Laterite is iron rich. Also the Grō Soil Tabs are high in iron (and other good stuff). I got some of them from the RAOK he did and I'm really impressed with them. It would be very easy to put them down first before the sand.
O crap , totally forgot about laterite !!!
I'm thinking if the flourite sand can hold ferts , placing the laterite underutilized could only help with iron treatment