Читать книгу Heart & Soil - Des Kennedy - Страница 13

Feat of Clay

I recently had a chance encounter with a young woman whose garden was encumbered with heavy clay soil. Daunted but far from defeated, she maintained an admirable stiff upper lip, as clay-based gardeners do, if they’re not to despair. I’ve long maintained that dealing with heavy clay soil builds character and that every gardener might be a finer person for having a patch of rough clay to contend with.

Needless to say, our place has one, a low-lying area at the north end of the garden where our standard sandy gravel gives way to lumpish and intractable clay. I imagine it was once a prehistoric pool in which tree ferns flourished and brachiosauruses wallowed. A couple of big rambling roses have thrived there for years, but otherwise the area’s been a bit of a disgrace, much of it given over to invasive lemon balm, hairy apple mint and variegated creeping dead nettle.

Heavy clay is almost always difficult to deal with. When wet it coheres like malignant gumbo; when dry it hardens to obstinate concrete. One looks for windows of opportunity for improving things, and indeed such a window opened for me when an uncharacteristic dry spell dehydrated the soil enough that it could be worked without compacting it badly. I first dug out the invasive ground covers—and received a welcome reprieve from what I had anticipated would be a protracted campaign. Revelling in surface moisture, the mint, dead nettle and lemon balm had each spread a thick matting of rootlets near the surface, but none had plunged deep into the clay. Using a heavy mattock I was able to rogue their roots out with relative ease.

Although a bully-boy attitude helps when tackling clay, care was required at this stage, as a number of creatures had settled in for the cold weather—a small greenish-brown tree frog hopped off unhappily over the disturbed earth, and between two sandstone blocks edging a bed I discovered a rough-skinned newt, torpid from the cold, its distinctive underside a gleaming yellow orange. These were good omens, as were the abundant pink earthworms wriggling in the surface soil.

I spaded the whole bed over, plunging the spade blade deep, then turning up great podgy clods of clay. This is what old-timers would call “four-horse land”—ground so tough it would require four horses to break it in spring plowing. Particle size is the villain here. Clay particles are minuscule—a mere 0.002 millimetres or less, as compared to sand, which has particles ranging from 0.25 millimetres up to 1 millimetre. The finest garden loams contain approximately 50 percent sand mixed with some silt and clay, so the diversity of particle sizes creates abundant pore spaces in which air, moisture and organic bits can be held.

Once wet, clay can retain moisture for a very long time—and once dry can remain impervious to moisture for just as long. Its moist cohesiveness is what makes clay perfect for nursery folk to use for balled and burlapped trees—the so-called B&B trees. I remember acquiring one such specimen years ago and heeding the expert advice of the day to simply plop the whole root ball into the ground. The tree lived all right, but for years grew at an unimaginably slow pace. Where it should have been seven metres high, it was still only two. Eventually I dug it out and, amazingly, most of the roots were still locked inside the clay ball. Only one adventuresome root had managed to break free and grow, and was probably the sole reason the tree was still alive. The problem here is that the extreme difference in soil texture between clay and the surrounding earth impedes water movement and inhibits root growth. Nowadays, whenever we acquire a B&B tree or shrub, I first bare-root it, systematically dissolving the imprisoning clay ball with a jet of water from a hose, before transplanting.

But back to our clay patch—which I suppose in hindsight we might have simply planted with compatible B&B trees and called it quits. The only alternative was to try expanding pore space by adding amendments. Organic matter being a cheap and effective soil conditioner for heavy clay, I first spread a thick layer of wood chips—about fifteen centimetres deep—over the whole surface. Some weeks earlier, we’d had a dump truck deliver eighteen cubic metres of shredded Douglas fir bark and sapwood from a pole yard on Vancouver Island. Ideally, you’d leave the stuff to decompose for a few years, but here I simply spaded it in, digging the bed over a second time, working it even deeper than the first dig. As attractive as it is to beneficial fungi and bacteria, decomposing wood waste sucks nitrogen out of the soil, so I next spread a layer of canola meal, followed by a thorough drenching of urine to boost available nitrogen.

Then began a veritable layer cake of amendments. Dolomite to sweeten the acid soil, as well as render it more porous and friable. A sprinkling of granulated bone meal for phosphorus (we’ve switched to rock phosphate in the meantime). A pinch of boron. Then some sand, though not as much as I would have liked. Ash and cinders are, along with sand, traditional amendments for breaking up clay soils, as well as a good source of potash and trace minerals, so I next spread a layer of wood ash saved from last winter’s wood-stove burning. Then some precious leaf mould from the bigleaf maples. Finally I turned the whole works in by spading the bed for a third time. Already, by this digging, the soil had loosened and lightened tremendously. I planned to add compost and more wood ash in a few months’ time.

For the moment, fortified with sufficient character building to get me through any other garden challenges the year might throw our way, I left the bed to the elements, for bacteria and fungi to set about breaking down the organic matter, opening up pore spaces and creating the crumbly, altogether gratifying state of good tilth.