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Potassium is most important macro-nutrient for crop growth. More than 30 Pakistani soils are low in available K and is inevitable for maximum crop yield. The methods for computing the K fertilizer recommendations, model based (Langmuir and Freundlich isotherms) recommendations are best for crop-specific and site-specific purposes under different textures. Therefore, a field experiment will be conducted to determine the K requirement of rice by studying the K retention capacity of soil using Freundlich and Langmuir adsorption model, in village Boopra Khird, District Gujranwala. The treatments will be: Control (recommended doses of K), Freundlich calculated potassium dose (F-K) and potassium dose calculated through Langmuir model (L-K). Treatments will be replicated thrice following randomized complete block design (RCBD). Composite soil samples will be collected from each plot at 0-15 cm soil depth for determining various chemical characteristics (ECe, pHs, OM, CaCO3, soluble cations and anions). The crop will be harvested at maturity stage and different growth parameters will be recorded. Plants and soil samples will be analyzed to compute external and internal K requirement.
Rice (Oryza sativa L) is considered one of the most important cereals of Pakistan and occupies second position after wheat. It is the most important summer cereal crop of traditional rice growing areas of the Punjab (61%), Sindh (31%), Baluchistan (4%) and high altitude valleys in the North (4%) (Awan et al., 2007). It is an important source of earning foreign exchange. Area under rice cultivation in Pakistan is 2571 thousand hectares with annual production of 6160 thousands tones and average yield is 2396 kg ha-1 (Economic Survey of Pakistan, 2011-12).
Potassium is a vital element for plant growth and is an exceptionally dynamic ion in the soil system. As an ion, potassium is highly mobile in the plant system but, only moderately mobile in the soil system. Potassium is a key nutrient and makes the plants tolerant to stresses such as cold / heat stresses, drought, diseases, and pest attack. Potassium catalyzes many of the enzymatic processes that are taking place in the plants (Ujwalaranade, 2011).
In Pakistan, mostly soils are sufficient in total K as Mica is the dominant clay mineral in most of soils (Soil Survey, 1985). Potassium is present as component of relatively insoluble mineral, however only a small fraction is present in available form to plants. In the soil, there is the equilibrium among soluble, exchangeable, non-exchangeable and fixed forms of the K and hence responsible for the K availability. Among these forms, soluble and exchangeable K is eagerly available to plants whereas the form which is slowly available is the non-exchangeable K. Due to less application of potassium fertilizer and extensive removal of rice straw from the fields, Pakistani soils are facing a K-depletion of 8.8 mg kg-1 soil on an annual basis and a total deficit of 0.27 million tones annually (Bajwa, 1994; Malik et al., 1989).
For the rice, the farmers use only N fertilizer and very often P and K fertilizers are applied at sub-optimal level. This practice of imbalance and inadequate fertilizer application affects the soil productivity and depletes the essential nutrients (Cassman et al., 1996). It is therefore pre-requisite to rationalize the use of fertilizers particularly the K. Oftenly; response to K fertilizer in Pakistan is sporadic which necessitates the site-specific recommendations for K.
Different adsorption equations/models are traditionally used for recommendation of fertilizers while keeping in view crop requirement and adsorption capacity of soils. Among these models Freundlich and Langmuir models are widely used for site-specific recommendations. These involve monitoring of adsorption of the nutrient in soil, development of the sorption isotherm which accounts for strength, amount and competence factors which have much importance for predicting the amount of nutrient required for the maximum growth of the plant.
The present study will be conducted:
Apply and compare the potassium levels in the field determined by Freundlich and Langmuir models.
V. REVIEW OF LITERATURE
Hannan et al. (2007) conducted a laboratory study to find out the potassium adsorption characteristics of four soils under potato-maize cultivation system in Punjab. Adsorption isotherm of Potassium were constructed by equilibrating 2.5 g soil samples with 11 levels of K (0- 250mg L-1) as KH2PO4 in 25 ml of 0.01M CaCl2 solutions. Then adsorption data were fitted to the Freundlich, Langmuir and the Temkin adsorption equations. Loam, sandy clay loam and silt loam soils arrived at equilibrium after 12 hours while clay loams soil reached in steady state after 18 hours of equilibrium period. On an average amount of K adsorbed ranged from 30.10 to 37.67 % of added K. The Freundlich equation explained the adsorption behavior of K better than the other two equations as higher correlation values ranging from 0.95 to 0.99 is evidenced. All the four soils with almost similar clay contents differed in their adsorption characteristics.
Hannan et al. (2011) conducted an experiment to investigate K-adsorption in the soil. For this, soil sample from the major potato growing areas of Pakistan was collected. The Freundlich and Langmuir are the adsorption models in which adsorption data were fitted. The results showed that the Freundlich adsorption model (R2= 0.96**) explains the data better than the Langmuir adsorption model. On the basis of Freundlich model fertilizer rates were calculated and the potassium levels of the soil solution were targeted at 0, 3, 6, 9, 12, 15, 18, 21, 24, and 27 mg K L−1. Then on the soil a field study was conducted to determine the effect of various potassium levels (0-27 mg L−1, having potassium fertilizer rates of 0, 24, 49, 75, 101, 128, 155, 182, 210, and 237 kg ha−1) in the soil solution on the yield and quality of potato tuber. At the time of sowing N and P2O5 are applied at the rate of 300 kg and 250 kg ha−1 respectively. Yield response models (linear plus plateau, quadratic, square root, quadratic plus plateau, and exponential) were used to estimate the fertilizer rate optimal for potato crop. Linear plus plateau model fit the data with less favoritism than the other yield response models. It was seen that there was a considerable effect of potassium use on the yield and quality of potato tubers. Application of potassium fertilizer @ 130 kg K ha−1, which is equal to the level 12 mg K L−1 in the soil solution, maximized the yield of potato tubers.
Hannan et al. (2012) Different soils have different adsorption capacity due to which an individual critical level of K is in sufficient for different textured soils as well as verity of crops. In a country, fertilizer recommendations for a specific region required prolonged investigational trails. Being a good indicator of soil fertility, sorption isotherm provides reliable fertilizer recommendations. Freundlich and Langmuir adsorption isotherms are worldwide used to measure the behavior of nutrient in soil and fertilizer recommendation for a specific site and crop. A few experimental trails required for this kind of approach. Initially within laboratory, target soil solution levels to be set with the help of previously discussed adsorption models. At the end of trail, a precise targeted yield to be achieved through evaluation of yield data by using various models (quadratic, exponential/Mitscherlich, Boltzman sigmoid, Extended logistic, linear plus plateau models).Maize fodder crop tested by using above said approach. As a result of 14.50 mg L−1 soil solution level which is equal to 144 kg K ha−1, the total fodder yield was 61.91 Mg ha−1.The result indicates that a higher soil solution level required for maximum and better quality fodder trait output.
Doula et al. (2008) studied the kinetics of potassium adsorption from solution to exchangeable phases on bentonite samples. Firstly, bentonite was saturated with calcium in the form of CaCl2. Potassium adsorption time was evaluated on Ca-saturated samples using 125, 150, 200, and 250 μg K mL-1 solutions equilibrated for 10, 15, 20, 30, 35, 45, 60, 75, 80 and 120 minutes. Samples pH varied between 4.0 and 9.0. Equilibrium in potassium exchange was reached faster in lower concentrations and higher pH values. First order rate, parabolic diffusion, Elovich and modified Freunlich equation were used to describe potassium sorption. Comparison of coefficients of determinations (r2) plots indicated that modified Freunlich and parabolic diffusion models provided the best fits of the adsorption data (r2 > 0.979) Constants for the Freunlich equation were estimated and the model was expressed as a function of pH.
Choudhury et al. (2003) investigated the K deficiency in different rice growing areas of Malaysia. So a study was carried out on K adsorption in three Malaysian rice soils (Guar, Hutan and Kangar series). They used six levels of K (0.00, 28.77, 33.57, 38.37, 43.16 and 47.96 mmol kg-1). The data on the K adsorption were fitted into Langmuir, Freundlich, and Temkin adsorption equations. Adsorption data were also correlated with pH, cation exchange capacity and organic matter content of the soils. Potassium adsorption increased linearly with increasing levels of added K in all the three soils. The rate of increase was the highest in Guar series followed by Kangar and Hutan series, respectively. Potassium adsorption in two soils (Hutan and Kangar) fitted into Langmuir equation while the adsorption data in Guar series did not fit into this equation. Adsorption data in none of the soils fitted well in Freundlich and Temkin adsorption equations. Correlation between K adsorption and pH was significant (r = 0.881), whereas, correlation of K adsorption with either organic matter content or cation exchange capacity was non-significant. The results of this study indicated that K adsorption is mainly dependent on soil pH. In soils with higher adsorption capacity, more potassium fertilizer may be needed to get immediate crop response.
Jibrin (2010) conducted an experiment to measure the status of potassium (K) and adsorption behavior of 12 cultivated Fadama surface soils across Nigeria. The soils had 0.01- 0.23 (median 0.07), 0.21-6.83 (median 2.70), 0.20 - 1.73 (median 0.91) and 0.09-0.26 (median 0.14) cmolc kg-1, water soluble, 0.5 M NaHCO3 extractable, 1 N HNO3 extractable, and exchangeable K concentrations, respectively. Five of the soils were low in exchangeable K , while the remaining Seven had medium range of exchangeable potassium concentration. The Freundlich sorption model conformed to the K characteristics of the soils with R2 values ranging from 0.630 to 0.852 and had in the range of 54.1 to 71.8 L kg-1 KF (Freundlich coefficients) value. The soils had several types of sorption sites confirmed by the poor fit indication of the Langmuir model. The results of this study indicated that exchangeable K pools in the soil respond to high nutrient-demanding crops, such as rice with K fertilizer application.
Y.J. Dua et al. (2004) presented a study in the exploration of some factors controlling the adsorption of K+ onto two soils (Ariake clay and Akaboku). The synthetic leachates were used i.e solution having KCl salt and the solution containing multi-salts of KCl, NaCl and CaCl2. For both soils the potassium adsorption depends upon solid/solution ratio but in case of Ariake clay soil the results were more significant as compare to Akaboku soil. The K+ adsorption decreased with the increased ratio of soild/solution. The soils were equilibrating before the preordained time of standard batch-type tests and the Freundlich isotherm equation was used to determined the relationship between adsorbed and equilibrium concentration. In case of KCl solution conditions, both soils were adsorbed larger amount of K+ relative to multi-salt conditions.