By Thorsten Severin, Reuters

FREISING, Germany (Reuters) - Pope Benedict said on Thursday even he could not do everything that was expected of him and advised German priests not to burn themselves out trying to make up for the growing shortage of Roman Catholic clerics.

In his first unscripted speech of the six-day trip, the Pope, 79, said the Church could not simply advertise for more priests but had to pray to God to send them.

The growing priest shortage, a trend that is leaving many parishes without pastors, has led to calls for the Vatican to drop its celibacy rule or allow women to be ordained. Benedict is adamantly opposed to any such reforms.

"There is so much to be done, I see that I can't do it," he told a group of priests.

"That is true for priests, I can guess to what extent. That is also true for the Pope: he has so much to do and my strength is not sufficient for it all. So I must learn to do what I can and to leave the rest to God and my colleagues."

His address, on the last day of his nostalgic trip to his native Bavaria, took place in Freising Cathedral, near Munich.

The Pontiff began his career as professor of theology at Freising's University of Philosophy and Theology. He and his brother were ordained in the town in 1951.

The Pope has looked well on his visit home, but held few public appearances and observed long rest periods between them.

Benedict has hinted this may be his last trip to Bavaria, saying he is old and does not know how much more time God would give him. He has not planned many arduous foreign journeys.

LACK OF PRIESTS

Benedict said he had faith in God to supply more priests.

"There are fewer workers. Ask God that he sends workers," Benedict said. "We cannot, as in other professions, recruit people with targeted management strategies," he said.

In the next 10 to 20 years, the church's aging clergy faces a dramatic drop in its ranks as priests die. The average age for Catholic priests is already over 60.

The number of Catholic priests in Germany fell 29 percent between 1992 and 2004, according to Germany's biggest liberal Catholic network, We Are Church. In 2004, just 112 priests were ordained while 311 died, 359 retired and 37 quit the priesthood.

Liberals argue that loosening celibacy rules for priests could ease the problem.

We Are Church say 78 percent of German Catholics favor an easing of the strict celibacy rules for priests and 77 percent hope the Church will allow female priests.

Prominent dissident theologian Hans Kueng told German broadcaster NDR he had not altogether given up hope of reforms under Benedict.

"With time, he must see that if he constantly disappoints people he will land himself in difficulty," Kueng said. "His predecessor made parishes desolate and just asked and prayed for more priests for 27 years with no success."