Mixed fertiliser typically refers to a fertiliser containing two or more of the elements of nitrogen, phosphorus and potassium (NPK) which are essential for promoting plant growth and high crop yields. They are obtained by thoroughly mixing the ingredients either manually or mechanically.NPK mixture fertilisers are formulated and recommended by agricultural scientists to enhance the output of crops by giving it specific and exclusive blend of plant nutrients. They are slow releasing by nature and remain in the field for a long time. They are tailor made as per the soil and are crop specific.

Mixed fertilisers have a number of benefits, a few of them being:

  • Use of mixed fertilisers results in reduction of labour costs as applying a mixture consumes lesser time as compared to applying the components separately.
  • Micro nutrients which help in increasing soil organic matter content are applied in small amounts to the soil. They can be incorporated in fertiliser mixtures. This facilitates uniform soil application of plant nutrients.
  • If a proper mixture suits a particular soil type and crop, the use of a fertiliser mixture leads to balanced manuring. It results in higher crop yield.
  • Being in granulated form, mixtures have a better physical condition and hence their application is easier.
  • Residual acidity of of fertilisers can be controlled by using neutralisers in the mixture.

Types of Mixed Fertilisers

A mixed fertiliser means a mixture of two or more straight fertilisers. For example, ammonium sulphate and single super phosphate may be thoroughly mixed to get a mixed fertiliser. Mixed fertilisers are marketed as wheat fertiliser mixtures, paddy fertiliser mixtures etc.

Mixed fertilisers are of two kinds

1. Open formula mixed fertiliser: In this case, the manufacturing firms disclose the names and quantities of the straight fertilisers that are constituents of the mixed fertilizer.

2. Close formula mixture: In this case, the manufacturing firms do not disclose the names and quantities of the straight fertilisers that are constituents of the mixed fertiliser.

Materials Used to Make Mixed Fertilisers

The following materials are required to make the mixed fertiliser:

(i) Straight fertilisers are the primary materials used for making the mixed fertiliser. They supply the fertiliser element required in the proper amount to the soil.

(ii) Some low grade organic materials are added @ 100 pounds per ton to the mixed fertiliser in order to prevent caking, and to maintain them in a good physical condition. These organic materials (peat, paddy husk etc.) are called conditioners.

(iii) Some basic material like Dolomite is added to neutralize acidity if the fertiliser mixture produces acids.

(iv) Some materials like finely ground ash, soil etc. are added to make up the difference between the weight of the fertiliser necessary to supply the plant nutrients and the desired quantity of fertiliser mixture.

Incompatibilities in Fertiliser Mixture

A detailed knowledge of the properties of fertilisers is essential for preparing the fertiliser mixture.

The following kind of fertilisers should not be mixed at all or only mixed just before use:

(i) Ammonium sulphate, ammonium nitrate etc. should not mixed with basically reactive fertilisers like basic slag, rock phosphate etc., because they may decompose to liberate ammonia gas.

(ii) Fertilisers like single super phosphate, ammonium phosphate etc. which contain water soluble phosphorus should not be mixed with fertilisers then contain free lime because water soluble phosphorus would be converted to water insoluble phosphorus as shown below:

(iii) Some hygroscopic fertilisers like urea and sulphate of potash tend to form lumps after mixing. Therefore, they should be mixed only just before application.

(iv) If urea is to be mixed with a phosphatic fertiliser, it should be mixed with mono or di-ammonium phosphate, or ammoniated single super phosphate or ammoniated triple super phosphate.

(v) Urea should not be mixed with ammonium nitrate because the mixture will readily liquefy.

Granulation of Mixed Fertiliser:

Recently the granulated mixed fertilisers are being manufactured to decrease the separation of the constituent straight fertilisers and to prevent them from caking. There are more conveniently applied in the field and are difficult to adulterate. Most urea is either pilled or pelleted in sizes too large to form stable dry formulations with most other fertilisers. At present, a granular form of urea is also available.

Advantages of Mixed Fertilisers

1. Two or more fertiliser elements are added together to make a mixed fertiliser to be applied to the field. Less labour is therefore required for the application of a mixed fertiliser.

2. The fertiliser elements can be more uniformly applied to the field especially when they are required in small quantities.

3. Mixed fertilisers can be easily drilled in the filed because they are in a good physical condition.

4. When a mixed fertiliser has been prepared on a scientific basis and compensates for the deficiency of nutrients in the soils of the region, the farmer is assured of the right proportion of plant nutrients in the soil.

Disadvantages of a Mixed Fertiliser

1. The use of mixed fertiliser does not permit application of individual fertiliser elements, which may be required by the crop at specific times.

2. The illiterate farmer cannot effectively control the quantity of plant food present in the mixture. He has to rely entirely on the grade advertised by the firms.

3. The firms charge for mixing the fertilisers.