The process for the production of a slow-release micronutrient fertilizer is described. The compound contains zinc, iron, manganese, and copper as micronutrients and is produced by polymerizing a system containing phosphoric acid, zinc oxide, hematite, pyrolusite, copper sulfate, and magnesium oxide followed by neutralization of the polyphosphate chain with ammonium hydroxide. Changes in temperature, density, and viscosity of the reaction system during polymerization were studied. Reaction kinetics was studied at three different temperatures. Rate curves revealed a multistage process with essentially linear rates at each stage. Thus, each stage displayed zero order kinetics. The product was crystalline and revealed ordering of P-O-P chains. It had low solubility in water but high solubility in 0.33 M citric acid and 0.005 M DTPA. Three different field trials showed significant yield increments using the slow-release micronutrient fertilizer compared to the conventional micronutrients. Yield increments in rice were in the range of 10–55% over control (with no micronutrient) and up to 17% over the conventional micronutrient fertilizers. There were significant increases in total uptake of zinc, iron, and manganese in the grain. Slow-release fertilizers also produced significant yield increases in potato as well as significant increase in vitamin C content of the tuber.
Micronutrients are essential components of proteins and enzymes and are vital for increasing crop yields as well as improving the nutritional quality of food. The bulk of micronutrients used all over the world today are water soluble salts that include mainly the sulfates or their chelated forms (EDTA, DTPA, etc.). Water solubility of these materials results in leaching and run-off, nutrient fixation by soil, high dosage requirements, and nonattractive cost economics. Slow-release fertilizers (which have low water solubility) are, therefore, considered as the solution to the problem. The focus of research on slow-release fertilizers has been on the macronutrients (NPK). Here slow-release functionality has been achieved by encapsulation of water soluble materials within a membrane or conversion to polymers of the urea aldehydes [
The slow-release fertilizer in this study has been developed with a different mechanism of nutrient release. Here, plant roots are able to “digest” certain insoluble compounds by ion-exchange with the root hairs or by extracellular organic acid secretions that extract nutrients by chelation. These compounds have low water solubility and high solubility in citrate and diethylene triamine penta acetic acid (DTPA) [
Whereas the earlier work mainly focused on individual cationic slow-release compounds [
Here, we report the process for production of slow-release multimicronutrient fertilizer of Zn, Fe, Mn, and Cu. Studies on polymerization kinetics, process parameters, changes in physical properties, and product characterization were done. The process was optimized on a small pilot plant. Finally, field trials were conducted with three different crops. Results indicate that the compound is a very promising fertilizer.
Based on the average levels of Zn, Fe, Mn, and Cu in plants [
Synthesis was done with commercial grade chemicals, namely, hematite, pyrolusite, zinc ash, copper sulfate, roasted magnesite, and phosphoric acid. All raw materials were analyzed prior to synthesis using analytical grade reagents. Iron in hematite was determined spectrophotometrically as the o-phenanthroline complex [
Fertilizer production was done in a pilot plant which was a 20 L acid-proof brick lined reactor vessel. 10 kg of phosphoric acid was taken in the vessel and 1.25 kg zinc ash was added to it. This was followed by 0.69 kg, 0.324 kg, 0.357 kg, and 0.096 kg, respectively, of hematite, pyrolusite, copper sulfate, and magnesite. The mole ratios of the cations added with respect to moles of phosphorus are thus Zn/
Changes in temperature, density, and viscosity in the course of polymerization reaction were recorded. Temperature was recorded
Since the reaction for formation of polyphosphates is a polycondensation reaction, kinetics is most conveniently studied by recording weight loss (water loss) of the system [
Chemical analysis of the fertilizer for P was done by fusion of 0.1 g with 1.5 g of NaOH beads and extraction in 0.2 N HCl. P was determined spectrophotometrically as described above. Cations were determined after digestion with concentrated H2SO4-H2O2 and the solution analyzed for Zn, Fe, Mn, Cu and Mg as described earlier.
Solubility of the fertilizer in water was determined by adding 50 mL water to 0.1 g fertilizer, agitating for 60 min followed by filtration. The solution was analyzed for Zn, Fe, Mn, Cu, and P. Solubility in 0.33 M citric acid and 0.005 M DTPA was determined by a similar procedure except that water was replaced by 0.33 M citric acid and 0.005 M DTPA.
Infrared (IR) spectra of the powdered sample were recorded on a Perkin Elmer Fourier transform infrared (FTIR) RX1 instrument with the scan range of 4500–450 cm−1 (resolution ± 5 cm−1) using KBr pellets. X-ray diffraction (XRD) was recorded on a Philips PW 1140 X-ray diffractometer using Ni-filtered CuK
Field tests were conducted with three crops at two different locations. These were at (i) Baruipur (South-24 Parganas), West Bengal, India, (fine, mixed, hyperthermic aeric endoaquept; new alluvium), pH 5.73,
Temperature optimization studies showed that the reaction was 30.3% faster at 200°C than at 185°C and 19.6% faster at 225°C than at 200°C. Temperature in the range of 175–185°C required very long periods for completing polymerization. Again, higher temperature (225°C) consumed more energy and was not much faster compared to 200°C. Thus, temperature of 200°C appeared to be the optimum.
Changes in temperature, density, and viscosity in course of polymerization of Zn-Fe-Mn-Cu-Mg polyphosphate systems are shown in Figures
Temperature change during the polymerization reaction.
Density increases almost linearly with temperature, reaching 2.13 g/cc after 280 minutes of heating (Figure
Density change during the polymerization reaction.
Viscosity change during the polymerization reaction.
Kinetics of condensation of Zn-Fe-Mn-Cu polyphosphates is shown in Figure
Kinetics of polymerization at 175, 200 and 225°C.
At the start of reaction, metal oxides would react with phosphoric acid to produce the respective dihydrogen phosphates. Subsequent (partial) polymerization to the polyphosphates may be represented as
The compound is a zinc iron manganese copper magnesium ammonium polyphosphate with a composition of 9.85% ZnO, 3.70% Fe2O3, 2.03% MnO2, 0.73% CuO, 0.49% MgO, 13.5%
Solubility, IR, and XRD characteristics of the slow-release fertilizer.
Chemical component | % of total solubilized in | IR absorption in wavenumber (cm−1) | XRD peaks | ||||
---|---|---|---|---|---|---|---|
Water | 0.33 M citric acid | 0.005 M DTPA | Wavenumber |
Absorbance | Basal spacing |
Intensity ( | |
Zn | ND | 93.7 | 83.9 |
447.7 |
42 |
6.29 |
16 |
Fe | ND | 93.2 | 91.8 | ||||
Mn | ND | 90.9 | 80.1 | ||||
Cu | 1.8 | 96.8 | 91.3 | ||||
P | 8.4 |
Infrared (IR) spectrum of the compound (Figure
IR spectra of zinc-iron-manganese-copper fertilizer.
X-ray diffraction (XRD) characteristics of Zn-Fe-Mn-Cu-Mg polyphosphate are shown in Figure
XRD of zinc-iron-manganese-copper fertilizer.
Results of field trials showed that yield of rice grains increased by 35–55% in slow-release fertilizer treated plots over the control in the experiment at Baruipur (new alluvium). Yields were also higher compared to the micronutrient sulfate treated plots by 51.1, 38.9, and 17% for treatments P1, P2, and P3, respectively, over the corresponding micronutrient sulfate treatments, S1, S2, and S3. Total uptake of Zn, Fe, and Mn in the polyphosphate treated plots was significantly higher than control and micronutrient sulfate treatments at 1% level (Table
Field trial using slow-release fertilizers on rice and potatoes: yields, nutrient uptake, and vitamin C.
Treatments |
Rice at Baruipur (new alluvium) |
Rice at Nalikul (old alluvium) |
Potato at Nalikul (old alluvium) |
|||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Yield (kg/ha) | Total uptake (g/ha) | Yield (kg/ha) | Total uptake (g/ha) | Yield (kg/ha) | Total uptake (g/ha) | Vitamin C (mg/100 g) | ||||||||||
Zn | Fe | Mn | Cu | Zn | Fe | Mn | Cu | Zn | Fe | Mn | Cu | |||||
C | 3242 | 70.6 | 246 | 140 | 21 | 3698 | 76 | 492 | 206 | 41 | 9900 | 217 | 764 | 69 | 91 | 6.21 |
S1 | 3076 | 70.6 | 350 | 158 | 22 | 3724 | 80 | 499 | 216 | 42 | 10700 | 239 | 948 | 107 | 97 | 4.53 |
S2 | 3596 | 75.6 | 303 | 162 | 26 | 3961 | 78 | 593 | 236 | 47 | 9500 | 216 | 915 | 93 | 97 | 5.33 |
S3 | 3748 | 85.7 | 341 | 179 | 23 | 3831 | 87 | 577 | 229 | 44 | 11000 | 277 | 756 | 85 | 103 | 5.94 |
P1 | 4647b,d | 105.1b,d | 472a,c | 236b,d | 33 | 4056 | 117b,d | 597 | 332b,d | 70 | 14075a | 432a,c | 1966a | 151a | 163 | 8.44a,d |
P2 | 4998b,d | 120.8b,d | 615b,d | 288b,d | 33 | 4397b,c | 122b,d | 876b,c | 351b,d | 76 | 12550 | 396 | 1756 | 140 | 148 | 6.31 |
P3 | 4383b | 110.6b,c | 492a | 229b | 34 | 4462b,d | 114b,c | 693 | 291b,c | 48 | 16025b,d | 560b,d | 2569b,d | 225b,d | 198a,c | 8.11c |
|
||||||||||||||||
CD (5%) | 758.3 | 22.4 | 215 | 559 | 373.2 | 208 | 241.6 | 596 | 3460.0 | 83.9 | 479.5 | 33.3 | 33.1 | 2.07 | ||
|
||||||||||||||||
CD (1%) | 1021.2 | 29.8 | 286.5 | 74.4 | 502.6 | 281 | 325.3 | 80.2 | 4687.5 | 113 | 645.6 | 44.9 | 44.6 | 2.81 |
C: Control; S1–S3: sulfates of Zn, Fe, Mn, and Cu; P1–P3: slow-release micronutrients; S1 and P1: 1 kg/ha Zn + 0.33 kg/ha Fe + 0.165 kg/ha Mn + 0.083 kg/ha Cu; S2 and P2: 2 kg/ha Zn + 0.66 kg/ha Fe + 0.33 kg/ha Mn + 0.165 kg/ha Cu; S3 and P3: 4 kg/ha Zn + 1.33 kg/ha Fe + 0.66 kg/ha Mn + 0.33 kg/ha Cu. aSignificant with respect to control at 5% level. bSignificant with respect to control at 1% level. cSignificant with respect to the corresponding water-soluble treatment at 5% level. dSignificant with respect to the corresponding water-soluble treatment at 1% level.
In conclusion, the slow-release micronutrient fertilizer reported here is a crystalline polyphosphate of Zn-Fe-Mn-Cu-Mg-NH4. It had low water solubility but was almost completely soluble in organic acids, namely, 0.33 M citric acid and 0.005 M DTPA which suggests good plant availability. Reaction kinetics showed complex pattern with several stages, each being nearly linear. Features suggested a condensation reaction of P-OH groups with zero order rates that changed with degree of polymerization. P-O-P linkages were evidenced by IR spectra. XRD showed long range order of polyphosphate groups. All three field tests resulted in significantly higher yields in the slow-release fertilizer plots over the control plots as well as over the conventional micronutrients. Plant uptake of micronutrients was also higher with the slow-release fertilizer. Potato tubers had significantly increased levels of vitamin C.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank Professor P Ray, Department of Chemical Engineering, Calcutta University, for his suggestions. They also thank NBSS & LUP, ICAR, Nagpur, for XRD scans and Professor A Patra, Department of Chemistry, Calcutta University, for the IR analysis.