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811: Agronomy Guide > Soil
Fertility and Nutrient Use > Micronutrient Fertilizers
Nitrogen fertilizer materials are available in dry or liquid forms. Although there are some limitations to the use of these materials (see Toxicity of Fertilizer Materials), in most cases, the different sources will produce equal yields. The choice of material should therefore depend on availability, equipment for handling and cost per kilogram of nitrogen, plus the cost of application.
First, calculate the cost per kilogram of nitrogen for various sources delivered to your farm. Using the rate of application, determine the cost per hectare. Add to this, the cost of application per hectare before deciding which nitrogen source to use.
Where separate additions of nitrogen are referred to in the recommendations, kilograms of elemental nitrogen, not kilograms of fertilizer materials, are used. Table 9-17, Fertilizer Materials - Primary Nutrients, and Table 9-18, Fertilizer Materials - Secondary and Micronutrients, show the percentage of fertilizer nutrient contained in different materials.
Various fertilizer companies have premixes available containing one or more micronutrients in addition to the micronutrient sources listed in Table 9-18, above.
A wide range of liquid fertilizers is used in Ontario. While these are generally more expensive per unit of nutrient than dry granular fertilizers, liquid fertilizers are easier to handle and can be metered and placed more precisely. Characteristics of the most commonly used liquid fertilizers are included in Table 9-19, Densities of Common Liquid Fertilizers.
High concentrations of water-soluble salts in soils can prevent or delay the germination of seeds and can kill established plants or seriously retard their growth.
Ontario soils are naturally low in soluble salts. Soluble salts therefore rarely cause a problem in crop production and are not routinely measured in soil tests.
Soluble salts in soils can result from excessive applications of fertilizers and manures, runoff of salts applied to roads and chemical spills on farmland. High concentrations of soluble salts in or near a fertilizer band can cause serious reductions in early plant growth without seriously affecting the salt concentrations in the remainder of the soil. A given amount of salt in a soil provides a higher salt concentration in soil water, if the amount of water is small.
1 Imperial gallon = 1.201 US gallons = 4.546 litres 1 US gallon = 0.8326 Imperial gallons = 3.785 litres
Soluble salts also interfere with the uptake of water by plants. For these reasons, plant growth is most affected by soluble salts in periods of dry conditions at planting.
Soluble salts can be measured readily in the laboratory by measuring the electrical conductivity of a soil water slurry. The higher the concentration of water soluble salts, the higher the conductivity. Table 9-20, Soil Conductivity Reading Interpretation, this page, provides an interpretation of soil conductivity readings.
All fertilizer salts are toxic to germinating seeds and to plant roots if applied in sufficient concentration near the seed. Fertilizers vary in toxicity per unit of plant nutrient due to:
Ammonium nitrate, monoammonium phosphate and ammonium sulphate are similar in toxicity and much safer than anhydrous ammonia, aqua ammonia or urea. Diammonium phosphate is more toxic than monoammonium phosphate 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 are extremely toxic fertilizers, they should not be placed near seeds. It is preferable to make preplant 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. Fertilizers containing more than half as much nitrogen as phosphate frequently contain urea.
Fertilizer Calculations - An Example
Based on soil test results and N calculator, a farmer requires 120 lb of N, 18 lb P205 and 30 lb K20 to grow a crop of corn. A liquid starter fertilizer (5 gal/acre 6-24-6) will be included.
Step 1: Determine starter contribution
Liquid fertilizer with a blend of 6-24-6 and a density of 13.3 lb/gal is applied at 5 Imperial gal/acre.
The fertilizer contains 6% N, 24% P205 and 6% K20. The application rate is 5 gal/acre x 13.3 lb/gal = 66.5 lb/acre.
N 66.5 lb/acre x 6/100 = 4 lb/acre
P205 66.5 lb/acre x 24/100 = 16 lb/acre
K20 66.5 lb/acre x 6/100% = 4 lb/acre
Step 2: Determine balance required
120 - 4 = 116 lb N/acre still required
Step 3: Determine supplemental fertilizer to meet balance of crop requirements
Fertilizer rates are determined by dividing the nutrient requirement by the percent nutrient in the fertilizer. If the supplemental fertilizer is to be a blend of urea plus potash, the calculations will be:
116 lb N/acre ÷ 0.46 (or 46%) = 252 lb/acre urea (46-0-0)
Total application rate = 252 + 42 = 294 lb/acre of blended fertilizer
For more details on calculating nutrient requirements and fertilizer blends, see OMAFRA Publication 611, Soil Fertility Handbook.
Phosphate fertilizers are usually low in toxicity due to a large portion of the phosphate being 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.
Diammonium phosphate is more toxic than other phosphate fertilizers. See Nitrogen Fertilizers, opposite page.
Muriate of potash (KCl) is the most common source of potassium in fertilizers and is less toxic per unit of plant nutrients than most nitrogen fertilizers.
Sulphate of potash (K2SO4) is slightly less toxic than muriate of potash. Sulphate of potash-magnesia has approximately the same toxicity per unit of potassium as muriate of potash. Potassium nitrate is one of the safer sources of potassium.
Table 9-21, Maximum Safe Rates of Nutrients, provides the maximum safe rates of nutrients for various crop scenarios. The safe rates listed in this table are for practices occurring singly. 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.
Fertilizers Containing Micronutrients
Fertilizers containing micronutrients (boron, copper, iron, manganese or zinc) are more toxic than the same grades without micronutrients; reduce maximum safe rates. Boron is particularly toxic and should not be banded.
Guidelines for Safe Rates of Nutrients Applied at Seeding
Fertilizer toxicity varies widely, depending on the amount of soil moisture. Injury will occur most frequently on light-textured (sandy or gravelly) soils low in organic matter and with dry weather. To ensure completely safe rates of banded fertilizer for all seeding conditions would require extremely low rates of application. The maximum safe rates suggested here will most likely reduce or delay germination, or retard growth in up to 10% of the cases where they are used. It is generally advisable to use lower rates of fertilizer at seeding than those listed in Table 9-21, Maximum Safe Rates of Nutrients.
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 toxic, and the safe rates recommended
will be lower than those shown in this table.
Sweet corn can be more sensitive to fertilizer placed with the seed. Do not apply fertilizer with the seed of super sweet hybrid sweet corn.
At higher rates, band at least 15 cm from seed. At row widths other
than 100 cm, the rate may be adjusted to provide the same maximum
concentration in the row (e.g., in 50-cm row, the safe rate = 100/50
x 55 = 110 N).
Up to 20 kg/ha phosphate fertilizer may be drilled with the seed
as superphosphate or mono-ammonium phosphate. Do not apply N (except
as mono-ammonium phosphate) or K with the seed.
Fertilizers containing more than half as much N as P2O5 (e.g.,16-16-16) often contain urea. Fertilizers containing urea are not suitable for banding at seeding in many cases.
Some growers use much higher rates of banded fertilizer than are listed here, with no apparent problem. Crops are able to tolerate much higher rates of fertilizer with adequate moisture, but it is impossible 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.
Excess fertilizer can harm seedlings because of effects from ammonia and salt. These effects are related to fertilizer nitrogen (N) and potassium (K) content. Toxicity varies widely depending on soil texture, moisture conditions, crop, fertilizer source and placement. Table 9-21, Maximum Safe Rates of Nutrients, provides guidelines that will most likely limit injury to less than 10% of the cases where they are used. Injurious effects include reduced or delayed germination or retarded growth. Weather, stress and other conditions that affect growth may increase the chances of injury.
Micronutrients can be supplied to crops through foliar fertilization,
particularly in instances where a deficiency is due to a tie-up
of these nutrients in the soil (i.e., manganese). Quantities of
nutrient that can be applied in this manner are limited because
of the danger of leaf burning. Take care when combining nutrients
that the resulting solution is not too concentrated. Check pesticide
labels before mixing foliar nutrients with any pesticide spray.
Calculate the mineral fertilizer required for optimum crop production by deducting the nutrients in manure and legumes from the total nutrients required. Further, it is often beneficial to separate the starter component of the fertilizer, which is generally high in phosphorus, from the balance of the nitrogen and potassium.
The choice of starter fertilizer will depend on the crop to be grown, the mineral fertilizer requirements and the equipment available. It is often equally efficient to apply part of the fertilizer as a starter and broadcast the rest, as it is to apply all the fertilizer through the planter or drill. The advantage to this program is savings in time and labour, and less risk of fertilizer injury to the seedling.
Deduct applications of starter fertilizer and side-dressed fertilizer
from the total mineral fertilizer requirement. Broadcast any balance
remaining. If only very small numbers remain, consider adjusting
the rates of one of the other nutrient sources, ignoring the small
residuals or planning a fertilizer application that will meet multi-year
requirements (P & K only).
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