polyacrylamides - potassium based - added to soilless media at a rate of 1 to 2 ounces per cubic yard will increase the water holding capacity of the growing media and also help retain higher levels of N-P-K in media. Tomatoes still need to be irrigated, but can reduce irrigation applications per week by 25% when growing tomatoes in the greenhouse.
Dr. Mike Orzolek, Prof. Emeritus, The Pennsylvania State University