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close this book Soils, Crops and Fertilizer Use
close this folder Chapter 9: Using chemical fertilizers
View the document What are chemical fertilizers?
View the document Are chemical fertilizers appropriate for limited-resource farmers?
View the document An introduction to chemical fertilizers
View the document Common chemical fertilizers and their characteristics
View the document The effect of fertilizers on soil pH
View the document Fertilizer salt index and "burn" potential
View the document Basic application principles for N, P, and K
View the document Fertilizer application methods explained and compared
View the document Troubleshooting faulty fertilizer practices
View the document Getting the most out of fertilizer use: crop management as an integrated system
View the document Understanding fertilizer math

An introduction to chemical fertilizers


Chemical fertilizers contain one or more of the "Big 3" (N, P, and K) along with varying amounts of calcium and sulfur. 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 sulfate (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.

Color as a likely nutrient indicator: The color 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 sulfate. However, potassium sulfate (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.


• 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 for foliar application.


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 manual 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:

P x 2.3 = P2O5

P2O5 X 0.44 = P

K x 1.2 = K2O

K2O x 0.83 = K

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?


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 sulfur, 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:

Fertilizer Formula

Fertilizer Ratio













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).