Hi,
Well the thing about aquatic plants is that you don't really need to search for the perfect sediment as if it were The Holy Grail. There are a lot of factors that are attributable to plant growth performance, of which the sediment is really only one. The performance regime makes a difference, i.e the low speed, low tech (non-CO2) versus high speed, high tech (CO2 enrichment).
In your experiments there are a couple of reasons that explain why your laterite and peat combination works better than using sand alone. Laterite itself is just another iron rich clay. In the early days, people became hysterical over this discovery of Iron rich clay, thinking that this was in fact The Holy Grail of plant growth. That might explain why so many people focus these days on Iron in the planted aquarium. It seems to be a residual mind set from those earlier days. It turns out however, that the fact that laterite is a clay, is actually much more important than the fact that it is Iron rich.
An important attribute of sediments, from a plants perspective is an electrochemical property called Cation Exchange Capacity (CEC) and Anion Exchange Capacity (AEC). Roots embedded in the sediment develop fine hairs that act, in a way as an electric plug which make contact with the sediment particles. Have you ever passed a comb through your hair and then used the comb to pick up small pieces of paper? This is a similar effect. The comb pulls negatively charged particles from the surface of your hair, giving it an electric charge.
Cations are positively charged atoms or molecules such as Calcium (Ca++), Magnesium (Mg++) or Ammonium (NH4+), while anions are negatively charged, such as Nitrate (NO3-). If the sediment particle has a sufficient electric charge on it's surface it can attract these cations/anions to its surface and then pass them on along the root hairs to the plant. So in fact the laterite clay has the ability to pass Iron on to the root hairs, but also other nutrient ions such as K+ or Nitrogen in the form of NH4+. Movement of these ions from the surrounding water to the sediment surface and then on to the root hairs depends on the local acidity as well as the relative amounts of those ions.
The peat itself acidifies the sediment which then facilitates the CEC/AEC and as it decays, it breaks down and provides carbohydrates for the bacteria to consume. These bacteria will then nitrify the ammonia that develops from decay and organic waste that finds it's way into the sediment.
In general, clay has a high CEC/AEC than does sand or synthetic aquarium gravel, which means that root hairs in contact with clay particles have a higher uptake of nutrient ions than root hairs in contact with sand or synthetic particles so it's not surprising that you might have have better growth using the clay.
It is for this reason that the bulk of aquatic sediments produce commercially for plants are clay based. It's also no surprise that many of them are laced with peat. The more expensive sediments, such as ADA Aquasoil, are also fortified with NPK.
This is all good and well, but to focus only on the sediment is to have tunnel vision and misses the point of aquatic plants, which are easily capable of feeding directly from the water column. So you could repeat your experiment using sand only and fortify the water column with nutrients. You would then get very similar results as you did earlier when fortifying the sediment with laterite and peat.
Cheers,
Im not that good at maths.