Hi,
leisure=horse_riding + natural=sand is not the recommended tagging for a riding arena like https://www.openstreetmap.org/way/1321924155. It should be tagged with leisure=pitch + sport=equestrian + surface=sand instead. 
leisure=horse_riding  should be used for the whole riding centre,
see
https://wiki.openstreetmap.org/wiki/How_to_map_a#Riding_centre_(equestrian)
https://www.openstreetmap.org/way/932566381#map=18/38.743241/-9.192668&layers=P (example)