Chemical Fertilizers

Chemical fertilizers contain one or more of the “Big 3” (N, P, and K) along with varying amounts of calcium and sulphur. Ordinarily, chemical fertilizers contain no magnesium or micronutrients unless these have been specially added. (Micronutrients are usually applied as separate fertilizers when needed).

The myth of “complete” fertilizer: Those fertilizers like ammonium sulphate (21% N) that contain only one of the Big 3 are called straight fertilizers. Others, like di-ammonium phosphate (18-46-0), contain two of the Big 3. Those such as 12-24-12 which contain N, P, and K are often called complete fertilizers, but this is misleading, because few of them contain all 12 plant mineral nutrients. However, some types may contain significant amounts of some secondary and micronutrients; check the label.

Some NP and NPK fertilizers are simple mechanical mixes of two or more fertilizers. Others are actual chemical combinations with every individual granule having the same nutrient content.

Colour as a likely nutrient indicator: The colour of a fertilizer’s granules is often a useful indicator of its general composition. Grey granules usually indicate an NP, NPK, or straight P fertilizer. White granules usually indicate a straight N fertilizer like urea, ammonium nitrate, or ammonium sulphate. However, potassium sulphate (0-0-50) and most forms of potassium chloride (0-0-60) are also white; some forms of potassium chloride are reddish due to impurities.

Physical Forms

• Most come as granules meant for soil application. Some granular fertilizers like ammonium nitrate and urea will also readily dissolve in water and can be sprayed on plant foliage in very dilute form or watered into the soil.
• Liquid formulations are available in some areas. Some can be used for soil application like granules. Others often contain NPK plus micronutrients and are meant for spray applications to the leaves (foliar applications); they are usually rather costly in relation to their nutrient content.
• Soluble powders containing NPK and/or micronutrients may also be available in your area and are meant for foliar application.

How to Read a Fertilizer Label

All reputable commercial chemical fertilizers carry a label giving their nutrient content, specifying not only the NPK content, but also the amounts of secondary nutrients and micronutrients.

The 3-Number Labelling System

With a few exceptions (notably those fertilizers that originate in South Africa), most countries use a universal 3-number labelling system that indicates the N, P, and K content in that order, usually in terms of N, P₂O₅, and K₂O. The numbers refer to percent. For example, a 12-24-12 fertilizer contains 12% N, 24% P₂O₅, and 12% K₂O; 200 kg of 12-24-12 contains 24 kg of N, 48 kg of P₂O₅, and 24 kg of K₂O. A 0-21-0 fertilizer contains 21% P₂O₅ but no N or K.

N-P₂O₅-K₂O vs. N-P-K

The N-P₂O₅-K₂O labelling system is traditional and dates back to the 19th Century when chemical fertilizers were first developed. The P and K contents were analyzed by burning (oxidizing) the fertilizer and then measuring the resulting P₂O₅ (called phosphoric acid or phosphorus pentoxide) and K₂O (potash, potassium oxide) that formed. The N-P₂O₅K₂O system is known as the oxide form of labelling.

In recent years, a few countries have switched over to to the elemental form (straight N, P, and K) for labelling and giving nutrient rates; in some cases, the label will give the fertilizer formula in both the oxide and the elemental forms.

Note that N content is given in terms of actual N in both systems.

Don’t be confused by this. It really makes little difference whether a fertilizer’s NPK content is expressed in the oxide or elemental form as long as the fertilizer labels and the nutrient rate recommendations given by the extension service both use the same form. A fertilizer’s true nutrient content is the same whether measured in the oxide or the elemental form, just as the distance between your village and the country’s capital is the same whether measured in kilometers or miles. Likewise, the sodium content of a pickle is the same whether measured as pure sodium or sodium chloride.

NOTE: Throughout this article we’ll use the N-P₂O₅-K₂O system since it’s still the most common. The terms “P” and “K” will often be used as a short form for phosphorus and potassium with no regard to either labelling system.

When the difference does matter: In some countries like the U.S., both systems are being used. In this case, you’ll want to double check and be sure whether the amount of phosphorus or potassium listed on a label or given as a fertilizer recommendation is in the oxide or the elemental form. This affects the actual amount of fertilizer needed, especially in the case of phosphorus. Here’s how to convert between the two systems:

Here are 2 practice problems to clear up any confusion:

Problem 1: Suppose soil test results recommend that Suheyla apply phosphorus at the rate of 30 kg of actual P (elemental P) per hectare. If the phosphorus content of the fertilizer is expressed in the oxide form (P₂O₅), how much P₂O₅ will be needed to supply 30 kg elemental P?

Solution:

Since P₂O₅ = P x 2.3, you’d multiply the 30 kg actual P by 2.3 to convert it to P₂O₅. The answer is 69 kg P₂O₅.

Problem 2: Suppose your country uses the elemental system in labelling fertilizers. You see a fertilizer with the formula 15-6.6-12.5 (N-P-K basis). What would the formula be in terms of N-P₂O₅-K₂O?

Solution: 6.6% P x 2.3 = 15% P₂O₅ 12.5% K x 1.2 = 15% K₂O

Therefore: 15-6.6-12.5 N-P-K formula equals 15-15-15 on an N-P₂O₅-K₂O basis.

Why Don’t the 3 Numbers Add Up to 100?

If you’ll look at the fertilizer composition table in Appendix D, you’ll notice that the percentages of N, P₂O₅, and K₂O don’t even come close to totalling 100. The main reason is that N, P, and K have to be combined with carriers like sulphur, calcium, oxygen, and hydrogen to become stable and usable.

Examples: Ammonium nitrate fertilizer (33-0-0) has the chemical formula NH4NO3. It contains 33% N with the rest being hydrogen and oxygen.

Single superphosphate (0-21-0) has the formula Ca(H2PO4)2CaSO4 . In addition to containing 21% phosphorus P₂O₅ basis), it has calcium, hydrogen, sulfur, and oxygen.

Another reason why the 3 numbers don’t total 100 is that some fertilizers have fillers like sand added so that they end up with a whole-number formula. In addition, conditioning agents are sometimes added to improve handling qualities.

Another Useful Term: Fertilizer Ratio

The fertilizer ratio is the ratio between the 3 numbers in a fertilizer’s formula and tells the relative proportions of N, P₂O₅ and K₂O (or N, P, K if the elemental system is used) in the fertilizer. Some examples:

Understanding fertilizer ratios is very useful when trying to match the kind of fertilizer to a recommendation. For instance, if soil test results recommend applying 30 kg N, 60 kg P₂O₅, and 30 kg KsO per hectare at planting time, this is a ratio of 1:2:1. It follows that any fertilizer with a 1:2:1 ratio could be used to supply the 3 nutrients in the right proportion and amount (i.e. 300 kg/ha of 10-20-10 or 250 kg/ha of 12-24-12).

Common Chemical Fertilizers and Their Characteristics

Nitrogen Fertilizers

Nearly all chemical N fertilizers contain either ammonium (NH4+) or nitrate (NO3-) nitrogen. The nitrate form is quicker acting because it’s more immediately mobile (leachable) and reaches the roots sooner if applied to a growing crop. But, remember that ammonium is rather quickly converted to mobile nitrate in warm soils (all of it within 7-10 days).

N fertilizers and soil pH: Most N fertilizers containing ammonium N have a gradual acidifying effect on the soil; this will be covered in detail farther along.

Loss of N by volatilization: All ammonium N fertilizers will release ammonia gas when applied to soils with pH’s above 7.0. If applied to the soil surface, significant amounts may be lost to the atmosphere. Urea fertilizer releases ammonia at any pH. Losses can be avoided by placing such fertilizers a few centimetres deep.

Common Nitrogen Fertilizers

Ammonium Nitrate (33-34% N)

• Contains half nitrate N and half ammonium N, so is quicker acting than straight ammonium fertilizers.
• Absorbs moisture and becomes slushy in high humidity; keep bags well sealed.
• Can become explosive if mixed with oil. Releases oxygen when exposed to fire which encourages combustion.

Ammonium Nitrate with Lime (26% N

• Same as above but is coated with dolomitic limestone to neutralize the acid-forming properties of regular ammonium nitrate and to reduce moisture absorption.

Ammonium Sulfate (20-21% N)

• In addition to N, it contains 23% sulfur (or 69% sulfate).
• Good handling and storage properties

Urea (45-46% N)

• The highest-strength solid form of N.
• Its N is initially in the amide form (NH2) but is converted to ammonium in moist warm soils within 1-2 days (a week or two in cooler soils) and then to nitrate by soil bacteria.
• Unlike ammonium N fertilizers, urea is mobile and leachable until its amide N has been converted to ammonium.
• Regardless of soil pH, some N will be lost to the atmosphere as ammonia gas if urea is left on the soil surface. Losses are highest above a soil pH of 7.0 and can reach 35% when urea is broadcast (spread) over grass pastures. Losses are minimal, however, if rainfall or irrigation occur within a few hours after such surface applications.
• Can “burn” (injure) seeds and seedlings if placed too close due to release of free ammonia.
• May sometimes contain excessive amounts of biuret ( toxic to plants) due to faulty manufacturing. Biuret is most toxic when urea is mixed with water and applied foliarly ( sprayed on the leaves).
• Tends to absorb moisture, but not as much as ammonium nitrate.
• Can be fed to ruminants like cattle as a protein source; the rumen bacteria convert the N to protein; BUT urea can be toxic at anything but very low levels and must be fed in combination with certain other feeds. Vinegar is the antidote.

Sodium Nitrate (16% N) (Chilean nitrate)

• Its nitrate N is readily leachable.
• Unlike most ammonium N fertilizers, it has a gradual basic effect on the soil.
• Can easily burn seeds and seedlings because of its very high salt content. (Fertilizer burn is covered farther along)
• Absorbs moisture and can become slushy in high humidity; keep bags well sealed.
• Expensive because of its low nutrient content relative to shipping costs.

Anhydrous Ammonia (82% N)

• Exists as a liquid under pressure and a gas when released into the soil.
• The highest-strength N fertilizer available.
• Must be injected into moist soil about 15 cm deep to avoid ammonia loss.
• Very dangerous; inhalation and facial exposure can cause blindness and fatal lung damage.
• Requires special storage and application equipment.

Aqua Ammonia (21% N)

• Made by dissolving ammonia gas in water. Has strong odor of ammonia. Unlike anhydrous ammonia, it doesn’t have to be applied or stored under pressure.
• Should be applied at least 4-5 cm below the soil surface to avoid loss of ammonia.
• Requires special storage and application equipment.
• Releases irritating fumes.

Potassium Nitrate (13-0-44: See under K fertilizers

Ammonium Phosphate Fertilizers: See under P fertilizers.

Time-Release or Slow-Release N Fertilizers: They’re coated with special substances that reduce their solubility and slow down the rate at which soil bacteria convert ammonium to nitrate. Leaching losses are much lower, but they’re usually too expensive to be cost effective for farmers.

Phosphorus Fertilizers

The phosphorus in most chemical fertilizers comes from reacting rock phosphate with sulfuric, phosphoric, or nitric acids or with anhydrous ammonia.

Water-soluble vs. Citrate-soluble vs. Insoluble P

A chemical fertilizer’s P can exist in several forms which should be listed on the label:

Water-soluble P: This type of P is soluble in water and moves quickly out of the granules into the soil. But, that doesn’t mean it will be 100 percent available to plants, because it’s still subject to the soil’s ability to tie up (fix) P. When P fertilizer is placed in a band, hole, or half-circle near the row, it’s recommended that at least half the fertilizer’s P be water-soluble. When P fertilizer is broadcast on soils below pH 7.0, water solubility isn’t important, because soil acidity helps dissolve the P.

Citrate-soluble P: This type of P isn’t soluble in water but will dissolve in a weak acid solution. Heat-treated rock phosphate contains largely citrate-soluble P which is usable only in acidic soils.

Insoluble P: This type of P isn’t soluble in water or a weak acid solution, so it has very limited availability to plants. Most of the P in raw rock phosphate is insoluble and only very slowly available, even in acid soils.

Common Phosphorus Fertilizers

Single Superphosphate (16-22% P₂O₅, 8-12% S): A common P fertilizer and also a good sulfur source. About 78% of its P is water soluble (see above). Made from rock phosphate and sulfuric acid.

Triple or Concentrated Superphosphate (42-48% P₂O₅): Has much more P than single super but only 1-3% sulfur. About 84% of its P is water soluble. Made from rock phosphate and phosphoric acid.

Ammonium Phosphate Fertilizers

There are 3 classes, all with 100% water-soluble P:

• Mono-ammonium phosphate (11-48-0, 12-61-0): Tends to work better than all-ammonium phosphate on alkaline soils. Low in sulfur. Less likely to cause burning than DAP.
• Di-ammonium phosphate (16-48-0, 18-46-0, 21-53-0): A good P source but can injure seeds or seedlings due to ammonia release if placed too close.
• Ammonium Phosphate sulfate (16-20-0, 13-39-0): Both are also good sources of sulfur (915% S in 16-20-0, 7% S in 13-39-0).

Miscellaneous NP and NPK Fertilizers: 20-20-0, 14-14-14, 12-24-12, etc.

Heat-treated Rock Phosphates: These vary a lot in P content and are made by heat treating rock phosphate which greatly increases its low availability. Its P isn’t water soluble but is citrate-soluble (see above) and will slowly become available in acid soils when broadcast. It may be a cheap P source in areas with phosphate deposits but is only recommended for acid soils or where organic matter is very high. It should be in a finely ground form and be applied by broadcasting to promote the release of its P through soil reaction. It doesn’t become available quickly enough to be used as the sole source of added P for short-term annual crops like maize. Much higher rates are needed than for more available forms. Where mycorrhizae soil fungi are abundant, they increase the availability of rock phosphate to plant roots.

Raw rock phosphate

Basic Slag (8-25% P₂O₅) A by-product of steel making. About 60-90% of its P is citrate soluble, so it’s best used on acid soils, much like heat-treated rock phosphate. It has a gradual basic effect on soils.

Potassium Fertilizers

The most common K fertilizers are:

• Potassium chloride (muriate of Potash): Contains about 60%-62% K₂O
• Potassium sulfate: Contains about 48-50% K₂O and 18% S.
• Potassium nitrate (13-0-44).
• NPK fertilizers like 10-20-10, etc.

NOTE: Tobacco, potatoes, and sweet potatoes are sensitive to high amounts of chlorides which affect crop quality. In this case, potassium chloride should be avoided or minimized.

Secondary Nutrient Fertilizers (Calcium, Magnesium, Sulfur)

Calcium and Magnesium

Even acid soils have enough calcium for most crops. Where liming is needed and magnesium is also deficient, dolomitic limestone (a mixture of calcium and magnesium carbonates) should be used. Liming with calcium only can also provoke a Mg deficiency. Gypsum has no effect on soil pH and is often used to supply calcium to crops with high needs, such as peanuts, without raising the pH. Magnesium sulfate (epsom salts; 9-11% Mg) and potassium magnesium sulfate (11% Mg) are other sources and have no effect on soil pH. The Mg content of fertilizers is often expressed in terms of magnesium oxide (MgO); the conversion is: Mg x 1.66 = MgO MgO x 0.6 – Mg

Sulfur

Some common fertilizers are good S sources like single superphosphate (8-12% S), ammonium sulphate (23-24% S), 16-20-0 (9-15% S), and potassium sulphate (17% S). Usually, the higher the NPK content of the fertilizer, the lower the S content (i.e. triple superphosphate contains only 1-3% S).

Sulphur deficiencies are on the increase in non-industrial areas, due to the growing use of high-analysis fertilizers with lower S contents. It’s usually a good idea to include a sulphur bearing fertilizer in a fertilizer program, especially on acid, sandy soils. Organic fertilizers are a good source of S. Appendix D lists the S content of chemical fertilizers. The S content of fertilizers is often expressed in terms of SO4 (sulphate). The conversion is: S x 3 = SO4

Micronutrient Fertilizers

Some NP and NPK fertilizers may have added amounts of micronutrients (check the label) but usually too little to correct deficiencies. If a meaningful amount of a micronutrient is present, it may be indicated by a fourth number in the fertilizer formula, referring to it.

Separate micronutrient fertilizers like copper sulphate, ferrous sulphate (iron), zinc sulphate, manganese sulphate, and borax can be used for soil or foliage (leaf) application. Remember that soil tie-up of added manganese and iron is often a problem on deficient soils.

Micronutrient chelates: Specially synthesized forms of micronutrients called chelates are available and used where soil tie-up problems are serious. A chelate has a special molecular structure that protects the micronutrient from being tied up.

Some fungicides like Maneb (containing manganese) and Zineb (containing zinc) can supply these micronutrients in conjunction with a disease control program.

Basic application principles for N, P, and K

Before covering the specific application methods for chemical fertilizers, let’s go over some important principles that affects how N, P, and K can be best applied.

Nitrogen Application Principles

Remember that nearly all chemical fertilizer N is mobile and leachable in the soil, because ammonium N is rapidly converted to mobile nitrate in warm soils. The sandier the soil and the higher the rainfall, the greater the potential leaching losses.

How to Combat Leaching Losses of N

If all the N is applied at planting or transplanting, much may be lost by leaching, especially since young plants have relatively small N needs. For annual crops, such as maize, tomatoes, and cabbage, it’s far better to “spoonfeed” the N by applying only 1/3-1/2 of the total (but no less than 30 kg/ha actual N) at planting or transplanting, usually as part of an NP or NPK fertilizer. The remaining 1/2-2/3 is applied in one to several sidedressings along the crop row, starting about 4 weeks after the initial NPK application. Sidedressings usually consist of a straight N fertilizer like urea or ammonium sulfate.

Guidelines for Side Dressing N

The number of sidedressings over which the remaining N is divided depends on 2 factors:

• The potential for leaching losses as influenced by texture and rainfall.
• The length of growing period for the crop.

Here are some examples:

Maize: Usually needs one side dressing around knee-high stage (about 4 weeks after planting in warm areas). Under high rainfall, especially on sandy soils, 2 side dressings are recommended: one at knee high, one at tasselling.

Vegetables: A very short season crop like radishes doesn’t need a side dressing. Leafy vegetables such as lettuce, pak choy, and amaranth may get one to several side dressings (at 3-4 week intervals), depending on whether the whole plant is harvested at once or picked a few leaves at a time over a longer period. Short-term cucurbits like summer squash and cucumber can use 1-2 side dressings, while longer-tare ones like melons and winter squash might need 2-3. Tomatoes will need from 2 to as many as 6 or more, depending on leaching conditions and length of production. A good interval between side dressings is 3-4 weeks.

Where to Place Side dressed N: We’ll cover this under application methods in a few pages.

How Deep to Place N: Since N is so mobile, it doesn’t have to be placed deep in order to reach the roots, but just enough (2-5 cm deep) to avoid being washed away by rain or losing N as ammonia gas.

Phosphorus Application Principles

The yield response obtained from applying fertilizer P to P-deficient soils depends a lot on how and when it’s applied. Learn these important guidelines:

• Apply P early: Young seedlings need a high concentration of P in their tissues for early growth and root development. One study showed that young maize seedlings take up 22 times more P per unit of length than plants 11 weeks old. P should be applied at planting or transplanting time.
• Remember that applying P in combination with N (if needed) helps stimulate P uptake.
• Application method has a big influence on the soil’s ability to tie up applied P. Broadcasting (spreading) fertilizer P usually results in far more tie-up than using a localized placement method (band, hole, or half circle) since it maximizes the contact of each fertilizer granule with soil particles than can cause tie-up. These methods will be explained in the upcoming section on fertilizer application
• Place broadcast P deep: It should be thoroughly mixed into the topsoil with a plow or hoe, except when spread around tree crops (this will be explained farther along under application methods).
• Don’t “spoon feed” P: Depending on application method, the mobility of P varies from nothing to very moderate. Leaching is never a problem, so all of the P can be applied in one application. There’s no advantage to making side dressings as growth proceeds unless P hunger signs develop.

Potassium Application Principles

K ranks midway between N and P in terms of mobility and leaching. As with P, all the K can usually be applied at planting or transplanting as part of an NPK fertilizer or as a straight K fertilizer. Where leaching losses are likely to be high, split applications of K may be needed. Split applications are also recommended for pastures to avoid “luxury consumption” of K.

Source: nzdl.org