Soil Science

Soil Analyses at the Crescent Farm at the Arboretum

By Lynn Fang, MS
October 2016

Soil Health Indicators

The Crescent Farm utilizes several methods of building healthy soil that minimizes inputs (such as water, fertilizer, pesticides) for ecological organic gardening and horticulture. Some of these methods include lasagna mulching and hugelkulture. While these methods have been practiced for centuries, there is little documentation on their effects on soil health indicators.

We took soil samples under lasagna mulching, bark mulch, and hugelkulture and analyzed them for their soil health indicators, including available water capacity, organic matter content, aggregate stability (the strength of the soil structure), soil protein index, soil respiration, active carbon, soil pH, and soil minerals. We compared these to a soil that was not amended with any kind of mulch or organic matter.

Results of these tests are shown in the graphs in Figure 1 and 2.

Standard soil testing typically only assesses soil mineral content. However, soil mineral content only reflects the chemistry of the soil, and does not provide a picture of the biological and physical health of the soil. Soil is much more than its mineral content. It is a matrix of life that constitutes the integration of physical structure, biological life and ecological relationships, as well as chemical interactions. Observing and nurturing the integrated, overall health of the soil is the key to growing plants and food without petrochemical fertilizers or pesticides. Ecology is the key here – ecology looks at the interactions of all organisms with their environment, integrating disciplines of biology, geography, physics, and chemistry. Healthy soil ecology translates to a healthy plant ecosystem, minimizing and even eliminating the need for synthetic inputs. Thus, we chose to assess these soil building processes not only for their mineral content, but also for their biological and physical characteristics. Because soil is much more than its mineral content, and is a structural matrix, it is a major reservoir for storing water.

Available Water Capacity is the maximum amount of plant available water a soil can provide, an indicator of how much water the soil can retain and make available to plants. Poor quality soils are not able to infiltrate or hold onto water for very long, causing most of the water that falls onto its surface to run off into storm drains and ultimately into the ocean, failing to adequately recharge groundwater. Whereas, high quality soils infiltrate water quickly and hold onto large volumes of water, before draining back into groundwater. This is an extremely significant property in times of drought, when we need to reduce our water use. The more we can maximize the available water capacity of our soils, the less we will need to irrigate, and the more we will recharge our groundwater. This allows plants grown in these soils to be more resilient in times of extreme drought or climate change.

Organic Matter content relates to soil biological life, sources of plant available nutrients, available water capacity, and aggregate stability. Organic matter refers to any plant or animal matter in decay, and is an estimate of the total organic carbon pool in the soil, including both active and stable types of carbon. Organic matter is the primary food source for the soil food web – the intricate web of bacteria, fungi, protozoa, nematodes, microarthropods, insects, worms, and other small animals. The soil food web transforms organic matter into plant available nutrients, a process known as mineralization. In addition to generating plant available nutrients, the soil food web also transforms organic matter into humus, complex chains of carbon, hydrogen, and oxygen that cannot be further decomposed. Humus provides many benefits to the soil, such as carbon sequestration, stabilizing heavy metals and pollutants, providing habitat for microbial life, and stores plant available nutrients that can be released slowly over time.

Organic matter encourages microbial life to flourish, which assist in creating aggregates in soil. Soil consists of various mineral particles. Microbes secrete an enzyme that glues these mineral particles together into small clumps known as aggregates. These aggregates act as a sponge to absorb and hold onto water, and also help the soil to become more stable and resilient to disturbances such as rain, wind, or tillage.

Aggregate Stability assesses how well the soil holds up to rainfall. Soils with poor aggregate stability will fall apart and erode easily under rainfall. Intensive cultivation of the soil without restoring its life is one of the reasons why we are losing topsoil every year, rather than building more of it. Loss of topsoil is the reason why civilizations have collapsed in human history. Strengthening soil aggregate stability allows the integrity of the soil structure to remain intact under heavy rainfall, wind, or other disturbance. This is particularly important in our climate and the current drought. Though we are in a drought and do not often receive rainfall, climate trends predict increasing amounts of monsoonal rains, especially as a result of global warming. In order to preserve the soil we do have and that we do create, it is important to ensure that it will retain its structural integrity under extreme weather events.

Soil Protein Index is the original form of nitrogen – microbes consume soil proteins and excrete plant available forms of nitrogen. Greater amounts of organic matter provide greater amounts of soil protein. Nitrogen is primarily found in protein, thus this is a good indicator for how much plant available nitrogen could be generated.

Soil Respiration is an estimate of microbial activity in the soil. Microbes breathe in oxygen and exhale carbon dioxide.

Active Carbon is an estimate of “labile” carbon, or carbon sources that are easily decomposed and transformed, such as fresh plant residues and living organisms. This is an active source of nutrition and a major food source for soil microbes. In contrast, stable carbon sources are not readily decomposed, and include large wood sources, as well as humus, which is the end product of decomposition.

Soil Mineral Content in ecologically managed soils is related to organic matter and microbial activity.

Soil Building Processes

Lasagna Mulching is the process of layering cardboard, green waste, and mulch on top of the soil. The cardboard blocks weed growth, the green waste provides nitrogenous organic matter, and the mulch provides sturdy carbon-rich organic matter that also infiltrates water quickly. The area sampled is irrigated several times a week.

Bark Mulching was used experimentally. A layer of bark pieces were set on top of a layer of cardboard. Much like our own skin, tree bark is unique in that it is the interface between the external and internal worlds of the tree, keeping out pathogenic organisms and filtering in necessities such as moisture and light. Tree bark typically consists of many grooves and edges, providing habitat for microbial colonization, such as fungi and lichen. The interior of the tree is mostly filled with dead cells that provide structure and passageways for circulating water and nutrients. The bark, however, is alive, actively working to feed and protect the tree. Thus, bark mulch may act differently on soil than undifferentiated wood chips. The area sample is not actively irrigated regularly, though it is near a spigot that may leak water into the bark mulch area regularly. It is also right next to an in-ground hugel trench that collects runoff from the nearby road.

Hugelkulture is an old practice of burying logs, branches and twigs, green waste, compost, and mulch, under a mound of soil. The logs provide several benefits – they decay gradually over time, providing a consistent source of long-term nutrients for plants, and because their circulatory pathways are in tact, they can hold onto and transport moisture. The branches also assist in acting like a sponge where water can be stored and accessed during times of drought. The logs and branches also provide aeration, assisting in the decomposition of the different layers of organic matter in the mound. Their unique mound shape maximizes surface area for planting, and also creates microclimates of full sun and partial sun for plants that have different light requirements.

Figure 1. Soil Health Indicators of Soil Building Processes - Lasagna Mulch, Bark Mulch, and Hugelkulture. (A) Available Water Capacity, (B) Organic Matter, (C) Aggregate Stability, (D) Soil Protein Index, (E) Soil Respiration, (F) Active Carbon

Figure 2. Soil Mineral Content of Soil Building Processes - Lasagna Mulch, Bark Mulch, and Hugelkulture. (A) Extractable Phosphorus, (B) Extractable Potassium, (C) Magnesium, (D) Iron, (E) Manganese, (F) Zinc

Results

Available Water Capacity

There is no compelling difference in available water capacity for the unamended soil and mulched soils. The bark mulch is the only process that showed highly enhanced available water capacity. This reflects the unique properties of tree bark that sets it apart from regular, undifferentiated wood chips.

Organic Matter %

Here, there is a major difference in organic matter between the unamended soil and mulched soils. The lasagna mulch is highest in organic matter, followed by the hugelkulture, and the bark mulch. This reflects the large quantity of green waste layered into the lasagna mulch. It is also interesting that the bark mulch consists only of cardboard and bark, with no other green waste, and yet displays as much organic matter as the hugelkulture, which was layered with abundant green waste.

Aggregate Stability %

There is a noticeable difference in aggregate stability between the unamended soil and mulched soils. The bark mulch showed the greatest level of aggregate stability, followed by hugelkulture, and the lasagna mulch. It is possible that the microbial community supported by the bark mulch is able to generate stronger aggregates.

ACE Soil Protein Index

There is an appreciable difference in soil protein content between the unamended soil and mulched soils. The lasagna mulch had the greatest amount of soil protein, followed by hugelkulture and bark mulch. This may be related to the amount of green waste that was originally added. The more green waste, the greater the soil protein content.

Soil Respiration

Lasagna mulch had the greatest rate of soil respiration, followed by bark mulch and hugelkulture. This indicates that lasagna mulch has the greatest amount of microbial activity and is more actively and quickly transforming organic matter into nutrients and humus.

Active Carbon

Lasagna and bark mulch had the greatest reservoirs of active carbon, followed by hugelkulture. This is consistent with the fact that hugelkulture primarily consists of more stable forms of carbon, such as logs and branches. Interestingly, bark mulch is comparable to the lasagna mulch. Bark would be expected to be less easily decomposed than lasanga mulch.

Soil Mineral Content

Both Phosphorus and Potassium showed similar patterns with lasagna mulch having the greatest concentration, followed by hugelkulture, and the bark mulch having a similar amount as the unamended soil.

Discussion

There is a consistent pattern between soil organic matter content and soil respiration – the greater % organic matter in lasagna mulch is reflected in its higher respiration rate. This pattern also extends to the soil protein index – greater protein content may reflect greater organic matter content.

Higher organic matter and respiration rate do not necessarily translate to greater aggregate stability – general microbial abundance is not necessarily correlated with aggregate stability. This indicates that there may be specific microbial communities, that perhaps secrete specific enzymes or compounds, which are important to aggregate stability.

With climate change, soil has the potential to sequester carbon from the atmosphere. The soil carbon cycle is dependent on a number of factors, including organic matter content, active carbon, and respiration rate. The more organic matter in the soil, the greater the pool of organic carbon. However, a greater respiration rate transpires greater amounts of carbon into the atmosphere.

Overall, the bark mulch provides the greatest amount of available water capacity and aggregate stability, pointing to enhanced soil structure, while minimizing the amount of carbon transpired into the atmosphere. It also has a comparable amount of active carbon as the lasagna mulch, yet it does not provide abundant minerals. The bark mulch is likely most excellent for building soil structure and potentially may sequester more carbon than other forms of mulching, but it would not provide enough minerals for growing vegetables. It may be sufficient for growing California natives and other low water Mediterranean horticulture plants.

Potentially, a lasagna mulch could be developed that primarily uses bark as the mulch, and this could provide the most optimal soil ecosystem for growing vegetables – improved soil structure with optimal available water capacity, aggregate stability, and mineral content.

Both the lasagna mulch and hugelkulture provide abundant minerals suitable for growing vegetables.