How did your bag of Roots look if I may ask? Do you know which variety you're using? I know they have a few bagged mixes.
It was the original mix I didn't like and thought was very inconsistent between bags that I did purchase (2-3 years ago when it first became available in my area). I opened it up to find what appeared to be a mass of mostly chunky perlite. There was other material in there too to some extent (it looked like compost mixed with perlite), but I could not get over the amount of perlite. I know some people think that perlite is uber important, but it's really not and you just don't need a god awful amount like that. It is also inexpensive and shouldn't be sold for $15 a bag.
I am curious if their product has improved, but with Fox Farm at least I can say that personally I've
always had consistency between bags of soil and not been dissatisfied. Usually I mix OF with another product called Sunshine Advanced Mix #4, which is really just a base mix of screened, limed sphagnum, coco coir and perlite. If I do this I also mix more compost or castings in, but I like the addition of coco coir and the texture of the screened mix (there's no large particles of anything).
Perlite isn't something you want too much of in a soil mix, because it is totally inert, and doesn't really hold water or nutrients. Other materials like compost, humus, earthworm castings, clay and even coco coir, sphagnum and vermiculite act to some extent like "magnets" for nutrients, or ions (charged particles) that plants absorb. These materials are referred to as having a "cation exchange capacity" as well as a "buffering capacity".
So for example, you may know that plants need nutrients like Nitrogen (N), Phosphorous (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S), Iron (Fe), even Sodium (Na), among others. These are absorbed by plants and supplied by fertilizers or soil ingredients/microbial activity as
ions.
- Cations are positively charged and include: Ca++, Mg++, Na+, K+, Fe2/3+, H+ (hydrogen)
- Anions are negatively charged and include: H2PO4−, SO42−, OH- (hydroxide anion)
- Nitrogen can actually be a polyatomic cation: NH4+ (ammonium ion) or anion NO3− (nitrate)
- Particles of clay, compost, humus, castings, sphagnum, etc. are small enough to be involved in electrostatic interaction
- They have "exchange sites", which are predominately negative, and so adsorb or latch onto cations (opposites attract)
- The attraction is strong enough to hold onto the cations so they don't wash away, but typically weak enough that plants are still able to absorb them
- There is such a thing as "anion exchange capacity" as well, but most plant nutrients are cations and CEC predominates
CEC relates to a soils pH and buffering capacity, or ability to resist changes in pH. E.g. Materials with high CEC generally need more lime less frequently. Materials with lower CEC generally need less lime more frequently. Good base cation saturation (K+, Ca++, Mg++, and Na+ are base cations) also keeps acidity in check.
Perlite, however, having virtually no CEC whatsoever, wont hold onto to nutrient cations. Excess amounts can also create gaps or pockets of air/dry spots in your mix, where roots wont really want to grow. So too much is basically like creating dead space in your mix.
And do not waste your time with Black Gold anything (besides perhaps the seed starting mix). I'm not sure the stuff they sell is even pure earthworm castings, I thought it was blended with other stuff like sphagnum (check the ingredients). I bought a bag of their "Garden Compost" one time, as it was inexpensive, but I was disappointed to find that it was actually a lot more like a mulch and derived from peanut shells, peat and forestry products.
For bagged castings, try VermiWorm.
