Healthy Soils for Healthy Vegetables

Written December 12, 2012, published in late December, 2012, all photographs by Kathleen Sayce

Soil health for vegetable gardens is more precise than for ornamental gardens and native plants. Most vegetables are annuals or biennials, living only one year, or over one winter.  All of are from other places and climates, with nutrient and soil needs considerably different than local soils can provide. Vegetable plants need:  Deep, open, well-aerated soils with soil carbon, diverse minerals, sunlight, warmth and regular water. With these, they grow quickly into tender, nutritious and edible foods; without them, vegetable plants struggle, easily fall ill, and fail to thrive. Vegetables generally are not shade plants, especially along the raincoast; warmth and regular watering are needed for vegetables to grow well. 

Well-grown vegetables are able to resist weather, diseases, insect pests, and have high levels of minerals, proteins and other plant compounds. This photo of Red Russian Kale was taken in Jim Karnofski’s vegetable garden by Kathleen Sayce.

Soil Carbon

As with other kinds of plants, vegetable plants need soil carbon. The forms that are the most usable for vegetable plants are not aged wood chips or forest debris, but well prepared compost with humus, and biochar (biologically activated charcoal). Vegetable plants use soil carbon throughout their root growing areas, so gardening practices for optimal plant nutrition incorporate carbon of several kinds throughout the soil profile. Gardeners work carbon into the soil with a rototiller or shovel, add layers to the surface, side dress plants, and amend planting holes. They also fallow garden sections every few years, planting cover crops to put more carbon back into the soil. 

Carbon promotes soil health by giving soil organisms food to eat (carbon) and places to live (cellulose scaffolding). The one drawback is that, being formed of cellulose (wood), most forms of compost break down quickly. So gardeners need to add compost regularly, year after year. Only humus, a brown, clean-smelling, somewhat sticky substance, persists for decades to centuries in soil. Compost piles can form humus if clay and local soil are added to each layer. 

Compost with charcoal added is dark colored, and is now ready to go into the vegetable garden.  Photo of one of Jim Karnofski’s compost bins.  

A second soil carbon material, biochar, is charcoal that has been activated with compost or soil microbes. Biochar has an advantage as a soil amendment: charcoal is stable in soils for centuries to millennia. When gardeners add biochar, this is a permanent improvement in the soil. Add biochar along with compost, and over time, you will have the same productivity with less compost. 

Making Biochar

When wood is burned, charcoal is formed during the burning process. If burning is complete, the wood goes to charcoal and then to ash. Starving the fire of oxygen (a process called pyrolysis) promotes charcoal formation and keeps the fire from consuming all the wood. Innovative pyrolysis burners are being developed at backyard and industrial scales to produce large amounts of charcoal with minimal amounts of ash. When the charcoal is wet and cold, it can be added to compost to be inoculated. See  HYPERLINK "http://www.biochar-international.org/" http://www.biochar-international.org/ for biochar producing devices. A short video for an introduction to home charcoal making is on You Tube at  HYPERLINK "http://www.youtube.com/watch?v=dqkWYM7rYpU" http://www.youtube.com/watch?v=dqkWYM7rYpU .

Freshly made charcoal is ready to go into the compost pile when it is wet and cold, and broken into small pieces. 

Mineral Nutrition

The second soil management practice for optimally healthy soils is to use soil tests to determine what minerals are needed, and then to add those missing minerals in the correct amounts. Soil tests are inexpensive, and a simple way to ensure a garden is not over-fertilized with some minerals and too low in others. It’s a good gardening practice to test soils in your vegetable garden and adjust your fertilizer program every year. Minerals can be added as rock dusts, algae extracts, and other forms.  The differences in terms of productivity can be staggering; I’m not talking ten percent increases or even twenty. At times, improvements can be on the order of multiples, as measured by plant weights or volumes, fifty pounds of potatoes instead of twenty, for example.  

Jim Karnofski, local vegetable gardener and retired nurse, has delved into soil mineral nutrition as a neighborhood soil analyst, and is wiling to teach anyone interested in learning the details how to decipher soil test results. He also makes custom nutrient blends for specific soils. I tested my soils a few weeks ago, after years of adding carbon, trace minerals, and organic fertilizer blends. I found that my soils are surprisingly low in boron, manganese, sodium, copper and sulfur. Jim composed a custom blend to meet the nutritional deficiencies based on the soil test. I’ll add a portion of these missing nutrients every few months, test again in coming years, and keep adjusting minerals to improve my soil. A new book by Steve Solomon, The Intelligent Gardener: Growing Nutrient-dense Food, goes into splendid detail about vegetable nutrition. 

The sum of all of these actions (adding carbon, testing soils and adding mineral nutrients) is to have optimally healthy soils. Healthy soils produce healthy plants, able to resist disease, drought and insect predation. In turn, healthy plants produce nutrient-dense vegetables and fruits, which are better foods for us.  Many chronic human health conditions go away when people make the change to eating fruits and vegetables grown on optimally healthy soils. I think we’d all like to live healthier lives, and my personal task for the New Year is to promote soil health, so as to promote human health.