Maximum Safe Rates of Fertilizers

Excess fertilizer can harm seedlings due to injury from ammonia and salt. Fertilizer toxicity causes tissue damage to seedling roots and may result in delayed emergence and reduced plant stands. These effects are most often related to fertilizer nitrogen (N), potassium (K), and sulphur (S) content.

Fertilizer toxicity varies widely, depending on soil texture and the amount of soil moisture. Injury will occur most frequently on coarse-textured (sandy or gravelly) soils low in organic matter, high in pH, and with dry weather. It is generally advisable to use lower rates of fertilizer at seeding than the maximums listed.  To ensure completely safe rates of banded fertilizer for all seeding conditions would require extremely low rates of application.

If fertilizer requirements are high, it may be better to broadcast most of the fertilizer required and to band only a small portion at seeding. Fertilizers containing the micronutrients boron, copper, iron, manganese, and zinc are more injurious than the same grades without micronutrients, and the safe rates recommended will be lower than those shown in this table. Boron is particularly toxic and should not be banded.

Some producers use much higher rates of banded fertilizer, with no apparent problem. Certain crops can tolerate higher rates of fertilizer without damage, and all crops are able to tolerate much higher rates of fertilizer with adequate soil moisture. It is impossible, however, to predict before planting when adverse conditions for germination will occur. Keeping below the maximum safe rates is the surest way to ensure a good start for the crop.

Table 1, Maximum safe rates of nutrients in fertilizer, provides the maximum safe rates of nutrients for various crop scenarios. It is generally advisable to use lower rates of fertilizer at seeding than are listed here – the maximum rates may reduce or delay germination, or retard growth in up to 10% of the cases where they are used. The safe rates listed in this table are for single applications. If two or more fertilizer applications are combined, the additive effect may cause damage to the crop even though the individual applications are below the threshold for injury.

Table 1. Maximum safe rates of nutrients in fertilizer

Crop	Fertilizer	N			N + K2O + S
		75 cm (30 in.) row	38 cm (15 in.) row	18 cm (7 in.) row	75 cm (30 in.) row	38 cm (15 in.) row	18 cm (7 in.) row
		kg/ha			kg/ha
Banded 5 cm to the side x 5 cm below seed (2 in. x 2 in.)
Corn1	urea	40	—	—	79	—	—
	other fertilizers	52	—	—	117	—	—
Soybean2, pea, dry beans	ammonium sulphate	30	60	—	NR	NR	—
	other fertilizers	NR	NR	—	90	180	—
With the seed3
Corn	other fertilizers	NR	NR	—	10	20	—
Winter wheat, triticale, barley	other fertilizers	—	—	15	—	—	40
Spring oat, barley, wheat	urea	—	NR	10	—	—	30
	other fertilizer – sand	—	NR	35	—	—	55
	other fertilizer – clay	—	—	45	—	—	70
Canola	ammonium sulphate – sand	—	—	22	—	—	11
	ammonium sulphate – clay	—	—	22	—	—	33
Broadcast, strip till
Corn	urea	200	—	—	250	—	—
LEGEND: NR = not recommended, — = no data, 100 kg/ha = 90 lb/acre

Fertilizer Placement

The higher the concentration of fertilizer material, the higher the likelihood of crop injury in response. Narrow rows provide more row length in a given area, allowing a higher rate of fertilizer to be used safely than wider rows. Broadcast application can reduce toxic fertilizer concentration nearby to seedlings, but may be less efficient in terms of nutrient efficiency.

Strip-till systems vary in how fertilizers are distributed and mixed in the soil. Research to determine safe rates in various placement systems is ongoing.

Toxicity of Fertilizer Materials

All fertilizer salts are toxic to germinating seeds and plant roots if applied in sufficient concentration near the seed. Fertilizers vary in toxicity per unit of nutrient due to:

• differences in the amount of salts contained in the fertilizer per unit of nutrient.
• differences in the solubility of the salts in the soil.
• the presence of specific materials or elements that are particularly toxic (e.g., ammonia and boron).

Many nitrogen fertilizers, despite relatively low salt index, release free ammonia into the soil.

Injury from fertilizer can cause a stand reduction, increased variability in emergence, and both root and foliar “burn” symptoms. These symptoms can be confused with plant diseases or insect damage, so scouting is critical for confirmation.

Nitrogen Fertilizers

Ammonium nitrate, monoammonium phosphate (MAP), and ammonium sulphate are similar in toxicity and much safer than anhydrous ammonia, aqua ammonia, or urea. Diammonium phosphate (DAP) is more toxic than MAP but less toxic than urea. Use lower rates of urea or increase the distance between the seed and fertilizer band, particularly with sensitive seeds such as beans or peas, and on coarse-textured soil (sand and sandy loam).

Because anhydrous ammonia and aqua ammonia release free ammonia, they should not be placed near seeds. It is preferable to make pre-plant applications crossways to the direction in which the crop will be planted. Stand reductions may still occur over the band in very dry soils, or if planting takes place too soon after application.

Urea is toxic when banded with or near the seed, but is safe when broadcast at rates normally used. Blends containing more than half as much nitrogen as phosphate frequently contain urea.

Phosphate Fertilizers

Phosphate fertilizers are usually low in toxicity because a large portion of the phosphate is precipitated in the soil before it can reach the plant roots. The concentration of phosphorus in soil solution at any one time is very low. No limit is normally set for the safe rate at which phosphates may be applied with or near the seed of field-grown crops.

DAP is more injurious than other phosphate fertilizers due to its nitrogen content.

Potash Fertilizers

Muriate of potash (KCl) is the most common source of potassium in fertilizers and is less injurious per unit of plant nutrients than most nitrogen fertilizers.

Sulphate of potash (K₂SO₄) has a lower salt index than muriate of potash. Sulphate of potash-magnesia has approximately the same toxicity per unit of K as muriate of potash. Potassium nitrate is one of the safer sources of potassium.

Sulphur Fertilizers

Liquid thiosulphate fertilizers, such as ammonium thiosulphate (ATS), pose a higher risk of seedling injury than other S fertilizers.

Sulphate fertilizers are relatively safe in comparison, although those containing N (e.g., ammonium sulphate) have a moderately higher risk of injury than those containing K (e.g., potassium sulphate). Calcium sulphate (gypsum) has a low salt index.

Elemental sulphur fertilizers are not utilised effectively in a band, due to reduced contact between S particles and soil microorganisms that oxidize S and convert into the plant available sulphate form. Instead, broadcast is recommended.