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Open Access
Influence of applied nutrients and stage of harvest on the yield and physicochemical properties of essential oil of palmarosa (Cymbopogon martini Stapf var. motia)
(Department of Soil Science and Agricultural Chemistry , College of Agriculture, Vellayani, Trivandrum, 1985) Chinnamma, N P; KAU; Aiyer, R S
Palmarosa, an essential oil crop introduced in Kerala, from Maharashtra, nearly two decades ago, is spreading steadily la the pialas and midland regions of North Kerala. Several agro-techniques have yet to he standardised for the commercial cultivation of this crop in the State. The present studies were undertaken at the Aromatic pnfl Medicinal Plante Research Station, Odakkaly during 1980-'8 4 to obtain information on the nutrition of palmarosa, optimum harvest intervals to ensure maximum herbage and oil yields and the factors influencing the quality of oil. The treatments in the major field experiment (1980-'82) consisted of three levels of N, and KgO, each at 25, 50 And 75 kg/ha along with six intervals of harvest at 40, 45, 50, 55, 60 and 65 days. The total number of treatment combinations wore 162 in a 3^ x 6 confounded asymmetrical factorial design. Tho main experiment was continued for another two yoaro limiting tho observations, to the yield of herbage and oil. This was then followed by an observation trial with intervals of harvest longer than tho maximum of 65 doye tried in the main experiment. In the main experiment the herbage yield vns significantly increased by application of P2°5 ^2°* Nitrogen did not have any eignifioant offeot on herbage yield poaeibly due to tha medium level etatua of soil N in the ABSTRACT ^erimental plots. The oil yield was Bignifioantly enhanced by P205 application at 50 kg/ha. Different levels and K showed no significant influence on the yield of oil. Harvest intervals showed significant influence on herbage yield, oil yield and oil content and the maximum value was recorded by the 65 day interval. A path analysis of the herbage yield with related cnaracters has shown that the height of the plant is the most important yield attribute influencing directly the yield of herbage. Path analysis of oil yield showed that oil yield is mainly dependent upon herbage yield. The direct effects on oil yield by yield attributes are found to be in the decreasing order of number of tillers with inflorescence, height of the plants and length of inflorescenoe. Maximum indirect affect via herbage yield is expressed by the height of the plant and length of inflorescence. All these directly and indirectly contributing factors arc soon to be markedly influenced by tho applioation of phosphorus and intervals of harvoat* Herbage yield and oil yield for various harvest lntorvalo obtained In the flrot two years wore fit tod In a Cobb-Dcughlua reoponoo function and tha expooted values «leulat«d were found to be very olooe to tho actual observed values• In thethird and fourth year of the experiment, the ghest herbage and oil yields were recorded by 60 day interval followed by the interval of 65 days, A quadratic Sanction for the data pooled over for four years was fitted to see if the data show a diminishing return with an increase in harvest interval and it was found that the optimum herbage yield and oil yield were obtained when the harvest was done at 62 and 63 days interval respectively. The final observational trial also indicated that the herbage and oil yields were maximum for a harvest interval of 65 days beyond which it decreases. Increase in the levels of both N and P tended to increase the content of geraniol and to decrease the content of geranyl acetate which are the price determining quality attributes of the essential oil. But the maximum interval of harvest viz., 65 days tried in the main experiment was not sufficient for moot of the physico-chemical properties of oil ouoh os specific gravity, refractive index, geraniol content, geranyl acotato content etc. to reach the minimum limit prescribed by 131. The observational trial showed that the oil obtained at intervale of 95 days and above satisfied the 131 specification with respect to all the phyalco-ohemioal properties of the oil. However, at thle herveat Interval the yield Itself wae considerably depressed compared to the yield at 65 day Interval. Th, eoonomloe worked out for different Intervale of harveab baaed on the ourrant market prloe of the oil paeein« as above the I SI limits as fe. 240/- per kg (oil from harvest intervals at and above 95 days) and those below them at Fs.220/- per kg (oil from harvest intervals of 55, 65, 75 and 85 days) it has been found that a wider cost benefit ratio and net return per rupee investment are obtainable for harvest intervals ranging from 65 to 85 days. However, 65 day interval has the advantage of early returns from the investment. Application of 50 kg PgO^/ha in view of its significant effect on herbage yield, oil yield and oil quality increase the net profit per rupee invested for the harvest interval of 65 days. The average removal o f II, P, K, Ca and Mg from the s o i l by palmarosa per h ecta re per year lias a lso been worked o u t . The optimum f e r t i l i s e r le v e l fo r palmarosa i s 25 kg II, 50 kg P?05 and 25 kg K20 per h ectare over an a p p lic a tio n o f spent g ra ss at 5 tonnes per h ectare per y e a r . The optimum h arvest in to r v a l i s 65 days. TIiIb would give maximum horbago y io ld , o i l y io ld ond ea rly return from in v e stm e n ts.
Open Access
Nitrogen losses from the rice soils of Kerala with special reference to ammonia volatilization
(Department of Soil Science and Agricultural Chemistry, College of Horticulture, Vellanikkara, 1989) Anila Kumar, K; KAU; Rajaram, K P
In order to get a deeper insight in to the N dynamics of selected submerged rice soils, an investigation entitled “Nitrogen losses from the rice soils of Kerala with special reference to ammonia volatilization” was carried at the Regional Agricultural Research Station, Pattambi during 1985 – 87 with the following objectives. 1. To estimate the magnitude of ammonia volatilization losses from submerged rice soils, representing major rice growing tracts of Kerala. 2. To study the factors which are responsible for accelerating the rate of ammonia volatilization under flooded soil conditions. 3. To evaluate the effect of submergence, organic matter application, complementary effect of P and K on ammonia volatilization from the rice soil ecosystem. 4. To identity suitable N carriers capable of reducing the loss of N due to ammonia volatilization from submerged paddy soils. 5. To find out the effect of continuous application of organic and inorganic manures in lateritic submerged paddy soils on the quantum of N loss through ammonia volatilization. 6. To find out the transformations and extent of mineralization of applied urea. With these objectives, in view, a serious of laboratory incubation studies, followed by pot culture trials were carried out and the results were finally verified under field experiment also. Besides these, the plots of permanent manorial trial (dwarf indica) were utilized for estimating the N loss through ammonia volatilization on long term application of organic manures and inorganic fertilizers. In the incubation study for estimating the magnitude of N loss though ammonia volatilization, eight rice soils of kerala viz., sandy, karapadam, kayal, kari, pokkali, kole, poonthalpadam and laterite soils representing the major rice growing tracts of Kerala were incubated with no N and 27 g N m-2 as urea. Air train and acid trapping device was utilized to collect the volatilized ammonia. The results showed that sandy soil collected from Onattukara region registered an increased N loss through ammonia volatilization, whereas in the kole soil of kattukampal, the process was retarded to the lowest level. More than 75 per cent of the volatilization loss was observed within 9 days after urea application. Significant negative correlation was observed between ammonia volatilization and organic matter content, clay fraction and cation exchange capacity of the soil, whereas the coarse sand fraction showed significant positive correlation. Soil sterilization had little influence on ammonia volatilization in any of the soil under study. Another incubation study to assess the impact of quantity of urea applied on the quantum of N loss through ammonia volatilization was carried out using four soil types (sandy, kayal, poonthlpadam and laterite soils) with four rates of N application (9, 18, 27 and 36 g N m-2 ). The results indicated that the N loss through ammonia volatilization had a positive relationship with increased rates of urea application, though not linear. The complementary effect of phosphorus and potassium on the extended loss of N through ammonia volatilization was estimated in another incubation study utilizing the same four soil types with treatment as N alone, N and P, N and K and N, P, K @ 27:13.5:13.5 g N, P, K m-2 respectively as urea, superphosphate and muriate of potash. The results revealed that combind application of urea and muriate of potash was found to be significantly better in reducing the volatilization loss to be significantly better in reducing the volatilization loss of ammonia compared to the treatments, N alone and N and P. The incubation study to find out the influence of depth of submergence on the rate of volatilization of ammonia was conducted using the same soil types and four treatments (soil saturation, 5,10 and 20 cm submergence). The results showed that the soil samples maintained at saturation point recorded double the values for ammonia volatilization, compared to samples kept under submergence of 20 cm depth. The effect of application of organic matter on N loss through ammonia volatilization was studied in the same four soil types with the treatments as no organic matter, 0.25, 0.50, 0.75 kg organic matter m-2 as farm yard manure. The results indicated that application of organic matter was found to reduce volatilization losses considerably in all the soils studied and the lowest value recorded was for the treatment receiving farm yard manure @ 0.75 kg m-2. The relationship between N sources and the extent of volatilization of ammonia was investigated in another incubation study employing the same four soil types and ten different N carriers to supply 27 g N m-2. The relative efficiency of different N carriers in reducing the ammonia volatilization loss was in the order sulphur coated urea > urea mudball > gypsum coated urea > rock phosphate coated urea = neem cake coated urea = ammonium sulphate = ammonium chloride > urea : coconut pith: soil = urea. The pot culture study to trace the pathway of transformation and extent of mineralisation of urea under flooded soil condition consisted of three soil types (laterite, kari and poonthalpadam soil) and two levels of N (no N and 90 kg N ha-1 as urea). The rate of mineralisation of applied urea followed the soil reaction and the mineralisation stopped at the stage of NH+4 formation and hence chances of N loss through denitrification is meagre, unless the soil is aerobic. The second pot culture experiment was conducted with a view to identify the different ways that result in minimum loss of N through ammonia volatilization in sandy and laterite soils. The study showed that the decreasing order of N loss through ammonia volatalization from different N carriers followed the order, urea basal = urea; coconut pith: soil = coaltar coated urea = gypsum coated urea = rock phosphate > coated urea > urea split > urea super granule > urea mudball > sulphur coated urea. The five treatments selected from this experiments viz., urea split, urea mudball, urea super granule, gypsum coated urea and rock phosphate coated urea on reduced ammonia volatilization and high grain yield were compared in another pot culture trial and finally it was verified under field experiments in trial and finally it was verified under field experiments in laterite soil. The results revealed that urea mudball placement in the anaerobic layer of soil was found to reduce the n loss through ammonia volatilization to negligible level. Treatments with surface application of rock phosphate coated urea and urea in split dose ranked second and third position respectively in reducing the volatilization losses. Treatment receiving split application (top dressing of urea at 20 and 40 DAT) reduced ammonia volatilization considerably. Significant positive correlation was found between the cumulative N loss through ammonia volatilization and flood water pH measured at 0800 hrs and 1400 hrs, flood water NH4 – N content and flood water bicarbonate content. The pH of flood water measured at 1400 hrs were significantly higher than the value recorded at 0800 hrs and highest diurnal variation was observed for treatment with urea super granule deep placement. The urea super granule deep placement treatment resulted in increased grain yield in both the pot culture trials and field experiment. However, in field experiment the effect of different N carriers on grain yield was found to be uniform. The periodical N uptake by plants as well as N accumulation in grain and straw at harvest were found to be higher in the case of treatments receiving USG deep placement and urea split application. The effect of long term application of organic and inorganic nitrogen sources in soil on the rate of n lose through ammonia volatilization was studied utilizing the permanent manorial experiments. Plots receiving combined application of cattle manure + green leaves + NPK @ 45:45:45 kg N, P2 o5, K2 o as ammonium sulphate, super phosphate and muriate of potash were recorded the lowest value of n loss via ammonia volatilization when compared to other treatment plots.
Open Access
Nutrient dynamics in the rice based cropping systems
(Division of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani, 1989) Sundaresan Nair, C; KAU; Subramonia Aiyer, R
The experiment consisting of five cropping sequences viz. rice - rice – rice (A1), sweet potato – rice – rice (A2) cowpea – rice – rice (A3) daincha – rice – rice (A4) and fallow – rice – rice (A5) and six treatments with varying doses of N P and K were conducted to study the performance of the sequences in relation to the nutrients required for optimising the out put from the sequences. The field experiment was laid out at R.R.S., Pattambi in 1980 – 81 and the experiment was conducted for two consecutive years ie. For six seasons. The experiment was started with the summer crop of 1981, namely summer rice (Triveni), sweet potato, cowpea, daincha and a summer fallow wherein the land was ploughed twice and left as such without any crops. The component crops were raised with five treatment variations modified from the recommended doses for each crop. The biometric observations for the summer crops, virippu and mundakan crops were recorded. The indications were that treatments have a significant effect on summer crops virippu and mundakan rice crops of 1981 and 1982. The yield shows that both treatments and sequences have a significant effect. The sequence daincha – rice – rice and the cowpea – rice – rice sequence gave the highest yield. The chemical analyses of plant parts of the summer crops, virippu and mundakan rice crops of both 1981 and 1982 show that the treatments have no effect on the NPK content. The soil study shows that the cropping sequences have a significant effect on soil pH. A pH decrease was noticed in all the sequences, the highest decrease being in the rice – rice – rice sequence. The organic carbon level of the soil is also affected due to the cropping sequence. The rice – rice – rice sequence shows a maximum decrease in organic carbon level and the daincha – rice – rice – rice shows a gain in organic carbon level of the soil. The total nitrogen of the soil shows a decrease in all the sequences and maximum decrease was noticed in sweet potato – rice – rice sequence. The available nitrogen level also was influence both by the sequences and treatments. A decrease in available nitrogen was noted to be a maximum in the rice – rice – rice sequence. The total P and available P levels show an increase in all the sequences and were high in daincha – rice – rice and cowpea – rice – rice sequences. The treatments also have a significant effect in maintain the P level in soils. The total K status of the soil as well as the exchangeable status of K shows a decrease after two year of cropping. The nutrient uptake studies reveal that the maximum NPK uptake takes place in the sequences sweet potato – rice – rice followed by rice – rice – rice and cowpea- rice – rice – rice followed by rice – rice – rice and cowpea - rice – rice. The balance sheet of nutrients reveals that nitrogen and available phosphorus in all sequences show a decrease and increases with decrease in fertilizer levels. The balance sheet of K shows that the soil maintains K levels. The sequence daincha – rice – rice is the best in maintaining a high K status in the soil. An analysis of the economics of cropping sequences reveal that the sequence sweet potato – rice – rice with full recommended dose of fertilizers gave the highest net return, which was followed by Cowpea in – rice – rice and rice – rice – rice. From nutrient balance studies, yield and economic analysis it is clear that any attempt in reducing the quantity of fertilizer for the component crops of the sequences affects the yield, besides deleteriously affecting the fertility of the soils. Any decrease in the fertilizer doses in the sequences will not be economical. With a long range view of enhancing crop output from cropping sequences and maintaining soil fertility, it becomes necessary to enhance and maintain higher fertility levels.
Open Access
Potassium supplying capacity of Neyattinkara- Vellayani soil association and its relationship with potash nutrition of major crops on them
(Department of soil science and agricultural chemistry, College of Agriculture, Vellayani, 1989) Valsaji, K; KAU; Subramonia Aiyer, R
Detailed study on the potassium supplying capacity of Neyattinkara-Vellayani soil association and its relationship to potash nutrition of major crops on them namely coconut and cassava has been made. This soil association represents the red loam soil type and consists of Neyattinkara series tentatively classified under Typic Eutropepts and Vellayani series under Typic Tropudalfs. Soil samples were collected from selected fields under coconut and cassava for the dominant soil types namely sandy clay loam and sandy loam soils under Neyattinkara and Vellayani series. To find the most suitable depth and location of soil sampling for coconut samples were also drawn at two different depths of 0-30 and 30-60 cm from basins and interrows. Empirical methods, quantity intensity studies, electro ultrafiltration studies and foliar diagnostic techniques were employed to decide on the most suitable method for plant available K. The various intensity, quantity and capacity factors which relate the readily available, difficultly available and storage or buffer capacity were assessed for a proper appraisal of the K status and supplying capacity. The various soil K parameters such as total K, water soluble K, exchangeable K, available K, nonexchangeable K, HNO3 extractable K, H2SO4 extractable K, sodium tetraphenylboron extractable K and percentage K saturation were found to be low. The water soluable K formed higher proportion of available K than exchangeable K. Interrelations showed that water soluble K, exchangeable K and available K are in dynamic equilibrium. Nonexchangeable K did not show any relationship with available K indicating that it is a poor source of available K. Quantity-intensity studies showed that the shape of the Q/I curve was similar in all types of soil since they belong to a group of related soils. The Q/I parameters like Are.K, Ko, Kx. KL and potential buffering capacity values were low. The KL values were higher than NH4OAc.K. The free energy values were found to be high indicating easy release rates of K. For both coconut and cassava, the leaf K did not show any consistent relationship with the soil K parameters. Among the Q/I parameters, KL had significant relation with leaf K of coconut. EUF 10 and EUF 35 showed significant relationship with leaf K of both coconut and cassava. Electro-ultrafiltration studies showed that the easily desorbed K was more than the strongly desorbed K indicating easy supply rate. The EUF 30-35 values were related to HNO3.K showing that this fraction included some initially nonexchangeable K. EUF 10 had significant relation with exchangeable K and EUF 35 had significant relation with exchangeable and available K. The buffer parameters BK (EUF 10/EUF 30) and EUF.Q (EUF 30-35/EUF 30) values were low indicating low buffer capacity. The EUF desorption pattern showed that the first peak was within 10-20 minutes and prominent than the second peak. This indicated low reserve K but easy supply rates. The EUF desorption curve of soils with high NH4OAc.K was above that of soils with low NH4OAc.K. This indicated that with increase in NH4OAc.K the easily desorbed K also increased. The yield of coconut and cassava were related to the various K parameters to evaluate the suitability of different methods for available K. In most of the cases NH4OAc method was found to be suitable for both coconut and cassava. Leaf analysis was also found to be suitable. For cassava the result obtained after 41/2 months is meaningless for the current crop. For coconut collection of index leaf is laborious. It was also found that the Q/I and EUF were suitable for coconut. But these methods cannot be followed in routine soil testing because of the high input of laboratory work involved. Considering these aspects it was found that NH4OAc method is the most suitable one because it is simple, cheap and easy to adopt. Based on the NH4Ac.k content the fertility status was found to be low to medium. The various intensity, quantity and capacity factors of soil K, Q/I and EUF parameters revealed that this soil association has low K supplying capacity. Hence heavy dose of K fertilizers is required. Split application is preferable especially in sandy loam soils to reduce leaching losses. NH4OAc.K which was found as a suitable indicator of plant available K varied significantly in basin and inter-row samples of sandy clay loam soils. This K fraction did not vary with depth in both sites. In general, the K nutrient index indicated that the basin samples had a higher level than inter-row and surface samples. Thus basin sampling at 0-30 cm depth was found to be the ideal site of soil sampling for coconut tree.
Open Access
Pedologic studies on lateritic catenary sequences occuring in Kerala
(Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani, 1980) Venugopal, V K; KAU; Koshy, M N
Three toposequences were identified at Varkala, Poruvazhy and Kalliaserry in the midland laterite regions of Kerala using soil maps prepared by the Soil Survey Wing of the Department of Agriculture. A systematic study of these catenary sequences along transects extending from crest to valley was made. Profile pits were dug at the different physiographic positions of the transects, viz., crest, upper midslope, nearly level lower slope and valley at each of these sites are examined for their morphological features. Soil samples representing the different horizons were also collected for laboratory studies. The physicochemical charecteristics, mineral assemblage of fine sand fraction and the minerology of the clays in the profile samples were studied with a view to relating these properties to the genesis of the four diverse groups of soils formed in the transects . Patterns of similarity or otherwise between the three locations of widely different conditions and the four physiographic positions were brought out .
Open Access
Assessment of the factors governing response to phosphorus in the rice soils of Kerala
(Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani, 1986) Harikrishnan Nair, K; KAU; Subramonia Aiyer, R
An attempt to asses the various factors and arrive at an integrated picture on response of rice to phosphorus application In rice soils of Kerala has teen made. To reach certain conclutions on some of the possible reasons for response of rice to phosphorus in the acid rice soils of Kerala, the following approaches have been resorted to. Categorising the rice soils with special reference to phosphorus and factors affecting them. Detailed studies on the various phosphorus fractions, both inorganic and organic as well as toe relationships between the p fractions and important soil parameters have been worked out. Elaborate studies have been undertaken on the p fixing capacity of rice soils and the factors governing it. Phosphorus transforation studies, both inorganic p fractions and available f due to waterlogging have also been conducted. Two rice varieties, a medium end a short duration have been tested under pot culture conditions -lO with graded levels of B and finally two p labelled sources of phosphorus, a water soluble monocalcium phosphate and water insoluble tricalcium phosphate have been evaluate in 7 soil types representing the major rice soil types of Kerala
Open Access
Release of soluble aluminum in soils under submerged conditions and its effect on rice
(Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani, 1984) Alice Abraham; KAU; Koshy, M M
A study has been made of the extent of aluminium toxicity in relation to other important characters in the acid Rice soils of Kerala with a view to evolving suitable ameliorative measures. The submerged wet-land rice soils of Kerala showing a pH value of less than 6.0 were found to contain appreciable amounts of exchangeable aluminium. The aluminium saturation of the total and effective CEC of these soils were also very high and above the critical limits for rice. Flooding of these soils in water resulted in a rise in pH and EC, as well as water soluble and exchangeable iron. Peak values for these parameters were obtained by 20-30 days of flooding, after which the pH remained steady while EC and iron showed a decreasing trend . Water soluble and exchangeable aluminium, on the other hand, registered a significantly lower content after the 6th day and the trend continued till the 30th day.
Open Access
Effect of potassium nutrition on the yield and quality of cassava
(Department of soil science and agricultural chemistry, College of Agriculture, Vellayani, 1983) Gopalakrishnan Nair, P; KAU; Aiyer, R S
Investigations were carried out as the College of Agriculture, Vellayani to find out the optimum dose and time of application of potassium to conserve in relation to yield and quality. For this purpose a seed culture experiment with nutrient solutions at varying duration of it application and a microplot field experiment with different levels of E were conducted during 1977-80. Before conducting the field experiment, two crops of cassavawere taken in the same plot without applying potassium and thus bringing down the available K to a low level.
Open Access
Studies on the Solubilisation of iron in submerged soils and methods to minimise its solubility and toxic concentration to paddy
(Department of soil science and agricultural chemistry, College of Agriculture, Vellayani, 1989) Ramasubramonian, P R; KAU; Koshy, M M
A study has been made of the extent of solubilisation of iron in the submerged acid rice soils of Kerala State where iron toxicity is likely to be a serious field problem during rice cultivation. The kayal, kari and karapadom soils of Kuttanad, brown hydromorphic soils of the midland lateric zone and the sandy soils of Onattukara were included in the study. Chemical characterisation of the soils and soil profiles in relation to forms of iron were investigated with a view to obtain a better understanding of the dynamic aspects of iron in these soils. The nature, and extent of periodical variations in soluble iron as influenced by levels of sea water, organic matter (farm yard manure) and ammonium sulphate added to kayal, kari, karapadom and brown hydromorphic soils under submerged conditions were also studied along with the influence of levels of applied lime on the amelioration of iron toxicity. Among the Kuttanad soils, kari soil was most acidic with a mean pH of 3.77, high organic carbon content and CEC compared to others. The water soluble iron ranged from 79 – 165 ppm in the Kuttanad soils. This form of iron and pH were negatively correlated. Electrical conductivity and water soluble iron were significantly and positively correlated. The exchangeable iron varied between 144 – 310 ppm and was positively correlated with CEC. Active iron ranged between 1460 and 5200 ppm. Active iron had a significant positive correlation with organic carbon and electrical conductivity. Kuttanad soils contained high contents of water soluble and exchangeable iron, together known to contribute towards the development of iron toxicity to transplanted rice in these soils. High contents of water soluble, exchangeable and active iron were noticed in the profiles of Kuttanad soils as well. Compared to these, the brown hydromorphic and sandy (Onattukara) soils had much lower contents of soluble iron. Total iron content decreased with depth in most of the profiles while water soluble, exchangeable and active iron, increased with depth studied upto 100cm. In brown hydromorphic soil the water soluble and exchangeable iron were found to decrease with depth. Incubation studies under laboratory conditions indicated that submergence of soils resulted in an increase in the soluble iron with time, reached a peak value on the 10th day in kari soil and 25th day in the other soils, after which the soluble Fe2+ decreased to lower values. Sea water submergence resulted in enhanced releases of Fe2+ with time to reach peak value around the 25th day followed by decrease. Kayal soil alone, however, needed 40 days for peak release of Fe2+. The release of Fe2+ was influenced by the dilution of sea water used. Kayal, kari and karapadom soils released significantly higher amounts of Fe2+ compared to brown hydromorphic soils. However, at the lowest level of 25 per cent sea water all soils behaved similarly. Presence of organic matter under the submerged conditions enhanced the Fe2+ release considerably depending on the content of organic matter in the soil. Kari soil on the 25th day and kayal soil on the 40th day of submergence released significantly higher amounts of Fe2+. Addition of ammonium sulphate to soils under submerged conditions resulted in increased releases of Fe2+ in the soil solution with time. Peak releases of Fe2+ were noticed on the 25th day in all the soils. Maximum release by kari soil was influenced by ammonium sulphate applied at 100 kg N/ha, in the karapadom and brown hydromorphic soils by 200 kg N/ha. The beneficial effect of lime on the suppression of iron release was clearly evident in the soils though to varying extents. In kayal soil 600 kg lime/ha suppressed iron release upto 10 days and 1000 kg/ha could suppress more soluble iron for 25 days. However, after the 40th day, soluble iron exceeded that of the control. In kari soil the iron suppressing effect of both the levels of lime was evident only up to the 10th day after which the release of soluble iron exceeded that of the control. In karapadom and brown hydromorphic soils, lime at 600 kg/ha was helpful in suppressing the release of Fe2+ till the 40th day. Lime at 1000 kg/ha, however, could suppress more of the soluble Fe2+ throughout the period of submergence. In kayal and kari soils, levels of lime upto 1000 kg/ha appear to be inadequate in controlling iron toxicity. Flooding the field for 25 days and leaching out the released Fe2+ just before planting of rice is suggested as an alternate solution to minimise iron toxicity to rice in Kuttanad soils.

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