| | How much fertilizer can be placed safely with seed?
Even though this may seem like a simple question, the answer is complex. Fertilizer source, seeding equipment, soil properties and conditions, crop type, seed quality, and risk factors must all be considered before this question can be answered.
Fertilizer source
Seed-placed fertilizer causes damage through ammonia toxicity and/or “salt effects”. Urea (46-0-0) and liquid UAN (urea ammonium nitrate, usually 28-0-0) are fertilizers that primarily cause ammonia toxicity in the seedrow. Salt effects are usually associated with ammonium sulphate (21-0-0-24 or similar analyses), potash, ammonium nitrate and ammonium phosphate.
Cereals, oilseeds, pulses, and grasses suffer permanent damage if exposed to high concentrations of ammonia. Small seeded crops like canola or flax are permanently damaged by high salt concentrations in the seedrow. Salt effects may delay cereal germination and emergence but with salt dilution through rainfall, these crops will recover and grow normally. Most seed-placed fertilizer guidelines focus on ammonia toxicity and are based on urea-nitrogen application rates. Unfortunately in the case of blended fertilizers that have both ammonia toxicity and salt effects, there is currently insufficient research to establish safe rate recommendations.
Seeding equipment
Seedbed utilization (SBU), the width of seed and fertilizer spread divided by the row spacing, reflects the relative concentration of fertilizer in the seedrow. With high SBUs, seed and fertilizer are spread over a larger area and seedrow conditions are usually safer than they are with low SBUs. Seeding depth and packing are other equipment factors that need to be considered. Ground speed during seeding may also be a factor as excessive speed may compromise seed and fertilizer placement as well as the overall quality of the seeding operation. Crop damage from seed-placed fertilizer is more pronounced when other stresses like uneven crop residue distribution, deep seeding and poor seed-soil contact compound the effects of the fertilizer.
Soil properties and conditions
The success of seed-placed fertilizer operations is highly dependent on soil moisture at seeding and in the first week or so after seeding. With soil at or near field capacity, the toxic effects of fertilizer are diluted and seeds can germinate and develop with few problems. Fine textured soils such as clay loams and clays, not only hold more water than coarse soils like sands and loamy sands, but clay surfaces also adsorb ammonium from the soil solution and further reduce the toxic effects of nitrogen fertilizers. Dry, eroded hilltops with low soil organic matter levels and presence of free lime (high pHs) are prone to injury from seedrow fertilizer. “Safe” seed-placed fertilizer rates in these exceptional areas may be much lower than the rest of the field and need to be managed accordingly.
Crop type
Cereals withstand more seed-placed fertilizer than oilseeds. Within the cereals, oats are slightly more tolerant than barley, which is slightly more tolerant than wheat. Similarly, in the oilseeds, flax is slightly more sensitive to seed-placed fertilizer than canola. Even though there are these minor differences, for safe seed-placed fertilization rates, the cereals can be treated as a group, and the oilseeds can be treated as another group.
Seed quality
If everything else is equal, seed with poor vigour will suffer more damage from seed-placed fertilizer than vigorous seed. Environmental conditions during grain filling and harvest, grain drying, over-winter storage conditions, seed borne diseases, and seed age may all affect seed vigour and the seed’s ability to tolerate the adverse effects of seed-placed fertilizer. Published guidelines for safe seed-placed fertilizer rates assume seed quality is good. See the table below or the links to additional information at the bottom of this document for access to these guidelines.
Risk factors
Seed-placed fertilization involves risk. The only rate that is safe in all situations is probably no fertilizer in the seedrow. In some conditions high rates of seed-placed fertilizer will cause minimal injury yet in other situations the same application rates will cause extensive damage. Seed or seedling mortality may not always lead to lower yield, but thin stands tend to tiller and branch, often delaying maturity by several days or more. With reduced crop competition because of seed-placed fertilizer damage, weeds often thrive, leading to higher dockage and harvest management problems.
How much nitrogen fertilizer can be placed safely with seed?
Guidelines for maximum safe rates of nitrogen with cereals or canola and flax using urea as the nitrogen source are given in the following table. These rates are based on soil moisture at or near field capacity and must be reduced if soil moisture is not excellent. Keep in mind that these are guidelines that in some situations are conservative and in other cases overestimate the maximum safe rates.
Maximum rates of actual nitrogen (lb/ac) that can placed in the seedrow of cereals and oilseeds using urea as nitrogen source
| Cereals |  | | | | | | |
| Seed Bed Utilization | 8% | 11% | 17% | 22% | 25% | 33% | 50% |
| Light (sandy loam) | 15 | 15 | 20 | 25 | 25 | 30 | 40 |
| Medium (loam to clay loam) | 20 | 25 | 30 | 35 | 35 | 40 | 50 |
| Heavy (clay to heavy clay) | 30 | 30 | 35 | 40 | 40 | 50 | 60 |
| | | | | | | |
| Canola and Flax | | | | | | | |
| Seed Bed Utilization | 8% | 11% | 17% | 22% | 25% | 33% | 50% |
| Light (sandy loam) | 0 | 5 | 10 | 15 | 15 | 20 | 30 |
| Medium (loam to clay loam) | 5 | 10 | 15 | 20 | 20 | 30 | 40 |
| Heavy (clay to heavy clay) | 10 | 15 | 20 | 30 | 30 | 40 | 50 |
Adapted from the Saskatchewan Agriculture, Food and Rural Revitalization document Guidelines for Safe Rates of Fertilizer Applied With the Seed
Links to additional information
Alberta Agriculture: Fertilizer Application and Placement
Saskatchewan Agriculture, Food and Rural Revitalization: Guidelines for Safe Rates of Fertilizer Placed With the Seed
Prepared by Ag-Info Centre, Alberta Agriculture and Forestry |
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