Out of 11 trials completed over a nine-year period, significant responses—increases as high as 31%—have been obtained from five out of six trials on Ripperdan soils. In the sixth trial on Ripperdan soil and in one trial out of three on Hanford soils, the response was only slightly below the degree of significance. No response was obtained in one trial each on a Ripperdan-Dinuba soil and on a Grangeville soil.

Forrest Fullmer of the American Potash Institute assisted in the above studies.