. Different types of disk and hoe seed openers have different degrees of separation of seed and pop-up fertilizer.

The effect of soil water content, N rate and N source on winter wheat germination at 41°F is shown in Table 2. A similar comparison at 59°F is shown in Table 3. Soil water content and N application rate played major roles in determining winter wheat tolerance to pop-up N fertilizer. In general, the drier the soil, the greater the reduction in germination. The degree of germination reduction was also affected by N source. Ammonium nitrate was the safest, followed by ammonium sulfate and then urea. Urea caused the greatest damage.

As soil temperature increased, germination was further reduced for most N rates, N sources and soil water contents. Mahler explains that higher soil temperatures caused more ammonia (NH₃) evolution which can damage germinating seeds. Soil temperature should have little effect on salt damage, however.

Table 1. Stand reduction required for a statistical yield decline in winter wheat varieties grown on a Palouse silt loam soil near Moscow, ID.

Table 2. The effect of soil water potential, N rate and N source on germination of Stephens winter wheat when Incubated at 41oF.

Mahler concludes that growers can use soil water content and soil temperature as management guides in helping to determine safe rates of pop-up N fertilizer. Seed fertilizer separation with the different seed openers must be considered as well. Liquid pop-up fertilizers also have less germination damage potential than dry fertilizers. Mahler estimates that about twice as much N pop-up liquid fertilizer could be used with seed compared to dry fertilizer.

Table 3. The effect of coil water potential, N rate and N source on germination of Stephens winter wheat when incubated at 590F.

Under favorable soil moisture and temperature conditions, pop-up fertilizer provides an additional fertilizer placement option. More research is needed to develop guidelines for pop-up fertilizer under other soil types, wheat varieties and growing conditions. Research is also underway on slow-release N fertilizer sources (such as sulfur-coated urea) where all of the crop N requirement could be supplied as a “with the seed application”.