Frequently Asked Questions - Safe Application of Fertilizer

Can I place fertilizer in the seed row?
If fertilizer can cause damage to the seedling, why is fertilizer place in the seed row?
Should all fertilizer be separated from the seed?
What is in the fertilizer that makes it so harmful?
What else affects the amount of fertilizer I can safely place with the seed?
What rate of a fertilizer blend can I place withy the seed?
I suspect that I have fertilizer damage.  What symptoms can I expect?
References

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Can I place fertilizer in the seed row?

Fertilizer placed in or close to the seed row is an effective way of meeting the crop’s nutrient requirements, provided it isn’t applied at rates that inhibit germination or seedling development.  Limited amounts of fertilizer can be placed in the seed row without creating damage to the emerging seedling.  The Farm Facts sheet Guidelines for safe rates of fertilizer applied with the seed should be consulted to determine the maximum rate of fertilizer that can be applied with the seed under good moisture conditions.

If fertilizer can cause damage to the seedling, why is fertilizer placed in the seed row?

 Some fertilizer—such as phosphate fertilizer—is applied in the seed row for the “starter effect.”  Starter fertilizer is particularly beneficial for early season growth when soils are cool.  Phosphorus does not move very far from the granule, so it needs to be placed close to where the developing roots will intercept it.  Also, seedlings require relatively high amounts of phosphorus in the early growth stages. 

Agronomically, fertilizer only needs to be applied with the seed to provide a “starter effect.”  Practically, some seeding systems do not allow separation of the seed and fertilizer.  Fertilizer can be applied with single shoot systems for convenience; however, rates are limited.  Single shoot seeding systems have limited amounts of fertilizer that can be placed in the seed row. If you have the machinery capabilities to apply the nitrogen fertilizer in a side-band or a mid-row band, that is your best option.  If fertilizer requirements are in excess of the recommended safe rates, then an alternative method of applying that fertilizer away from the seed must be found.

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Should all fertilizer be separated from the seed?

Early season nutrient availability is important for proper crop development.  Nutrients are required shortly after germination to ensure plant vigour and maintain yield potential. 

Immobile nutrients such as phosphorus (P) and potassium (K) don’t have to be in the seed row, but should be close to the seed row, within 1.5 inches of the seed. Early access to P and K is required for vigorous growth of the seedling.  This is often called the “pop-up” or “starter effect” of fertilizer and is most pronounced on cereal in nutrient deficient, cool, and wet soils.

Urea, anhydrous ammonia and ammonium forms of nitrogen (N), once converted to the nitrate form in the soil, and sulphur (S) in the sulphate form, will move in the soil solution and don’t have to be placed in or very near the seed row.

What is in the fertilizer that makes it so harmful?

There are two major factors that can affect seedling development: the salt effect and the free ammonia damage.  The salt effect will prevent soil moisture that is near the seed from being accessed by the germinating seed and young seedling.  Damage to the developing seedling - or fertilizer burn - is caused by the ammonia released from urea-nitrogen fertilizer as it converts from urea-N to ammonium-N form.  All fertilizer has some salt effect.  The salt effect of fertilizers is measured as an index as indicated in Table 1. 

Table 1. Fertilizer Salt Index

Ammonia toxicity occurs when the free ammonia ion (NH₃⁺) is released when urea-N fertilizer converts to the ammonium form (NH₄⁺).  Urea fertilizer releases free ammonia ions, ammonium sulphate doesn’t.  High pH, low CEC (cation exchange capacity), and dry soil conditions increase the free ammonia levels in soil.  Highly calcareous soils are more susceptible to ammonia toxicity than non-calcareous soils.

In Saskatchewan, fertilizer separated from the seed row by a minimum of one inch to the side and one inch below will provide a soil bridge that will provide significant protection against fertilizer under most conditions.  Damage to the seedling can occur if fertilizer is placed with less than one inch of separation. 

Side–band openers will separate the seed and fertilizer. However, caution should always be taken to maintain seed and fertilizer separation.  Worn openers or faulty equipment, excessive ground speed and fan speed or excessive rates of fertilizer can cause mixing of the seed and fertilizer and reduce seed emergence. 

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What else affects the amount of fertilizer I can safely place with the seed?

There are many variables that affect the safe placement of seed row fertilizers.  Besides the type of fertilizer, the main factors that determine the safe rate of fertilizer placed with the seed are: crop grown, seedbed moisture at seeding, seedbed utilization (SBU) and soil texture. 

Crop Type:
Crop tolerance to seed placed nitrogen fertilizer varies.  Generally, cereals can tolerate more seed placed urea-nitrogen than canola or flax.

Soil Moisture at Seeding:
The guidelines for the safe rates of seed placed fertilizer are based on good to excellent soil moisture conditions.  If moisture conditions are not good to excellen, the rates of seed-placed fertilizer should be dropped by at least 50 per cent. 

Seedbed Utilization (SBU):
Seedbed utilization (SBU) is the amount of seedbed over which the fertilizer and seed can be spread.  SBU is calculated by dividing the width of spread of the seed behind the opener by the row spacing multiplied by 100.  For example, if the seeding implement has six inch row spacing and spreads the seed and fertilizer over two inches, the SBU would be two divided by six, multiplied by 100 = 33 per cent.

Soil texture:
Coarse textured soil (sand) holds more soil water per foot of moist soil than a fine textured soil (clay).  Sandy soils also have larger pore spaces for air.  The higher surface area of finer textured soils means a higher Cation Exchange Capacity that allows for more sites for the ammonium cations (NH₄⁺) to be held on the soil exchange. Higher levels of soil water held by clay soil also help dilute the salt, reducing the salt effect of the fertilizer.

What rate of a fertilizer blend can I place with the seed?

When using a blended fertilizer, calculate the amount of nitrogen going on per acre and use the Safe Rates Tables to determine the correct rate of nitrogen.

When ammonium-sulphate fertilizer is placed with the seed, add the pounds of N from the rate of ammonium-sulphate to the pounds of urea-nitrogen being placed with the seed.  The total nitrogen should not exceed the maximum safe rate of seed placed urea-N found in the guideline Tables.

Safe rates of phosphorus and potassium are also comparable.  If both of these nutrients are applied, the rate of each nutrient should be added together. The combined total should fall within the safe level of phosphate applied with the seed.

 The safe rate of phosphate is in addition to the safe rates of nitrogen. The safe rates of urea-nitrogen found in the Tables were developed based on the assumption that the safe rate of phosphate will be seed-placed.  Also, the small amount of nitrogen found in phosphate fertilizers (monoammonium phosphate) does not have to be added to the amount of urea-nitrogen when using the urea-nitrogen Tables in the Guidelines for Safe Rates of Fertilizer Applied  With the Seed fact sheet.  

I suspect that I have fertilizer damage.  What symptoms can I expect?

Excessive seed row fertilizer reduces germination and weakens seedlings.  The effects are often undistinguishable from other causes of seedling damage.  Other stresses to the seedling can compound the damage from seed row fertilizer.  Seeding a check strip without seed-placed fertilizer is the only way to positively identify whether fertilizer is contributing to the emergence problem. It is always better to be on the conservative side when placing fertilizer with the seed.  Fertilizer can be added with another operation; it is more difficult to reseed. 

References:

Guidelines for Safe Rates of Fertilizer Applied With the Seed
Westco Co-operative Fertilizers Ltd.
Potash and Phosphate Institute Canada