(1)South China Agricultural University,510642,Guangzhou,Chinaª (2)Guangdong Academy of Agricultural Sciences,Guangzhou,China

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1 Scientific registration n 1579 Symposium n :14 Presentation : Poster Study on relationship between quantity-intensity characteristics of potassium in paddy soils and potassium application Etude des relations entre caractéristiques et doses de potassium dans la fertilisation des sols de rizière LI Huaxing (1), LI Guoful (2), MA Manzhuang (2), HUANG Xiaohong (2)ª (1)South China Agricultural University,510642,Guangzhou,Chinaª (2)Guangdong Academy of Agricultural Sciences,Guangzhou,China Summary This paper discusses the Q/I characteristics of potassium in paddy soils.the results show that PBC k is significantly correlated with clay content of soil. K ã has positive correlation with soil exchangeable K and is significantly correlated with the relative percentage of rice yield.areª k is negatively correlated with clay content of soil and closely related to the frequancy of potassium application. Since the Q/I concept was put forward by Schofield(1947),Beckett (1) used the Q/I curves and its relevant parameters to evaluate the status of soil potassium.areª k could be used to measure the potassium availability,- K could be used as an index for avaluating the amount of soil available potassium,while potential buffering capacity of soil potassium could be estimated by PBC k.some researches shown that PBC k was significantly correlated with CEC (2,5),and - K was significantly correlated with soil exchangeable K (5). Lin (3) reported that PBC k was significantly correlated with the total amount of potassium absorbed by paddy rice under pot culture without potassium fertilizer for continual four crops.the aim of this study is to find out the retationship between Q/I charateristics of potassium in paddy soils with different texture and potassium application. Materials and methods The paddy soils used were sampled from 13 counties in Guangdoug Provinces and their properties are listed in Table 1. 1

2 Table 1 Soil physical and chemical properties of the investigated soil samples N Texture Organic matter (g/kg) < mm clay (%) K* (mg/kg) 1 Clay loam Clay loam Clay loam Clay loam Clay loam Clay loam Clay loam Clay loam Clay loam Loam Sandy clay loam Sandy clay loam Sandy loam Sandy loam *Exchangeable K: 1mol/L ammonium acetate extration and flame photometer method. Field exeperiment Experiment 1:The experiment with two treatments,one received potassium (potassium chloride,225 kg/hm²)and the other had no potassium application,was conducted in 15 soils.each experiment was carried out in four repetitions and each treatment received enough nitrogen and phosphate.paddy rice was the tested plant transplanted in eary season and late season seperately. Experiment 2:The experiment with 5 treatments of split application of potassiun and three repeats was conducted in sandy loam soil and clay loam soil.the treatments were as follows: 1.No potassium fertilizer % potassiumm(225kg/hm²) was applied as basal dressiug. 3.50% of the total potassium as basal dressing and 50% as topdressing on 18th days after transplanting. 4.50% as basal and 50% at panical initiation. 5.50% as basal,30% on the 18th day after transplanting and 20% at panical initiation, respectively. Analyses Five grams of soil were equilibrated with 50ml mixed solutions containing 0.01 mol/l CaCl 2 and 0,10,20,30,50,70,90,100 mg/l KCl separately.the suspensions were shaken on a rotary shaker at 25 C for 30 minutes.potassium was determined in the filtrate,using flame photometer,while calcium and magnesium were determined by atomic absorption spectrometer. K can be calculated according to the difference between initial and final equilibrium potassium concentrations.the 2

3 potassium activity ratio AR k is equal to a ak ( Ca+ Mg) 1/2. The Q/I curve was plotted by using K as ordinate and AR k as abscissa.- K, PBC k and AR k can be got from the Q/I curve (1,4). Results and discussions The Q/I curves in different soils Figure 1 shows that the soils with different clay content present different Q/I curves.the curves represent different potassium availability and potassium status.the parameters of Q/I curves of different soils and its relative percentage of paddy rice yield are shown in Table 2. Fig.1Q/I curves of the soils with different texture Soil potassium potential buffering capacity(pbc k ) The PBC k ( K ) is an index of soil potassium capacity PBCk =. The figure 2 shows that PBC k ΑRk is significantly correlated with the clay contents of the siols(r=0.665**).zhu's study (5) indicated that PBC k and CEC are significantly correlated,and the same is true for CEC and the clay content of soil.this means that the value of PBCª K can be measured by the amount of soil clay.lin (3) has proved that PBC k is significantly correlated with the amount of potassium absorbed by paddy rice in continual four crops under pot cultrue without potassium fertilizer.but this study shows that PBC k has no correlation with the amounnt of potassium absorbed by paddy rice and relative percentage of rice yield in one crop (Table 2).The result indicates that the value of PBC k can only reflect the long-term potential ability of soil potassium supply. 3

4 Table 2 The main parameters of Q/I curves of the soil in relation to the relative percentage of rice yield N Texture - K (mg/kg) Ar k ex10-4 PBC k x10 4 (mol/l) 1/2 [(mg/kg)/(mol/l) 1/2 ] Relative percentage of yield (%) 1 Loam clay Loam clay Loam clay Loam clay Loam clay Loam clay Loam clay Loam clay Loam clay Loam Loam Sandy clay loam Sandy clay loam Sandy clay loam Sandy clay loam Increase rate of rice yield (%) The results of this study show that the value of K is significantly correlated with the soil exchangeable potassium (Y (- K ) = X (k),r=0.956**).the mean value of - K is not different from that of soil exxhangeable potassium and the result is different from that in upland soils (5).The result also indicates that both - K and exchangeable potassium have positive correlation with relative percentage of rice yield(r (- K ) =0.874**,r (k) =0.830**).Therefore both - K and the soil exchangeable potassium can reflect the soil potassium supply ability and can be used as the main parameters for estimating the amount of soil potassium supplement during rice growing season. - K has significantly negative correlation with the increase rate of rice yield increased by potassium fertilizer(y (- K ) = X (increase rate of rice),r=-0.874**).when the value of - K is at the level of 92.3 mg/kg,rice yield may not be increased by potassium fertilizer. 4

5 Soil liable potassium(are k ) Figure 3 shows that ARe k has significant negative correlation with the amount of soil clay(y (ARek) = X (clay),r=-0.541*),but has no significant correlation with relative percentage of rice yield. ARe k,as an intensity index of soil liable potassium,is of impotance in determining the potassium fertilizer distribution during the rice growing period.table 3 shows that rice yield is the highest in sandy loam with an ARe k value of 29.9x10-4 (mol/l) 1/2 when potassium was applied in a three-split application,i.e.50%,30% and 20% of the total potassium fertilizer were applied befor transplanting,18 days after transplanting and at panical initiation respectively.in clay loam with an ARe k value of 12.2x10-4 (mol/l) 1/2,the rice yield is increased by potassium fertilizer when potassium was applied in one or in two-split application,namely,100% of the total potassium fertilizer was applied before transplanting, or 50% before transplanting and the remainder on the 18 day after transplanting(or at panical initiation).it is obvious that the sandy soils with a high value of has a strong supply intensity of potassium and the frequency of potassium application could be increased to meet the demand of the rice plant,but the clay loam is on the contrary,thus the frequency of potassium application could be reduced. Table 3 The effect of split application of potassium on the rice yield in the soils with different ARe k values Treatment n ARe k = 29.9x 10-4 (Saddy loam, Guangzhou) ARe k = 12.2x 10-4 (Clay loam, Guangzhou) Yield Relative percentage Yield Relative percentage Conclusion 5

6 The paddy soils with different clay content show different Q/I curves which indicate soil potassium status. PBC k, parameter of soil potassium capacity,has positive response to soil clay(r=0.665**),and can reflect the long term potential ability of soil potassium supply. The value of - K has positive correlation with soil exchangeable potassium and is significantly correlated with the relative percertage of rice yield(r=0.874**),and has significantly negative correlation with the increase rate of rice yield increased by potassium fertilizer(r=-0.834**).when - K value is at 92.3mg/kg,paddy rice may has no response to potassium fertilizer. ARe k has a significant negative correlation with the soil caly (r=-0.541*).the value of ARe k may be used as a parameter for determining the frequency of potassium application. References 1. Beckett,P.H.T.,1964: Studies on soil potassiumi and II. J.Soil Sci.15,1 2. Huang, Changyong and Xie Zhengmiao and Jiang Qiuyi,1987: The correlation of soil buffering power and potential buffering capacity with soil K critical level.acta Agriculturae University Zhejiangensis.13(1): Lin, Zhongyan,1987: Application of thermodynamic Parameters in dianosing potassium requirement of paddy soils.acta Pedologica Sinica,24(1): Sparks,D.L.and Huang P.M.,1985: Physical Chemistry of soil potassium.potassium in Agriculture,R.D.Munson(ed.)201 «276.ASA.CSS and SSSA,Madison,WI 5. Zhu, Yongguan and Luo Jiaxian,1993: Q/I characteristics of potassium in soils in South China.Tropical and Subtropical Soil Science,2(2),73-80 Key words:q/i curve,potassium potential buffering capacity,exchangeable K,clay content,relative percentage of rice yield Mots clés : courbe Q/I, pouvoir tampon potentiel, K échangeable taux d'argile, pourcentage relatif du rendement en riz 6