Abstract: A field experiment was conducted at Tamil Nadu Agricultural University, Coimbatore, in deep clay soil (Vertic ustochrept) to study the effect of real time N management using Leaf Color Chart (LCC) on biomass production, nutrient uptake and soil available nutrient status in short duration rice (Var. CO 47) under direct wet (drum) seeded condition. The study was conducted in factorial randomized block design with three replications. The treatments included maintaining three LCC critical values (cv.) viz., LCC 3, 4 and 5 with different rates of N application (20, 25, 30 and 35 kg N ha-1 each time, besides absolute control (Zero-N), Blanket N (120 kg N ha-1in four equal splits from 21 Days After Sowing (DAS) and Manage N practices (120 kg N ha-1 in four unequal splits). The results of the experiment suggest that rate of biomass production and NPK uptake were highest during panicle initiation to first flowering stages for the tested variety CO 47 and LCC based N management have effectively saved the N fertilizer. The lack of difference in soil available nutrient between LCC based treatments and conventional blanket N treatment suggest that LCC based N management could be best option for farmers to save fertilizer N besides maintaining soil fertility. The results suggest further scope to standardise the LCC based N management for different intensively cultivated cultivars at specific location and season.
INTRODUCTION
India’s food supply has kept pace with rapid population growth during recent decades; primarily because of wide spread use of modern rice varieties with high yield potential. Long term fertilizer experiment at IRRI indicated that after several consecutive years of intensified cropping with constant and high fertilizer inputs, grain yield begin declining in modern varieties (Cassman and Pingali, 1995). Rice crop usually take half of the applied N to yield above ground biomass. The other half of the N is dissipated in the wider environment causing environmental and ecological problems (Balasubramanian, 2004). The efficient N use is critical to produce sufficient food for feeding the burgeoning population and avoid large scale degradation caused by excess N (Tilman et al., 2001). Adoption of blanket dose of N fertilizer recommendation alone causes improper usage in some cases leading to wastage through leaching, volatilization, run off and denitrification and in other cases it may prove insufficient if the soils are poor in N fertility (Balasubramanian et al., 1999). In this connection, nitrogen fertilizer has received due attention in order to obtain the required yield targets through rational fertilizer use with the aid of portable diagnostic tools such as chlorophyll meter (SPAD 502) and Leaf Colour Chart (LCC) that can measure crop N in situ (Alam et al., 2005).
Optimal nitrogen management strategies seek to match crop N demand with fertilizer N supply, thus reducing losses and maximizing crop N utilization. Adequate leaf N should be maintained throughout the growing period of rice, which is critical for achieving high yield (Olfs et al., 2005). Assessing the leaf N concentration is one of the techniques to decide this and maintaining particular N concentration of different physiological stages is one way of increasing the rice yield. This is because N concentration in leaves is highly correlated with the concentration of CO2 fixing enzyme in photosynthesis and hence, the potential maximum rate of photosynthesis could be linearly related to the N concentration in the leaf (Black, 1993). Since the plant growth reflects the total N supply from all sources, plant N status will be a good indicator of N availability to crops at any given time. LCC is a simple, easy-to-use and inexpensive tool to monitor the crop N status in the field and to determine the timing of N top dressing to rice.
Rice plant take up nutrients from the soil environment (native and applied) and the ratio of nutrients taken up by the rice plant from either source is affected by soil properties, cropping seasons, time and amount of fertilizer application. Native N supply in intensive irrigated rice systems is not sufficient for higher yields; additional N supply through fertilizers is inevitable. Matching seasonal crop demand with supply from soil would be the ideal N management and this requires an understanding of the dynamics of crop requirement and native N supply. The LCC based N management forms an integral part of the Site Specific Nutrient Management (SSNM) strategy. Some soils of South India were able to supply N equal to the demand of a sufficiently fertilized crop during initial crop growth and in such cases, basal N can be skipped (Thiyagarajan et al., 1997). Exploring nutrient availability in soil forms an important tool for assessing the demand and supply of nutrients for the growing crop.
A considerable increase in the yield of the economic product is usually dependent on an increase in total dry matter produced, though this is not an absolute relationship and always true. Some amount of carbohydrates formed before flowering are stored in culms and leaf sheaths and later retranslocated to the grain. The response of a crop to an imposed treatment will be reflected by the biomass production. Though nutrient uptake depends on the plant’s need and soils ability to supply them, uptake of the nutrients by plants need not be in the same ratio as they occur in soil. Consumption of the harvested produce by domesticated animals and human beings results in the continuous removal of nutrients from the soil. Result of exploitative intensive agriculture has been the progressive occurrence of nutrient deficiencies in soils and crops. For efficient nutrient management, complete knowledge of the nutrient uptake by crops is necessary (Rattan and Goswami, 2002).
This study describes the changes in soil N, P and K, variation in biomass production and N, P and K uptake by rice crop at various stages under real time N management in rice.
MATERIALS AND METHODS
A field experiment was conducted at Tamil Nadu Agricultural University, Coimbatore,
India in vertisol belonging to Noyyal series (Vertic ustochrept) during
2002 to study the effect of real time N management using LCC on soil available
nutrient status, biomass production and nutrient uptake in short duration rice
(Var. CO 47) under direct wet (drum) seeded condition. The experimental design
was Factorial Randomized Block Design (FRBD) with three replications. The treatments
include maintaining three LCC critical values (cv.) viz., LCC 3, 4 and 5 with
different rates of N application viz., 20, 25, 30 and 35 kg N ha-1
each time, besides absolute control (Zero-N), Blanket N (120 kg N ha-1
in four equal splits at 21 Days After Sowing (DAS), Active Tillering (AT), Panicle
Initiation (PI) and First Flowering (FF) stages respectively) and Manage N practices
(120 kg N ha-1 in four unequal splits of 1/6th, 1/3rd, 1/3rd and
1/6th at 21 DAS, AT, PI and FF stages, respectively) (Table 1).
Nitrogen was applied as per treatment schedule based on the LCC critical value
assessed at weekly intervals from 21 DAS. A uniform dose of 38 kg P2O5
ha-1 (all basal) and 38 kg K2O ha-1 (in two
splits at 21 DAS and panicle initiation stage), 25 kg ZnSO4 ha-1
(all basal) and 500 kg gypsum ha-1 (all basal) were applied to all
the treatments. The initial analysis of the soil of the experimental site revealed
that soil was neutral (pH = 7.87) with low soluble salts (EC = 0.79 dS m-1),
medium organic carbon content (0.72%), medium in KmnO4-N (290 kg
N ha-1), high in Olsen-P (41 kg ha-1) and high in NH4OAc-K
(796 kg ha-1). Soil samples collected during panicle initiation and
post harvest stages (PH) were processed and analysed for available N (Subbiah
and Asija, 1956), P (Olsen et al., 1954) and K (Stanford and English,
1949). Separate area was demarked for plant sampling at different growth stages.
Whole shoot samples were collected during AT, PI, FF and HT stages and analysed
for total content of N (Piper, 1966), P (Pemberton, 1945) and K (Toth and Prince,
1949).
| Table 1: | Treatment details of the field experiment |
The observations collected from the field experiment and the data on the results of analysis of soil and plant samples were subjected to statistical scrutiny as per the procedure of Gomez and Gomez (1984). The treatments T4 to T15 were treated as FRBD along with three checks (control, blanket N and manage N) and statistically analysed.
RESULTS AND DISCUSSION
Biomass Production
Highest biomass production at FF and HT associated with 210 kg N ha-1
was an evidence for a judicious supply of N for reducing vegetative growth.
The mean biomass yield of 561, 1525, 8858 and 11316 kg ha-1 as recorded
at AT, PI, FF and HT stages indicated that the increase in biomass was very
slow (16 kg day-1 ha-1), slow (46 kg day-1
ha-1), rapid (261 kg day-1 ha-1) and moderate
(79 kg day-1 ha-1) during sowing to AT, AT to PI, PI to
FF and FF to HT stages respectively. Mir Zaman Hussain (1999) and Sudhalakshmi
(2002) observed a steady increase in biomass between PI to FF stage, whereas
Bindhu (2002) observed a vigorous dry matter production during active tillering
to panicle initiation. Blanket N and manage N recorded almost comparable biomass
at all crop growth stages indicating that the differential rate of split N application
have no influence on biomass production. The comparable biomass under different
LCC levels at AT was due to the uniform dose of N applied in all LCC levels
upto AT. While at the latter stages of crop growth viz., PI, FF and HT, LCC
cv. 5 recorded markedly higher biomass yield over LCC cv. 3 and 4 (Table
2) which was due to higher N dose applied to maintain LCC cv. 5 (Table
1).
The failure of different rates of N application to create any appreciable variation
in the biomass yield at AT, FF and HT stages was due to similar cumulative dose
over weeks among the treatments under the LCC levels (viz., 25-40 kg for LCC3,
50-70 for LCC 4 and 120-210 for LCC cv.5) (Table 1). The biomass
yield of 120 and 210 kg N ha-1 applied treatments were comparable.
Bindhu (2002) also observed comparable biomass yield in hybrid rice for N application
from 135-200 kg ha-1 which confirm that N application beyond optimum
dose not increase the biomass yield.
| Table 2: | Soil available nutrient (kg ha-1) and biomass production (kg ha-1) at various growth stages of rice as influenced by different N regimes |
| *PI: Panicle Initiation stage, PH: Post harvest stage, AT: Active tillering stage, FF: First flowering stage, HT: Harvesting stage, NS: Non significant at p<0.05 | |
Biomass production at harvest was positively correlated with soil available N at harvest (r = 0.532*). The over all result on the biomass production at different stages and its relationship with soil available N urge for an optimal N was an management technique to meet the crop demand at appropriate stages. The results of the present investigation emphasize the possibility of keeping the biomass production at considerably higher level by resorting to the latest N management techniques involving LCC for achieving a higher yield with out wasting fertilizer N.
Nutrient Uptake
The rate of N, P and K uptake followed a similar trend as that of biomass
production over crop growth stages (Table 3). The mean N uptake
of 11.5, 34.2, 81.2 and 94.2, mean P uptake of 1.90, 8.58, 18.4 and 22.1, mean
K uptake of 8.66, 31.2, 90.8 and 96.0 kg ha-1 at AT, PI, FF and HT
stages respectively, indicated the increase in the rate of N P and K uptake
was low between sowing to active tillering (0.33 kg N day-1, 0.05
kg P day-1 and 0.25 kg K day-1) and also between first
flowering to harvest (0.44 kg N day-1, 0.12 kg P day-1,
0.17 kg K day-1) and moderate between active tillering to PI (1.08
kg N day-1, 0.32 kg P day-1, 1.07 kg K day-1)
and high between PI to FF stages (1.67 kg N day-1), 0.35 kg P day-1,
2.13 kg K day-1). Nutrient uptake is a function of biomass production
at the respective stages. The rapid increase in biomass between PI and FF has
demanded more nutrients, thus resulting in a higher rate of uptake during this
growth phase. The result draws buttress from Sivasamy et al. (1994),
Aruna Geetha (1998), Mir Zaman Hussain (1999) and Sudhalakshmi (2002). Mir Zaman
Hussain (1999) contradicted the above results and reported a lower N uptake
rate between PI to FF than AT to PI due to seasonal influence on ASD 19 variety.
The comparable nutrient uptake under different LCC levels at AT was due to the
uniform dose of N applied in all LCC levels upto AT. The concomitant increase
in NPK uptake with increasing LCC levels at PI, FF and HT stages was due to
differential N dose applied. The N uptake results of this experiment in comparison
with earlier studies suggest that LCC could be an indirect tool to measure the
N demand of rice varieties in specific season and location.
| Table 3: | Nutrient uptake (kg ha-1) at various growth stages of rice as influenced by different N regimes |
| *AT: Active tillering stage, PI: Panicle Initiation stage, FF: First flowering stage, HT: Harvesting stage, NS: Non significant at p<0.05 | |
Increase in P and K uptake with increased dose of N added evidence to the fact that N application has synergistic effect on the uptake of other nutrients besides N which was due to increase in biomass of the crop with increased N application. From the nutrient uptake rate it could be concluded that crop requires higher nitrogen between PI and FF. The increase in N uptake of different rates of N applied under LCC cv. 5 over blanket and manage N has proven that the rice crop required lower N at the early stages; more N during its grand growth period (PI to FF) and a comparatively lower N during the later stages of crop growth. Thus LCC could help in promoting need based nitrogen application for increased nutrient uptake and yield in rice.
Soil Nutrient Status
In the present study there was a marked decline in the available NPK status
of the soil from PI to HT stage (Table 2). The decline was
due to the removal of nutrients by growing rice crop, various losses in the
soil-water-plant-atmosphere system and to certain extent by fixation. A marked
decline in the soil available N with the advancement of crop growth was reported
by Velu (1989) and Bindhu (2002). Although the crop N uptake in control plot
was only 50.1 kg ha-1, the decline of soil available N at HT was
maximum in control plot (76 kg over the initial N level) indicating the higher
magnitude of losses of nitrogen in paddy field. Similarly the overall mean value
of available N status of the soil at the end of the experiment showed a decline
of 37 kg N ha-1 over the initial N status.
Though the crop N uptake was increasing at decreasing rate, the soil available N was still found to decline with increasing rate of N application indicating higher loss of applied N at higher rate of N application as reported by Velu (1989) and Bindhu (2002). The marked depletion in the available nitrogen status of the soil with increasing LCC critical values at PI stage indicated that more nitrogen is demanded by the rice crop to maintain high chlorophyll content viz a higher LCC values and biomass production. However at post harvest stage, there was no significant variation in available N status of soil with the different levels of LCC or different rates of applied N indicating that application of excessive N does not add soil N fertility and again revealing the benefits of N fertilizer saving under LCC based N management. Mahajan and Tripathi (1992) observed that available N content determined after harvesting rice crop did not show any variation due to levels as well as sources of N suggesting the possibility of loss of excessive N.
The available phosphorus in soil also faced a similar decline as that of nitrogen. Inspite of the application of P2O5 at the rate of 38 kg ha-1 as SSP, the overall decline was around 50% from initial status of the soil (41 kg ha-1) which could be due to the fixation of the applied phosphorus in submerged soil. Application of N in increments decreased the available P considerably. As the applied N being the primary nutrient factor, the rice biomass and P uptake in rice got increased considerably with increasing rate of N application and LCC levels, ultimately leading to a proportional decline in soil available P. This confirms the findings of Bindhu (2002) and Selviprabha (2002). Apart from the release of P from Fe and Al phosphates, P is also released from occluded forms under anaerobic conditions. Under such circumstances the added P2O5 at the rate of 38 kg ha-1 could not be expected to create a marked variation in the P availability.
There was only a marginal decline observed in available K status of soil at PI and post harvest stages compared to the initial K. Neither LCC levels nor the different rates of applied N influenced the available K at PI stage. However, there was a marked depletion in available K with increasing levels of LCC at harvest. Higher N application at higher level of LCC enhanced biomass and K uptake of rice there by resulting in a decline in the soil available K. The rates of applied N had little impact on the soil available K at harvest due to the insignificant variations observed in the yield and K uptake in rice under the different rates of applied N. This is due to higher initial K availability (796 kg ha-1) of the experimental field and addition of 38 kg K2O ha-1 have little or no influence on the K availability in soil at PH. Among the three major nutrients, K is always in a dynamic equilibrium with different forms. Once the available K starts depleted, it is automatically replenished from the reserves (De Datta, 1981).
CONCLUSIONS
The results of the experiment suggest that rate of biomass production and NPK uptake were highest during PI to FF stage for the tested variety CO 47 and LCC based N management have effectively saved the N fertiliser. The lack of difference in soil available nutrient between LCC based treatments and conventional blanket N treatment suggest that LCC based N management could be best option for farmers to save fertilizer N besides maintaining soil fertility.