Effect Of Phosphorus And Potassium Nutrition On Yield And Quality Of Alfalfa(Medicago Sativa L.)and Related Mechanisms

Alfalfa (Medicago sativa L.) is an important high quality forage crop with ecological and economic benefits. With rapid development of animal husbandry and re-adjustment of cropping systems in China, alfalfa is receiving more and more attention. Nitrogen, phosphorus (P) and potassium (K) are not only essential nutrients for normal plant growth, but also important components in alfalfa forage quality evaluation. Therefore, the status of P and K nutrition has significant effects on alfalfa yield and quality. Although there are research results reported in the literature on effects of P and K use on yield and quality of alfalfa, little work has been done to study the effects of P and K nutrition on physiological processes of yield formation and related mechanisms.In 2003, one field experiment with 5 different P application rates was established in Lingxian County of Shandong province to study the effect of P application rate on yield and quality of alfalfa; and 4 field experiments with 3 levels of P and 3 levels of K combinations were established in Jingtai County of Gansu province, Ledu County and Huangzhong County and Xining of Qinghai Province to study P and K application and their interaction on yield, quality and mineral nutrients absorption. In 2004, the above 5 field experiment were continued, and additional three pot experiments were conducted in Beijing to study the influences of P, K fertilizer application and their interactions on yield and quality of alfalfa, on mineral nutrients absorption and related mechanism. The results and main findings are as following:I. Results from pot experiments indicated that average plant height, the number of lateral branches and forage yield were increased with increase of application rates of P and K, until to an optimal level, over which, further increase of P and K application rate resulted in negative effects on plant growth and yield. Significant interactive effects of P and K on alfalfa plant growth and yield were also found. The optimum rates of P₂O₅, K₂O and proper combination were P₂O₅ 1.06g/10kg soil, K₂O 0.7g/10 kg soil, and P₂O₅ 0.53-1.07g/10 kg combined with K₂O 0.4g - 0.8 g/10 kg soil, respectively. These treatments with optimum rates of P, K or their combination resulted in maximum yield, maximum plant height, and largest number of lateral branches. Compared with CK treatment, the optimum treatments increased plant height by 3.3-4.3 cm. increased number of lateral branches by 3-3.8/plant, increased yield by 10.4-23.3%.The two years results from the field experiment in Lingxian County of Shandong province with 5 different P levels indicated that application of P fertilizer significantly increased average plant height, the number of lateral branches and the forage yield. The optimum application rate of P fertilizer in 同 the first year was 85.2 kg P₅O₂/hm² with yield of 19875.1 kg/ hm². In the 2^nd year, alfalfa yield increased with increase of P rate with the highest rate of 120 kg P₅O₂/ hm² resulted the highest yield of 88333.3 kg/hm².The two years results from the 4 field experiments of P and K combination in Gansu and Qinghai Province indicated that P and K application and their combined use significantly increased yield andimproved quality, and significant interaction was found. Within the proper range of application rates, P and K application increased average plant height, the number of lateral branches and the yield. By using the Logistic equation, y=k/(l+aebx) and binary quadratic equation, the maximum yield, the highest increased rate (HIR) of plant growth (plant height), appearance time of HIR (days after planting when the HIR of plant height was observed), and the average growth rate were estimated for different P and K combinations. Results indicated that the optimum rate of P and K to obtain the highest yield, the highest increase rate of plant growth and its earlier appearance, and highest average growth rate were P60K90 in year one and P120K0 in year two in Jingtai of Gansu; P6oK9oin year one and PW)K9o in year two in Ludu of Qinghai; P60K90 in year one and P120K.90 in year two in Xining of Qinghai; and P60K90 in year one and P120K90 in year two in Huangzhong of Qinghai.II. The contents of crude protein, crude fat and Ca were increased with increase of P and K fertilizer application rates till an optimum level. Further increase of P and K rates reduced the contents of crude protein, crude fat and Ca. No significant differences among various P and K treatments were observed for ash and non-nitrogenous extract contents. Absorption ofN,P>K> Cu -. Fe ?, Mn -. Mg by alfalfa plants increased by P and K fertilizer application, with average increase ranged from 4.5 tolO.1%. Application of K fertilizer resulted in increased absorption of Zn. Proper use of P and K improved alfalfa quality, but over-application of P and K had negative influence on alfalfa forage quality. In all experiments, the highest alfalfa yield and the best quality were observed from the same treatment, indicating that proper use of P and K can increased alfalfa yield and at the same time to improve the forage quality.III. Proper use of P and K increased absorption of N, P and K by alfalfa plant, increased photosynthesis rate, improved activity of NR, GS, GOGAT, GDH, GOT and GPT, which were enzymes involved in carbin and nitrogen metabolism.IV. Phosphate and K fertilizer use increased absorption of Mg* Fe^ Mn> Cu, increased chlorophyll and carotenoid of alfalfa leaves and improved photosynthetic rate. These processes would stimulate photosyn thesis, improve C metabolisms, and then improve yield and quality.V. Compared the results from experiment sites located at different altitudes with different annual average temperature, but with same soil type and similar soil nutrition level, it was found that altitude and annual average temperature has significant effect on alfalfa forage yield and quality. Compared with the result from Ledu site with relatively low altitude of 1920 m and higher average annual temperature of 6 °C, alfalfa in Huangzhong site at an altitude of 2630 m and average temperature of 3.7 °C showed slower growth rate, lower forage yield, lower crude protein and N, P, K contents, but higher crude fiber. According to the current world alfalfa forage quality standard, the forage produced from the higher altitude site can only meet the standard of 2nd grade with total yield of 12424 kg/ha in two years, 28.4% increase than CK treatment, and average content of crude protein in two years,3.13...