A tale of two North Island gardens’ soils:
The two gardens are at different stages of development. The backyard garden is developed on land which had been massively bulldozed and reformed about 40 years ago, worked for a time and then abandoned to coarse grasses and broadleaf plants.
As a consequence of bulldozing there was a wide variety of soils in the garden when we started there. We created raised beds and gradually, through application of compost, have made the soils more uniform.
An initial chemical analysis showed a pH of 6.4, a reflection of the application of lime, and deficiencies of all the major nutrients, but the application of compost has undoubtedly changed this, since we now have excellent crops of more than thirty varieties of edible plants including broccoli, cabbage, strawberries, potatoes, currants, herbs, garlic, beans, and Jerusalem artichokes.
The Grassroots Garden contrasts with the yard garden in many respects, but it too was bulldozed about 40-50 years ago after removal of the natural forest, then left to become alder woodland. The alders and shrubs were removed, the stumps were bulldozed and the land was then ploughed, leaving behind a fairly uniform sandy loam soil with many boulders.
Now beds are being made, many of them raised, in the 1/4 acre (of 2.7 acres) selected for initial field crops. Chemical analyses of the bared soil showed a pH of 4.6 as expected, too low for most vegetables, while all major nutrients except nitrogen and most micronutrients were deficient. Lime to improve the pH and organic slow release amendments were applied.
Clearing and ploughing resulted in exposure of the previously protected soil to heavy rain so the initial high nitrogen content (courtesy of fixation by the alders) was rapidly leached out and areas previously drained by alder roots were periodically waterlogged. Also the soil now exposed to the sun lost organic matter.
The results have been mixed. Perennials and a few annuals have done very well; other annuals have been disappointing. For most vegetable and many fruit crops the solution is to gradually and methodically improve the soil by adding compost, possibly mulch, and both slow-release and readily available nutrients. Over time, with patience, we expect the Grassroots Garden to catch up with our backyard garden.
Healthy plants, like healthy people, need a nutritious diet. There are seventeen nutrients essential to healthy, high-yielding plants. In organic agriculture, we want these nutrients to be available to the plant roots in the soil, so we are working continually to improve the soil, and thus the quality and quantity of the harvest.
In conventional agriculture, the nutrients are often sourced from a bag or bottle of synthetic substances which are poured or spread on or around the plant, along with “cides” to kill other living creatures and plants/weeds. This is a temporary fix for the farmer, the unsustainable, long term effects of which are to deplete and ruin the soil, pollute our waters, and kill pollinators and other living creatures.
In a subsequent column we will consider plant nutrition in detail, and explain why “Compost trumps all!”
Dawn Moorhead and David Lang are longtime practitioners of organic agriculture. They welcome your comments or questions at email@example.com