PhD

A study entitled “Effect of thermochemical organic fertilizer on soil carbon
pools, nutrient dynamics and crop productivity in Ultisols” was conducted from 2018
to 2020 at the Department of Soil Science and Agricultural Chemistry, College of
Agriculture, Vellayani with the objective to study the effect of thermochemical
organic fertilizer on soil carbon pools, nutrient dynamics, their retention and leaching,
and crop productivity in comparison with conventional organic fertilizers in Ultisols
using tomato - amaranthus cropping sequence.
The study includes production and characterization of organic fertilizers,
leaching study using soil columns, an incubation experiment and field experiments
using tomato - amaranthus cropping sequence. For the leaching study and incubation,
the treatments were addition of FYM, ordinary compost (OC), vermicompost (VC),
microbial compost (MC), unfortified (TOF) and fortified thermochemical organic
fertilizer (F-TOF) @ 50 g per soil column/ pot and an absolute control. For the field
experiment on tomato-amaranthus cropping sequence, the treatments were T 1 – FYM
+ POP recommendation of NPK, T 2 - FYM + soil test based recommendation of NPK
(STBR), T 3 - OC + STBR, T 4 - VC + STBR, T 5 - MC + STBR, T 6 - TOF + STBR, T 7 -
F-TOF + STBR, T 8 - F-TOF alone and T 9 - absolute control.
The organic fertilizers required for the study viz., OC, VC, MC, TOF and F-
TOF were produced from bio-waste from vegetable markets and food waste from
college hostels and FYM was purchased. The physical and chemical properties of
organic fertilizers were in accordance with the standards of FCO. VC, OC, MC and
FYM were neutral to slightly alkaline in reaction while TOF and F-TOF were slightly
acidic. The lignin content (27.9 %) and the carbon pools viz., TOC (43.90 %),
WSOC (1642 mg kg -1 ), labile carbon (1776 mg kg -1 ) and recalcitrant organic carbon
(32.45 mg kg -1 ) were highest for TOF followed by
F-TOF. The N content was
391highest for MC (2.61 %), P for VC (1.36 %) and K (2.56 %), Ca (1.12 %), Mg (0.78
%), Zn (254 mg kg -1 ) and B (4.64 mg kg -1 ) for F-TOF while S (550 mg kg -1 ), Fe
(9580 mg kg -1 ) and Pb (4.16 mg kg -1 ) for FYM. All the organic fertilizers contained
Pb, but within permissible limit, while, Cd was not detected in any of them.
The soil column study was conducted to assess the extent of leaching loss of
nutrients from F-TOF amended soil in comparison with other organic fertilizers and
their nutrient retention ability in soil. Soil columns amended with organic fertilizers
in the surface layer were subjected to leaching on 1 st , 4 th , 8 th , 12 th , 16 th , 20 th and 24 th
weeks with double the pore volume of water. During the leaching, the mineralized
nutrients moved downwards and deposited at different depths in the soil column in
accordance with their mobility and the rest was lost in leaching water. Leachates from
organic fertilizer amended soils showed slightly acidic pH, which decreased up to 8 th
week followed by an increase towards 12 th week. EC was highest at first week
followed by a decrease towards 24 th week. The highest cumulative loss of N (172.34
mg L -1 ), Ca (273.86 mg L -1 ) and Mn (3.97 mg L -1 ) was from VC while that of P (7.22
mg L -1 ), K (333.36 mg L -1 ), Mg (144.41 mg L -1 ), Cu (0.080 mg L -1 ) and B (0.166 mg
L -1 ) was from F-TOF. For S the loss (4.19 mg L -1 ) was highest from FYM, and Fe
(4.71 mg L -1 ) and Zn (4.58 mg L -1 ) from OC. The leachate did not contain Pb and Cd.
The leached soil in the soil columns maintained a higher level of nutrients
compared to the level prior to the addition of organic fertilzers even after leaching for
24 weeks. In the surface soil, highest quantity of total N was retained by MC; P, Mn
and Cu by VC and K, Ca, Mg, Zn and B by F-TOF while FYM retained highest
quantity of S and Fe. Evaluating the available nutrient status of the leached soil, it
was found that F-TOF had highest availability for K, Mg, S, Fe, Zn, Cu and B.
Availability of P and Mn was highest in VC amended soil and Ca from MC.
Availability of Pb and Cd were not detected in the leached soil.
392The incubation study for a period of 24 weeks revealed the nutrient release
pattern of organic fertilizers. The peak release of most of the nutrients from organic
fertilizers was from 8 th to 16 th week and for S it extended up to 20 th week. The
availability of K, Ca, Mg, Fe, Zn and B was highest from F-TOF amended soil while
VC maintained the highest values for P, Mn and Cu and FYM for S. The different
fractions of B were highest for F-TOF amended soil and the peak was during 12 th
week of incubation. Available Pb and Cd were not detected in the incubated soil.
Organic fertilizers amended soil maintained a higher microbial count and exhibited a
higher dehydrogenase activity compared to the control and the highest value was
observed with F-TOF amended soil.
During the field experiments, in the first cropping sequence, VC amended
treatment T 4 (VC + STBR) recorded significantly highest fruit yield (40.97 t ha -1 ) for
tomato followed by T 7 (F-TOF + STBR) while in the second cropping sequence F-
TOF gave the highest yield which was statistically on par with VC. While for
amaranthus, F-TOF recorded the highest yield during both the cropping sequences
(24.62 t ha -1 and 26.89 t ha -1 , respectively) followed by VC and both the treatments
were statistically on par in the second cropping sequence.
The quality parameters of tomato and amaranthus were highest for the
treatment T 7 (F-TOF + STBR) but was statistically on par with other treatments
which received organic fertilizers along with POP or soil test based NPK fertilizers.
Evaluating the economic benefits, the performance of VC was the best for the first
tomato followed by F-TOF while for second tomato F-TOF was the best. For
amaranthus, F-TOF performed best during both the sequences. When the overall B:C
ratio for the whole cropping sequence was taken F-TOF was the best followed by
VC.
Uptake of N, P, K, Ca, Mg, S, Zn and B in tomato was highest for F-TOF
while that of Fe, Mn, and Cu was for VC. For amaranthus, the uptake of N, P, K, Mg,
Fe, Zn, Cu and B was highest for F-TOF for both the cropping sequences. Among the
393heavy metals tested only Pb was detected in plant parts and that too in roots only. For
tomato, it was detected only in FYM treated plants while in amaranthus, all the
treatments receiving organic fertilizers showed the presence of Pb. But the Pb content
in the root was within the safe limit. The availability of Pb in the post-harvest soil
was trace and there was no significant difference between the treatments. Cd was not
detected in soil as well as plant samples.
The continuous application of organic fertilizers had improved the physical,
chemical and biological properties of the soil. At the end of the second cropping
sequence, the lowest bulk density and highest water holding capacity was recorded by
the treatment receiving F-TOF + STBR. The highest value for TOC, labile carbon,
microbial biomass carbon and recalcitrant organic carbon in the post-harvest soil
during the cropping sequence was maintained by F-TOF. The N pools, NH 4 -N, NO 3 -
N, organic N and total N at the end of second cropping sequence was also highest for
F-TOF. In the case of availability of P, S, Fe, Mn and Cu in the post-harvest soil,
better performance was showed by VC while F-TOF showed higher availability for
K, Ca, Mg, Zn and B. Biological properties are a better indication of soil health and
application of VC, MC and F-TOF maintained a higher microbial load in soil. The
highest dehydrogenase activity was maintained by F-TOF. Continuous application of
F-TOF and TOF increased the carbon stock of surface and sub-surface soil than other
organic fertilizers.
F-TOF is superior to VC, OC, MC and FYM in terms of increasing carbon
pools and carbon stock of the soil. The nutrient release was highest from F-TOF for
most of the nutrients compared to other organic fertilizers, which might have resulted
more leaching losses. However, the nutrient retention was also highest for F-TOF,
even after the leaching for 24 weeks, suggesting a revisit on the rate and mode of
fortification for F-TOF, popularly marketed in the trade name “Suchitha”. For crop
production, the performance of F-TOF found equally good as that of vermicompost
and was superior to FYM, OC and MC.