• Crystal pearls retain water in soil, but other products can work.blue pearls on black image by laurent dambies from Fotolia.com
  
  Crystal pearls are a soil substrate alternative made of water-retentive polymers. The polymers resemble tiny beads and can be used to assist in hydroponics, promote the health of cut flowers or retain water in soil. You can order crystal pearls from Chinese manufacturers or use an equivalent product.
  
  

Hydrophilic Gels

• According to the University of Florida IFAS Extension, hydrophilic gels were introduced in the United States during the past 20 years. These gels absorb more than 150 times their dry weight. Because of the cost, they primarily are used in greenhouse and commercial growing. Hydrophillic gels are formulated to look like granules or flakes that can easily be mixed into potting mixes. After the granules absorb water, they swell and become gel-like in their consistency. As the gel swells, it increases the aeration of the potting soil.
  
  

Polyphenolic Foam

• Polyphenolic foam is made by the same manufacturer as a florist foam. This product is open-celled foam sold in pieces 3/8 inches wide and of variable lengths. Incorporating polyphenolic foam into potting soil helps increase aeration in the soil. At the same time, the open cells of the foam absorb and retain water. Polyphenolic foam is used in potting mediums that grow orchids.
  
  

Perlite

• Perlite is made of volcanic silicates that have been heated to a high temperature. As the silicates heat, they pop like popcorn. The resulting product is a coarse, porous material that is lightweight and holds water well. Perlite is often an ingredient in potting soil because it lightens and aerates the soil while still retaining moisture. Perlite is also frequently used as a substrate in hydroponic systems. There is some risk associated in breathing perlite dust. Gardeners should only use horticulture-grade perlite and wear breathing protection when using the product.
  
  

Rockwool

• Rockwool is manufactured from basalt that is heated and spun in a process resembling the manufacturing of cotton candy. The resulting fiberous product is filled with open holes that make it good for both insulation and growing plants. For horticulture purposes, rockwool is cut into blocks or slabs and used for growing seedlings or hydroponics. In potting soil mixes, loose and granulated rockwool provide many of the same benefits as crystal pearls by aerating the soil and retaining water.