Potassium is a macroelement essential for plant growth and its importance in agriculture is well understood. Potassium in clay minerals is an important source of potassium for plants in many soils. No precise information is available on the impact of lime on potassium uptake by plants. Therefore, this study was conducted to investigate the effect of lime on K availability form phlogopite mineral in alfalfa rhizosphere.
Materials and Methods
This study was conducted using a completely randomized design with factorial arrangement. Alfalfa was grown in media containing a mixture of quartz sand and phlogopite, different levels of lime (0, 2, 5, 12, 25%) and under two types of nutrient solutions (complete and potassium free) with three replications for a period of six months. Alfalfa shoots and roots were harvested and their potassium concentration was measured. The Data obtained from this experiment was statistically analyzed using SAS software and comparison of means was performed with the LSD test.
Results and Discussion
The results showed that both shoot and root biomass was significantly affected by lime. Potassium concentration in shoot and root significantly reduced as the level of lime increased. The amount of soluble calcium increases as the level of lime in medium increases. This is mainly due to the hydrolysis of lime which also reduces the ratio of potassium to calcium and magnesium. This causes a great decline in potassium uptake by plant. Reduced uptake from the root surfaces of the plants in medium containing lime can also be caused by calcium oxalate precipitation on the root surfaces due to the abundance of calcium ions in the the root zone. Despite the fact that the amount of potassium supplied by different media has been the same, plants were not able to absorb equal quantity of potassium. Plants grown in lime-containing pots were indirectly deficient in potassium. Indirect exposure of plant to potassium deficiency means that, despite the high soil available K level, due to physiological reasons, potassium uptake by roots and its transfer to shoots is restricted. This is attributed to the negative effects of high concentration of magnesium and calcium compared to that of potassium in soil. Potassium concentration of plant roots was less than that of the shoots. This is mainly due to higher demand for K in shoots as compared to that in roots. Besides, the ability of plants to transfer potassium from root to shoot is very high. Shoot dry weight in plants with a complete nutrient solution major than the plants with a potassium free nutrient solution. Maximum dry weight was found in plants supplied with the complete nutrient solution with no lime added. In contrast, plants supplied with potassium free nutrient solution grown in pots containing 25% lime had the least dry weight. Lower root biomass could be caused by differences in physical characteristics of the root environment due to the presence of lime. Under potassium free nutrient solution, the amount of potassium uptake was significantly influenced by the amount of lime as such that the least potassium uptake of shoot and root occurred in treatments with %25 lime. The maximum K uptake (145.84 mg/pot) was obtained in plants treated with the complete nutrient solution. There was a highly significant correlation between the shoot dry weight and potassium uptake and also between the shoot dry weight and potassium concentration indicating that the plant shoot yield increases as the K concentration and uptake increase.
The results obtained in this study clearly indicate that the presence of lime in the root zone could be a limiting factor for potassium uptake by plants. Although the concentration of potassium in plants was in the sufficient range defined by standards, but potassium uptake significantly reduced as the level of lime in medium increased. The decreasing trend was more obvious in the plants treated with potassium free nutrition solution. Plants treated with potassium free nutrient solution with no lime added to the medium have been able to take up a high amount of potassium. It appears that plant roots can influence on clay minerals to release potassium by the secretion of H+. But the presence of lime in the medium could release high level of Ca2+ ions into the solution, which, in turn, could reduce the release of potassium from phlogopite and its uptake by plants. In general, in media containing micaceous minerals as the only source of potassium, the presence of lime can have a negative impact on potassium release from minerals and its uptake by plants. Therefore, in calcareous soils with high potassium storage, the level of lime should be considered when potassium fertilizer is recommended. Besides, the amendment of highly calcareous soils by organic matters is suggested to improve the soil physical properties in order to have a better K uptake.