The Mosel Valley has a glorious range of Rieslings, partly due, some say, to the magic of the soil

Three lumps of rock sit on a shelf in my office. The rocks travelled back with me from a trip to the Mosel Valley. Before that they formed part of three vineyards, one rock from each patch of steeply sloping earth. And, if you believe Ernst Loosen, whose guest I was, they largely explain why the Mosel is such a gloriously diverse source of Riesling.

Ernie Loosen is winemaker for his family's Dr Loosen estate. And he is a man with a mission: he wants to bring Riesling to an ever-shrinking audience. "Riesling will always be a niche market," he believes. "It has a special taste and character not all people like. You have to stay with its character and traditions and only use it in places where it works."

As owners of the JL Wolf estate, Dr Loosen also makes wine in the Pfalz. (Check out its beautifully appley Pinot Gris 1999, on sale at, £4.99 from £5.99.) But the Mosel is the source of its greatest wines. When Ernie took over in 1988, his senior staff left abruptly before the harvest - they didn't like the sound of his proposed improvements in vineyard practise. Without them, Ernie didn't know which plots were his. "I found out by waiting until everyone else had harvested. What was left belonged to us." The smallest parcel consisted of three vines.

The details are important because these vineyards are minutely differentiated by aspect - this is very hilly country and the river bends like a snake. They're also differentiated by soil types, which is where the rocks come in. Loosen's principal holdings are in six vineyards: Erdener Pralat, Erdener Treppchen, rziger Wurzgarten and Wehlener Sonnenuhr, Bernkasteler Lay, and Graacher Himmelreich. The first four are the greatest of all. Slate dominates the soil, but it can be red or blue or grey. Sandstone is also found, but not everywhere. When those rocks are on the ground and not on my shelf, they profoundly affect the taste of the wine.

Others disagree. Some New World winemaker or other - I've heard it attributed to one in California and one in Australia - has quipped that "soil is dirt". He means (crudely summarising) that wine gets its character from sunlight and skill, not from the medium in which the vines grow. But when you have the privilege of tasting wines from Loosen's vineyards, you think differently. I will not bore you with details from my tasting notes, but just one observation: rziger Wurzgarten (the name means "The Spice Garden of Urzig") has a powerful spicy quality not found in the other vineyards. It is also the only vineyard where there's sandstone as well as slate. A coincidence? Maybe. But the differences between vineyards pertain throughout the wine categories, from Kabinett to Auslese and TBA.

Top Loosen wines are not the easiest to find. Its entry-level Dr L Riesling is sold by Waitrose, Booths and at £5.99. The greatest wines are confined to independent merchants, except for a brace from Wehlener Sonnenuhr at Waitrose from £15.99 to £17.99. The vineyard-designated wines start at £9.99 and end in mild panic. Information from the importers, Walter Siegel (01256 701 101).

But fine German wine is expensive by nature, apart from special purchases like rziger Wurzgarten Spatlese, Christoffel Berres, at Majestic for just £4.99. On the whole, cheap equals awful - and hard to sell. Ernie Loosen told some hair-raising stories of what desperate producers will do to flog their wares in the domestic market. If you've seen the movie Paper Moon and remember the bible-selling scam, you have at least one of the pictures.

The Loosen wines are not cheap, but they aren't overpriced. Think of the work that goes into tending those dizzily steep slopes. Remember that quantities are not huge. And think about the soil. It may be dirt, or it may be magic. Buy a bottle and decide for yourself.