Twenty experiments between 1970 and 1974 tested the effect of spring applications of all combinations of five amounts of potassium (0–333 kg K/ha) and two amounts of sodium (0, 150 kg Na/ha) on sugar-beet yield, profitability and the amount of K and Na in the crop in mid-summer. The experiments were in commercial sugar-beet crops where soils contained little exchangeable potassium, most having less than 100 mg K/l in top soil. On average, sugar yield was greatest when 150 kg Na/ha plus 167 kg K was used, the fertilizer increasing yield by 0·71 t/ha. Response to this combination of K and Na fertilizers was 1·45, 0·57 and 0·22 t sugar/ha when the soil contained less than 60, 61–120 and more than 120 mg K/l respectively.

From results of earlier experiments, it was thought that response to even a large amount of K without Na would be less than to Na without K because usually there is insufficient rainfall in the spring for maximum response to K. In fact, 333 kg K/ha increased yield by about the same amount as Na, probably because a large proportion of these experiments were made in wetter-than-average springs.

The concentration of K in dried tops and roots in mid-summer increased linearly with soils containing from 40 to 120 mg K/l but when there was more K in the soil the concentration of K in the plants did not increase further. By contrast, the concentration of Na in plants did not reach a similar plateau. Whole plants in mid-summer contained between 128 and 251 kg K/ha and 42–102 kg Na/ha, depending on the fertilizer treatment. Recovery of applied K varied from 90% when small amounts were used on fields containing least K to about 20% when large amounts were given on fields containing moderate amounts. Recovery of Na varied from about 50 to 23%.

The comparison of the financial return from using K alone or K plus Na showed that Na was essential for maximum profit. On average, the most profitable dressing of K gave a return of £14/ha whereas K plus Na gave up to £46/ha. On fields with least K, the corresponding results were £50 and £115/ha respectively. In relation to soil K, the most profitable application was 150 kg Na/ha plus about 150 kg K/ha on soils with 60 mg K/l or less, and 150 kg Na/ha plus about 75 kg K/ha on most other soils.