🎯 Controlled-Release Fertilizers

Controlled-release fertilizers improve nutrient-use efficiency by synchronizing nutrient availability with crop demand. This lesson focuses on zeolite-based systems and nitrification inhibitors.

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Controlled-Release Fertilizers Using Zeolites

The U.S. Geological Survey (USGS) has experimented with zeolites to help

control the release of fertilizer nutrients in soil. The use of soluble fertilizers can lead to

water pollution and to wasted nutrients. Nitrogen, for example, can leach into ground and

surface waters, especially in sandy soils, and phosphate may become fixed and

unavailable to plants, especially in tropical soils. Zeolites are porous minerals with high

cation-exchange capacity that can help control the release of plant nutrients in

agricultural systems. Zeolites can free soluble plant nutrients already in soil, and may

improve soil fertility and water retention. Because zeolites are common, these unique

minerals could be useful on a large-scale in agriculture.

Controlled-Release Nitrogen Fertilizer

Urea is one of the most common nitrogen fertilizers. It is very soluble in water, and can

be leached through the root zone. In addition, urea is converted into ammonium ions by

an enzyme found in most soils. Soil bacteria then convert these ammonium ions into

readily leachable nitrate ions. Using zeolitic rocks in fertilizer can help prevent these

nutrient losses.

Controlled-Release Phosphorous Fertilizers

Phosphate (H 2 PO 4 ) can be released to plants from phosphate rock (P-rock) composed

largely of the calcium phosphate mineral apatite by mixing the rock with zeolite having

an exchange ion such as ammonium. The approximate reaction in soil solution is as

follows: (P-rock) + (NH 4 -zeolite) = (Ca-zeolite) + (NH 4+ ) + (H 2 PO 4- ).

The zeolite takes up Ca 2+ from the phosphate rock, thereby releasing both phosphate

and ammonium ions.

Growth responses to controlled-release fertilizers

Most controlled-release fertilizers are N-based, and most of the research involving

them has evaluated plant responses to N application. Additionally, because most

controlled-release N sources cost several times more per pound of N than the soluble

sources, most of the evaluation has been conducted on higher-cash-value crops such as

ornamentals, vegetables, citrus and turfgrasses. Little research has been conducted on

agronomic crops because their use in this sector is not considered economically

feasible. Technologies currently under development may reduce the cost of controlled

release products to the point that they can be used on agronomic crops, but such is not

yet the case.

Nitrification inhibitors

It should be non-toxic plants, soil microorganisms,animals,fish and mammals

It should block the conversion of NH3 to NO3 by inhibiting Nitrosomonas activity

It should not interfere with the transformation of NO3 (nitrite) by Nitrobacter

It should be able to move with the fertilizer so that it will be distributed uniformly through

the soil zone contacted by nitrogen fertilizer

It should be stable for the inhibitory action to last for an adequate period of time

It should be relatively in expensive, so that it can be used on a commercial scale

There are various nitrification inhibitors, of which N-Serve or nitrapyrin and AM

are most important.

N-Serve- It is 2-choloro-6(trichloromethyl) pyridine and also referred to as nitrapyrin

AM- Chemically it is a substituted pyrimidine (2-amino-4-chloro-6- methyl pyrimidine)

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Summary Cheat Sheet

Topic	Key exam point
Main concept	Fertilizer technology designed to release nutrients at a controlled rate over time
Examples	Polymer-coated, resin-coated, zeolite-based, and inhibitor-assisted fertilizers
N focus	Controlled-release nitrogen fertilizers are the most common exam examples
P focus	Controlled-release phosphorus systems are also discussed in specialized formulations
Main advantage	Better synchronization between nutrient release and crop demand
Nitrification inhibitors	Delay conversion of ammonium to nitrate and help reduce nitrogen loss
Agronomic effect	Can improve growth response and nutrient-use efficiency
Key distinction	Controlled-release is more precisely engineered than generic slow-release fertilizers
Typical use	High-value crops, nursery media, and situations with high nutrient-loss risk
Trap	Do not confuse coated fertilizer with inhibitor-based fertilizer; both control availability differently

References

3 sources • [1] [2] [3]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook

Book

[3]

Department of Agriculture & Farmers Welfare

Website