Evaluation of ammonium and soluble salts on grass sod production in compost. I. Addition of ammonium or nitrate salts

Inhibitions in seed germination and in plant growth in some composts have been associated with high concentrations of ammonium or soluble salts in the media. This experiment was conducted to determine changes in ammonium and soluble salts in fertilizer‐amended compost with time and their impacts on plant growth. Turfgrass (Lolium perenne L.) was seeded into an ammonium‐depleted municipal solid waste (MSW) or leaf composts or into MSW or leaf composts with 1,500 or 2,300 mg N/kg (dry weight) from (NH4)2SO4 or Ca(NO3)2 added to simulate immature composts. Seeding occurred on the day that the composts were treated and applied to flats. Ammonium‐nitrogen (N) and nitrate‐N concentrations and electrical conductivity were measured on the day of seeding and after 3, 7, 14, 21, and 28 days. Germination or growth was assessed after 7, 14, 21, and 28 days. Ammonium‐N in the compost declined with time, whereas nitrate‐N and electrical conductivity initially increased then decreased with time. Ammonium‐N from (NH4)2SO4 added to the compost declined by half within seven days, and as the compost ammonium‐N declined, germination and growth of grass increased. Electrical conductivity indicated that initial soluble salt levels in the composts with 1,500 or 2,300 mg N/kg from Ca(NO3)2 were sufficient to inhibit seed germination and plant growth. In composts with 1,150 mg N/kg from Ca(NO3)2, germination and growth of grass improved after 14 days, whereas growth in composts with 2,300 mg N/kg from Ca(NO3)2 was inhibited for at least 28 days. Ammonium salts appear to be lost from the compost more rapidly than nitrate salts, which have a prolonged inhibitory effect on germination and growth.