Loam
Loam, strictly speaking, comes from turfs which have decomposed. This is the organic component of the potting mix. Here in the UK we have a standard, loam based potting compost, which has been developed over many years by the John Innes Institute. I don't have the exact composition of it, but the vast majority of it is loam. It also contains peat, gravel and fertilizer. These are "John Innes" #1, #2 and #3, depending upon the amount of fertilizer that has been added. Below is shown the raw loam component of the soil mix, before removing the larger components.
I manually remove any large pieces of peat, or large stones if I see any. I am informed that John Innes composts can vary greatly, depending upon the producer, but they all follow the same list of ingredients. I am sure that an Internet search will reveal all the information you could want about John Innes composts.
Sieved Loam
I sieve the loam to remove all components larger than 1--2mm. This is the same loam as shown in the photograph above, but this has been passed through a kitchen flour sieve.
