Dissolved gypsum and calcium nitrate each increased infiltration rates over the control for each of the 15 irrigations to which they were added. An equivalent amount of gypsum spread on the surface at the beginning of the experiment had the same beneficial effect for only 10 irrigations.

The authors thank Gary Thatcher of Domtar Gypsum Company, Oakland, for providing the gypsum and dissolving machine, and Robert Millaway of Wilson and George Meyer Company, San Francisco, for providing the calcium nitrate. They also thank Brad Hall, Glenn County Extension Assistant, and Dan Moore and Shannon Sadler, UC Davis graduate students, for their assistance in making field measurements.