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Seed-placed fertilizer safety

How much seed-placed fertilizer is safe? Pat Beaujot says 
any seed-placed fertilizer is risky business

The cold, late spring this year brought many farmer questions regarding seed-placed fertilizer and how much starter fertilizer including phosphate (P) could be safely seed-placed. My answer is that with today’s seeding equipment and optimally placed side-band fertilizer, any amount of seed-placed fertilizer is too much! Why risk fertilizer toxicity and jeopardize germination and emergence when you don’t have to?

I think the right question to ask is whether we need to risk seedling germination with seed-placed starter fertilizer at all, or can we put all the starter fertilizer in the side band close to the seed and still get the “pop-up effect.” Putting all the fertilizer in the side band simplifies seeding logistics, speeds seeding and can eliminate the risk of seedling toxicity.

Is safe really safe?

In canola, the seed-placed challenge is especially difficult as it uses high rates of P but can only tolerate low levels of seed-placed fertilizer. A 35-bushel per acre canola crop takes up 46 to 57 pounds of phosphate (P205) per acre. However, safe-rate guidelines published by Saskatchewan Agriculture caution that only 25 pounds actual P205 per acre (divide by 0.51 to get pounds of 12-51-0 per acre) can be safely applied in the seed row with a knife opener with a one-inch spread on nine-inch row-spacing. These recommendations are for good to excellent soil moisture and do not include other fertilizers like potassium or sulfur.

In order to get the crop out of the ground fast with starter phosphorus and still satisfy the crop’s nutritional requirements, the choice is to either cut back on phosphorus fertilizer to meet safe seed-placed guidelines, which will have negative implications on long term fertility, or put some or all of the starter fertilizer in a sideband. From the research I have seen, and what I have seen in the field, I believe that all of the starter fertilizer can be put in a sideband if the sideband is optimally positioned.

First, you need to look at the risk of seed-placed phosphorus on canola germination and emergence. In the Canola Council of Canada’s Growers Manual, research from Agriculture and Agri-Food Canada at Beaverlodge, Alta. showed that even small rates of 10 to 20 pounds of P205 reduced days to emergence and per acent emergence. Just 10 pounds of seed-placed P205per acre increased days to emergence by five days, and 20 pounds increased days to emergence by 10 days. In moist soils, germination percentage dropped from 100 per cent with no seed-placed fertilizer to slightly over 80 per cent with 10 pounds and to 60 per cent with 20 pounds P205. But with dry soils, germination dropped to 40 per cent at the 20-pound P205 rate.

In the field

I have seen the same results in the field. One farmer trial at Bradwell, Saskatchewan used a starter blend for canola of 16-19-0-19 at a rate of 106 pounds per acre. The field was split with the blend either seed-placed or side-banded three-quarters of an inch below and 1.5 inches to the side of the seed. The field had received 100 pounds nitrogen as anhydrous ammonia the previous fall. In this trial, no differences were observed between the two treatments on overall yield, showing that starter fertilizer doesn’t necessarily have to be with the seed.

However, the same farmer used the same fertilizer blend in a seed-placement on a different field with rolling topography. On this field, the seed-placed fertilizer caused germination and seedling damage on the mid- to upper-slopes where the organic matter was lower and the soil drier.

Another trial that we looked at compared seed-placed and side-banded P in a barley crop. Barley is more tolerant of seed-row fertilizer, and safe guidelines are set at 50 pounds actual P2O5 per acre. On this field at Aylsebury, Saskatchewan, the barley crop was seeded on a sandy loam soil and both treatments received 63 lbs of N and 35 lbs of phosphorus. In this trial, the side-banded phosphorus produced a two-bushel per acre advantage over the seed-placed P treatment. The farmer has since moved to side-banding all of his nutrients. †

One of the pioneers of fertility research in Saskatchewan, Harry Ukrainetz with Agriculture Canada and Agri-Food Canada at Saskatoon, looked at the effect of phosphorus placement on canola yield in the ’70s.

Ukrainetz compared P placement one-inch below and one-inch to the side of the seed; one inch below the seed; and with the seed. His research clearly showed that seed-placing phosphorus provided some benefit at lower rates of 15 pounds P2O5 per acre, but yield decreased at higher seed-placed phosphorus fertilizer rates.

However, when phosphorus fertilizer was banded below and to the side of the seed, yields were about two bushels per acre higher than seed-placed phosphorus fertilizer, and yields continued to rise with higher rates, providing about five bushels per acre more canola at 35 pounds and six bushels per acre more at 55 pounds P2O5 per acre.

In my experience, I think fertilizer placement 1-1/2 inches to the side and 3/4 of an inch below the seed is the sweet spot. This placement helps to ensure that in any soil condition, you can get the seed on firm soil but still be close enough to the fertilizer band. If you look at the research on how much fertilizer phosphorus to put with the seed and how it can affect the time of emergence and germination, it makes very little sense to me to put any significant amount of fertilizer with the seed if you can place starter fertilizer in an optimum side-band position. †

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