Physico-chemical Changes of Paddy Soils under Long-term Intensive Fertilization
M.S.H. Khan ,
M.J. Abedin Mian ,
The main objective of the experiment was to evaluate the impact of intensive fertilization and cropping on some physical and chemical properties of soil. Low bulk density and high pore volume remain in surface layer than the subsurface layer due to long-term fertilization. Laboratory experiment was carried out with soil samples (Sonaltola silt loam) from different depths of a permanent manurial experimental field of the Department of Soil Science, Bangladesh Agricultural University (BAU) farm, Mymensingh. Relatively low bulk and particle densities were found in FYM treated soil. Results showed that P and FYM are more effective in maintaining large aggregate in soils. Lowest amount of >2mm size aggregate and highest amount of 0.1-0.01 mm size aggregates were obtained in NS plot due to solubility of gypsum in water. Soils of surface layer were slightly acidic to neutral and the sub-surface layer was alkaline having pH values ranging from 6.81 to 7.11 and 7.48 to 7.85 in 0-10 and 10-20 cm layers respectively. The status of total organic matter, total nitrogen, available P and S also decreased with increasing depth although the status varied widely among the treatments and decreased due to treatments in which P was not included.
Fertilizers are essential parts of modern farming, with about 50% of the worlds production being attributed to fertilizer use. Fertilizer nutrients use in different countries of the region has increased considerably with maximum (509 kg ha-1) in the Republic of Korea as against only 102 kg ha-1 yr-1 in Bangladesh. The importance of fertilization and manuring of our land is increasing day by day and farmers are supposed to use these inputs intensively for sustained crop production. Practices of intensive fertilization and manuring undoubtedly bring some changes in the physical and chemical properties of soil as well as biological properties.
Chemical properties that prevail under aerated conditions undergo a drastic change upon submergence. Flooding of water logging reduces the soil and consequently, the oxidized material viz., SO4-2, NO3¯, Fe+3 and Mn4+ are replaced by their counterparts S2-, NH4+, Fe2+ respectively. The concentration of P, Fe2+ and Mn2+ increases and decreases at the initial and final stages of submergence respectively.
Long-term application of FYM and GM increases the organic matter and total nitrogen content in soil[4,5]. The status of P, K and S, in soil is enhanced due to intensive application of these fertilizers and manures. This increase in nutrient reserve of soils due to intensive fertilization and manuring may ultimately lead to decline in crop yields as well as limit the use of this natural resource by creating nutritional imbalance.
The deficiency of primary nutrient elements like N, P, K and the beneficial
effects of single and combined application of these nutrients in rice production
have already been established. But the deficiency of sulfur and zinc has been
established recently and its area and severity are increasing day by day at
alarming rate particularly in HYV rice cultivation. The cause for this increased
deficiency of nutrients in paddy soils are many, among which the intensive cropping,
use of HYVs and chemical fertilizers are thought to be the main factors.
In addition to higher uptake by intensive cropping with HYVs, high temperature
accompanied by high rainfall also enhance the loss of nutrients through weathering
of soil materials. Ali stated that an amount of
1054 thousand tons of nutrients (N, P and K) is being lost every year from arable
land of Bangladesh. The phenomenon of nutrient depletion or mining from soils
risking the prospect of increased food production has been highlighted and well
documented by the FAO in recent years. Khan et al.
stated that application of TSP fertilizer increased the availability, total
and soil solution content of Zn, Cu, Fe and Mn. If the problem of nutrient depletion
is not rectified, it will result in remarkable damage to the soil and the welfare
of the farmers who depend entirely upon the land. Since fertile soil is the
fundamental resource for higher crop production, its maintenance is a pre-requisite
for long-term sustainable crop productivity. In view of above facts, the present
research was undertaken to see the impact of long-term fertilization on physical
characteristics and to see the status of nutrients in paddy soils.
MATERIALS AND METHODS
A laboratory experiment was carried out with soils collected from a permanent manurial experimental field of Department of Soil Science, Bangladesh Agricultural University Farm, Mymensingh, Bangladesh during the period of 1998-99 (after two decade of starting the permanent experiment). This farm belongs to the Sonatola Series under the general soil type of non-calcareous dark grey floodplain alluvium. This may be correlated with Aeric Heplaquapt of USDA soil taxonomy and Eutic Gleysol of FAO-UNESCO soil unit.
This manurial experimental plot was started in the year 1978. The initial soil was silt loam in texture having pH 6.8, organic matter 2.16%, total nitrogen 0.06%, available phosphorus 9.0 mg kg-1, available potassium 0.20 cmol kg-1 as mentioned in Table 1. The treatments include control, N, NP, NPK, NS, NPKSZn, NFYM and NPKFYM. The fertilizer doses used in the experiments were 60 kg N ha-1 from urea, 20 kg P ha-1 from TSP, 15 kg K ha-1 from MP, 30 kg S from gypsum.
Collected soil samples from various depths (0-10 and 10-20 cm) were collected, grind, sieved and preserved in polithyne bag for analysis. Particle size analysis of soil was done by hydrometer method and the textural class was determined by plotting the values for sand, silt and clay% in the Marshalls triangular co-ordinate following USDA system. Particle density and bulk density was determined by volumetric flask method and core sampler method respectively. Total porosity of soil calculated from bulk and particle density after Vomocil. Water stable aggregate were determined following the method described by Savvinov. Total soluble salts was estimated from electrical conductivity of aqueous soil extracts from 1:5 soil:water suspension after Biswas and Mukherjee. Soil pH was measured by glass electrode pH meter as outlined by Page. Total nitrogen was determined by semi micro kjeldahl distillation method. Available P and available S was determined after Olsen et al.. Amount of organic matter, CEC, exchangeable K and exchangeable Ca was determined after Page.
||Physical and chemical properties of initial (1978) soils *(Results
expressed on oven dry basis)
|Initial soil was collected and analyzed just before starting
the permanent experiment in 1978 (HSTL Report 1. 1983)
RESULTS AND DISCUSSION
Texture: Results on particle size distribution of soil of the plots
under study were medium in texture and contained highest amount of silt ranging
from 70-80% in surface layer and 74-78% in sub-surface layer (Table
2). Considering the vertical distribution of different sized particles,
it appears from Table 2 that in general the sand content of
top soil was higher than sub soil (10-20cm depth) whereas in case of clay the
sub surface soil content was higher than surface soil. However, the silt content
did not show any definite trend between the layers. It may be speculated from
vertical distribution of different sized particles that the finer particles
were transported from surface layer to sub surface layer with leaching water
and deposited there. Result of mechanical analysis showed decrease of clay content
in surface layer and increase in the sub-surface layer when compared with initial
soil. From the lower sand content in the sub-surface layer of fertilizer treated
plots it may be further speculated that in addition to physical weathering,
strong chemical weathering of sand particle also occurred in sub-surface layer.
This might be possible for the organic and inorganic acids produced in the surface
layer from the residues of applied fertilizers as well as decomposition of organic
matter. These acids are leached down from surface and accumulated in the sub-surface
layer for relatively long period and helped in chemical weathering.
Bulk density: Bulk density of 0-10 cm layer varied from 1.0 to 1.2 g
cm-3 whereas in 10-20 cm layer it varied from 1.35 to 1.43 g cm-3
irrespective of seasons (Table 3). The lower values in the
surface layer are attributed to the higher content of organic matter in comparison
to sub-surface (10-20 cm depth) layer. Moreover, frequent cultivation of land
made the soil loose and ultimately contributed to the lowest density in this
layer. Results did not show remarkable variation in wet or in dry season although
some variations existed between the treatments. In the sub-surface layer, on
the other hand, bulk density showed a wide variation among the treatments. The
treatment NS tended to show its superiority over other treatments. The value
of the treatments N, NP, NPK and NPKSZn were almost identical and followed the
|| Results of mechanical analysis of different depth
|| Bulk density, particle density and porosity of soils in different
|| Distribution of different sized aggregates of soils of different
The values of the treatment NFYM, NPKFYM were identical and lower than any
other treatment under study, both in wet as well as in dry period, specially
in the surface layer. The high bulk density in the sub-surface layer indicated
the presence of compacted sub-surface layer. Brammer also reported
the presence of compacted (plough pan) sub-surface layer due to accumulation
of outwash from the upper horizon and the pressure produced from the animals
and machineries used for the cultivation of lands. It also appears that there
was a wide seasonal variation in bulk density of soils. The reason(s) for such
seasonal variation in bulk density is not clearly understood. However, the relatively
too low bulk density in wet season indicated that the soil contained expanding
type of clay minerals which upon submergence of the field expanded and resulted
in the low bulk density.
Particle density: The values of particle density varied from 2.34 to 2.46 g cm-3 and 2.45 to 2.61 g cm-3 at 0-10 and 10-20 cm depths respectively (Table 3). The different fertilizer treatments did not show remarkable variation in their effect on particle density of soil. However, the relatively low particle density as obtained due to FYM containing treatments might be possible for the increase in organic matter. The variation in particle density between the layers is mainly related to the variation in organic matter, although the presence of heavy metals could not be ignored.
Porosity: Data on soil porosity varied from 54.15 to 60.17% (vol.) and
45.78 to 51.22% (vol.) at 0-10 and 10-20 cm depths respectively (Table
3). The higher amount of pore space in the surface is attributed to the
higher amount of organic matter as well as loosening of soil material during
cultivation/puddling of soil. This is also evidenced from the higher pore space
in FYM treated plots the lower amount of pore space in the sub-surface layer
on the other hand is associated with higher bulk density due to compactness
as well as presence of lower organic matter.
The porosity decreased with increase of soil depth probably due to lower organic
matter content and higher bulk densities and compactness of soil.
Aggregate stability: The amount of aggregates of <0.1 mm size in
general followed the >2 mm size except the control and NS treatments whose
amount of 0.1 mm size particles was next to >2 mm size particles (Table
4). Considering the effects of different treatments it was noted that P
and FYM containing treatments were effective in maintaining larger aggregate
in soils. However, among these treatments the effect of NFYM was the best which
was followed by NPK in surface soil whereas the treatment NPKSZn was more effective
than any other treatment. It may be mentioned here that the calcium sulfate
(gypsum) is a cementing material and it was expected to obtain higher amount
of larger aggregate in NS treated plot. But unexpectedly the lowest amount of
<2 mm sized aggregate was obtained in this plot. The highest bulk density
and lowest amount of pore space also indicated the cementing effect of gypsum.
It is rather difficult to explain the reason(s) for such a low content of larger
aggregates and higher content of fine aggregate (0.1 to <1 mm). However,
it seems that the residual sulfur of applied gypsum remained in soil as water
soluble sulfate salts which after shaking with water was dissolved as a consequence
the aggregates were destroyed resulting in lowest amount of larger aggregate
and lowest amount of fine aggregate. The higher amount of larger aggregate found
in P and FYM treated soil was probably due to calcium as well as organic matter
respectively which also has strong cementing effect in soil. The relatively
low amount of larger aggregate and higher amount of finer aggregate compared
to other treatments (except NS) are attributed to the absence of cementing materials
in these treatment. The amount of aggregates of 1, 0.5, 0.25 and 0.1 mm did
not follow any definite trend due to different treatments and depths.
Soil pH: The data on soil pH at different depths appears that the pH value of soils varied considerably due to different treatment combinations but it did not follow any definite trend (Table 5). The pH values of all fertilizer treated soils varied from 6.82 to 7.11 and 7.48 to 7.85 at 0-10 and 10-20 cm depths respectively. In general, the pH of soil increased with increasing soil depth irrespective of treatments under study. An increase in soil pH was found due to long-term application of NS and NPKSZn treatments over all other treatments as well as original pH of 6.8 (Table 1). The values for the rest of the treatments were almost identical and equal to original values although some variations existed among the treatments. The higher values are found in NS and NPKSZn treatments might be possible for the relatively high content of Ca2+, supplied with gypsum. The pH of sub-surface (10-20 cm) varied from 7.51 to 7.85 i.e., slightly alkaline in reaction. This is probably due to decrease of clay and organic matter with the increase of soil depth. Chowdhury observed that the pH of Sonatola series ranged from 6.8 to 7.2, which is very close to the results obtained under the present study.
The low pH in surface layer may be attributed to the oxidation of ammonium to nitrate during air drying of soils which produce H+ as stated by McLaren as follows:
In addition Fe2+ also produces H+ ion during its oxidation to FeOOH.
The sulfur is reduced to sulfide under wetland condition when the sols are
strongly reduced and forms Metal Sulfide (MS). When the soils
dry out during the dry season these sulfides are oxidized to sulfate and become
|| Total N, available P, exchangeable K, available S and Ca
of soils of different depth
After dissociation of sulfate salts in solution the sulfate ions may produce
acids as H2SO4. The lowering of surface soil pH due to
S and Zn may be explained according to Schachtschabel et al.
Organic matter content: Soil organic matter content was higher in the
0-10 cm layer which is similar to Sood and Kanwar and Rajamannar
et al.. In the surface layer lower value was found in
control plot which did not receive any fertilizer for the last 19 years (Table
5). The addition of N, NP, NPK, NS, NPKSZn, NFYM and NPKFYM gradually increased
the organic matter status over control although it was not remarkable except
the NFYM and NPKFYM treatments. Application of FYM with N and NPK for period
of 19 years increased the organic matter content of soils over all of the treatments
under study. This increase in organic matter was mainly due to higher biomass
production through manuring (FYM). In general, the status of organic matter
even in control plot was higher than the initial status. This might be possible
for the undecomposed roots and crop residues as soil sampling was done immediately
after harvest of crop. USDA Soil Survey Staff also observed decreasing
trend of organic matter with increasing depth. On the other hand, low organic
matter content in sub soils might be possible for the presence of compact plough-pan
in the sub-surface layer of the field. This compact layer restricted the penetration
of plant roots to the deeper depths contributing to high status of organic matter
in the surface layer[15,24,25]. In addition to this the high microbial
population and crop residues also contributed to the high organic matter status
in the surface layer.
Electrical conductivity: In all treated soils the electrical conductivity
was found higher than control plot where the conductivity was the highest in
the S-containing treatments (Table 5). In other treatments
the EC although showed higher values over control and it did not follow any
definite trend in surface as well as sub-surface soil. The higher EC found in
S-treated plots is attributed to the presence of higher amount of water soluble
sulfate salt produced from the residues of applied fertilizers. Such increase
in EC due to sulphur fertilization was also noted by Abedin Mian (1991).
Cation Exchange Capacity (CEC): The CEC of soils did not show any remarkable variation among different treatments as well as depths (Table 5). The continuous cropping for a period of 19 years with fertilizers slightly decreased cation exchange capacity of soil in control plot when compared with other fertilizer treated plots. It appears from the data that long-term use of chemical fertilizers in different combinations had no influence on the CEC both in surface and sub-surface soils. Similar results were also found by Shinde and Ghosh. A considerable increase in CEC was obtained in NFYM and NPKFYM treated plots due to continuous application of FYM[26,27]. It appears from the results that CEC of soils slightly decreased with increasing the depth of soils. Similar decreasing results were also reported by Chowdhury.
Total nitrogen: The total N content of soils showed a wide variation between the depths (Table 6). The soils from surface 0-10 cm layer always showed dominance over the soils from 10-20 cm depth. This variation in total N content between the depths is mainly associated with the variation in organic matter content between the layers. Considering the effect of different fertilizer treatment it appears from the result that all the fertilizer treatments tended to show their superiority over the control plot in both the depths. The variation of total N content of soil due to different fertilizer treatments were very negligible. However, the slightly lower values found in P and S treated plots might be possible for the replacement of a part NO3¯ by PO43¯ and SO42¯ by NH4+ and additional calcium supplied with TSP and gypsum fertilizer.
Available phosphorus: The data on phosphorus at different depths appears that the continuous cropping for long period without P fertilizer remarkably decreased the P-content of soil in the P control plots when compared to other fertilizer treated plots (Table 6). The treatments NP, NPK, NPKSZn and NFYM were found almost equally effective in maintaining t he high availability P in soil due to the residues of applied P mainly. The treatment NPK showed its superiority over all other treatments in maintaining the highest P level in surface layer. This higher concentration of P in NPKFYM treated plot indicates that in addition to P fertilizer the application of organic matter (FYM) over a period of 19 years also contributed to some extent on available P contents. This result is in accordance with the previous results of Khan et al.. It is further apparent from the data that the level of P also increased in both surface and subsurface layers over the initial value of 9.0 ppm due to the treatments without P. This may be possible for relatively high organic matter produced from higher biomass production mainly.
Exchangeable potassium: Results on exchangeable K content of soils did not show any remarkable variation between the treatments as well as soil depths (Table 6). The treatments containing K fertilizer tended to show a high status but practically it did not vary from the other treatments. This almost uniform K status of different treatments indicated that the urea-N applied with fertilizer has some adverse effect on soil K due to the increase of NH4+ concentration produced from hydrolysis of urea in soil. Khan reported a higher adsorption of K soils from NK treated plot than control plot due to displacement of K+ by NH4+ mainly. However, this finding of the present study is in confirmation with the above finding.
Available sulphur: Result pertaining to the available sulphur content
of soils showed wide variation among different treatments and depths (Table
6). The continuous cropping for a period of 19 years without S-fertilizer
slightly decreased the S-content of soil in control plot when compared to other
fertilizer treated plots. However, a significant influence of S-fertilization
over a period of 19 years brought about remarkable increase in available S content
of surface soil. A considerable increase of available S was also obtained in
NFYM and NPKFYM treated plots. This increase was probably due to continuous
application of FYM, which is one of the main sources of sulphur in soils.
In a sulphur fractionation study Abedin Mian also found an increase
in sulphur status due to S-fertilization. The result obtained in the present
study might be possible for intensive fertilization. The status of sulphur in
control treatment was just at or below the critical level of 14 ppm as reported
by BARC. The variation as noted between the surface and sub-surface concentration
was associated mainly with organic matter.
Exchangeable calcium: Results on exchangeable Ca content of s oils from
different fertilizer treated plots showed almost a similarity irrespective of
depths (Table 6). The concentration of Ca varied from 5.2
to 6.6 me 100-1g and 4.1 to 6.0 me 100-1g soil at 0-10
and 10-20 cm depths, where the lowest value was obtained in control plot. In
general, it is apparent that the P and S treated plots tended to show higher
concentration of Ca than of other plots. The Ca supplied by P and S containing
fertilizers might be one of the reasons for this high status.
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