This blog article is for readers who may have heard of subsoil, but don't really know what it is, or how it may or may not affect their gardening efforts. 

What is subsoil?

There are several layers (also known as “horizons” to soil scientists) that can be found when we dig deeper and deeper down into the soil. We can imagine all the individual layers of a soil stacked one on top of the other like a layer cake, this is called the soil profile. The surface soil is the uppermost layer of the soil profile, and the one we are most familiar with, because this is where most of the action takes place. Soil mixing with tillage, compost and fertilizer application, irrigation, plant root growth, and animal activity (including microbes) are mainly concentrated within the soil's surface layer. Additionally, decomposition of plant and leaf litter occurs most rapidly in the surface soil, this eventually leads to the formation of new soil organic matter. In comparison, the subsurface soil, or subsoil, is composed of one or more soil layers that lie below the influence of surface soil activities. There is not a consistent depth at which every surface soil layer changes into the subsoil layer(s), rather the subsoil occurs at a different depth from place to place depending on numerous factors, including some of the factors mentioned previously. This is the reason why it is difficult to determine where the subsoil layer begins in the soil profile.  

What does subsoil contain?

Subsoil is found deeper in the soil profile and does not directly experience a lot of the processes that occur in surface soil. As a result, the subsoil has less soil organic matter, less insect and animal activity, slower air and water movement, and a slower rate of decomposition compared to the surface soil layer. However, the subsoil underlies the surface soil and is indirectly influenced by the activities that occur at or near the soil surface. Therefore, the subsoil layer(s) has the excess fertilizer nutrients, herbicides, pesticides, and other dissolved salts that were lost from the surface soil due to heavy rainfall or irrigation. As surface soil begins to erode and age, the water moving into the subsoil can also have a lot of clay that accumulates in distinct regions of the subsoil.

What does subsoil look like? 

It is possible to identify subsoil by recording when and where you notice distinct changes in color, texture, and/or structure of the soil. The surface soil is often enriched in organic matter, giving it a dark appearance, typically dark brown or black. The movement of smaller soil particles from the surface soil into the subsoil increases clay content. The subsoil also lacks many of the rooting channels and biological pores that help build and support soil structure due to the limited presence of plant roots and animal activity. Therefore, as you dig deeper in the soil profile, if you see the soil color begin to fade from a dark brown or black to lighter shades of gray or brown, you notice it becomes harder to dig deeper into the soil, and/or you notice the soil is no longer loose and crumbly but becomes denser and lacks plant roots and worm channels, it is very likely you have reached the subsoil layer.    

What is subsoil used for? 

In general, the subsoil is a less suitable medium for plant growth compared to surface soil because of some of the factors mentioned previously. There are properties of subsoil however that make it suitable for other uses such as a source of “fill soil” for “cut-and-fill” construction operations, as a source of clay for building materials, and as an absorption layer for on-site wastewater disposal.

What can home gardeners do to keep their subsoil in great shape year after year?

Healthy soils are effectively balanced among the chemical, physical, and biological processes that allow soil to function properly and achieve a desired outcome, such as being an optimal medium for plant growth. Therefore, to build or support healthy subsoil, it is important to consider each of these processes, keeping in mind that what occurs in the surface soil layer can, at some point, be reflected by what is observed in the subsoil. With this in mind, to encourage soil health in the subsoil it is important to recognize that certain plant nutrients move with rainfall or irrigation water, such as nitrogen, and may be enriched in the subsoil. Effectively balancing plant nutrient fertilization to avoid excesses can limit the amount of plant nutrients transferred from the surface to the subsoil. The subsoil often has a poorly developed structure due to the lack of plant rooting channels and animal activity (including microbes). Growing a diverse group of plants that have distinctly different rooting depths, ranging from shallow- to deep-rooted, can encourage the formation of rooting channels that extend into the subsoil. The burrowing activity of earthworms also creates channels that deep-rooted plants can easily use and follow into the subsoil. Additionally, the presence of actively growing plant roots acts as a food source in the subsoil that stimulates microbes. The physical structure of subsoil can be improved with deep-rooted annual or perennial plants. As soil structure improves and more of these biological pore spaces are created, the rate of water and air movement into the subsoil layer will increase, helping sustain the balance among chemical, physical, and biological processes.           

What is the substratum layer of soil? Does that layer affect gardening at all, and if so how?

Stratum is a term that is commonly used to identify a distinct layer of rock or soil. Substratum then is a term used to indicate the relative underlying position of a distinct rock layer within stratified rock formations or a distinct layer of soil that lies below the surface soil. The substratum layer of soil has some of the same properties that are common to the subsoil. Although, many of the chemical, physical, and biological processes are further diminished in the substratum layer compared to the subsoil. Specifically, growing roots are not likely to be found in the substratum, so the properties of the substratum layer are unlikely to directly influence plant growth. The limited amount of water and air in the substratum layer will however help breakdown larger solid rock fragments, forming a loose and very coarse structure that closely resembles the mineral composition of the underlying layer(s) of the soil profile.      

What is the bedrock layer? Does that layer affect gardening at all, and if so how?

Bedrock is the bottom layer of the soil profile layer cake. The bedrock layer consists of solid rock that has not yet been exposed to the chemical, physical, and biological processes of the surface soil and subsoil. In some places, bedrock is the foundation from which the overlying soil layers developed, while in other places, the bedrock layer may have become “buried” by windblown sand or sediment. Bedrock does not directly influence plant growth, but it can determine the type of clay minerals found in different layers of the soil profile. 

Dr. Anthony Fulford is the Area Nutrient Management and Soil Quality Advisor for Stanislaus, Merced, and San Joaquin Counties.

UCCE Stanislaus County Master Gardeners Rhonda Allen and Denise Godbout-Avant are looking forward to sharing their enthusiasm and knowledge about monarch butterflies with you!

You'll learn about butterflies in general, about monarchs, their life cycle, their incredible migration, the importance of nectar plants and milkweed to their survival, and how you can help these endangered butterflies. Free milkweed seed packets will be given to attendees! Use the link below to register.

Date: Saturday, October 15, 2022
Time: 9:00 am – 10:30 am
Where: Stanislaus Agricultural Center, 3800 Cornucopia Way, Harvest Hall Rooms D&E.
Register: http://ucanr.edu/monarchs/2022

If you see this post and can't or forget to register, please come anyway!

Starting around mid-August through September, both the Western Monarch and Eastern Monarch butterfly populations (Danaus plexippus) begin their long journey south from southern Canada and northern USA to their overwintering sites in coastal California and Mexico. These magnificent, colorful creatures are the world's most recognized and beloved butterflies.  Yet, they are increasingly in danger of becoming extinct.

Life Cycle

Like all butterflies, Monarch butterflies have four life stages: 

• Egg: The female Monarch lays her eggs on their sole host plant, the milkweed (Asclepias). She generally lays one egg per plant.  Each egg is about the size of a grain of salt. 
• Larva (Caterpillar):  After 3 – 5 days, the Monarch egg hatches to a larva, also called a caterpillar, eating only milkweed leaves. They go through five instar stages over a period of 10 – 14 days as they grow from 1/16^th of an inch to about two inches, molting their exoskeleton at each stage.
• Pupa: The caterpillar finds a protected place to develop its chrysalis for the pupal stage. During the next 11-15 days the pupa will change to an adult by liquifying its body while inside the chrysalis, ultimately emerging to the adult butterfly.
• Adult: In the final hours before emergence, the chrysalis becomes translucent, a crack will appear, with the Monarch butterfly freeing itself from the case. Hanging from the now-empty chrysalis case, it will spend the next few hours pumping fluid into its wings until they are firm enough to fly. Eventually it will take flight and start seeking out nectar for its 1^st meal. Adult Monarch butterflies feed on flowers, which makes them pollinators. The nectar provides energy for flight, mating, and migration.

Milkweed

Milkweeds are herbaceous, perennial, flowering plants, named for their latex, a milky substance, which is exuded when the plant is damaged. It is potentially toxic to humans and many other species.  Monarchs have evolved the ability to store the toxins. Caterpillars feeding on milkweed retain the poison as chrysalids and adults. Birds and other Monarch predators dislike the toxins and learn to avoid them. (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19440).

Migration

The Monarch migration is extraordinary with none quite like it in the butterfly world. A Monarch butterfly begins an epic one-way journey south up to 2,800 miles to a specific place where they have never been to before, where their great-grandparent spent the previous winter. It remains largely a mystery how successive generations know the route and where to spend the winter months.

No single Monarch makes the entire round trip journey north and south. There are four to five generations each year. Three to four generations make the journey north in the spring and summer months, with each generation moving further north.  Each northward generation consists of adults feeding on nectar, mating, females laying eggs on milkweed, the parents dying, their offspring hatching from eggs, going through the larval and pupal stages, ultimately becoming adults, and continuing the cycle. The adults in these generations live two to six weeks.

Late in summer or early fall, a final generation emerges.  Triggered by changes in temperature and sunlight, this generation will migrate south. Known as the “Methuselah” generation (after the biblical patriarch said to have lived 969 years), they can live up to 6 – 8 months. They do not emerge as sexually mature butterflies, being in a “sexual diapause,” so their energies can be focused on developing flight muscles and storing lipids for their long journey south and surviving the winter months.

Come spring, with warming temperatures and longer days, these butterflies will become sexually mature, feed on nectar, mate, and start moving northward, laying eggs, which will hatch to continue the annual migration cycle.

You may see migrating Monarchs this fall in your garden feeding on nectar flowers, resting on flat flowers or rocks, or drinking from water sources.  Admire them, but leave them be, since they still have some distance to go to reach their wintering grounds. In California, their wintering spots are along coastal areas from Monterey area (Pacific Grove) to San Diego.

Monarchs in Trouble

On July 21 of this year, the International Union for Conservation of Nature (IUCN) announced the Monarch butterfly has been put on its "Red List of Threatened Species as Endangered” due to habitat destruction, climate change, and pesticides, with the primary reason being reduction in milkweed plants that are so vital to their survival.

In temperate areas like the Central Valley, the tropical milkweed plant (Asclepias currasavica) does not go dormant. A parasite that lives on the plant is ingested by developing caterpillars and is linked lower migration success and reductions in lifespan. Choose milkweed species that goes through winter dormancy such as narrowleaf milkweed and showy milkweed (A. fasicularis and speciosa.) 

The IUCN announcement states: "The western population is at greatest risk of extinction, having declined by an estimated 99.9%, from as many as 10 million to 1,914 butterflies between the 1980s and 2021.” In addition, "The larger eastern population also shrunk by 84% from 1996 to 2014. Concern remains as to whether enough butterflies survive to maintain the populations and prevent extinction."

Sign up for our Free Class!

On Saturday, October 15, UCCE Stanislaus County Master Gardeners Rhonda Allen and Denise Godbout-Avant will give a talk on these marvelous butterflies that so many love. Learn about butterflies in general, more about Monarchs, their life cycle and their incredible migration, the importance of nectar plants and milkweed to their survival, and how you can help them. Free milkweed seed packets will be given to attendees.  Use the link below to register.

Date: Saturday, October 15, 2022
Time: 9:00 am – 10:30 am
Where: Stanislaus Agricultural Center, 1800 Cornucopia Way, Harvest Hall 
Register: http://ucanr.edu/monarchs/2022

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Resources:

• Art Shapiro's Butterfly Site   https://butterfly.ucdavis.edu/butterflies 
• Butterflies in Your Garden  https://ucanr.edu/sites/CEStanislausCo/files/345791.pdf
• Xerces Society - Pollinator Plants: California  https://xerces.org/publications/plant-lists/pollinator-plants-California
• UC Davis Arboretum – Larval Hosts for Butterflies https://arboretum.ucdavis.edu/blog/larval-host-plants-butterflies
• California Native Plant Society – Native Planting Guides https://www.cnps.org/gardening/choosing-your-plants/native-planting-guides
• Tropical Milkweed - a no grow  https://xerces.org/blog/tropical-milkweed-a-no-grow 
• Calscape CA native plants     https://www.calscape.org/
• UC ANR Bug Squad Blog     https://ucanr.edu/blogs/bugsquad/
• International Union for Conservation of Nature (IUCN)  https://www.iucn.org/
• The Monarch: Saving Our Most Beloved Butterfly, by Kylee Baumle, St. Lynn's PressMilkweed Poisoning: https://www.poison.org/articles/milkweed-can-cause-serious-poisoning-204

Curious about composting but not sure where to start? Tried composting in the past but gave up for various reasons? In this free class from the UC Cooperative Extension Stanislaus County Master Gardeners, you will learn how to turn kitchen and garden waste into "gold" that you can use in your garden. Adding compost to your houseplant pots, landscape, or garden is extremely beneficial. We will review common problems and their solutions. The main thing to remember that no matter what you do, "Compost happens!" One lucky person from each class will take home a free composter!

Stanislaus County Library Composting Classes

Tuesday, September 6, 2022 at 6:00 p.m. – Salida Library
Saturday, September 10, 2022 at 2:00 p.m. – Riverbank Library
Monday, September 12, 2022 at 6:00 p.m. – Ceres Library
Tuesday, September 20, 2022 at 6:00 p.m. – Oakdale Library
Monday, September 26, 2022 at 6:15 p.m. – Modesto Library
Wednesday, September 28, 2022 at 6:00 p.m. – Turlock Library

Stanislaus County Agricultural Center, Harvest Hall Composting Class

Tuesday, September 27, 2022 at 6:00 p.m. rooms D&E

Turlock Community Gardens

We taught composting at this location in spring, so this month we are offering vermicomposting, the fun and easy way to compost kitchen scraps using red wiggler worms. Children are welcome!

Saturday, September 17, 2022 at 9:00 a.m.

Patterson Library

We are offering a Fall Vegetable Gardening Class at the Patterson Library, in case you missed this class last month at      other locations. Class is Wednesday, September 14, 2022 at 2:00 p.m. Composting will be taught at the Patterson Library on October 12 at 2:00 p.m.

Never miss a class, bookmark our online calendar: https://ucanr.edu/sites/stancountymg/Calendar/