> these rocks get in the way and they need to be removed [...] you will periodically/frequently come across old "abandoned" stone walls [made] from the stones t

> these rocks get in the way and they need to be removed [...] you will periodically/frequently come across old "abandoned" stone walls [made] from the stones that were handy because they were removed from their nearby fields You didn't mention what size these rocks/stones are (to my non-native ears, a rock is at least several centimetres), but if you can build walls from them, I take it they're not sand-sized. If you click the link in the first paragraph of the article, leading to the source, it mentions in the introduction > Crushed metabasalt and olivine soil amendments were used to investigate changes in soil pore water alkalinity as a proxy for bicarbonate production. The metabasalt (median grain size: 102 ± 22 μm) was selected because of its benign elemental composition and modest weathering potential (the parent volcanics includes felsic facies, yielding an overall silica content in the intermediate-to-felsic range). The olivine (median grain size: 83 ± 12 μm), in contrast, was chosen for its potential to weather rapidly TL;DR the remains of the crushed rock they tested is measured in micrometers