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Influence of Potassium-Fertilization Rate and Form on Photosynthesis and N₂ Fixation of Alfalfa¹

This study reports a growth chamber experiment conducted to investigate the influence of potassium (K) fertilization level (control, medium, high K) and form (KCI and K₂SO₄) on photosynthesis, carbohydrate concentration, and N₂ fixation of alfalfa (Medicago sativa L.). Fertilization with high levels of K as K₂SO₄ increased shoot number per plant by 51% and increased shoot dry weight by 20% over the unfertilized control. Control plant herbage contained 1.28% reducing sugars (RS) compared with 0.78% RS in herbage of the high K-high S treatment, indicating that K deficient plants accumulated these compounds in photosynthetic tissues. Chlorophyll concentration increased linearly in response to K application and net carbon exchange rate (CER) on a per plant basis was increased by average of 28% over the control. Nodule numbers per plant ranged from 28 for the control to 90 for the medium K-high S treatment. Nitrogen-fixation rate was enhanced by K fertilization and ranged from 12.3 nmoles ethylene/plant/minute for the control to 56.8 for the medium K-medium S treatment. These results suggest that the increases in N₂ fixation of alfalfa in response to K and S fertilization are a result of a number of factors including increased CER, increased nodule numbers, and enhanced carbohydrate movement from the shoot.

Key Words: Nodulation • Medicago sativa L. • Carbohydrates • Sulfur

² Assistant professors of agronomy.

Received for publication July 14, 1980.

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