Ammonium and nitrate in the nitrogen economy of some conifers growing in Douglas-fir com-

Hydration of a commercial hydrophilic polyacrylamide gel in 20 meq Ca(NO 3)2/ liter was reduced to <10% of the maximum hydration in deionized water. Repeated soaking with deionized water to remove soluble salts restored hydration to ≈ 30% of maximum. Incorporating KNO3 at concentrations ranging from 5 to 40 meq·liter -1 with the Ca(NO3)2 in the hydration solution partially reversed the Ca 2+ inhibition of hydration following repeated soaking. Potential hydrogel hydration increased to 50% of maximum with 40 meq K/liter. Potassium nitrate supplied separately following hydration in Ca(NO3)2 was much more effective at reversing Ca 2+ inhibition of hydrogel hydration than joint application. Potential hydrogel hydration (following repeated soaking) was doubled after treatment with 5 meq KNO3/liter and reached 77% of maximum at 40 meq KNO3/liter. Hydrophilic polyacrylamide gels (hydrogels) are marketed as amendments to increase the water-holding capacity of container media and field soils, based on their ability to absorb up to 1500 times their weight in water (Johnson, 1984). The cross-linked polyacrylamide hydrogels commonly available in the horticultural trade are capable of absorbing >400 ml of water per gram of hydrogel. The presence of salts in solution, however, severely restricts hydrogel hydration (Bowman et al., 1990; Foster and Keever, 1990; James and Richards, 1986; Lamont and O'Connell, 1987; Wang and Gregg, 1990). Solutions containing salts of the monovalent cations K+ and NH4 + or the divalent cations Ca2+ and Mg2+, at 20 meq·liter-1, reduced hydrogel hydration by >75% and 90% of maximum, respectively (Bowman et al., 1990). The inhibition of hydration caused by KNO3 was completely reversed by rinsing the hydrogels with deionized water (DI), whereas the inhibition caused by Ca(NO3)2 was only partially reversible. We found no reports of methods by which the Ca2+ inhibition of hydrogel hydration might be removed. We report here the results of two Received for publication 22 Oct. 1990. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. experiments examining the effects of K+ on a calcium-inhibited hydrogel. Experiment 1 examined the effect on hydrogel hydration of K+ supplied in solution with Ca2+. Samples (1 g) of a commercial polyacrylamide hydrogel (Broadleaf P-4, Broadleaf Industries, San Diego, Calif.), with a particle size of 1 to 2 mm, were each added to either 1 liter of DI or 1 liter of 20 meq Ca(NO 3)2/liter, plus KNO3 at 0, 5, 10, 20, or 40 meq·liter-1, with three replicates per treatment. After 24 h, the hydrated hydrogels were collected on a fine mesh screen and the excess solution was removed by blotting the bottom of the screen with a moist sponge. After weighing the samples, each was transferred to 1 liter of DI for 24 h, then collected and reweighed. The samples were transferred daily for 3 days to fresh DI and reweighed to estimate the maximum potential hydration of the hydrogels. Experiment 2 examined the ability of K+ to reverse the calcium inhibition of hydrogel hydration when the KNO3 solution was supplied after hydration of the hydrogel in a solution of Ca(NO3)2. Samples (1 g) of the hydrogel were hydrated for 24 h in 1 liter of 20 meq Ca(NO3)2/liter. The samples were then collected, weighed, and transferred to 1-liter solutions of KNO3 at 0, 5, 10, 20, and 40 meq·liter-1. The hydrogel samples were weighed again after 24 h, then transferred to 1 liter of DI. As in Expt. 1, this