Right rates for seed-placed fertilizer

One-pass systems that put down all seed and fertilizer at the same time are common on the Prairies. For canola seed and seedling safety, most of that fertilizer has to go in a side band or mid-row band. Because fertilizer can damage seed and seedlings, the safest is to put nothing with the seed. 

One exception is phosphorus, which can provide an establishment benefit for canola seedlings, especially in cool soils. In soils with low to medium levels of phosphorus, the recommendation is to put a starter rate (15 to 20 lb./ac. of P2O5, which is ~30-40 lb./ac. of monoammonium phosphate, for example) in the seed row. Soil tests will often recommend a higher rate of phosphorus for canola, so the balance is best added into the fertilizer blend placed outside the seed row.

Damage explained

The risk of seed-placed fertilizer comes from the nitrogen component of ammonium phosphate, ammonium sulphate and all nitrogen fertilizers. Ammonia can damage seedlings through direct toxicity while nitrate will damage seedlings by desiccation through the salt effect. These salts, when dissolved in soil water, have an osmotic effect that can hold back moisture from germinating seeds and seedlings. When soil moisture is lacking, this effect is worse for seed and seedlings. Canola is sensitive to both the salt effect and direct toxicity, which is why there is no distinction made between products containing straight ammonium and those containing a mixture of ammonium and nitrate nitrogen when it comes to safe seed-placed nitrogen rates for canola. Nitrogen products with a polymer coating (e.g. ESN) can allow for higher safe-rates of seed-placed nitrogen by lowering the concentration of ammonia that is in contact with the seedling. Anhydrous ammonia should never be placed in the seed row.

Potassium fertilizers, which have a high salt index, can also damage seedlings and are best kept out of the seed row. Canola has a much lower tolerance to seed-placed potassium than cereals, and stands can be reduced if seed-placed potassium rates exceed safe rates, especially with drills that have low seedbed utilization.

Test the safety of seed-placed rates

To test the effect of seed-placed fertilizer, turn off seed-placed fertilizer runs for a few 100-foot strips. Mark these areas and then go back and do plant counts early in the season to compare treated and untreated strips. 

NITROGEN

Factors that increase or reduce the risk from seed-placed nitrogen are:

Soil texture. The lighter the soil texture, the higher the risk to emergence damage and yield loss. Sandier soils are more risky than clays. Clay soils hold more nitrogen in the ammonium (NH4+) form. In sandy soils, more ammonium converts to ammonia (NH3), which is the form that causes seed and seedling damage because it is more toxic to plant tissues.

Seedbed moisture conditions at seeding. With moist soil conditions, water dilutes the concentration of nitrogen molecules around the seed and seedling. Water also disperses nitrogen molecules throughout the soil, reducing concentrations around the seed. In dry conditions, seed-placed nitrogen fertilizer tends to produce higher concentrations of ammonia and ammonium that can damage young seedlings. Seed-placed nitrogen rates should be lowered when conditions are drier than normal.

Fertilizer source. Anhydrous ammonia (NH3) is the most damaging form, which is why anhydrous ammonia should never be placed in the seed row. Polymer coating or urease inhibitors slow the release of ammonia and ammonium from urea, which can allow for higher safe-rates of seed-placed urea.

Application rate. The higher the nitrogen rate, the higher the risk of emergence damage and yield loss resulting from lower plant counts.

Crop type. Generally, small seeded crops such as canola are more sensitive to seed-placed nitrogen. Do not use the same rates for canola as for wheat, which is less sensitive to ammonia toxicity and salt effect.

Soil pH. Safe rates will be even lower in fields with high pH soils. At higher pH, more of the nitrogen from urea is in the free ammonia form (NH3) vs. ammonium (NH4+). Ammonia is more damaging.

–Seed bed utilization. SBU is the seed placement spread as a percentage of total soil area. Narrow rows and wide openers will increase SBU. A 1″ knife opener on 12″ centres has a low SBU of eight per cent, whereas a 3″ opener on 6″ centres will have a high SBU of 50 per cent. High SBU means the same rate of seed and seed-placed fertilizer is spread over a greater proportion of the soil; because separation is higher, this reduces the risk from putting higher rates of fertilizer with the seed. (*SBU is also called row-width utilization (RWU), which is the term used in the graphic below.)

Examples of how openers can change seed-bed utilization (a.k.a row-width utilization)
Click table to enlarge. See more table notes below.

Important notes about the table: 

  • These guidelines show approximate safe rates of urea placed with the seed if seedbed soil moisture is good to excellent. Safe rates will be lower if soil moisture is lower. If moisture conditions are dry, the safe amount of seed row nitrogen should be reduced by half.
  • All rates are in pounds of actual N per acre. Divide by 0.46 to get pounds of urea (46-0-0) per acre. 
  • When selecting seed placed nitrogen rates, note that the rates in Table 1 are IN ADDITION to the rates seed-placed MAP (11-51-0 or 12-52-0). MAP is usually safe at rates below 40 lb./ac. of product, or 20 lb./ac. of actual P2O5. Some growers also use diammonium phosphate (DAP) for starter phosphorus, but this form has a higher nitrogen component. Safe seed-placed rates for DAP will be slightly lower than for MAP. 
  • Numbers in brackets are safe rates for Manitoba. The first number is for the rest of the Prairies. This suggests that there is a line at the Manitoba border where canola suddenly becomes more sensitive. Of course this is not true. This distinction is based on soil characteristics. Manitoba soils tend to have high soil pH, and high levels of free lime or carbonates. Under these conditions, urea tends to release more of its nitrogen in the free ammonia form (NH3), which is more damaging than the ammonium form (NH4+). While more common in Manitoba, these conditions are not exclusive to Manitoba. Average pH in Manitoba is not much different from many other regions of the Prairies. 

Canola Encyclopedia on nitrogen placement

PHOSPHORUS

Phosphorus supply during the first two to six weeks of canola growth is critical to achieve optimal yield. Given that soil supply is reduced under wet, cold conditions, canola benefits from a seed-placed supply of fertilizer phosphorus. Cold soil decreases phosphorus solubility and diffusion in the soil solution, slowing phosphorus movement to roots and root uptake rates. 

A seed-placed rate of 20 lb./ac. of P2O5 (~40 lb./ac. of monoammonium phosphate, or MAP) strikes a balance between seed proximity and seed safety. At that rate, fertilizer prills or droplets will be close enough for each seedling to access the supply.  The rate can be safely increased somewhat with good moisture conditions and with increases in seed-bed utilization (wider openers or narrower row spacing or both).

Formulation: Diammonium phosphate (DAP) has a higher nitrogen component than MAP, so safe seed-placed rates for DAP will be slightly lower than for MAP.  When comparing phosphorus products, often the best approach is to choose a source that provides the target rate at the lowest cost, with consideration for farm logistics.

Canola Encyclopedia on phosphorus placement

POTASSIUM

If canola requires potassium fertilizer, include potassium in the blend placed outside the seed row. Soils with higher clay and organic matter will reduce the damage from seed-placed potassium, but potassium deficiency is more likely to occur in sandy soils – which tend to increase the risk scenario for seed-placed potassium. In favourable soil moisture/seedbed conditions, the best move is likely to increase seed-placed phosphate rates closer to the maximum safe rate, and continue to put all other fertilizer outside the seed row.

Canola Encyclopedia on potassium placement

SULPHUR

Ammonium sulphate applied in the seed row along with monoammonium phosphate can increase seedling toxicity and reduce plant stand. Because sulphur uptake peaks later in the season, it doesn’t need to go in the seed row.

Canola Encyclopedia on sulphur placement

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