Agriculture for Engineers 4 (3+1)

Lesson 16. Chemical fertilizers

16.2 Fertilizers

16.2.1 Straight Fertilizers

When a fertilizer contains a single nutrient, it is called a straight fertilizer. Such fertilizer is called nitrogenous or phosphatic or potassic e.g. urea.

16.2.2 Complex or Compound Fertilizers

Two or more nutrients in one compound is known as complex or compound fertilizers. These fertilizers is granular and easy to apply. Examples are:

16.3 Nitrogenous Fertilizers

The nitrogenous fertilizers are broadly classified into 4 groups (Table 16.1).

Table 16.1 : Classification of Nitrogenous Fertilizers

Sources of nitrate	Name of fertilizer	Percentage of nitrogen	%P	Reaction (Acidity/Basicity)
Nitrate	Sodium nitrate (NaNO3) Calcium nitrate Ca(NO3)2 Potassium nitrate (KNO3)	16.0 15.0 13.0		Basic   -do-   -do-
Ammonium	Ammonium sulphate (NH4)2 SO4   Ammonium chloride (NH4Cl) Diammonium phosphate (NH4)2HPO4	20.6     25.0       18.0	_         46	Acidic     -do-         -do-
Nitrate and Ammonium	Ammonium nitrate (NH4NO3)   Ammonium sulphate nitrate (NH4)2 SO4, (NH4NO3)   Calcium ammonium nitrage	33.5       26.0       20.5		-do-         -do-       Neutral
Amide	Urea CO(NH2)2   Calciumcynamide (CaCN2)	46.0   22.0		Acidic   Basic

 

16.3.1 Characteristics of nitrogenous fertilizers

According to characteristics, nitrogenous fertilizers are classified in following four groups:

16.3.1.1 Nitrate fertilizers. The most common nitrate fertilizers in use is sodium nitrate or calcium nitrate.

(a) Nitrate group of fertilizers are soluble in water and hygroscopic (absorbs moisture from the atmosphere to become sticky).

(b) They are alkaline in nature. Constant use of sodium nitrate, creates deflocculation of clay particles and poor drainage.

(c) Contains less percentage of nitrogen than other groups so that its use is diminishing at a fast rate.

(d) They are completely soluble in water and readily available for the use of plants as such, without any chemical change in the soil.

(e) The nitrate is not retained (adsorbed) by the soil and is liable to fast leaching.

(f) They are applied in small doses and repeated at intervals on standing crops (top dressing).

(g) It is not suitable for rice crop in early stage of growth. Because rice plants take nitrogen in ammonical form.

16.3.1.2 Ammonium fertilizers. This group of fertilizers are in wide use particularly ammonium sulphate.

(a) Ammonium fertilizers are water soluble but non-hygroscopic.

(b) They are acidic in nature.

(c) High level of nitrogen than nitrate fertilizers.

(d) They are less readily available to plants than nitrate fertilizers. The ammonical nitrogen has to nitrify in the soil and be converted into nitrate before it can be taken up by plants.

(e) The ammonia in ammonium sulphate is fixed (adsorbed) by the soil immediately after application and it is not leached away like nitrates. Repeated and heavy doses of ammonium sulphate without adequate supplies of lime in the soil will lead to acidity in the soil.

(f) This group of fertilizers may be used in basal application and top dressing.

16.3.1.3 Nitrate and Ammonium Fertilizers

(a) These group of fertilizers are soluble in water and slightly hydroscopic (e.g., Ammonium Nitrate – highly hygroscopic ;Ammonium Sulphate Nitrate – slightly ; Calcium Ammonium Nitrate – slightly).

(b) In this group both nitrate and ammonium are available.

(c) Readily available to plants. With its nitrate-nitrogen, the plant drives it immediately. Ammonium form of nitrogen provides a steady source of N.

(d) Availability of ammonium reduces leaching loss.

(e) Acidic in nature but exception is calcium ammonium nitrate which is neutral in reaction.

(f) Used in top dressing and basal dressing.

16.3.1.4 Amide Fertilizers

(a) Amide fertilizers are soluble in water and hygroscopic in nature.

(b) These fertilizers are converted to ammonium carbonate and then to nitrates due to action of microorganism. The conversion of amides into ammonical and nitrate from takes about 6-7 days.

(c) Leaching loss is very less because once amide is converted to ammonical form it is adsorbed by soil colloids and slowly released and nitrified to nitrates.

(d) Amide fertilizers i.e., urea and calcium cynamide are synthetic organic fertilizers.

Urea: It is slightly acidic in nature. Urea application to soil creates a small loss of calcium from the soil. Urea is highly concentrated nitrogenous fertilizer containing 46% nitrogen. Therefore, its higher concentration may injure the plant roots or germinating seeds. It is desirable that urea be mixed with ashes or small quantity of soil to facilitate an even distribution to avoid risk of injury to plants. The toxic ingredient present in urea is biuret. It is cheapest or economical fertilizer. It can be used in liquid form as foliar application.

16.4 PHOSPHATIC FERTILIZERS

Phosphorous fertilizers are classified into: (A) water-soluble, (B) Citrate-soluble, and (C) Insoluble.

16.4.1 Fertilizers containing water-soluble phosphorus characteristics

1. Phosphate is water-soluble and quick acting (available). (Table 16.2)
2. Very less leaching loss in this group of fertilizers.
3. Should be applied in neutral and alkaline soils.

Table 16.2 : Fertilizers containing water-soluble phosphorus

Name of fertilizer	Chemical composition	Percentage composition	Acidity of alkalinity
Sulperphosphate  (ordinary)         (single super phosphate)	Ca(H2PO4)2	16-20	Neutral
Superphosphate   (concentrated) (triple super phosphate)	3Ca(H2PO4)2	40-48	Neutral or acidic
Diammonium phosphate	(NH4)2HPO4	46-48	Acidic

 

16.4.2 Fertilizers containing citrate-soluble phosphorus characteristics

(1) Insoluble in water but soluble in citric acid, so that it does not become readily available to plants (16.3).

(2) No leaching loss.

(3) These are slow acting fertilizers therefore, applied in the soil 15-30 days before sowing.

(4) These fertilizers should be used in natural and acidic soils.

Table 16.3 : Fertilizers containing citrate-soluble phosphorus

Name of fertilizer	Chemical composition	Percentage composition P2O5	Acidity or alkalinity
Basic slag (Indian)	(CaO)3. P2O5.SiO2	3-5	Alkalinity
Dicalcic phosphate	CaHPO4	35-40	Acidic

 

16.4.3 Fertilizers containing insoluble phosphorus characteristics

(1) Because of their insolubility and their slow availability, fertilizers of this be applied in the soil about 2 months before sowing, phosphorus is available in the form of tricalcium phosphate (Table 16.4).

(2)  Generally, deep placement is done in highly acidic soils.

Table 16.4 : Fertilizers containing insoluble phosphorus

Name of fertilizer	Chemical composition	Percentage composition     P2O5                  N	Acidity or alkalinity
Rock phosphate	Ca3(PO4)2	20 - 30
Bone meal	--	18 - 20                 3	Alkaline

 

16.5 POTASSIC FERTILIZERS

(1) Potassium fertilizers are water-soluble but not hygroscopic in nature (Table 16.5).

(2) They are readily available to plants.

(3) There is not leaching loss.

(4) As they are neutral in reaction so have little or no effect on the soil pH.

Table 16.5 : Potash containing potassic fertilizers

Fertilizer	Chemical form	Percentage composition of       K2O	Reaction
1. Potassium chloride        (mutriate of potash)	KCL	60	Neutral
2. Potassium sulphate	K2SO4	50	Neutral

 

16.6 Sulphur containing fertilizers

(1)  Sulphur containing fertilizers are mostly water-soluble.

(2)  Sulphur from these fertilizers is readily available to plants except S from elemental S, because it required sequential oxidation to convert SO₄^-2 from S (Table 16.6).

(3)  Continuous use of S containing fertilizer decrease pH of soil.

Table 16.6 : Sulphur containing fertilizers

Fertilizer	Sulphur content (%)
Ammonium sulphate	24
Elemental S	95-98
Single super phosphate	12

 

16.7 Micronutrients fertilizer

The inorganic sources of micronutrient fertilizer are following:

16.7.1 Sulphate (salts)

The sulphate form of micronutrients such as: Cu, Zn, Fe and Mn, represents a water-soluble form that is plant available. Borate is the equivalent plant available form for B.  Sulphates are the most commonly used form for field crops.  Sulphates can be applied to the soil or foliage.  Sulphate products, applied at agronomically recommended rates, can provide long term residual value. e.g. zinc sulphate, ferrous sulphate, manganese sulphate, copper sulphate etc.

16.7.2 Chelate

Chelates are complexes of certain micronutrients (the positively charged ones like iron and zinc) with organic molecules, to form compounds that can hold the micronutrients in a form that is available to plants.  Chelated micronutrients are much slower to form unavailable compounds, or to leach away.  Chelating compounds occur naturally in organic material in the soil, or can be synthesized to make chelated fertilizers.  Plant roots are able to take the micronutrient directly from the chelate, and release the chelating compound back to the soil. For example, a synthetic chelating agent is EDTA, and a natural chelating agent is citric acid. Chelates are generally many times more expensive than the sulphate or oxide forms, but this is partly compensated for in the low recommended rate of chelate product needed to supply the micronutrient. e.g. EDTA –Zn, EDTA –Fe,  EDDHA – Fe. 

16.8 NUTRIENT CONTENT OF CHEMICAL FERTILIZERS         

SN	Fertilizers	Nutrients (%)
		N	P2O5	K2O	S	Other
1	Urea	46	-		-	-
2	Ammonium sulphate	21	-		24	-
3	Ammonium chloride	26	-		-	-
4	calcium ammonium nitrate	25	-		-	8.1% Ca
5	Single super phosphate	-	16		12	20% Ca
6	Potassium chloride (muriate of potash)	-	-	60	-	Cl
7	Potassium sulphate (sulphate of potash)	-	-	50	18	-
8	Diammonium phosphate	18	46	-	-	-
9	Rock phosphate	-	18-20	-	-	Ca
10	Phosphogypsum	-	-	-	16	21% Ca
11	Magnesium sulphate	-	-	-	13	16% Mg
12	Borex	-	-	-	-	10.5% B
13	Copper sulphate	-	-	-	13	24% Cu
14	Ferrous sulphate	-	-	-	12	19% Fe
15	EDTA-Fe	-	-	-	-	12% Fe
16	Manganese sulphate	-	-	-	15	30.5% Mn
17	Zinc sulphate	-	-	-	11-16	21 % Zn
18	EDTA-Zn	-	-	-	-	12% Zn