Soil Building in the Americas

Soil in the Amazonian rain forests is notably infertile. The continual rain leaches nutrients out of the soil, carrying the runoff downstream to the ocean. A notable exception is the fertile soil—called terra preta—found all over the Amazon basin in large patches near ancient population centers. Farmers till these soils today, producing crops without fertilizers.

With Amazon rain forests being cleared, evidence is emerging that leads archeologists to think that large civilizations have managed the rain forest for the last 11,000 years. In 1542 Spanish explorer Francisco de Ornella traveled the length of the Amazon River, and reported large, sophisticated civilizations inhabiting the land all along the way. Later explorers found the river barely populated except for hunters and gathers living in primitive conditions, which cast doubt on the accuracy of Ornella’s reports. Today archaeologists are considering that the Amazon area may have supported as many as 5-6 million people in the 1500s. European diseases may have wiped out these large population centers leaving the area barely populated. They are also challenging the idea of a "pristine rain forest."

Nutrient poor soil (left) and rich terra preta soil (right) - photo by Bruno Glaser (Creative Commons) 

How could such a large population be supported on such poor soil? Fascinating to me is the discovery of black soil, or terra preta near population centers. Thought to be anthropogenic, the soil is dark and oily, and composed of organic matter, small bones, unfired pottery chards, and ash and charcoal from earth ovens (called biochar). The result is fertile soil that does not break down, and does not require modern fertilizers. The Kuikuro Tribe people, living in the upper Xingu region in Brazil, create this earth today. They toss organic waste behind their homes, and in three years the soil is very fertile. Scientists are also interested in the soil because biochar takes carbon out of the atmosphere, and puts it to work improving soil nutrition. More interesting ideas to consider from American gardening history.

Terrace Farming in the Americas

Terrace farming is another interesting gardening history topic from the Americas. The Wari and Inca civilizations of Peru provide fine examples of using this technique for large scale food production in challenging dry conditions. Terraced farming was mastered 1,500 years ago in Peru, and continues today as archeologists reintroduce farmers to this old technique.

Terraces at Tambomachay, Peru. Photo by McKay Savage. Creative Commons.

Steep mountainsides are cut back in large steps, and rock retaining walls, up to eight feet tall, are built forming a raised bed. The base of these steps are typically clay soil, topped with layers of gravel and sand, and up to a yard (or meter) of top soil. The rocks heat during the day keeping the soil warmer at night, enabling crops to be grown at high altitudes. Additionally, the layers of gravel and sand ensure drainage of water down to the next terrace, with the base layer of clay retaining some water. Archeologist Dr. Ann Kendall, who has studied terrace farming near Cusco, Peru, reports that soil is still moist six months after irrigation. Organic matter breaks down in this damp environment and is recycled upward, so that no fertilizers are needed. Typical crops include potatoes, maize, beans, barley, and quinoa.



Terraces at Ollantaytambo, Peru. Photo by Bernard Gagnon. Creative Commons.

Dr. Kendall formed the Cusichaca Trust in 1977 to educate modern farmers to the ancient techniques of terrace farming and irrigation, and restore Incan agricultural practices throughout Peru. This is another brilliant idea for sustainable farming in harsh conditions, worth considering as the world fills ups and innovative solutions for food production are needed.

Mushrooms

Winter in the San Francisco Bay Area typically brings rain. Native plants in the area are adapted for the long dry seasons of summer and fall, and the rainy season of winter and early spring. During this brief wet period, the dry hills turn green and vegetation becomes vibrant. Mushrooms emerge, and are a familiar sight in winter gardens or regional parks from November to January. They sprout up in the middle of the lawn, compost pile, or planting bed, leading some people to wonder if their soil has been infected with some disease.

Delicate capped mushroom emerging in November.

Pale mushroom muddy from early November rain.

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Mushrooms can be harbingers of bad news, such as the clusters of brown-capped mushrooms close to the trunk that may indicate Sudden Oak Death, or the decomposers that may indicate below-ground decomposition of dead roots. But they can also simply be the fruiting body of fungi growing in healthy soil. We learned about mycorrhizae in a past post, the fungus that lives in symbiotic relationship with tree roots. The mycelia of the fungus extends the reach of tree roots to moisture and minerals, while the tree uses photosynthesis to create nutrients mycorrhiza can use but not produce.

Gilled mushroom emerging from moss in December.

Bright orange mushroom cluster in late December.

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Mushrooms are members of the Fungi kingdom, in the Ascomycota or Basidiomycota phyla. In the past, fungi were thought to be plants without chlorophyll, but research showed the cell walls to contain chitin (similar to a crab) rather than cellulose. This makes them more closely aligned to animals than plants, so have their own kingdom. Mushrooms are composed of densely packed mycelium, with gills or sacs that produce asexual spores. They come in all shapes and sizes, and are fascinating to study. Caution should be used, since some are poisonous.

Soil Compaction Recovery – Step 3

Step 3 of my soil compaction recovery plan may seem like a repeat of step 2. Not really. I think it is important to acknowledge nature’s part in the process. Sure, I prepared the soil and planted the seeds, but while I was traveling in the fall, celebrating Thanksgiving and Christmas, watching weird plant movies in January, working several major deadlines at work, and studying trees during the spring semester at Merritt College, nature was quietly doing the real work.

Cover crop in November.

I checked progress over time, in between rain storms and when it was light enough (difficult when I was leaving for and arriving from work in the dark). Each time I was thrilled at the progress, and excited to see a whole different palette of plants grow. I grew up in Southeastern Alaska, so I’m not exactly a city girl. But we didn’t have crops. What can I say - it’s a thrill! I rooted around the soil a bit also. I can’t say that I can report anything major, but the soil remained broken up and porous.

Cover crop in late May.

However, things did get out of hand between March and May, when I was steeped in travels to Southern California and Spokane, deadlines at work, and my Tree ID class. Basically, I did not plough under my crop in February as I should have (or in March, April, or May). I’m afraid Farmer David would be aghast, and lecture me that I’ll be pulling pea and vetch weeds for the next couple of years. But my real goal is improving the soil – that part may still be a success.

Soil Compaction Recovery – Step 1

About a year ago, I came up with a plan for recovering from soil compaction in a section of my garden. Recall we had removed an overgrown pine tree and then never planted anything to protect the healthy soil. The clay soil had become compacted, unsightly, and unusable. Part of my plan to fight soil compaction was to break up the soil and add organic material.

The soil has been broken up in a 10' x 10' section of the yard.

In September, I started to execute my plan. The patch of dirt is relatively small, so I broke up the soil manually with a shovel. I did wet the soil lightly with a hose, and then waited a day or so. Once the dirt had softened, I turned the soil, breaking the large chunks into smaller ones to increase pore space. If this had been a larger area, I would have borrowed a tool from the tool lending library to help with the process.

Organic chicken manure amendment.

Next, I added some organic matter, in this case chicken manure, and allowed it to set for a couple of weeks. I also created a small path of wood chips to the compost bin, on the border of the area I was restoring, as a buffer against further soil compaction.

A bark path to the compost helps prevent
additional soil compaction.

Succession

In Designing California Native Gardens, Alrie Middlebrook describes her use of succession (moving through a series of simple-to-complex planting stages, culminating in a final stage) in her garden designs. For slower growing shrubs and trees, she recommends planting them with ample spacing, so they can grow to their mature size without overcrowding, and filling in with shorter-lived grasses, bushes and flowers. She gives the example of establishing an oak woodland garden in a hot area of a yard. While the oaks were getting started, she planted chaparral species that could tolerate the heat, and then started to introduce oak woodland understory plants as the oaks became established and their watering needs changed.

I saw succession at work while growing up in Southeast Alaska. A marker at the Mendenhall Glacier Visitor Center indicates the location from which the glacier had receded only a hundred years before. The glacier left behind barren, ground bedrock; over time moisture- and air-borne soil particles began to accumulate. Early plant species—such as fireweed, grasses, and alders—moved in. Alders can live in poor soil, and fix nitrogen, so they contribute to building the soil. Over time, spruce trees replaced alders – shading them out. A new understory of shade-tolerant plants emerged, such as fern, horse tails, moss, devil’s club, and Indian rhubarb. During this process, organic material accumulates and decomposes, further enriching and developing the soil.

Succession at work - this peninsula at Mendenhall Glacier used
to be barren rock just several decades ago.

The key ideas for using succession in the home garden are: (1) enrich the soil naturally, and (2) design an attractive short-term garden that allows slow growing trees and shrubs the space and time they need to get established.

Caring for Oaks

In nature, oak trees can live 350 to 500 years, well over the human lifespan. They grow through several stages – seedling, juvenile (2-5 years), adolescence (15 – 50 years), maturity (50 – 200 years), and senescence (200 plus years) – as they form their particular shape and character. Oak trees require space and have particular soil, watering, and nutrition requirements. Caring for oaks in the garden requires basic understanding of these needs, and a vision that extends beyond the gardener’s own lifespan:

A Quercus agrifolia seedling. This acorn was most likely
"planted" by a squirrel in preparation for winter.

• Seedlings require lots of water to become established. This may mean deep, weekly watering (15 gallons) for the first two years.
• Once established, oaks require much less watering. Many California native oak species are adapted to the summer-dry climate. After three years, they should not be watered in summer, except during extreme or prolonged drought.
• Oaks establish a deep tap root, as well as many shallow feeder roots at the surface within the drip line. These feeder roots should not receive any direct water, to avoid root rot. This means clearing away lawn or plants under the oak.
• If you do plant under an oak, select species with compatible water needs, preferably within its natural plant community.
• Protect oak roots during construction projects. Problems may manifest in an oak tree long after the project is completed if roots are damaged during construction.
• Mulch with the oak’s own organic matter (fallen leaves, acorns, and catkins), up to, but not touching, the trunk. Do not use other fertilizers.
• Prune toward the end of summer, when the weather is still dry, since oaks seem to be susceptible to water borne pathogens. Work with an arborist to develop a strong limb structure through pruning, while the tree is young.

Urban oaks face continual menace from damage, overwatering, space constraints, improper pruning, and disease. With some care, they can live into maturity, giving pleasure to people, and providing habitat for birds, insects, and wild life.

Fighting Soil Compaction

I have done some research into how to fight soil compaction, so I can come up with a game plan for restoring my compacted soil to healthy, productive soil. The primary thing I learned is that it is difficult to restore compacted soil, and the best solution is prevention. Great. But there are some guidelines and principles I can use:

• Break up the ground to start the process of adding pore space.
• Don’t work with wet compacted clay soil. The molecules can slip into even more compacted configurations. Let the soil dry out a bit.
• Add organic matter, such as compost, to the soil. Work the organic matter into the top few inches of soil. Organic matter breaks down quickly, so this is an iterative process of adding organic matter, working it in to the soil, letting it break down.
• Use mulch to cover soil if you do not plan to plant immediately.
• Keep people from walking over the soil, to prevent further compaction. Ideally provide paths where needed.
• Use a cover crop to protect the soil, and even develop it. In agriculture, farmers plant a cover crop, such as alfalfa, between other crops. A crop like alfalfa, a legume from the Fabaceae (formerly Leguminosae) family, can actually grow in poor soil, and fix nitrogen in the soil to enrich it. Cut the foliage at the end of the growing season, but leave the roots in the soil to improve aeration.

Agriculture uses cover crops to hold the soil in place,
 and enrich the soil. The same principle can work
in the personal garden.

I learned a lot about soil from the Soil Management class I took from Dr. Ed Brennan, and from a soil compaction project I worked on with team members in the Arboriculture class I took from Jocelyn Cohen, both at Merritt College. Stay tuned to see if this research and knowledge pays off!

Soil Compaction

Healthy soil is a blend of decomposed minerals, organic material, microorganisms, and pore space, which provides oxygen. Soil characteristics include color, structure, and texture. Soil color is influenced by the parent mineral, for example, a red soil may indicate iron. Soil structure describes how the soil arranges itself in aggregates, for example, clay soil dries in hard, dry clumps. Soil texture is the composition of sand, silt, and clay, for example, sandy soil drains quickly, where clay soil holds on to water. Healthy soil should be teaming with life to provide a home for trees, shrubs, and plants.

Soil can become compacted, which means pore space is reduced or eliminated. This may happen purposefully during construction of a house, where soil is compacted to stabilize the soil. This can also happen when raindrops strike bare ground over a period of time, ground is used as a path or road, or ground is left fallow with nothing growing in it. When soil becomes compacted, water runs off and does not penetrate the soil; roots cannot move through the soil as easily; and the water and oxygen exchange between soil and roots cannot take place. The microorganisms that are required for healthy soil cannot live in this condition. The soil becomes unusable.

Soil has become compacted in a section of the yard
where a tree was removed.

My garden has an area where the soil has become compacted. We took out a pine tree several years ago that had grown out of scale for the property, and was growing close enough to the house to become a fire hazard. We cleared out the tree roots, but never replanted anything in the area. The soil has become compacted, with nothing growing on it except for a colony of moss which revives during the winter rains. The rest of the year, the soil is like a clay brick. I’d like to restore this patch of ground to life.