Soil Management and Fertilizer Use:
Fertilizer Materials

Excerpt from Agronomy Guide for Field Crops (Chapter 2)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Introduction
   • Fertilizer Materials - Primary Nutrients - Table 2-26
   • Fertilizer Materials - Secondary and Micronutrients - Table 2-27
   • Characteristics of Common Liquid Fertilizers - Table 2-28
2. Soluble Salts in Farm Soils
   • Soil Conductivity Reading Interpretation - Table 2-29
3. Toxicity of Fertilizer Materials
   • Nitrogen Fertilizers
   • Phosphate Fertilizers
   • Potash Fertilizers
     • Maximum Safe Rates of Nutrients - Table 2-30
   • Fertilizers Containing Micronutrients
   • Guidelines for Safe Rates of Nutrients Applied at Seeding
4. Foliar Fertilizers
5. Calculating Fertilizer Requirements
   • Example Calculation for Fertilizer Application Requirements - Table 2-31
6. Updates on Soil Management and Fertilizer Use: Fertilizer Materials
7. Related Links...

Introduction

Nitrogen fertilizer materials are available in dry or liquid forms. Although there are some limitations to use of these materials (see the section 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, the cost per hectare can be determined. 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 (N), not kilograms of materials, are used. Table 2-26. Fertilizer Materials - Primary Nutrients, and Table 2-27. Fertilizer Materials - Secondary and Micronutrients, show the percentage of fertilizer nutrient contained in different materials.

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¹Expressed per unit (100 lb) of nutrient.
²Liquid under pressure.

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Various fertilizer companies have available, in addition to the micronutrient sources listed in Table 2-27. Fertilizer Materials - Secondary and Micronutrients, premixes containing one or more micronutrients.

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 2-28, Characteristics of Common Liquid Fertilizers.

1 Imperial gallon = 1.201 US gallons = 4.546 litres
1 US gallon = .8326 Imperial gallons = 3.785 litres

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Soluble Salts in Farm Soils

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. 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 2-29. Soil Conductivity Reading Interpretation, provides an interpretation of soil conductivity reading as read in Ontario field soils in a 2:1 water:soil paste, the procedure used by the OMAFRA accredited soil testing program.

For greenhouse soils, the OMAFRA accredited soil test uses larger soil samples and measures conductivity on a saturation extract. For greenhouse crops using this method, conductivity readings up to 3.5 millisiemens/cm are acceptable.

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Toxicity of Fertilizer Materials

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:

• differences in the amount of salts contained in the fertilizer per unit of plant nutrient
• differences in the solubility of the salts in the soil
• the presence of specific materials or elements that are particularly toxic (for example, ammonia and boron). Many nitrogen fertilizers, although they have a relatively low salt index, release free ammonia into the soil.

Nitrogen Fertilizers

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. More care should be taken, particularly with sensitive seeds such as beans or peas, and on coarse-textured soil (sand and sandy loam), than is required with ammonium nitrate or monoammonium phosphate.

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.

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Phosphate Fertilizers

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 the section Nitrogen Fertilizers.

Potash Fertilizers

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 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 2-30, 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.

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100 kg/ha = 90 lbs/ac

¹Fertilizers containing more than half as much nitrogen as phosphate (e.g., 16-16-16) frequently contain urea. Note that fertilizers containing urea are not suitable for banding at seeding in many cases.

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Fertilizers Containing Micronutrients

Fertilizers containing micronutrients (boron, copper, iron, manganese or zinc) are more toxic than the same grades without micronutrients, and maximum safe rates should be reduced. 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 2-30. 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.

Some growers are using 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.

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Foliar Fertilizers

Micronutrients can be supplied to the crop 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 this way are limited because of the danger of burning the leaves. Care must be taken when combining nutrients that the resulting solution is not too concentrated. Check pesticide labels before mixing foliar nutrients with any pesticide spray.

Calculating Fertilizer Requirements

The mineral fertilizer required for optimum crop production is calculated 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.

An example of this type of calculation is provided in Table 2-31. Example Calculation for Fertilizer Application Requirements, right. In this example, the total broadcast fertilizer required is 66 kg N/ha, or 143 kg/ha of urea. The starter fertilizer application could also have been reduced and still meet the crop requirements.

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Crop to be grown: corn
Previous crop: barley with red clover
Manure applied: solid dairy manure (22.4 t/ha or 10 t/ac) spring incorporated
Other organic nutrient sources: none
Starter fertilizer: 157 kg/ha (140 lb/ac) 8-32-16
Supplemental N: none

To use this table, enter the nitrogen, phosphate and potash requirements on the top line. Deduct the nutrients from legumes, manure or other organic sources to determine the amount of mineral fertilizer that will be required to add to meet the total requirements.

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. Any balance remaining should be broadcast. If only very small numbers remain, you may want to 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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Updates on Soil Management and Fertilizer Use: Fertilizer Materials

No updates available at this time.

Related links...

• Soil Fertility & Crop Nutrition

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