Seed safe fertilizer applications and phosphorus starter

By Sheri Strydhorst, PhD, Agronomy Research Specialist and Jeremy Boychyn MSc. P.Ag., Agronomy Research Extension Specialist | Alberta Wheat and Barley Commissions

Seed safety is an important consideration as we head into planting. High rates of seed placed fertilizer (urea, phosphorous and potassium) can damage seed viability and emergence. Under dry soil conditions, seed safe rates of fertilizer should be reduced by 50% to improve emergence.

Secondly, the application of phosphorus fertilizer is important for cereal grain production. However, the probability of a yield response depends on soil phosphorus levels. Farmers should consider their soil phosphorus levels, crop requirements and crop response and to plan their phosphorous fertilizer management program.

Fertilizer injury can damage seeds in two ways:

The first is through ammonia (NH₃) toxicity. As urea breaks down, it naturally releases NH₃. Excess NH₃ can be toxic and can impact seed germination when urea is close to the seed. The impact of NH₃ toxicity is higher in dry and acidic soils. Higher pH soils (> 7.5 pH) develop ammonia toxicity as less of the NH₃ is converted to non-toxic NH₄⁺. The reduced conversion of NH₃ to NH₄⁺is due to fewer H⁺ ions being present in the soil solution of higher pH soils. In moist soils, the H⁺ ions from water rapidly attach themselves to NH₃ and convert it to NH₄⁺ (ammonium), avoiding toxicity.

The second way fertilizer injury occurs is through salt injury. Different fertilizer sources have different salt indices. If there is excess fertilizer in the seed row, the salt index will increase. If the salt index in the seed row 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 desiccate 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/

What are safe rates of urea fertilizer for wheat and barley?

Urea is the most common seed placed fertilizer. Rigas Karamanos has developed a simple excel calculator to minimize the risk from seed row placed nitrogen fertilizer. The calculator requires growers or agronomists to know the:

• spring soil moisture conditions: dry, borderline or moist
• soil test nitrogen based on a 0-24” deep sample: High (90 ± 40 lbs N/ac); Low-Medium (46 ± 22 lbs N/ac); Low-Medium (43 ± 26 lbs N/ac)
• seed bed utilization (SBU): based on seed spread and row spacing
• soil organic matter: value based on previous soil tests
• soil texture: light (sandy loam); medium (loam to clay loam); heavy (clay to heavy clay)

After inputting this information, the calculator provides a guideline for safe seed row placed nitrogen. It also provides an estimated percent reduction in plant stand based on applying various rates of nitrogen fertilizer that exceed the safe rate. For example, the calculator indicates a dry soil with 5% organic matter and 10% SBU, should not exceed 12 lbs N/acre in the seed row. However, if the grower ignored this recommendation and choose to put 25 lbs N/ac in the seed row, the estimated plant stand is suggested to be 70% of the target. If the grower was targeting 25 wheat plants/sqft, the seed row placed fertilizer could reduce the plant stand to 17.5 wheat plants/sqft. This example shows how excessive seed row placed fertilizer and the corresponding reduction in plant stand will have season-long implications on agronomic management and yield.

If growers or agronomists are looking for more general guidelines, Table 2 lists seed safe rates of urea fertilizer based 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 Seed Row Spread x 100 / Row Spacing

Example: SBU = 3 inch wide seed row spread x 100 / 9 inch row spacing = 33% SBU

For most farmers, the amount of nitrogen required by the crop exceeds the safe amount of fertilizer that can be applied in the seed row. Therefore, the balance of the crop’s nitrogen fertilizer will have to be side-banded, mid-row banded or broadcast as an enhanced efficiency fertilizer product.

What are safe rates of phosphorous and potassium fertilizer for wheat and barley?

For phosphorus (P2O5), the seed safe 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.

When will phosphorus in my starter fertilizer help?

The response of wheat or barley to phosphorus differs from nitrogen. With nitrogen, one generally expects any nitrogen application to provide a yield benefit. With phosphorus, the crop response, in the year of application, is highly dependent on the amount of soil available phosphorus. 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 (Table 3), 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, phosphorus applications should still be considered on soils that test ‘high’ for phosphorous. If phosphorus is application is ignored for multiple years, soil availability of phosphorus can quickly decline and decrease 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. Therefore, the year following a 50 bu wheat crop, 30-32 lbs of P2O5 should be applied to the field.

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

For more information on phosphorus applications in Alberta see here.

Lastly, it is highly recommended that farmers keep a log of phosphorus application and estimated crop removal from each field. Applying this technique can 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 and removal.