Photo: Thinkstock.com

Spanish authorities who raided what is thought to be the country's largest outdoor marijuana plantation didn't have far to look for the prime suspect. He was in jail.

The 47-year-old Spaniard arranged for others to cultivate a three-hectare (7.4-acre) olive grove he had purchased while on prison leave, Interior Ministry spokesman Federico Pozuelo said Saturday.

The prisoner relied on a 59-year-old Lithuanian assistant and a staff of five to tend 75,000 cannabis plants, which weighed five metric tons and were valued at 5 million euros ($5.6 million), police said.

Special plants that only grow to a height of 40 centimetres (15.7 inches) and look more like ordinary weeds than normally tall marijuana bushes were sown between rows of well-established olive trees and watered by a sophisticated drip-irrigation system, police said.

The arrests were made when the plantation was raided just as crop harvesting had begun and 5,000 plants were drying in a building containing a clandestine laboratory and packing facility.

Police said the farm in the central Albacete region was surrounded by a protective perimeter wall of dung and garbage that masked the plants' telltale aroma.

Two Spaniards, three Moroccans and the Lithuanian were arrested, Pozuelo said. One suspect was armed with a loaded handgun at the time of the arrests, police said.