Studying on potassium nutrient balance for sugarcane in lam son sugarcane belt thanh hoa province

; amount of K loss by erosion and leaching 3.2.1. The capacity of K supply of soil 3.2.1.1. Experimental soil properties The results of analysis of soil agrochemical targets of typical gray ferralit in Lam Son hilly region, used in experiment about the K supply capacity of soil for cane showed that soils with light mechanical composition 1,05 g/cm3; acidic soil (pH 4.64); poor organic matter content (1.22% OM); N, P, K compound are poor level (0.11% N, P2O5 0,05% K2O: 0.08%); phosphorus, exchange potassium are poor (P2O5 4,15 mg/100 g soil, K2O 5,75 mg/100 g soil exchange); low cation exchange capacity (CEC 11,31 mg/100 g soil). 3.2.1.2. The situation of sugarcane growth Results of monitoring the sugarcane growing criteria presented in Table 3.7 show that: In irrigated condition, the growing indicators of sugarcane increased compared with no irrigation in both cases, no fertilizing and N, P fertilizing. As N, P fertilizing, growing indicators increased compared with no N, P fertilizin in both no irigation and irigation cases, but the increasing level was lower than in irrigated conditions. 10 Table 3.7. Effect of irigation and N, P fertilizing on the sugarcane growth N o Formulas Germinat ive time (date) Tillering ratio (time) Plant height (cm) Stem diameter (mm) Plant volume (g/plant) Number of plant (plant/jar) 1 No irigation – no N, P 15 1,33 145,1 16,4 103,0 4,0 2 No irigation- N, P fertilizing 15 1,47 159,8 17,8 115,5 4,3 3 Irigation-no N, P fertilizing 10 1,63 186,5 20,7 116,7 4,5 4 Irigation - N, P fertilizing 10 1,90 225,5 24,6 153,3 5,4 LSD0,05 - 0,06 10,2 1,4 7,14 0,3 Irigation combined with N, P fertilizing, growing indicators of sugarcane increased compared with the cases that are irigation - no N, P fertilizing and no irigation – N, P fertilizing. Compared N, P fertilizing - no irigation formula with irigation - no N, P fertilizing, tillering ratio of sugarcane in the formula of N, P fertilizing increased 29.3 % and 16.6 %; plant height increased 41.1 % and 20.9 %; sten diameter increased 38.2 % and 18.8 %; plant volume increased 32.7 % and 31.4 %; plant density increased by 25.6 % and 20 % , respectively . 3.2.1.3. Amount of K2O accumulated in the plant Table.3.8. Effect of irigation and N, P fertilizing on amount of K accumulated in the plant No Formulas Fresh volume (g/jar) Dry matter content (%) Dry volume (g/jar) Content K2O (%) Amount of K2O accumulated (g/jar) 1 No irigation – no N, P fertilizing 568,0 27,4 153,5 0,19 0,34 2 No irigation - N, P fertilizing 684,3 28,7 197,5 0,22 0,51 3 Irigation - no N, P fertilizing 766,3 22,9 171,1 0,22 0,45 4 Irigation - N, P fertilizing 1059,9 25,3 264,4 0,280 0,870 The results in Table 3.8 show that compared with no irigation, irigation formulas have reduced dry matter content in all parts of the tree in both cases, no fertilizing and N, P fertilizing. However, due to living mass , dry mass and K2O content increased high result in that mass of K2O accumulated in tree increased high: 32.4 % and 70.6 % compared with no irigation, in the cases both fertilizing and N, P fertilizing respectively. N, P fertilizing increased living mass, dry matter content and K2O content result in that K2O cumulative volume in plant increased 50 % and 93.3 % compared to no N, P fertilizing in the cases of irigation and no irigation respectively . Irigation combined with fertilizer N , P , K2O cumulative volume reached the highest value (0.87 g / jar), up 93.3 % compared with the case of only irigation, no N, P fertilizing and 70,6 % compared with the case of only N, P fertilizing, no irigation. 3.2.1.4. Effect of irigation and N, P fertilizing on K supply capacity of land for plant K supply capacity of land for plant in conditions of irigation and N, P fertilizing differently is determined by the total K2O cumulative volume in all of the sugarcane after deducting the amount of K2O in cutting when planting. K2O volume in land able to provide for plant (kg K2O /ha) are converted from the land mass in the jar and the land mass in the range of 90% active sugarcane roots (depth of 0- 40 cm, natural weight of land 1.05 g/cm3). The research results are presented in Table 3.9; 3.10. Table 3.9. K2O volume in the cuttings when planting Number of cuttings (cutting) Cutting volume (g/cutting) Dry content (%) K2O content (%) K2O volume (g/jar) 3 11,5 31,2 0,47 0,05 11 Table 3.10. K2O supply capacity of land for sugarcane No Formulas K2O accumulation in plant (g/jar) K2O plants take from the soil (g/jar) K2O soil ables to supply (kg K2O/ha) 1 No irigation – no N, P fertilizing 0,34 0,29 40,6 2 No irigation - N, P fertilizing 0,51 0,46 64,4 3 Irigation - no N, P fertilizing 0,45 0,40 56,0 4 Irigation - N, P fertilizing 0,87 0,82 114,8 LSD0,05 0,03 0,05 6,9 The results in Table 3.9; 3.10 showed: Sugarcane in irrigated conditions, the K2O volume that soil ables to provide for plant increased 37.9 % and 78.3 % compared with no irigation condition in cases of no fertilizing and N, P fertilizing respectively. Sugarcane in conditions of N, P fertilizing, K2O volume that soil ables to provide for plant increased 58.6 % and 105 % compared with N, P fertilizing in cases of irigation and no irigation respectively. Sugarcane in irrigated conditions combine with N, P fertilizing, the soil ables to provide the highest K2O volume for sugarcane: reach 0.82 g / jar (kg / ha), an increase of 105 % (0.42 g / jar) higher than in the case of irigation - no N, P fertilizing, and 78.3 % ( 0.36 g / jar ) higher than in the case of N, P fertilizing - no irigation. From the above results showed, irigation and N, P fertilizing have a significant impact on the K supply capacity of typical gray ferralit soil. Irigation combined with N, P fertilizing, soil has able to provide the highest level of K2O volume (114.8 kg K2O /ha). In the case of only irigation - no N, P fertilizing or N, P fertilizing - no irigation, K2O volume, that is provided by soil, is reduced to 56 kg K2O /ha (down 48.8 %) and 64, 4 kg K2O /ha (down 56.1 % ) compared with the case of irigation combination fertilizer N, P. 3.2.2. K due to rainwater From the hypothesis that, in the area of sugar mills, K content in rainwater is higher than other areas due to the manufacturing process, the sugar mills use bagasse as a feedstock for boiler and emit into the atmosphere a considerable amount of furnace ash dust. Besides, sugarcane leaf tip burn practices after each harvest also contributes to increased levels of ash in the air. When it rains, dust rainwater enters the soil, thereby returning to land a certain amount of K. Rainfall K2O Content K2O volume Hình 3.2: K due to rainwater supply for soil in the Lam Son region of Thanh Hoa (2010 - 2013) K2O amount due to rainwater supply for soil (kg K2O /ha/year) are determined through rainfall and K2O concentrations in rainwater over the months of the year. Results monitoring of precipitation, and the amount of K2O, K2O content in rainwater over the month in 3 years (2010-2012) in the Lam Son presented in Figure 2 show that the climatic conditions of the Lam Son, the June, July, August, September, October, November have the highest total rainfall (average 1684.3 mm, accounting for 87.5 % of the total rainfall for the year). K content in rainfall is low (average 0.38 mg/l). The amount of K2O give land 5.81 kg K2O /ha/year, accounting for 70.8 % of K2O by rainwater supply in years. In contrast, December , January, Febuary, March, April, May K content in rainfall is high (1.18 mg / l), 3.1 times higher than the average of Rainfall (mm) Month K2O content (mg/lít ) Month K2O volume (kg/ha) Month 12 June , July, August, September, October, November. However, due to low rainfall (only 12.47 % of the total rainfall in the year) K content provides for soil only 2.4 kg K2O /ha/year. 3.2.3. K loss by erosion Table 3.11. K loss by erosion in Lam Son region of Thanh Hoa (2010 - 2012) No Monitoring criteria Basis Basis + 100 K2O Basis + 150 K2O Basis + 200 K2O Basis + 250 K2O Basis + 300 K2O 1 K loss due to erosion Erosive water (m 3 /ha) 713,45 535,09 442,34 385,26 299,65 278,24 K2O content (mg/l) 2,91 3,32 3,46 3,40 3,49 3,48 K2O loss due to erosive water (kg/ha) 2,08 1,78 1,53 1,31 1,05 0,97 2 K loss by suspension soil Erosive suspension soil (kg/ha) 1.555 1.211 1050 926 846 1.123 K2O content (mg/l) 1,38 1,48 1,51 1,52 1,55 1,30 K2O loss by soil suspension (kg/ha) 21,46 17,92 15,86 14,08 13,11 14,60 3 K loss by sediment Soil sediment erosion (m 3 /ha) 773,16 602,43 543,94 472,45 408,01 520,43 K2O content (mg/l) 0,58 0,61 0,61 0,62 0,63 0,50 K2O loss by soil sediment (kg/ha) 4,48 3,67 3,32 2,93 2,57 2,60 4 Total K2O loss by erosion (kg/ha) 28,02 23,37 20,71 18,32 16,73 18,17 Research results K2O amount loss due to erosion (3-year average 2010-2012) presented in Table 3.11 shows that: K fertilizing had a positive influence to limit the amount of K loss by erosion. Water, soil suspension and erosive sediment soil tends to decrease when increasing the amount of K fertilizing, in contrast, K2O content in them tends to increase. However, due to the decline in water quality, suspension soil and erosive sediment soil is faster than the rate of K2O content in them, leading to loss of K according to the amount of ingredients as well as the total amount of K loss by erosion reduces gradually. However, the difference is only evident when comparing the formula of K fertilizing with formula of no K fertiling, even among the formulas of K fertilizing, the difference only shows unclear. The total amount of K loss due to erosion of volumes average from 100 to 300 kg K2O /ha/year is 19.5 kg K2O /ha, in which 77.7% (15.1 kg K2O /ha) loss by erosive suspension soil. The amount of K loss by erosive sediment accounted for only 15.5% (3.0 kg K2O /ha). The amount of K loss under erosive water accounted for only a small fraction (6.8%, 1.3 kg K2O /ha respectively). In the case of no K fertilizing, the amount of K loss by erosion is 28.0 kg higher 8.56 kg K2O /ha (44.0%), compared with average of fertilizing formulas from 100 to 300 kg K2O /ha. The amount of K loss due to erosion of the average three fertilizer formulas 200, 250 and 300 kg K2O /ha is 17.7 kg decrease 5.6 kg K2O /ha (down 23.4%), compared with the amount of fertilizer 100 kg K2O /ha. From the above results, the amount of K loss by erosion suggest to use in evaluating nutrient balance and determine the amount of K fertilizer suit for sugarcane cultivation on typical gray ferralit soil, no irigation, MY 55-14 variety, base fertilizer 200 kg N + 100 kg P2O5 in the Lam Son rgion is 17.7 kg K2O/ha. 3.2.4. K loss by leaching Research results K2O amount loss by leaching (average 3-year 2010-2012) presented in Table 3.12 show that changes in water, leaching suspension soil, K2O content in them and the amount of K2O loss by leaching at different levels of K fertilizing also occur similar to the case of erosion. The total amount of K loss by average leaching of the fertilizing levels from 100-300 kg K2O /ha/year is 29.5 kg K2O /ha, in which loss 99.2 % according to leaching water. The amount of K loss by leaching 13 suspension soil comprised up only 0.8 %. In the case of no K fertilizing, the amount of K loss by leaching is 36.25 kg K2O /ha higher 6.71 kg K2O /ha (22.7 %) compared with the average of the fertilizing formulas from 100-300 kg K2O /ha. The amount of K loss by average leaching of three formulas 200, 250, 300 kg K2O /ha is 26.7 kg K2O /ha, 8.54 kg K2O/ha decrease (down 24.3 %), compared with the amount of fertilizer 100 kg K2O /ha. Table 3.12. K loss by leaching in the Lam Son region of Thanh Hoa (2010 - 2012) No Monitoring criteria Basis Basis + 100 K2O Basis + 150 K2O Basis + 200 K2O Basis + 250 K2O Basis + 300 K2O 1 K loss by leaching water Leaching water (m 3 /ha) 9.186 7.257 6.522 6.063 4.961 4.593 K2O content (mg/l) 3,92 4,82 4,94 5,02 5,06 5,18 K2O loss by leaching water (kg/ha) 36,01 34,98 32,22 30,44 25,10 23,79 2 K loss by suspension soil Leaching suspension soil (kg/ha) 10,67 8,43 8,11 7,79 7,90 7,36 K2O content (mg/l) 2,23 2,76 2,83 2,89 2,92 2,96 K2O loss by suspension soil (kg/ha) 0,24 0,23 0,23 0,23 0,23 0,22 Total K loss by leaching (kg/ha) 36,25 35,21 32,45 30,67 25,33 24,01 From the above results, the amount of K loss by leaching annually proposed to use to assess nutrient balance and determine the appropriate amount of K fertilizing for sugarcane on typical gray ferralit soil, no irigation, MY 55 -14 variety, fertilizing 200 kg N + 100 kg P2O5/ha in the Lam Son region is 26.7 kg K2O/ha. 3.3. Relationship between the amount of K fertilizer with yield, quality of sugarcane, sugar yield, and K loss per harvedted products 3.3.1. Effect of K fertilizing volume on growth, yield, and quality of sugarcane, sugar yeild Results of the study affect the amount of K fertilizing on growth, yield, and quality of sugar cane, cane sugar yield of MY 55-14 variety on typical gray ferralit soil, no irigation, basic fertilizing 200 kg N + 100 kg P2O5 /ha in the Lam Son region presented in Table 3.13 show: - For the growth of sugarcane: K fertilizing had a positive influence tillering situation, rising and spending diameter of plant, plant weight, plant density at harvest. However, the increase in the growth indicators is evident only in low amount of fertilizing (100-150 kg K2O/ha) and stopped in the amount of fertilizing 200 kg K2O /ha. Comparison between the K fertilizing formula with no K fertilizing formula showed a significant difference in the growth indicators of sugarcane: tillering average coefficient of the fertilizing volume from100 kg K2O /ha to 300 kg K2O /ha increased 0.31 times; plant height increased 14.3 % (33.3 cm); stem diameter increased 12.4 % (0.30 cm); plant volume 16.5 % (0.17 kg/tree); effective plant density at harvest increased 13.2 % (0.70 plant/m2). Between the amounts of K fertilizing, the growth indicators increase in the amount of fertilizing 100 kg K2O /ha to 150 kg K2O /ha, then stopped at 200 kg K2O. - For cane yield: K fertilizing increased the growth indicators lead to increasing cane yield, but the difference is only evident when comparing the K fertilizing formula with no K fertilizing formula. Among the fertilizing levels, cane yield increased only in the amount of fertilizing 100 kg/ha and stopped in the amount of fertilizing 150 kg K2O /ha. No K fertilizing, cane yield reached 57.85 tons/ha. Fertilizing 100kg K2O/ha, yield was 66.37 increase of 14.7%, (8.52 tonnes / ha), exceeding the limit significant differecnces at 95% probcapacity (LSD0, 05 = 7, 17 tons/ha). The amount of fertilizing 150 kg K2O/ha, sugarcane yields continue rising, compared to fertilizing 100 kg K2O /ha, but increasing rate is low (3.76 tons / ha) and lies within the experimental error. Thus, although the growth indicators increase the amount of fertilizing to 150 kg K2O /ha, the increase is not large enough to be able to increase productivity with certainty at this fertilizing level. 14 Table 3.13. growth, yeild, quality of sugarcane, sugar yeild No Monitoring criteria Basis Basis + 100 K2O Basis + 150 K2O Basis + 200 K2O Basis + 250 K2O Basis + 300 K2O LSD0.05 1 Growth Tillering ratio (time) 0,97 1,17 1,27 1,31 1,30 1,31 0,13 Plant height (cm) 232,75 253,66 264,09 268,87 270,45 273,15 28,73 Stem diameter (cm) 2,45 2,67 2,76 2,77 2,77 2,80 0,26 Plant volume (kg/plant) 1,02 1,13 1,18 1,2 1,21 1,22 0,14 Plant density (plant/m 2 ) 5,34 5,68 5,99 6,17 6,16 6,21 6,62 2 Sugarcane yield (ton/ha) 57,85 66,37 70,13 71,77 71,08 71,83 7,17 3 Sugarcane quality Bix ( 0 ) 18,40 19,16 19,43 19,97 20,23 20,63 - The rich sugar (Pol %) 13,39 15,95 16,39 17,15 17,32 17,67 - Purity (AP %) 81,62 84,44 86,76 88,22 88,14 89,90 - Reducing sugar content (RS %) 2,59 1,44 1,23 1,10 0,97 1,00 - 4 Commercial Cane Sugar (CCS) 8,98 10,12 10,70 11,11 11,13 11,39 0,27 5 Sugarcane yield (ton/ha) 5,19 6,71 7,49 7,97 8,16 8,18 0,73 - For cane quality: K fertilizing affects significantly to improve the sugarcane juice quality and sugar content in plants. Indicators: Brix, Polarization (Pol), the purity of sugarcane juice (AP) increase continuously in the amounts of fertilizing from 100 kg K2O /ha to 300 kg K2O /ha, while reducing sugar content (RS) decreases. Compared with the no K fertilizing, Brix (average of the amount of fertilizer 100-300 kg K2O/ha) increased 1.48 degrees; Pol increased 3.51 degrees; AP increased by 5.87%; RS decreased 1.44 %. - For commercial cane sugar: sugar content of commercial sugarcane increased continuously in the amount of fertilizing from 100 kg to 250 kg K2O/ha and stop fertilizing at 300 kg K2O/ha. Compared with no K fertilizing, the average sugar content in the fertilizing formulas from 100 kg to 300 kg K2O/ha up 21.7%, corresponding to 1.95 CCS. Comparison between K fertilizin levels showed: increasing level of sugar content between 150 kg K2O /ha compared with 100 kg K2O /ha is 0.58 CCS and between 200 kg K2O /ha compared with 150 kg K2O /ha is 0,41 CCS. The difference is very reliable, exceeding the limit significant differences at 95% probcapacity (LSD0, 05 = 0.35 CCS). The difference in sugar content between the amount of fertilizer 250 kg K2O /ha compared with 200 kg K2O /ha, although it is not beyond the scope of laboratory data, it is still at high levels (up 0.22 CCS). - For sugar yield: K fertilizing causes increasing sugarcane yields, especially sugar content lead to increasing sugar yield. Compared with the control no K fertilizing, yield averages of the amount of fertilizing from 100 kg K2O/ha to 300 kg K2O/ha reached 7.7 tonnes/ha, increased of 48.4% (2.51 tonnes/ha). Among the levels of K fertilizing, sugar yield gap between the amount of fertilizing 150 kg K2O /ha compared with 100 kg K2O /ha (0.78 tonnes / ha) was reliable (LSD0, 05 = 0.73 tons/ha ). The difference between the amount of fertilizing 200 kg K2O /ha compared with 150 kg K2O /ha is 0.48 tons/ha, although it is not beyond the scope of experimental error, it is high increasing rates (0.48 tonnes / ha). In a word, the K fertilizing level, that brings benefits for growth and cane yield, is 150 kg K2O /ha. However, due to increased levels of sugarcane fertilizing continuously from 100 kg K2O /ha to 200 kg K2O /ha and only stopped when applied to K2O /ha, so the amount of K fertilizing for sugar yield the best was determined at 200 kg K2O /ha. 15 3.3.2. Effect of the amount of K fertilizing to sugarcane pest situation Table 3.14. Effect of the amount of K fertilizing to sugarcane pest situation No Formulas Ostrinia nubilalis Ceratovacum lanigera Rate (%) Levels Rate (%) Levels 1 Basis 26,2 4 47,6 4 2 Basis + 100 K2O 19,7 3 34,5 3 3 Basis + 150 K2O 18,4 2 25,2 2 4 Basis + 200 K2O 16,6 2 23,3 2 5 Basis + 250 K2O 13,8 2 22,7 2 6 Basis + 300 K2O 13,4 2 22,8 2 Monitoring results of Ostrinia nubilalis and Ceratovacum lanigera situation at K fertilizing volumes presented in Table 3.24 showed that: K fertilizing could decrease damaged rate of Ostrinia nubilalis and Ceratovacum. If K fertilizing volume increases, damaged rate decreases. On the 200 N + 100 P2O5 fertilizing and no K fertilizing, Ostrinia nubilalis ratio on the sapling period period is at 26.2 % (level 4), the rate of Ceratovacum lanigera in ripening period - harvest (September in last year to April in next year ) is 47.6 % (level 4), up 9.82 % and 21.9 % (up1 level) compared with the average of fertilizing volumes from 100 kg K2O/ha to 300 kg K2O/ha for Ostrinia nubilalis and Ceratovacum lanigera rate respectively. Between the level of K fertilizing, Ostrinia nubilalis rate decreased from 19.7 % to 13.4 % , Ceratovacum lanigera rate decreased from 35.5 % to 22.8 % (down from level 3 to level 2) in the amount of fertilizing 100 kg K2O/ha and 300 kg K2O/ha respectively. This is also one of the reasons for increasing quality while increasing cane quantity as K fertilizing 3.3.3. K productivity at different fertilizing levels Table 3.15. K productivity at different fertilizing levels No Formulas Yield Difference with basis K productivity Sugarcane (ton/ha) Sugar (ton/ha) Sugarcane (ton/ha) Sugar (tấn/ha) Kg sugarcane/ kg K2O Kg sugar/ kg K2O 1 Basis 57,85 5,19 - - - - 2 Basis + 100 K2O 66,37 6,71 8,52 1,52 85,20 15,20 3 Basis + 150 K2O 70,13 7,49 12,28 2,30 81,87 15,33 4 Basis + 200 K2O 71,77 7,97 13,92 2,78 69,60 13,90 5 Basis + 250 K2O 71,98 8,16 14,13 2,97 56,52 11,88 6 Basis + 300 K2O 71,83 8,18 13,98 2,99 46,60 9,97 Table 3.25 show: K productivity reaches the highest at fertilizing volume 100 kg K2O/ha (85.2 kg cane/kg K2O) and fertilizing volume 150 kg K2O /ha for sugar volume (reaching 15, 33 kg sugar /kg K2O), and then gradually reduce the amount of fertilizing at higher (from 150 K2O/ha for sugarcane and 200 K2O/ha or more for sugar). Compared to 100 kg K2O/ha fertilizer, fertilizing amount 100 kg K2O/ha in which performance begins to decline, for cane is 250 kg K2O/ha (33.7% reduction, respectively 28.68 kg cane/kg K2O), and for sugar is 300 kg K2O/ha (34.9% reduction, respectively, 5.36 kg of sugar/kg K2O). 3.3. 4. The relationship between K fertilizing volume and sugarcane yield, cane yield Based on the basis of 200N + 100 P2O5 fertilizing, MY 55-14 variety grows in typical gray ferralit soil, no irigation in the Lam Son region, among cane yield, sugar yield and K fertilizing volume (0-300 kg K2O/ha) correlated quadratic as follows (Figure 3.3) 16 Cane yield – planting cane Sugar yield – planting cane Ư Cane yield – ratoon cane1 Sugar yield – ratoon cane 1 Cane yield – ratoon 2 Sugar yield – ratoon 2 Figure 3.3. The correlation between the K fertilizing volume and cane yield, sugar yield On the basis of the correlation equation, the K fertilizing volume maximum about techical and optimum about economics for sugarcane yield, sugar yield is determined and presented in Table 3:16 Table 3.16. K fertilizing volume maximum technique and optimum economics Cane types K fertilizing volume maximum technique (kg K2O/ha) K fertilizing volume optimum economics (kg K2O/ha) Cane Sugar Cane Sugar Newly cane 252,14 299,27 209,70 269,19 Root cane 1 254,82 316,15 213,14 286,91 Root cane 2 261,75 285,26 198,29 251,30 Average 255,70 301,00 204,80 267,90 Note: cost of fertilizer KCl (60% K2O): 12.000đ/kg; cost of sugarcane 10 CCS: 950đ/kg) From the results in Table 3.16 show: the purchase price of sugarcane 10 CCS is 950 VND / kg; price of KCl (60% K2O) is 12,000 VND / kg, fertilizing volume optimum economics for sugarcane yield was 204.8 kg K2O /ha lower than 50.9 kg K2O /ha compared to the amount of fertilizer maximum techniques. The fertilizing volume optimum economics for sugar yield was 267.9 kg K2O /ha less than 33.1 kg K2O /ha compared to the amount of fertilizing maximum techniques. Theoretically, K fertilizers can moderate K2O /ha 255.7 kg to a maximum cane yield and 301 kg K2O /ha to achieve maximum sugar yields. NS mía (kg/ha) Lượng bón K2O (kg/ha) Lượng bón K2O (kg/ha) Lượng bón K2O (kg/ha) Lượng bón K2O (kg/ha) Lượng bón K2O (kg/ha) Lượng bón K2O (kg/ha) NS mía (kg/ha) NS mía (kg/ha) NS đường (kg/ha) NS đường (kg/ha) NS đường (kg/ha) 17 3.3.5. Relationship between K fertilizing volume and K loss according to harvested product Table 3.17. Effect of K on K2O volume in harvest products No Content Basis Basis +100 K2O Basis +150 K2O Basis +200 K2O Basis +250 K2O Basis +300 K2O LSD0.05 1 Stem of cane Fresh volume 57,85 66,37 70,13 71,77 71,98 71,83 7,17 Dry matter content (%) 22,58 23,69 24,08 24,35 24,42 24,38 2,28 K2O content (%) 0,38 0,52 0,57 0,63 0,67 0,68 0,046 Cummulative K2O volume (kg/ha) 49,49 81,78 96,06 109,31 116,07 118,35 5,26 2 Cane foliage Fresh volume 21,33 24,56 25,98 26,69 27,72 26,66 2,72 Dry matter content (%) 22,35 24,37 24,49 24,51 24,57 24,55 2,43 K2O content (%) 0,45 0,71 0,74 0,77 0,81 0,83 0,054 Cummulative K2O volume (kg/ha) 21,39 42,40 47,13 50,32 52,57 54,08 3,54 The research results about effect of the amount of K fertilizing on K2O amount loss follow harvested products (plant cane, cane foliage) presented in Table 3.17 shows: - For the amount of K loss by product of sugarcane: K fertilizing increases the volume of sugarcane, dry matter content and K2O content lead to an increased amount of K2O accumulation. Compared with no K fertilizing, the average amount of sugarcane in the fertilizing formula from 100 kg K2O /ha to 300 kg K2O /ha is 21.7% (12.57 tonnes / ha); dry matter content is higher than 1,6%; K2O content is higher than 0.23%, result in that the volume of K2O increases 110.9% (54.82 kg K2O /ha). Among the levels of K fertilizing, the accumulative K2O volume in sugarcane increased continuously at the amount of fertilizing from 100 kg K2O /ha to 250 kg K2O /ha and stopped at 300 kg K2O /ha. Compared with 100 kg K2O /ha, cumulative K2O volume in sugarcane at the amount of fertilizing 150 kg K2O /ha, 200 kg K2O /ha and 250 kg K2O /ha increase respectively: 17.5% (14.28 kg K2O/ha), 13.8% (13.25 kg K2O/ha) and 6.2% (6.76 kg K2O/ha). The difference in K2O volume is very reliable, exceeding the limit significant differences at 95% probcapacity (LSD0, 05 = 5.26 kg K2O /ha). - For the amount of K loss under sugarcane foliage after harvest: similar to sugarcane, foliage mass, dry matter content, K2O content in foliage increased with increasing the amount of K fertilizing. Compared with no K fertilizing, the average amount of sugar cane foliage of the formulas 100-300 kg K2O/ha increases 22.5% (higher than 4.79 tons/ha); dry matter content is higher than 2.5%; K2O content is higher than 0.32% result in that a K2O volume increases 30.5% ( higher than 27.91 kg K2O/ha). The increase of the targets tracked is very reliable, exceeding limits LDS0.05 (fresh sugarcane foliage mass = 2.72 tons/ha, dry matter content = 2.43%, K2O content = 0.054 %, cumulative K2O volume = 3.54 kg/ha). Compare among the different levels of K fertilizing shows the volume of sugarcane foliage and dry matter content tends to increase continuously in the amount of fertilizing from 100 kg to 250 kg K2O /ha. K2O content and accumulative K2O volume increased continuously for up to 300 kg K2O/ha. However, only the difference in quality is evident K2O fertilizer at 150 kg compared with 100 kg K2O/ha K2O /ha (K2O /ha increased 4.73 kg), 200 kg K2O /ha or more, the difference between the K fertilizer rates have not exceeded the scope of experimental error (LSD0,05 = 3.54 kg K2O/ha). Reciprocal internal efficiency of K (Riek), harvest index K (HIK) and K recovery efficiency of mineral fertilizer (REK) in different levels of K fertilizer are presented in Table 3:18 . 18 Table 3.18. Effect of K on agronomic performance, harvest index, and K use efficiency No Formulas Acumulative K (kg K2O /ha) Thedifference compared with basis (kg K2O/ha) RIEK HIK REK (%) Plant cane Foliage Total 1 Basis 49,49 21,39 70,88 - 0,12 0,70 - 2 Basis+100K2O 81,78 42,40 124,18 53,30 0,19 0,66 45,09 3 Basis+ 150K2O 96,06 47,13 143,19 72,31 0,21 0,67 42,73 4 Basis+ 200K2O 109,31 50,32 159,63 88,75 0,22 0,68 40,27 5 Basis+ 250K2O 116,07 52,57 168,64 97,76 0,24 0,69 35,82 6 Basis+ 300K2O 118,35 54,08 172,43 101,55 0,24 0,69 31,11 For RIEK: K fertilizing increases the volume of sugarcane, cane foliage volume after harvest as well as dry matter content, content of K2O and accumulative K2O amount in them, leading to increased RIEK. RIEK average fertilizing level of 100 -300 kg K2O/ha is 0.22% higher than 0.83 times the formula that does not fertilizer K. Among K fertilizing levels, RIEK continuous increase in levels from 100 kg K2O /ha to 250 kg K2O /ha (up from 0.19% in the amount of 100 kg K2O /ha, up 0.24% in the amount of 250 kg K2O /ha) and stop fertilizer at 300 kg K2O/ha. From the above results, the linear correlation between the amount of K fertilizer and Riek in different sugarcane crop is determined and presented in Figure 3.4. Planting Ratoon 1 Ratoon 1 Average of 3 crops Figure 3.4. Relationship between cane yield and reciprocal internal efficiency of K (RIEK) For HIK: K fertilizing affects simultaneously increase in K2O volume both product of sugarcane and leaves of sugarcane as harvest. However, due to the increase in the amount of K2O in sugarcane higher than the increase in the amount of K2O in foliage, resulting in that HIK in the K fertilizing formulas decreased compared with no K fertilizing. HIK average of the amount of fertilizing from 100 kg K2O /ha to 300 kg K2O /ha is 0.68 while HIK in fertilizing formula K is 0.70. However, when comparing the level of K fertilizing noticed, HIK have tended to increase with increasing the K fertilizing volume: increase from 0.66 at 100 kg K2O /ha up 0.70 at 300 kg K2O/ha. From the results showed no significant variation about HIK between the K fertilizing volumes. Thus, it can use average data (HIK = 0.68) to calculate the equilibrium and set equatation to determine K fertilizing suit for sugarcane on the basis of nutrient balance. - For REK: fertilizing at 100-300 kg K2O /ha, REK ranged 31-47% and in the direction of decreasing with increasing amount of fertilizer applied at 100 kg K2O /ha is 45.9%. Increase the amount RIEK(% ) RIEK(% ) RIEK(% ) RIEK(% ) Cane yield (ton/ha) Cane yield (ton/ha) Cane yield (ton/ha) Cane yield (ton/ha) 19 of fertilizer to 250 kg K2O /ha and 300 kg K2O /ha, REK fell to 35.82% (down 9.27%) and 31.11% (down 13.98%), respectively. To simplify and facilitate the application of research results in practical production, REK introduced to use in balanced calculation and determine the amount of K fertilizing on the basis of nutrient balance is 40%. 3.4. K balance and determination of suitable K fertilizing volume for cane base on nutrional balance in the Lam Son sugarcane area. 3.4.1. K balance for cane at different K fertilizing levels On the basis of the research results about quantity and the relationship between nutrient input and nutrient outputs, K balance for sugarcane at different K fertilizing levels, base 200N + 100 P2O5 fertilizing, MY 55-14 sugarcane varieties grown on typical gray ferralit soil, no irigation in the Lam Son region is determined and presented in Table 3.19. Table 3.19. K balance for cane in the different K fertilizing volume No Nutrient source Basis Basis + 100 K2O Basis + 150 K2O Basis + 200 K2O Basis + 250 K2O Basis + 300 K2O 1 Input 8,21 108,21 158,21 208,21 258,21 308,21 Mineral fertilizers - 100 150 200 250 300 Rain water 8,21 8,21 8,21 8,21 8,21 8,21 2 Output 135,2 182,86 196,34 208,63 210,65 214,56 Loss by stem removal 49,49 81,78 96,06 109,31 116,07 118,35 Loss by foliage removal 21,39 42,4 47,13 50,32 52,57 54,08 Loss by erosion 28,07 23,45 20,7 18,35 16,69 18,14 Loss by leaching 36,25 35,23 32,45 30,65 25,32 23,99 3 Balance -126,99 -74,65 -38,13 -0,42 47,56 93,65 The results in Table 3:19 shows that no K fertilizing, not buried back cane foliage, negative K balance at the average 127 K2O/ha/vụ. Mineral K fertilizing, K balance decreased from 74.6 kg K2O/ha at fertilizing level 100 kg K2O/ha to 0.4 kg K2O/ha at fertilizing level 200 kg K2O/ha K balance achieved at fertilizing level 250 kg K2O/ha upwards, so to ensure K reserves in the soil is not decline, the K fertilizing volume needs a minimum of 200 K2O/ha/vụ. 3.4.2. K balance for cane in exist productional condition Figure 3.5. Diagram of K balance for cane in exist productional conditions in the Lam Son sugarcane area Đầu vào Cân bằng Đầu ra Trung gian Kali cung cấp từ phân khoáng (IN1) Kali cung cấp từ nước mưa (IN 3) Kali cung cấp từ phân hữu cơ (IN2) Dinh dưỡng kali dự trữ trong đất Kali mất do rửa trôi chiều sâu (OUT 4) Kali mất theo ngọn lá mía sau thu hoạch (OUT 2) Kali mất theo sản phẩm mía nguyên liệu (OUT1) Kali mất do xói mòn bề mặt (OUT 3) Cân bằng kali theo sản phẩm (IN1 + IN2 + IN 3) – (OUT1 + OUT 2) Cân bằng kali hoàn toàn (IN1 + IN2 + IN 3) – (OUT1 + OUT 2 + OUT 3 + OUT 4) 20 - K nutrient input from fertilizer: analyzing results in chemical compostions, organic material and determine K due to NPK presented in Table 3.20; 3.21. Table 3.20. Chemical composition, organic materials produce NPK Lam Son Materials pH(KCl) Dry (%) N P2O5 K2O (% dry weight) Sludge of sugar mill 7,1 31,5 1,76 1,91 1,28 Table 3.21. Nutrient volume N, P2O5, K2O is provided by NPK Lam Son Supplying source Volume (kg/ha) N P2O5 K2O (kg/ha) NPK Lam Son 2.000 5,8 6,3 4,2 - K nutrient source was lost by harvested products: the average cane yield 62 tonnes/ha, K2O volume loss by product of sugarcane and cane foliage when harvest determined through reciprocal internal efficiencyof K (RIEK GY = 0.007 - 0.327) is 107 Kg K2O/ha. - Balance: K balanced calculation results presented in Table 3:22 show, in the current sugarcane production, the amount of fertilizer to 2,000 kg / ha NPK Lamson, average cane yield 62 tonnes / ha, no buried cane foliage returns, K balance is 7 Kg K2O/ha/vu. Table 3.22. Cân bằng K cho mía trong điều kiện sản xuất mía hiện tại Nutrient source K Symbol Describe Volume (kg K2O/ha) Input IN 1 K mineral in NPK Lam Son 132,0 IN 2 K organic in NPK Lam Son 4,2 IN 3 K from rain water 8,2 Total 144,4 Output OUT 1 K loss by cane stem removal 107,0 OUT 2 K loss by residue removal afer harvesting OUT 3 K loss due to erosion 17,7 OUT 4 K loss due to leaching 26,7 Total 154,4 Balance - 7,0 3.4.2. The equation determines the suitable K fertilizing volume for sugarcane on the basis of nutrional balance - General equation: on the basis of research results the relationship between the amount of K fertilizing to yield and quality of sugar cane, the amount of K loss by harvested product and the amount of K nutrient elements inputs and outputs in the Lam Son condition, the suitable K fertilizing volume for sugarcane varieties MY 55-14 on typical gray soil, no irigation, fertilizing base 200N + 100P2O5, has the form: FK= [(GY x RIEK - KCR - KR + KE + KL) x FM] + (GY - GY0K) x RIEK/REK). where: FK: mineral K needs to fertilizer to achieve targets of yield (kg K2O/ha) GY: expected cane yield (ton/ha) GY0K: cane yield in condition of no K fertilizing (ton/ha) RIEK: K reciprocal internal efficiency(%) KR: K supply due to rain water (kg K2O/ha/year) KE: Amount of K loss by erosion (kg K2O/ha/year) KL: Amoun of K loss by leaching (kg K2O/ha/year) 21 KCR: Amount of K returns to soil via returning cane foliage (kg K2O/ha). KCR = GY x RIEK x (1 - HIK) x CRR, where: HIK harvested index of K; CRR: crop residue retained (%); REK: Recovery efficiency of K mineral fertilizer FM: maintain coefficient K reserves in the soil compared to the first crop (FM> 1 potassium reserves in the soil are enhanced, FM <1 potassium reserves in the soil decline, FM = 1 potassium reserves in the soil are maintained). - Experimental equation: compared with results and parameters have been identified in the research content: GY0K = 57,85 ton/ha; RIEK = 0,007 GY - 0,327; KCR = GY x RIEK x (1 - HIK) x CRR; HIK = 0,68, CRR = 1; KR = 8,2 kg K2O/ha/year; KE = 17,7 kg K2O/ha/year; KL = 26,7 kg K2O/ha/year; REK = 40%, experimental equation determines K fertilizing volume to reach yield targets and maintain K reserve volume in the soil (FM =1), equation form: FK = 10 GY x RIEK + 25 (GY – 57,85) x RIEK - KCR + 36,2. Where: RIEK = 0,007 GY - 0,327; KCR = GY x RIEK x 0,32 CRR With the goal of increasing productivity of sugarcane is estimated 70 tonnes / ha, K2O amounts should fertilizer through empirical equation is 163,3 kg / ha in case returned 100 buried cane foliage (CRR = 1; KCR = 36.5 kg K2O/ha) and 199,8 kg kg K2O/ha in the case no return sugarcane foliage (CRR = 0; KCR = 0). At 199,8 kg K2O/ha of fertilizer in the absence of cane foliage, if calculated at the economic optimum fertilizing level, the correlation equation cane yield was determined at 73.1 tons / ha . The difference in yield between the two methods was 3.1 tonnes/ha, within the experimental error (LSD0,05 services cane = 7,06 tons / ha). Since the results determine K fertilizing volume according to nutrient balance method is consistent with the method of determining the amount in accordance with the correlation equation is commonly applied. The difference is that the amount of fertilizer K determined on the basis of nutrient balance for monitoring the status of K reserves in the soil, the K nutrient input, output, especially in the K input from harvest sugarcane foliage. Thereby, we can adjust the amount of K supplied from mineral fertilizers and identify technical solutions to implement sustainable management of K nutrition in accordance with the specific conditions of each field. 3.5. Effect of K nutrient sustainable management base on basis of nutrional balance Sustainable management model of K nutrition in sugarcane production in the Lam Son region is done for the purpose of verifying the accuracy, relicapacity and efficiency of the method of determining the amount of K fertilizing according to nutrient balance. The specific contents of the model include: goals of cane yield 70 tons / ha, buried return 100 % sugarcane foliage, using Lamson NPK fertilizer (NPK - HC: 6.4 to 3.2 - 6.6 HC 15), the amount of fertilizing 2,000 kg/ha, and added enough fertilizing volume of mineral fertilizers 200 N + 100 P2O5 and K2O amount determined from empirical equations after subtracting the amount of N, P2O5, NPK fertilizer K2O in Lam Son, specifically: The amount of fertilizer K2O determined from empirical equations with the aim of cane yield 70 tons/ha; 100 % return is 163.3 kg K2O/ha cane foliage. Then, the amount of fertilizing N, P, K in the test formula is determined and presented in Table 3:23. Table 3.23. Fertilizer volume in the model of K nutrient sustainable management N o Types of fertilizer Volume Nutrient content (kg/ha) N P2O5 K2O 1 NPK Lam Son (*) 2.000 129,8 74,3 136,2 2 Urea 152,6 70,2 - - 3 Superphosphate 160,6 - 25,7 - 4 KCl-postassium chloride 45,2 - - 27,1 Total 200,0 100,0 163,3 22 3.5.1. Technical specification of the model Table 3.24. Tagets of growth, yield, cane quality, and sugar yield in the model Growth Treatment Germinative rate (%) Plant density in the end of germination (plant/m 2 ) Tillering coefficient (time) Plant density in the end of tillering (plant/m 2 ) 1. Control 48,13 5,78 1,28 7,40 2. Test 53,17 5,96 1,36 8,11 Components of yield and yield Treatment Plant height (cm) Plant diameter (cm) Plant volume (kg/plant) Density (plant/m 2 ) Cane yield (ton/ha) 1. Control 267,00 2,69 1,25 6,12 63,45 2. Test 298,00 2,84 1,34 6,26 69,67 Quality, commercial cane sugar, sugar yield Formula Brix Pol AP RS CCS Sugar yield (ton/ha) 1. Control 20,15 15,41 82,27 1,12 10,14 6,43 2. Test 23,18 16,67 88,12 1,06 11,97 8,35 The results in Table 3:24 shows: - Growth: due to weather conditions in winter season 2011 - 2012 are prolonged drought and cold weather, which have affected the germinative situation: Cane germinates slow, germinative time is prolonged, and germinative rate is low (aveerage reached only 50 %). Compared with the control formula, germination, tillering coefficient, density of trees sprouting end, end of tillering in sugarcane formulas have higher test: germination rate reached 53.1 % (higher than 5.04 %), tillering coefficient is 1.45 times (0.13 times higher) result in that a density of tillering tree finished up 16.8 % (1.24 plant/m2). The cause of this difference is that the quiz of sugarcane planting foliage was effective in keeping heat, moisture retention for soil in low temperatures and low air humidity condition, so promoting the germination of sugarcane, tillering, increased tree density. - The components of cane yield and productivity: the growth indicators increased, resulting in yield components and cane yield of test formula increased compare with the control formula. Height of sugarcane while harvest in test formula is 298 cm (up 11.6%), 0.15 cm diameter (up 5.6%), tree volume of 0.09 kg/tree (an increase of 8,0%), tree density 6.26 plant/m2 (up 6.7%), cane yield reached 69.76 tons/ha (an increase of 9.8%, corresponding to 6.22 tonnes/ha) , compared with the control formula, respectively. - The quality indicators, sugar content and sugar yield : compared with the control formula , criteria and quality of cane sugar in the recipe testing high rise : dry matter content (Brix) increased 3.03 degrees, the sugar -rich (Pol) increased 1.26 degrees, the purity of cane juice (AP) at 88.12 % (up 5.85 %), reducing sugar content decreased by 0.09 %, CCS sugar reached 11.97, an increase of 13.8 % (1.65 CCS) . Yield and quality of sugar cane, especially CCS is higher in test formula lead to increasing sugar yield, reaching 8.35 tons / ha, an increase % (ton/ha) compared to the control formulas. Summary: The specifications of the cane in the sustainable management model of K nutrients are reached higher than current fertilizing techniques and achieve productivity goals. This is the basis of efforts to assess the feasibility and practicality of the technical model. 3.5.2. Economic effect of the model The results in table 3.25 shows that sustainable management model of K nutrient, fertilizing according to nutrient balance reaches economic efficiency is much higher than the fertilization process is applied in the area: 23 Table 3.25. Economic effect of the experimental model No Monitoring criteria Units Control formula Test formula Quantity Price Cost Quantity Price Cost I Total cost - - 9.000 - - 15.164 1 Fertilizer cost - - 9.000 - - 11.631 1.1 NPK Lam Son kg 2.0000 4,5 9.000 2000 4,5 9.000 1.2 Urea kg - - - 152,6 10 1.526 1.3 Superphosphate – SSP kg - - - 160,6 3,5 562 1.4 KCl- postassium chloride kg - - - 45,2 12 524 2 Additional increase of labor labor - - - 26 - 3.533 2.1 Collecting and stripping foliage cane labor - - - 20 130 2.600 2.2 Increased cane harvest ton - - - 6,22 150 933 II Additional increase of productional cost - - - - - 6.164 III Harvested products - - - - - 1 Cane yield ton/ha 63 - - 70 - - 2 Commercial cane sugar CCS 10,14 - - 11,97 - - 3 Sugar yield ton/ha 6,43 - - 8,35 - - IV Product value 1.000đ - - 61.085 - 79.325 V Additional increase of product value 1.000đ - - - - - 18.240 VI Marginal benefit cost ratio (MBCR) time - - - - 2,96 Compared with the control formula, the production cost of test formula increased 68.5 % (by 6.166 million VND/ha), of which increased 39.3 % cost of collected labor and buried sugarcane foliage when planting and harvesting products more increased (3.533 million VND/ha), the cost of purchasing mineral fertilizers (N, P, K) increased by 29.2 % (2.633 million VND/ha). Values of crop yields obtained from the increased productivity and quality of sugarcane, sugar productivity in testing formula is 18.240.000 VND, up 29.9 %, compared to the control formula. MBCR reached 2.96 times. With this message, the technical measures in the model to explicitly accept and sugarcane growers applied in large-scale production. This is the basis of contributing confirmed the economic vicapacity of the model. 3.5.3. Effect of the model on cane soil property The results of soil analysis after model building showed that in general, the soil agrochemical targets without significant changes when comparing before with after model building and between the control formula with test formula. The difference in the amount of targets at low levels is not really clear. For K, total K content to keep the same level compared to the pre-built models. Private K exchange increased from 5.75 to 5.82 mg/100 g soil (0.07 mg/100 g soil). For the soil texture, due to the impact of soil erosion, the rate of soil particles in the direction of reducing the volatility ratio clay particles, sand particles increase (1 %) compared to the prior model building. In a word, empirical models for sustainable K nutrient management maintain K concentration in soil after each sugarcane crop. 24 CONCLUSIONS AND RECOMMENDATIONS 1. Conclusions 1.1. The sugarcane region in Lam Son-Thanh Hoa locates in the area of tropical climate, influenced by the cold dry northeast monsoon and hot dry southwest monsoon. Over 80% of the standing cane is grown annually on typical gray ferralit soil with no irigation and the main sugarcane variety MY 55-14. Regional average yield is 62 tonnes/ha. The main fertilizer used is NPK Lamson (6,4 - 3.2-6.6 HC 9.5), the average amount of fertilizer is 2,000 kg/ha. K in sugarcane foliage after harvest is not returned to the soil. The factors used to evaluate K nutrient balance in the current production including three K input sources: mineral fertilizers, organic ingredients in Lam Son NPK fertilizer, rainwater and four sources of K output: cane stalks and foliages, erosion, leaching. 1.2. In case of non-irrigated sugarcane, typical gray ferralit soil is capable of providing 64.4 kg K2O/ha/crop. The amount of K provided by rainwater on average is 8.2 kg K2O/ha/year. The amount of K loss by erosion is 17.7 kg K2O /ha/year. The amount of K loss by leaching is 25.3 kg K2O /ha/year. 1.3. K has a positive impact on productivity and quality of sugarcane, sugar yield and the cumulative amount of K2O in harvested products. On the basic fertilization of 200 N + 100 P2O5, with MY 55-14 variety grown on typical gray ferralit soil and with no irrigation, the highest K fertilizer use efficiency was achieved at K fertilization rate of 100 kg K2O /ha (85 kg cane / kg K2O, 15 kg of sugar / kg K2O), Reciprocal Internal Efficiency of K (RIEK) varied from 0.19 to 0.24%, K harvest index (HIK) was in a range of 0,68 to 0,74, recovery efficiency of K in mineral fertilizers (REK) varied from 45 to 39%, equivalent to amount of 100-300 kg K2O/ha. 1.4. With an average cane yield 62 tonnes/ha, the amount of K added by application of Lamson NPK fertilizers was133.8 kg K2O /ha, K balance in the current sugarcane production in the Lam Son, Thanh Hoa has a negative value at 7.5 kg K2O/ha/crop. On the basic fertilization of 200 N + 100 P2O5, no cane foliage return after harvesting, to reach cane yield of 70 tons/ha or more and at the same time maintain K reserves in the soil, application of K at a minimal rate of 200 kg K2O /ha /crop is needed. 1.5. Based on the relationship between the amount of K fertilizer to yield and quality of sugar cane, sugar yield and nutrient inputs, outputs of K balance, the empirical equation determining the amount of K fertilizer according to productivity goals with MY 55-14 sugarcane variety grown on typical gray ferralit soil with basic fertilization of 200N + 100 P2O5 is shown as below: FK = 10 GY + 25 x RIEk ( GY - 57,85 ) x Riek - KCR + 36,2 Where: - GY: target yield (tons/ha). - RIEK: Reciprocal Internal Efficiency of K. RIEK = 0,007 GY - 0,327. - KCR: K inputs with retained crop residues KCR = GY x RIEK x 0,32 CRR (CRR % crop residue retained in soil). 1.6. Sustainable management model for K nutrient: determine the amount of K fertilizer needed based on nutrient balance with the goal is to increase cane yield to 70 tons/ha with basic application of 200N + 150P2O5, 100% cane foliage are incoporated to the soil (CRR = 1). This will not only efficiently increase cane productivity, quality and production efficiency but also maintain K reserves in soil. Compared with current fertilization techniques in the area, cane yield in the model increased 9,8% (6.22 tonnes/ha), sugar yield increased 29,9% (1.92 tonnes/ha), cost-benefit ratio reached 2,96 times, while the concentration of K reserves was maintained in the soil after each sugarcane crop. 2. Recommendations The empirical equation determining the amount of K fertilzer was set based on research results on the relationship between productivity and quality of sugarcane, sugar yield at different K application rates with K inputs and outputs in K balance for sugarcane in the specific conditions of Lam Son region. The level of relicapacity of the model proved in theory and empirical model building shows that the model has a high economic efficiency and help to maintain K reserves in the soil. The model of sustainable K management in sugarcane production should be broadcast and recommended in Lam Son region and other cane production regions which have similar conditions with Thanh Hoa province./.