Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Sand is packaged in several forms.
'Play Sand' is a mix of fines and not very coarse particles. This blend of sizes means the pore spaces pretty much get clogged with the fines, so there is very poor water movement through this type of sand. Think of a box of ping-pong balls, with marbles, and dried peas, and flour. (I have no idea why this is 'play' sand- I sure would not want my kids to play in it- they would track it into the house all too easily. Coarser sand could be brushed off before they come in)
'Pool Filter Sand' is almost always sifted so all the particles are the same size, usually 20 or 30 mesh. These same size particles maintain good space between the particles. Think of a box of ping-pong balls, with just a little flour (from shipping and handling).
'Blasting sand' is also graded for size. For example, one size of the Black Diamond material available at the local Tractor Supply is 20-40 mesh. Think of ping-pong balls and large marbles.
Many other sizes of particles are available. Check with local masonry stores and similar- rock yards, soils, and so on. Lapis Lustre is one brand name that packages sand in different sizes.
The one thing they all have in common is this: The particles of sand are so coarse they do not hold onto fertilizers in a way plants can get them.
Finer materials like clay (a certain soil particle size) and humus (very finely decomposed organic matter) are so fine they can show a charge, like a polar material. Charged fertilizer molecules can cling to these polar materials until the plants remove them. Good sources of this size particles include the montmorillonite clays such as Safe-T-Sorb, Turface and similar things.
Garden soil (Walstad concept tanks) will have a blend of materials, and often include some clay sized particles. The soil particles (especially the finest ones) are clumped together in a way that makes them act like coarser materials, with the pore space between the particles that will allow good water movement. Think of a popcorn ball- small particles are held together into larger balls. Water flows between the larger balls, and between the popcorn pieces that make up the balls. Yet the finer material retains its polar properties and the right kind of garden soil can be a good material for an aquarium.
Some commercial substrates are also like this.