Phosphorus Requirements of Cereal Crops and Fertility Build up Factor in a Typic Camborthid Soil
Field studies were conducted to arrest Phosphorus requirements of wheat, sorghum fodder and rice crops at 95% relative yield; Phosphorus Fertility Buildup Factor (PFBF) and level of phosphorus buildup in a Typic Camborthid (Sultanpur series) soil of rice tract of the Punjab, Pakistan. Sorption isotherms were constructed in the laboratory and data fitted into linear form of modified Freundlich Model and doses were computed against soil solution P levels. Different theoretical P doses were applied in the field to develop soil solution P level of 0.01, 0.02, 0.03, 0.04, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40 and 0.50 mg LG1 along with a control (native soil solution P). Phosphorus was also applied at the rate of 60 and 90 mg P2O5 kgG1 to subsequent sorghum fodder and rice crops, respectively. Wheat grain and straw, fresh sorghum fodder, rice paddy and straw yields were recorded at the harvest. Plant sampling was done at booting stage of wheat and rice and at the harvest of each crop. Soil sampling was also done at the harvest of each crop. The results indicated that maximum wheat grain yield was 4.05 mg haG1; sorghum fresh fodder yield was 43.83 mg haG1 and rice paddy yield was 4.43 mg haG1. Total P uptake by wheat, sorghum and rice was 22.73, 37.69 and 20.48 kg haG1, respectively. Mean phosphorus fertility build-up factor (mg P required to build 1 mg P kgG1 soil) was 16.23 and the level of P buildup (mg P kgG1 built-up in soil for each mg P kgG1 soil applied) was 0.062. Level of P depletion was 0.141 mg P kgG1 for NPK check plots while 0.162 mg P kgG1 for P check plots. Internal P requirement for wheat was 0.255% at booting stage and 0.281% for grain stage. For sorghum fodder, internal P requirement was 0.233% and for rice, it was 0.146% at booting stage and 0.266% for paddy stage.
The soils in the rice tract of the Punjab, Pakistan are deficient in available phosphorus (P) and the native P supply cannot cope with P mining due to continuous cropping. According to NFDC (2003), 93% of the soils of Pakistan contain less than 10 mg kg-1 Olsen extractable P and are thus deficient in plant available P. Soils of Pakistan are alkaline (pH > 7.0) and mostly calcareous (CaCO3 > 3.0%) in nature. When phosphatic fertilizers are added, part of them go to soil solution and is taken up by plants, while rest goes to exchange sites and is either adsorbed or precipitated and is a serious problem (Sharif et al., 2000). Fox and Kamprath (1970) constructed phosphate sorption isotherms to describe the relationship between levels of P in solution established after fertilization in laboratory and the quantities of P sorbed at the end of equilibrium. This approach has an advantage over conventional method of soil testing in that it integrates P intensity, capacity and quantity aspect of the soil, all of which play important role in controlling the P flux to most of the growing plants. Moreover, fertilizer requirement can be estimated directly from P sorption curves. Soil solution P is an immediate source for plant P uptake (Holford, 1989) and standard solution P concentration (0.2 mg L-1) provides adequate P for many crops if it is continuously maintained in the medium (Beckwith, 1965). Many equations e.g., Langmuir, modified Freundlich, Gunary, Tempkin and mechanistic models have been used to describe P adsorption in most recent study and have successfully confirmed P adsorption relationships (Huang, 1998). The Langmuir and the Freundlich equations described the adsorption phenomena satisfactorily (Boschetti et al., 1998).
The nutrient requirement of a crop can be expressed in several modes and channels.
The internal requirement is defined as the concentration of nutrient in the
plant (an intensity factors in plant nutrition) that is associated with near
maximum yield usually named as the critical concentration (Fox, 1981). External
P requirement of crop may be defined as the maximum concentration of P in soil
solutions equilibrated with soils associated with near maximum attainable yield
of crop. The quantity of nutrient frequently taken up is of great significance
if it is considered in relation to the capacity of the soil to hold nutrients;
thus both the internal and the external requirement can be expressed in terms
of P quantity, intensity and capacity factor.
Residual effect of P by the criteria of crop yield and soil analysis for P
content showed a significant increase in the yield of subsequent wheat crop
without P fertilizer application in the rice-wheat rotation (Saeed et al.,
1992). Puno (1991) studied the P fertility build-up factor (PFBF) and Olsen
P content in relation to P fertilization up to 32.8 mg kg-1 (150
kg P2O5 ha-1) in a silt loam soil and found
that PFBF ranged from 11.8-49.3 mg kg-1. Firat and Gumus (2001) observed
the residual effect and continuous application of P fertilizer in an alluvial
soil increased the amount of available P in the soil solution and P concentration
in plants. Keeping in view all the facts, present studies were planned in alluvial
soils of rice tract with the following objectives:
||To determine the internal phosphorus requirement of wheat,
sorghum (fodder) and rice.
||To see the level of P build-up and depletion following wheat-sorghum (fodder)-rice
MATERIALS AND METHODS
These investigations were carried out on a typic Camborthid (Sultanpur series) soil of the rice-growing tract of the Punjab, Pakistan during 2003-2004. Soil was selected on the basis of calcareousness and clay contents. A composite soil sample was collected from 0-20 cm depth, air-dried, sieved through a 2 mm sieve and stored for analyses. All basic analyses were performed for chemical characteristics of the soils based on methods described in Methods of Soil Analysis (Bigham, 1996) and U.S. Salinity Lab. Staff (1954) except texture by Moodie et al. (1959), available phosphorus by Watanabe and Olsen (1965) and Murphy and Riley (1962), Organic matter by Walkley and Black (1934).
Adsorption isotherms were constructed by using modified Freundlich equations
as proposed by Le Mare (1982). The simple form of the Freundlich model is as
where, a is the amount of P adsorbed (μg g-1), when the concentration
C is 1 μg mL-1 and b (mL g-1), is the buffer power
defined by the slope of the sorption curve at the point where P/C = 1 (mL g-1).
The value of P = C at which this point occurs varies between soils. The modified
Freundlich model used to describe the soils in this study is as follows:
Field experiments: The experiments were conducted in permanent lay out which included three crops i.e., wheat sorghum fodder and rice and fourteen treatments arranged in Randomized Complete Block Design (RCBD) with three replications having plot size of 6x4 m. All the agronomic and recommended cultural practices were followed for normal growth of the crops. Wheat crop cv. Inqalab-91 was sown during the second week of November. Half of the recommended nitrogen (70 kg ha-1) and potassium (K2O) at the rate of 70 kg ha-1 along with phosphorus (P2O5) doses (Table 1) were applied at sowing time in the form of urea, potassium sulphate and single super phosphate, respectively by broadcast method and mixed in soil. Second half of nitrogen was applied at first irrigation. Plant sampling (20-25 above ground portion of plants) at booting stage was done to observe the P concentration (Jones et al., 1991). The crop was harvested at maturity. Grain and straw yields were recorded by harvesting the whole plot. The grain and straw samples were analysed to calculate the P uptake by the crop.
Post harvest soil sampling was also done immediately after harvest of wheat
crop for the determination of Olsen P. After wheat harvest, sorghum cv. Hegari
was raised to reap the benefits as animal fodder. The recommended dose of N
(65 kg ha-1) and P2O5 (60 kg ha-1)
was applied to sorghum. Fodder was harvested after 55 days of growth and fresh
fodder yield was recorded. Plant samples were collected and oven dried at 70°C
for analysis to measure oven-dried weight of fodder and P uptake. Post harvest
soil samples were also collected for Olsen P analysis. After sorghum fodder,
fine rice cv. Super Basmati was transplanted in the second week of July. Half
of recommended N (55 kg ha-1), all P2O5 (90
kg ha-1) and K2O (70 kg ha-1) were applied
as basal dose and remaining half N with ZnSO4 at the rate of 12.5
kg ha-1 were applied after 30 days of transplanting.
|| Computed P doses to be applied in the field
Plant sampling at booting stage was done from 25 above ground portion of rice
plants. At maturity, paddy and straw yields were recorded and their respective
samples were collected, oven dried and analysed for P contents. Soil samples
were also taken at harvest for Olsen P determinations.
Relative yield (%): The relative yield is a measure of the yield response to a single nutrient when other nutrients are supplied adequately but not in excessive amount and was calculated by the formula
|| Yield at zero level of x
|| Point of maximum response to x
||Rate of nutrient (P) applied.
Phosphorus Fertility Build-up Factor (PFBF): PFBF was calculated by
the total amount of P added (mg kg-1 soil) divided by Olsen-extractable
P (mg kg-1 soil) after the last crop.
Level of phosphorus build-up: It was calculated by the net increase in Olsen P (mg kg-1 soil) after the third crop divided by the total P added (mg kg-1 soil).
Internal phosphorus requirement of crops: Internal P requirement of each crop was determined from the regression equation by plotting the relative yield (95%) against the P concentration (%) in the respective plant part.
External phosphorus requirement of crops: External Olsen and solution P requirement of crop was determined from the regression equation by plotting the graph between 95% relative yield and solution P level after the harvest of each crop.
Statistical analysis: Analysis of variance (ANOVA) technique was used and Duncans multiple range test (DMR) was applied to see the significance of difference among treatment means (Duncan, 1955). Regression and correlation was applied to compute build-up and depletion in soil P status in alluvial soils used in these investigations by the procedures as described by Steel and Torrie (1980).
RESULTS AND DISCUSSION
The physical and chemical characteristics of the soil used in this study given in Table 2 indicated that the soil was non-saline and non-sodic, moderately calcareous in nature, deficient in organic matter and available phosphorus but medium in extractable Potassium.
Crop yields (Mg ha-1)
Wheat grain and straw yield: Wheat gain and straw (Table 3)
yields increased with an increase in solution P level and maximum grain and
straw yield was obtained at solution concentration of 0.20 and 0.30 mg P L-1
which were developed by adding 197.17 and 242.92 kg P2O5
ha-1, respectively. The yields were significantly increased at lower
level of solution P. The yields at native solution P level where only NK was
applied were significantly higher than where no NK was applied. Memon (1982)
described a solution level of 0.009 mg P L-1 for 75% of maximum yield
of wheat and 0.028 mg P L-1 for 95% of maximum yield.
Iqbal et al. (1998) observed a marked response to P in different soil series of Pakistan. Tandon (1987) also found wheat yield response to P were highest on low P alluvial soils and progressively lower on soil of higher soil fertility. Among major crops grown in sequence, wheat is more responsive to P application in low P soil and at lower level of P application. Brar et al. (2000) found similar wheat grain yield results in response to P application.
Sorghum fresh fodder yield: The maximum sorghum fodder yield of 48.95
Mg ha-1 was harvested from the plot hading 23 mg P ha-1
as residual P after the harvest of wheat crop and also supplied with 60 kg P2O5
ha-1. The yield also increased significantly over native P
solution (NK control). The yield in control plot (0 NPK) was 18.09 Mg ha-1.
The results are in line with those of Bhatti (1996) who obtained sorghum green
fodder yield of 30.87 Mg ha-1 with traditional technology and 64.22
Mg ha-1 with improved technology and his data revealed that there
was good compromise of fodder yield and fodder quality at 50% flowering stage
of cutting. Khan et al. (1996) compared green fodder yield of sorghum
varieties in India (44.41 Mg ha-1) with Pakistan (42.59 Mg ha-1).
|| Original soil analysis of the selected soil series
|| Crops yields (Mg ha-1)
|Means sharing same letter(s) are statistically at par at 5%
level of probability
In Pakistan, sorghum is more responsive to fertilizer without lodging due to
its medium height, thicker stem, more leaves per plant and leaf area. The results
also commensurate with those of Akmal and Asim (2002) who emphasized adequate
P fertilization to sorghum fodder on the basis of soil tests to get better yields.
Phosphatic fertilizers are seldom applied to sorghum but it is an exhaustive
crop and remove large amount of NPK.
Sorghum dry matter yield: The dry matter yield of sorghum is a good quality parameter as animal feed. The residual P after wheat harvest was not enough to increase yield which was enhanced with fertilizer P. The sorghum dry matter yield (Table 3) significantly increased in ascending order of treatments i.e. lowest in T1 and maximum in T14. The maximum dry fodder yield was 10.72 Mg ha-1. Das et al. (1996) observed response of dry matter yield up to 80 kg P2O5 ha-1. Akmal and Asim (2002) also obtained similar results regarding dry matter yield of sorghum.
Rice paddy and straw yield: The paddy and straw (Table 3) yields increased significantly from T3 to T14. Maximum response was obtained to T10 in case of paddy and T14 in case of straw. The yield at native P level (T1) was significantly low. Phosphorus application promoted plant growth through an adequate supply and helped in better development of roots. Intensive cropping has drastically reduced native soil fertility to such an extent that addition of fertilizers has become necessary to increase and maintain crop yields. These results are supported by the work of Nelson (1980) and Nadeem and Ibrahim (2002).
Phosphorus concentration (%) in crops
Booting stage: The P concentration in a plant reflects the adequacy of P
for plant growth. Phosphorus concentration in wheat plants at booting stage
(Table 4) increased with increase in P solution level and
maximum P concentration was observed at 0.10 mg P L-1 solution which
was obtained by adding 137.95 kg P2O5 ha-1.
However, the higher levels showed a luxury consumption of P at this stage. Tandon
(1987) stated that modern high yielding grain varieties continue to absorb P
till maturity and almost 70-80% of absorbed P ends up in the spikes or panicles.
Phosphorus concentration below 0.05 mg P L-1 is in the deficient
range indicating that wheat roots were unable to absorb sufficient P from soil
solution below these levels for good crop growth. The results also correlates
with the findings of Delong et al. (2001).
Wheat grain and straw: Different soil solution levels of P showed significant effect on P concentration in harvested wheat. Maximum P concentration was obtained at 0.50 mg P L-1 (developed by adding 316.02 kg P2O5 ha-1) both in grain and straw. Minimum P concentration was observed in the treatment where no P was applied. Singh et al. (2002) observed grain yield and seasonal accumulation of P by wheat were higher for higher P rates. The results are supported by the findings of Brar (2000) and Delong et al. (2001).
Sorghum fodder: The phosphorus concentration of forage crops is important
since low P content in the animal diet leads to aphosphorosis which is a serious
disease in livestock. At less extreme levels of P deficiency, the fertility
of livestock may be quite seriously affected (FAO, 1984). The maximum P concentration
(0.32%) was recorded at T13 (residual P 21.95+13.65 mg kg-1 added
P). Minimum P concentration was observed in the control plot where no NPK were
applied. As the sorghum is a C4 plant and has extensive deep root
system, which can explore larger volume of soil for P extraction. Similar results
were reported by Meelu et al. (2000).
Booting stage: The maximum level of P concentration was 0.23% in the rice
plants in T14. Minimum P concentration (0.07%) was observed at native soil solution
Paddy and straw: The maximum P concentration was examined in paddy and
straw grown under T14 (residual P 13.75 mg kg-1 + 19.65 mg kg-1
added P). Minimum P concentration was found in the control plots where no NPK
fertilizer was added. The data also revealed that sufficiency level of 0.20%
was achieved in T9 (residual P 9.50+19.65 mg kg-1 added P), which
leads to the conclusion that the crop roots extracted residual P in a better
way. Similar results were found by Singh et al. (2002) who found mean
P content of rice grain from 20 soil series ranging from 0.41 to 0.55%, respectively.
Phosphorus uptake (kg ha-1) by different crops
Wheat grain and straw: The data of P uptake by wheat grain and straw is
depicted in Table 5, which showed that maximum P uptake in
grain and straw was noted at 0.50 mg P L-1 soil solution and this
level was developed by adding P fertilizer at the rate of 316.25 kg P2O5
ha-1. Minimum P uptake was seen in the control (without NPK) and
each additional level of P had profound effect on its uptake. It is stated that
the amount of P removed on an average yield of harvested grain varies from 7-15
kg P ha-1 (16-34 kg P2O5 ha-1) and
from this; 2-8 kg P ha-1 is returned back to the soil in the form
of crop residues, which are left in the field (Hanway and Olsen, 1980). The
present results are supported with the findings of Singh et al. (2002)
and Delong et al. (2001).
|| Phosphorus concentration (%) in plant
|Means sharing same letter(s) are statistically at par at 5%
level of probability.
|| P uptake (kg ha-1) by crops
|Means sharing same letter(s) are statistically at par at 5%
level of probability
Sorghum fodder: Phosphorus uptake by sorghum fodder is product of fodder
yield and concentration of phosphorus in fodder. The data regarding P uptake
by sorghum fodder is given in Table 5. The maximum P uptake
(34.67 kg ha-1) was recorded at T14 (residual P 23.00 mg kg-1
+ added P 13.65 mg kg-1) and was significantly more than T13 (residual
P 21.95 mg kg-1 + added P 13.65 mg kg-1). This P uptake
was found minimum in control plot (without NPK) and was 3.96 kg ha-1.
The data also showed that P uptake was progressively increased with increasing
treatment numbers from T1 to T14.
Paddy and straw of rice: The data of P uptake by paddy and straw is given in Table 5, which showed that maximum P uptake was recorded at T14 (residual P 13.75 mg kg-1 + added P 19.65 mg kg-1) and was significantly better than T13 (residual P 12.85 mg kg-1 + added P 19.65 mg kg-1). This P uptake was found minimum in control plot (without NPK) and was 1.55 kg ha-1. The increase in P uptake was increased with the increase in corresponding rates of P application.
Internal phosphorus requirement of crops under field conditions
Booting stage: The phosphorus concentration in wheat plants at booting stage
was plotted against relative yield of wheat grain by Boundary Line Technique
(Webb, 1972) and is shown in Fig. 1a. The value of P concentration
(%) for 95% relative yield was determined by regression equation and the value
obtained was 0.249%, which was critical phosphorus concentration for 95% of
maximum wheat yield at booting stage. The results are in line with those of
Memon and Fox (1983) who demonstrated that phosphorus content of two wheat varieties
(Pak-70 and Pavan) increased at flag leaf stage as the phosphorus concentration
in the solution increased.
Maturity stage: Internal requirement of wheat was determined by making graph of P concentration in grain and maximum attainable 95% relative yield as shown in Fig. 1b). The value obtained was 0.277%. This means that as the crop passed through reproductive phase, P was shifted to the seed. Rashid et al. (1992) found critical P concentration in wheat grain as 0.22% and in maize grain as 0.27% under green house conditions.
Sorghum fodder: The internal requirement of P of sorghum fodder which
is dependent on the P concentration at maximum growth (vegetative) was determined
and plotted as shown in Fig. 2. The graph revealed that maximum
internal P requirement of 0.326% was found for sorghum in this soil. Similar
results were reported by Chaudhry et al. (2003).
|| Internal P requirement of wheat
|| Internal P requirement of sorghum fodder
Booting stage: Data regarding P concentration at booting stage of rice was
plotted against 95% relative yield of rice as shown in Fig. 3a
which illustrates that internal P requirement of rice was 0.165%.
Maturity stage: The plot of P concentration in paddy versus 95% relative
paddy yield is shown in Fig. 3b which revealed that maximum
internal P requirement of 0.220% was found for rice in this soil
|| Internal P requirement of rice I) Solution P (mg L-1)
|| External P requirement of wheat
External (solution) phosphorus requirement of wheat: The solution levels
developed for wheat growth were plotted against 95% relative yield of wheat
for the determination of P requirement by the Boundary Line Technique (Webb,
1972) as shown in the Fig. 4. The graph revealed that solution
P requirement of 0.252 mg P L-1 was required for 95% yield in this
soil. Memon et al. (1991) found that 18-29 kg P ha-1 is required
to develop a solution level of 0.032 mg P L-1 in calcareous soils.
The concentration at the root surface of young plants need about 0.03-0.3 mg
P L-1 and older plants require about 0.03 mg P L-1 or
less but the concentrations which have been shown by many workers to be required
in bulk soil solution are little higher (0.06-0.68 mg P L-1) and
this would be expected because uptake reduces the phosphate concentration at
the root surface when plants are grown in static systems e.g. soils (Kamprath
and Watson, 1980). Similarly, Beckwith (1965) suggested a standard concentration
of 0.2 ug P mL-1 as adequate for most plant species. Memon (1982)
showed that P solution associated with 95% maximum yield of wheat was 0.025
μg mL-1, particularly 0.029 μg mL-1 for Pak-70
and 0.023 μg mL-1 for pavon variety. Memon et al. (1991)
determined external P requirement of wheat grain at near maximum yield as 0.032
mg L-1 as obtained from composite yield response curve.
|| Olsen extractable P (mg kg-1 soil) after harvest
|Means sharing same letters are statistically at par at 5%
level of probability
Phosphorus availability status in soils after wheat harvest: It is widely
recognized that the efficiency of applied P increases if P application rates
are tailored to the initial P fertility of the soil. After the harvest of wheat,
soil samples were collected from all the plots and analyzed for Olsen P to determine
the P status. The data presented in Table 6 reveal that P
application rates significantly increased the available P status 5.90 to 23.00
mg kg-1 in the soil. However the native P status decreased in all
the soils (5.20 to 5.10 mg kg-1 where no fertilizer was applied and
where NK was applied, this native P further decreased to 5.00 mg kg-1.
Decline in available P without P application is understandable because there
is only P output without any input. Similarly Tandon (1987) summarized the results
of 18 experiments and concluded that when half of the optimum rates of P were
used, available P declined or showed little change at 60% sites and when applied
optimum rates of P, a significant improvement in available P status was found.
It was observed in the present study that to raise P fertility to a given level,
different amounts of P are required. It was also noted that the higher P rates
become uneconomical for resource constrained farmers.
|| Phosphorus fertility build-up factor (PFBF) and level of
Phosphorus availability status in soils after sorghum fodder harvest:
Soil P status which was built after the wheat decreased to some extent with
the passage of time and contributed to the P uptake by plants or adsorption
on soil (Table 6). Native P of soils decreased to 4.65 mg
kg-1 in control plots of soil while in the plots receiving NK, Olsen
P was further reduced to 4.80 mg kg-1. However, the overall status
of soil available P ranged from 6.00 to 13.75 mg kg-1 in this soil.
The results are in line with that of Tandon (1987) who described that the continuous
application of recommended levels of P resulted in a significant increase in
the soils available P status.
Phosphorus availability status in soils after rice harvest: Available
phosphorus determined after harvesting of rice is depicted in Table
6 which revealed that soil P status was little bit improved after rice crop.
The possible reason might be that the rice was grown under submerged conditions
and more phosphorus became available for plant uptake and this P availability
status reflected that wetting and drying conditions changed the available P
status of soil i.e., after wheat it was built up due to high P application,
after sorghum it decreased and after rice, it either maintained or slightly
increased. The native P status decreased to 4.65 mg kg-1 in the plot
where no NK was applied and where NK was applied, it further reduced to 4.45
mg kg-1 in the soil. These data confirmed that where no fertilizer
was given, the rate of P depletion was less as compared to that where NK fertilizer
was added. The application of NK improved the growth and increased root growth
extracted more P from the native source. Tandon (1987) pointed out that soils
which are medium to high in available P, when cropped using N alone become deficient
in P within a couple of years and yields drop drastically unless P fertilizers
are applied. Similar results were obtained with rice by Ittyarial (1979).
Phosphorus fertility build-up factor and level of phosphorus build-up: PFBF exhihibited that PFBF was maximum (44.74) in T3 (0.01 mg P L-1) and minimum (11.00) in T14 (0.50 mg P L-1) with a mean value of 18.51 (Table 7). As regards level of P build-up, the maximum value (0.091 mg kg-1) was achieved in T14 (0.50 mg P L-1) and minimum value (0.022 mg kg-1) in T3 (0.01 mg P L-1) with mean value of 0.054 mg kg-1. There was a net decrease in control plots at the rate of 0.144 mg kg-1 in NPK check and at the rate of 0.164 mg kg-1 in P check plots, respectively. Similar results were obtained by Tandon (1987) and Puno (1991).
Main conclusions drawn from this piece of study are as under
||Olsen P can be built-up to nearly desired level in the soils
considering the phosphorus fertility build-up factor (mg P required to build
1 mg P kg-1 soil).
||External P requirement was greater for sorghum fodder being heavy P feeder
followed by rice and wheat. External P requirement showed significant correlation
with 95% relative yield of wheat, sorghum fodder and rice. However higher
P rates needs to be tested for sorghum fodder for getting maximum yields.
||Taking plant samples at booting stage than at crop maturity for obtaining
95% relative yield better correlated with internal P requirements of crops.
||Soils are being mined per annum at the rate of 0.144 mg P kg-1
if no fertilizers are applied and at the rate of 0.164 mg P kg-1
if only NK are applied in fine textured soils, respectively. So balanced
fertilization is a pre-requisite for harvesting maximum crop yields.
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