Soil compaction

When our children were little, we used to read them a small book called “Noisy Nora”. Her refrain was, ‘ “I’m back again,” said Nora, with a monumental crash.’

I am back again, dear Readers, although perhaps not with the monumental crash.

It is straight into the serious stuff. Soil compaction, which has been the topic of our most recent conversations on the gardening front here. Being on friable volcanic soil, it has not, I admit, featured large before. Pity those who garden in heavy soils and on clay – they have to deal with it all the time.

We have tended to work on the principle of planting well into cultivated soil (roots need friable, soft surrounds to reach into as plants establish) and then mulching mulching mulching each year or two with compost to keep soil fertility. It is not enough in the long term, and that has become apparent in recent years. Even our wonderful soils compact. They just become more and more solid and lose the airy fluff . As that happens they become less hospitable, even to established plants. Presumably the worms too start to despair and move on to more welcoming habitats where they don’t have to work so hard.

Readers with Sky may have caught “A Year At Kew” and their discussions on soil compaction. In their case it is exacerbated by millions of feet tramping over ground. In one programme from the famed botanic gardens in London, they talked about a surprise effect of the 1987 cyclone which caused such devastation that it may remain the worst ever weather incident for an entire generation. Amidst the trail of destruction were a large number of trees which had been loosened by rocking, but stayed standing. To their surprise, these trees thrived and made rapid new growth to look better than before the cyclone. This was attributed to the rocking motion breaking up compacted soil. In an attempt to replicate this effect, Kew staff were embarking on a systematic approach to try and beat the compacted soil around other trees. With a powerful compressor, they were blasting air in around the drip line of the trees. You could hear the whoomph and see the ground lifting as the air penetrated underground. The drip line, by the way, is the outer perimeter of the tree’s canopy.

No, we are not heading out with our compressor to undertake such drastic remedial action. But we are taking more notice of the state of our soil beneath all that mulch and no longer assuming that a combination of compost and worms will do the work.

There is of course a fine line between causing damage to the roots of established trees and shrubs and aiding them by aerating the soil. In high density traffic areas in public domains, boardwalks are often built to prevent damage to tree roots and excessive compaction. I recall hearing that the mighty Tane Mahuta is now reached by board walks. It is not just to keep the shoes of visitors clean. Boardwalks are an expensive solution and we don’t have enough foot traffic to justify it under our trees. But we are taking more interest in the areas of cultivated garden, particularly where we have noticed that the plants are just sitting or not thriving.

Aerating the lawn is hardly a new concept. One of those hilarious unsolicited mailorder catalogues had some lawn aerator shoes (think strap on soles with big spikes on them) which I believe were useless. Dreamed up, perhaps, by someone who had never tried to aerate his lawn. Using a fork to penetrate and loosen the lawn is the traditional method but this probably belongs more to the “It’s a good idea in theory” school of thought rather than common practice. Although I must bow to one friend. Somewhat incapacitated and currently on crutches, she was undeterred (and probably bored by an enforced period indoors). Using a garden fork as a crutch, she exercised by traversing the lawn and aerating it in the process.

It is likely that gardeners in very cold climates do more to cultivate their soils. When you are lifting plants like dahlias and tree ferns every winter in order to keep them alive, then you are replanting into freshly tilled soil. But in our soft climate, the plant and leave approach is the norm. And when weed control is carried out with Round Up, not even the soil surface is getting tilled. You can see it in gardens where this has been carried out over time because the ground starts to look very caked and solid.

In a large garden like ours, it is going to take more than one season to get around but it is more satisfying than everlasting weeding. I spent Sunday lifting a bed of Ariseama speciosum. I knew the soil was heavy in this area but I hadn’t worked out that over the decade or so that it had been planted and left undisturbed, the soil had gradually compacted and the patch become congested to the extent that it was pushing the big chunky arisaema tubers out onto the surface. Now replanted, I am left with a large supermarket bag of leftovers. I have a place in mind for these but I am a little concerned as to what I am going to do with the surplus when I get around to lifting the other two well established patches of the same plant. There is a definite limit as to how many drifts one wants, even of this unusual and choice Indian tuber.

I am feeling under pressure. Mark will persist in pointing out that spring is only about six weeks away and time is running out for lifting and dividing. The plants will soon be in growth. Winter is not a time of rest for the gardener.