Couldn't manage to find a duplicate to this, so here goes:
Urist halted halfway through bending over to pick the plump helmet, his body frozen at a strangely uncomfortable angle that he didn't seem to notice. Under his coarse red beard his lips began to split into a grin that showed his yellowing, beer-stained teeth. Suddenly he let forth an exultant whoop, the volume of which was sufficient enough to startle several of his fellow workers into falling flat on their rumps out of surprise. He quickly reached down and snatched the plump helmet out of the ground then darted as fast as his stubby legs could carry him down the nearest hallway into the lower levels of the fortress. His dirt-covered farmer's clothes left a trail of dust in his wake.
"What in Armok's name's gotten into that fool?" asked one of harvesters.
"Dunnae. If yer ask me, feller's always bin' a few hairs short of his beard," another replied with a grumble as she stood back up and rubbed her backside.
Urist, meanwhile, had made it all the way down to the lowest levels of the fortress. The guards posted at the entrance to the underground cave complex (just discovered last month) arched their brows as the dwarf went tearing past them. They scarcely had time to ask him about his business, and he only ignored them and ran into the darkness. After a moment of considering whether they should follow, the pair decided that if the crazy farmer wanted to land himself in the belly of a giant toad, that was his own business.
He darted past the first few cavernous rooms, eventually rounding the edge of a great underground lake before he came to a halt on its southern shore. Despite his mad efforts he was tireless, and he immediately began to look around for something as if he already knew it would be there; soon he spotted a small copse of fungiwoods and tower-caps, causing him to produce another lively and wordless exclamation of joy. The dwarf ran to the center of the small grove and began to dig into the soft red muck, his gnarled hands lifting out head-sized amounts of the gloppy soil with ease. Once the hole was deep enough he gingerly set the plump helmet into the bottom.
But he was not yet done. Without pause, he went scrambling off into the dangerous and unexplored caverns for the things he needed; first the webs of the cave spiders that haunted the gloomy depths. Next several varieties of berries and weeds and shrubs that grew here under the earth - he knew exactly the types he needed. He even made a trip back into the fortress to fetch a piece of beautiful willow-wood, a copper bar, and a softly glowing red gem, offering no explanation to either the confused citizens or the guards, who had no time to interrogate him.
Once back in the cave, he set all his ingredients into a pile and picked up a rock half his weight. Such an exertion should have tired a dwarf quickly, but Urist was bothered no more by the physical strain to his body than he would have been by a gnat's bite. Using the unwieldy rock, he began to pound all of the ingredients into a fine paste, simply smashing them into a dust so thin that the original objects would have been almost impossible to guess.
"YES! FERTILIZER!" he screamed when he was finally finished, tossing the rock aside to land with a great splash in the nearby lake.
The strange mixture was slowly and carefully scooped, handful by handful, on to the plump helmet. Afterwards he filled the hole with some of the excavated soil and gently patted down the top. The feverish light in his eyes suddenly flickered and disappeared, and the dwarf stood there in the cave for a moment with a confused look on his face.
"What in the bloody 'ell... where in the 'ell am I?"
Urist glanced around and shuddered, then began to pick his way back towards the entrance to the fortress, trying to make as little noise as possible to avoid the attention of any of the strange creatures that inhabited this place. By Armok, I've been sleepwalking again, haven't I? he thought.
A few months later, the first reports came. An exploratory military party had discovered an absolutely huge mushroom covering an entire segment (and multiple levels) of the caverns. It was apparently some bizarre strain of plump helmet, perhaps the grandfather of the species, for it bore a striking resemblance to the smaller variety that the dwarves were fond of; however, this giant artifact of a mushroom was connected and was tough enough to be walked and climbed over, but it produced clusters of edible plump helmet-like material that could easily be picked. Amazingly, this single plant could provide as much food as two or three dwarven farms running at full capacity. The thing was gigantic, growing from the bottom of the spacious caverns all the way to the ceilings multiple stories above, and oddly formed in a way that allowed dwarves to access most of it simply by ascending the stalks (of course, even the areas that could not be reached could always simply be accessed by constructed stairways).
Even more amazing, though, was the fact that this strange plant occasionally organically produced what appeared to be biologically-manufactured rubies, lumps of worked copper, and pieces of willow wood! Many dwarves also swore that they had found beautiful silken webs growing from the plant, or odd handfuls of tasty berries and brewable underground varieties of weed. The great dwarven thinkers of the fortress had pondered over this bizarre turn of events but could offer no explanation for the plant's strange ability to create useful materials. But from then on, any dwarf who collected food from the great mushroom would sometimes find a piece of perfectly smelted copper, or a long and flawless length of wood ready for a carpenter's skilled hands. And the luckiest swore that they had taken a bite of their food only to come close to chipping their beer-stained teeth on a huge and beautiful ruby.