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Seed safe fertilizer applications and phosphorus starter

By Jeremy Boychyn

Seed safety is an important consideration as we head into spring. As fertilizer rates increase and yields follow, being aware of safe fertilizer application rates is important. Research has shown that the application of phosphorus is important to cereal grain production. However, crop response is dependent on soil phosphorus levels. Farmers should consider their soil phosphorus levels, crop requirements, crop response and use this to plan their fertilizer management program.

What are seed safety rates for wheat and barley?

Fertilizer injury impacting seedling emergence and plant stand can be avoided.

Fertilizer injury can occur in two ways:

The first is through ammonia (NH3) toxicity. As urea breaks down, it naturally releases NH3. If the urea is very close to the seed, excess NH3 can be toxic to the seed and can impact germination. The impact of NH3 toxicity is higher in dry soils. In moist soils, the H+ ions from water rapidly attach themselves to NH3 and convert it to NH+4 (ammonium), avoiding toxicity. Higher pH soils (> 7.5 pH) are at a higher risk for developing ammonia toxicity due to fewer H+ ions being present in the soil solution to allow for the conversion of NH3 to non-toxic NH+4.

The second way fertilizer injury occurs is through salt injury. Fertilizers, depending on the source, have a different salt index. If there is excess fertilizer in the seed row, the salt index will increase. If the salt index is greater than the salt levels in the germinating seed, moisture is pulled from the seed into the seed row through osmotic pressure. The higher the salt index, the higher the osmotic pressure on the seed. This osmotic process can dessicate seeds. Putting salt on a slug does the same thing. Below is a table from the University of Wisconsin that demonstrates the salt index of various fertilizer sources.

Table 1: Salt index of different fertilizer sources

Sourced from: https://extension.soils.wisc.edu/wcmc/understanding-salt-index-of-fertilizers-2/

Seed safe rates of urea depend on soil texture and seedbed utilization (SBU). On fine-textured soils, seed safe rates of urea are 0lbs, 15lbs, 25lbs, and 30lbs for SBU of 5%, 10%, 25%, and 33%, respectively. On medium texture soils, those values jump to 10 lbs, 25 lbs, 35 lbs, and 40 lbs for SBU of 5%, 10%, 25%, and 33%, respectively. If soil conditions are dry, cut these values by half.

Table 2: Seed safe rates of urea fertilizer (lbs/ac of product) applied with the seed relative to soil texture and soil moisture

To calculate Seedbed Utilization (SBU) follow the formula below:

SBU = Width of SeedrowSpread x 100 / Row Spacing

Example: SBU = 3 inches x 100 / 9 inches = 33% SBU

For phosphorus (P2O5), the seed safety rate for wheat and barley is 50lbs of P2O5 for 10% SBU. If you are combining potassium (K) with your P2O5, the combined rates of the two products should not exceed the total rate recommended for P2O5 alone (50lbs/ac).

For more detail, Top Crop Manager provided a great breakdown of seed safety here: (https://www.topcropmanager.com/determining-safe-rates-of-seed-placed-fertilizer-16268/#:~:text=The%20safe%20rate%20for%20cereal,P2O5%2C%20respectively.)

When will phosphorus in my starter fertilizer help?

The response of a wheat or barley crop to phosphorus application differs from nitrogen response. With nitrogen, one would expect the application of any nitrogen to provide a yield benefit the year of application. One caveat to this would be if there is already enough nitrogen available to reach peak yield assuming nitrogen is the only limiting factor. With phosphorus, the response of a crop to application the year of application is highly dependent on the amount of available phosphorus already within the soil. Soils with higher available phosphorus levels are less likely to see a response to the application of phosphorus.

In soils that test ‘low’ or ‘medium’ for soil available phosphorus, the probability of crop yield response is 60-100%. Soils that test ‘high’ for available phosphorus may see a yield response only 30-50% of the time.

However, this does not mean that the application of phosphorus on high phosphorus soil is not required. If phosphorus is application is ignored for multiple years, soil availability of phosphorus can quickly decline and decrease field yield potential. Therefore, farmers should take crop P2O5 removal into account when planning P2O5 applications. For reference, a 50 bu wheat crop and an 80 bu barley crop each remove approximately 30-32 lbs of P2O5.

You can calculate your crop's nutrient uptake and removal here.

For more information on phosphorus applications in Alberta see here.

Lastly, I highly recommend that all farmers keep a log of phosphorus application and removal from each field. Applying this technique to your farm management will help garner a stronger understanding of whether your application techniques are leading to decreases or increases in soil available phosphorus. See here for more details on how to start tracking your phosphorus application/removal.