Lithops soil

Soil and Germination:

This post is some of my research around soil water, and soil structure. I don't guarantee accuracy especially with some terminology I am probably using wrong.

With greenhouse germination there are several routes:

1. Geoponics
• Growing plants in substrate (Peat, Coco noir, pumice, clay)
2. Hydroponics
• Growing plants in water
• Flooding and empty cycles
• Trickle flow
3. Aeroponics
• Growing plants suspended in air, spraying with water

When thinking about the needs of your plants you should know a bit of soil science.

1. Hysteresis - how a soil wets and drys
2. Stratification, do I put gravel at the bottom of my pot
3. Nutrients in a soil and soil pH
4. Organic, do I even need it?

Soil science is not my forte, but I've tried to research this thoroughly. If you see mistakes, especially in terminology feel free to comment or email me with suggestions (no time for sass!)

1. Hysteresis

You have a cup of water with a pot in it. The water level in the cup we'll call zero cm of water (in black below, we'll measure the level of water in the soil/substrate from this level.

On the below left a course soil, mostly gravel (>2 mm), on the right a finer soil with sand ( 0.05 - 2 mm). Water is effectively pulled through a soil by it's adhesion to substrate and it's held back by cohesion to itself.

If you drop a bead of water on a surface, it can stay as a bead as long as the water surface tension, i.e. cohesion, is greater than the water's adhesion to substrate. If you drop a bead of water on a wood table it stays a little dew drop because of cohesion. If you drop a bead of water on cotton fabric, the water adheres to the fine network of fibres and spreads out.

In the illustration below we see capilliary action as a balance between adhesion and cohesion. Gravel has bigger pores, bigger spaces between the gravel particles which allows a balance of cohesion to adhesion such that there is only a small capillary action drawing water up. In sand there is so much negatively charged surface area the water clings to more particles through adhesion and overcomes its surface tension, i.e. cohesion, getting drawn up into the pot higher.

What does this mean in developing a soil to grow lithops? Well in my case I thought it best to test out different grades of porous and non porous substrate to understand the wetting and drying curves of a substrate.

Black (UV) light pen

Fluorescence under UV pen light.

The first step is trying to find a way to measure hydrostatic pressure of a soil if you can't see how high water is drawn up a black soil.

My solution was to dilute 4 yellow highlighter ink's in water, and to use a UV pen light to make the soils fluoresce to see how high they were drawn up black lava rock of various grades. I took course 3 cm grade lava rock and put it through a Blake jaw crusher set to a roughly 2-3 mm setting. The Blake crusher breaks rock into a grade that can pass through it's 2-3 mm aperture.  I sifted both crushed lava rock and crushed granite into 2 mm or greater (i.e. gravel), 1- 2 mm, 0.5 - 1 mm, and 0.3 - 0.5 mm sands. I tested these pure mixes and combinations to find out the hydrostatic pressure in a 3x3x18 cm soil column.

Five soil samples in 3x3x18 cm soil columns being wicked in water with fluorescent dye.
Green rubber band noting fluid level. On the far left 2 mm grade granite, centre 0.5 mm lava rock, right 2 mm lava rock. 

Equation 1: Forces involved in pore water pressure determination.

In the above equation we can see that gravity acts on the fluid, we control for the fluid being water by using the density. The units of pore water / hydrostatic pressure are in cm, using cm for height we can see that with a constant density of water, and a constant gravity height is the reactive variable which determines pore water pressure. 

Table 1: Experimental Hydrostatic pressure

A fairly imprecise method, but I was surprised at the results, first of all without research I thought the water would not be drawn very high and limited my soil height to ~16 cm, 2cm under water. Several times fluorescence was seen at the soil surface indicating the recorded value is smaller than the correct value if water was allowed to rise through more substrate. Large grade granite was drawing water to the surface of the soil column, as was fine grade and a mix of fine grade granite and basalt. 

Granite is composed of hydrous aluminum phyllosilicates, that degrade into negatively charged colloids, and when these fine particles are massed together we know it as clay. These particles are negatively charged and this charge profile may lead to increased forces of adherence. 

On the other hand, porous basalt had very low hydrostatic pressures. This may be due to the balance of adhesion to cohesion being lower in the column due to the very high surface area of the porous medium. 

To summarise soils exert a pressure on water by adhesion and by capillary action. If the soil has a high hydrostatic pressure -- like peat bogs -- water is drawn closer to the soil surface. If a soil has a low hydrostatic pressure -- like sand dunes -- the soil can only draw water slightly upwards to the surface from the water table. We understand this draw and push as "hysteresis".

Hysteresis and Uncertainty in Soil Water-Retention Curve Parameters, William J. Likos (2014)

Soils can become fully saturated if the hydrostatic ( suction ) pressure -- measured in cm water above water table -- is strong enough that air is pushed out by higher pressure water. When soil is drying water exerts enough pressure that there has to be suction into the soil to allow air entry. As a soil begins to wet, starting at the top left of the graph above, the pressure pushing it to wet becomes less and less as it reaches its maximum wetness, i.e. high volumetric water content. At one key point air bubbles enter the equation and exert a pressure on the rising water. The water must have a high enough adhesion pressure sucking it upwards to expel air. Conversely as a soil drys there is a key point where air is drawn into a soil overcoming adhesion of the water to the substrate and cohesion to itself.

As in the graph above the air-expulsion pressure is separate from air entry pressure resulting in unique wetting and drying curves of substrates/soils.

Flip the graph above and you get this graph showing how gravel and sand should react.

A study of infiltration on three sand capillary barriers Hong Yang, H. Rahardjo, E.C. Leong, and D.G. Fredlund. Can. Geotech. J. 41: 629–643 (2004)

So this paper in particular gives us some idea of elevation in a soil column that you'll have a specific volumetric water content, and an idea on how stratification works.

2. Stratification

Many garden centres suggest putting rocks at the bottom of your pot. Why? If you're wicking water from the bottom of the pot, then the moisture level at the top of the course material is as high as the fine material can get. See this fine sand over medium grade sand:

A study of infiltration on three sand capillary barriers Hong Yang, H. Rahardjo, E.C. Leong, and D.G. Fredlund. Can. Geotech. J. 41: 629–643 (2004)

We see that there is a courser soil underneath a finer soil. The volumetric water content drops as you wick water upwards toward the interface between soils and then maxes out, but is drawn through capillary action to the surface.

What happens if you do what most people do and water from the top?

Well as you can see, the waters adhesion to the fine material is overcome by gravity and water accumulates in the fine material, more and more as the sum of draining substrate increases, until it reaches gravel where the water is quickly drained by gravity.

So if you wick water upwards you can control the maximum moisture that reaches your plants, the same with watering from the soil surface except you can expect higher volumetric water content in the surface soil than with wicking.

3. Nutrients and pH

Soils are a store of nutrients, but how does it work, and what does pH have to do with it?

Positively charged nutrients are adsorbed or released into water as the pH changes. Many soils have a cation exchange capacity which act as storage sites for nutrients when they are in high concentration, and act as a source of nutrients when nutrients are low in solution or in alkaline situations.

Negatively charged nutrients can interact with anion exchange capacity (AEC) of specific soils. Many soils only have a AEC at very low pH such as organic soils from peat. In these soils negatively charged nutrients are adsorbed to the soil at high pH and released at low pH.

As in the above two images you can imagine when we are making a soil we have to consider adding substrate that can utilizes cation and anion exchange sites to have a small buffer of nutrients available between fertilization. In testing my soils for hydrostatic pressure I used a porous basalt, as well as a non-porous granite. By combining these soils the different substrates should allow for additional synergy of CEC and AEC at one pH level. 

4. Organic 

With testing these mineral soils we have to wonder why adding organic is any benefit. Well when adding peat there are two benefits:

1. Hyaline cells - Peat moss has cells that can store 20x their weight in water, which adds a water sink in the soil
2. Organic soils have many organic carbon moieties that can adsorb cations and anions and resist decomposition. This can sink heavy metals and provide nutrients storage and release, as well as buffer pH.
3. Organic is known to provide structure and cohesion to loose soil, and act as a binding agent between mineral particles.

Summary:

Wicking is probably best for desert plants as it prevents too much moisture mid-pot in stratified soils as in gravity watering. By wicking we ensure appropriate soil aeration for plant health and fungal resistance. 

pH controls which nutrients can be adsorbed to the soil substrate. By having a mix of soil substrates we can allow both anion and cation exchange sites at one pH. 

Organic material can bind mineral particles together this gives more stability for roots, and sites for roots to draw nutrients from, but organics can store much more water than mineral soils, making fine root-level moisture control more challenging. 

At this point I have some ideas about an ideal soil for Lithops which I'll be explaining in my next blog post about Lithops Germination.

 Lithops aucampiae v aucampiae "Storm's Snowcap" C592A © Atreyu 2018

3 Month Seedling Cotyledons in 25% Compost (Bark, Chicken Waste, Municipal Compost) 75%  Washed Sand

Lithops biology

Living Stones: Growing Lithops 
Part I Biology: Introduction

Lithops, when a person first sees them, are ugly plants. But it's bizarre appearance and more bizarre desire to seemingly never want water, surviving in the wild for 20+ months between rain, is endearing. Lithops can complete an entire deciduous cycle of it's single two succulent leaves dying, and two new leaves growing to adult size without any external water. (Water recycling in leaves of Lithops (Aizoaceae). Otto et al. 2013). This unique adaptation, in addition to a contractile tap root which draws the drying plant into the soil, lends the plant to its common name "Living stones".

It was first seen by Europeans in 1811: "An picking up from the ground, what was supposedly a curiously shaped pebble, it proved to be a plant, and an additional new species to the numerous tribe of the Mesembryanthemem but in colour and appearance bore the closest resemblance to the stones between which it was growing" (Burchell 1821).

I was first introduced to a package of seeds of Lithops spp. while working at a greenhouse in secondary school. I was told of the difficulties people seemed to have germinating them, and though the idea of "living stones" was interesting, the thought of failing put me off these plants for several years.

Interest was sparked again this year via joining the succulent enthusiasts go-to-forum "reddit.com/r/succulents". As well, I was fortunate enough to acquire the excellent, and well researched volume by a prolific Lithops enthusiast and macro photographer Harald Jainta, and his extremely skilled partner Anja Jainta, which suggested the grassland species L. aucampiae and L. lesliei can tolerate living with grasses, which made me hopeful to experiment with growing them.

The first Lithops you may see might be labelled "Lithops", if you're lucky. Species are very rarely marked at garden centres, or hardware stores, and even experts seem stumped and have to resort to cutting the adult plants in half to speciate. Lithops are considered by most to be challenging to speciate. They have very limited morphological characteristics (seeds, capsules), and the flowers, a common morphological tool for taxonomists are nearly identical between species.

In 1922 the Lithops were placed in a larger group Mesembryanthemums (cf. Conophytum, Dinteranthus, Titanopsis). Even now these plants are affectionately referred to as mesembs. The initial speciation of the genus "Lithops" differentiated the genus based on flower colour (Xantholithops vs. Leucolithops viz. Schwantes 1957), and the presence of light conducting windows on the flat tops of the succulent leaves (Fenestratae vs. Afenestratae viz. Nel. 1946). Window pattern analysis ballooned the number of species to over 90. However Cole (1979) considered the windowing and leaf colour to be more of a selection pressure by herbivores, and Lithops to be limited to no more than 40 species.

Today it's believed that different species can appear nearly identical in leaf colour. Lithops that mimic endemic rocks/soil are more robust to predation. Recall the classic biology story of the absence of white moths in soot stained London, while black varieties had a survival benefit, and were well and good in London. A professor of African Languages, Desmond Cole, and partner Naureen Cole collected extensively and published many papers on Lithops, and attempted to resolve the taxonomy of Lithops. Robert Wallace (1988) did his Ph.D. dissertation on a systemic review of Lithops that avoided subjective pattern recognition on Lithops leaves.

Lithops karasmontana varieties (Kellner et al. 2011)

Lithops Biology:
Summary of Wallace 1988 Dissertation in regards to taxonomy

Lithops cross-section (Wallace, 1988)

TI: Colourations which can occur in various places, CL: Cleft, CP: thin walled water storage tissues with large central vacuole, PV: Primary vein, CHL: photosynthetic tissues, P: apical and axilliary meristem, R: Root

Lithops are composed of two fleshy succulent leaves which are fused or bifurcated, at the base. The stem cells, the meristem, is located above a single tap-root. "Windows" on the upper leaf surface, have thinner waxy cuticle than the rest of the plant, and the cell outer walls are thinner. These epidermal cells are convex at the air/plant interface which may help concentrate light deeper in the plant.

Populations found today are very cryptic, in that they are nearly identical to surrounding rocks. They are eaten by Springbok antelope, and grazing sheep or goats. Luckily, the position of the meristem at the base of the plant provides them the opportunity to grow new sets of leaves and even flower after their adult leaves are eaten.

In general the body is underground with only the flat leaf tops visible. They grow in gravelly or rocky medium derived from granite, sandstone, limestone, quartzite, gneiss, pegmatite, or schist, etc.  Plants that are covered by soil also lose less moisture to transpiration, and are known to be frost resistant in dry or porous soils.

Oxygen and carbon dioxide are exchanged through pairs of stomata guard cells which are on all skin tissue, but most densely concentrated near the base of the plant. Only one species is unique in regards to stomata in that it has three stomata guard cells, Lithops dorotheae. If the top of the leaves were entirely green, light would not penetrate very deeply. Lithops has adapted to dessication pressure by limiting the exposure to air, and developing its photosynthetic tissue in the underground portions adjacent to it's transparent windows. Nel described the plant as a mine shaft with light entering the mouth of the mine and lighting the interior (1946).

The main xylem and phloem tissues often curve from the base, around the central region into the top most central cleft, and downward. This may represent an evolutionary origin of the current morphology. Cross-sections of the plant suggest stem facing epidermal tissues are greatly reduced or adsorbed to facilitate light penetration to chloroplasts deep in the plant. In the evolution of Lithops, the proximal tissues could have fused, folded, and the surface then truncated to the current flat leaf surface.

The main contractile root is permanent and will pull a drying plant into the ground while secondary and tertiary roots die-back then rapidly regrow when moisture is present. This is similar to a convergent evolutionary strategy in cacti, which also have deciduous secondary roots with high salt concentrations to collect water by osmosis. Experiments in the early 20th century show that the entire plant, including old roots are nearly impervious to water. A plant weighing 23.95 g absobed 3.3 g water in 15 days. This same plant absorbed 7 g of water when a small cut was made at the base of the plant and immersed in water.

Lithops flower cross-section (Wallace, 1988)

S: Style, ST: Stamen, PT: Petal, N: Nectary, SE: Septal, I: Colouration and calcium oxalate raphides, R: Recepticle, PE: Pedicel, FS: False septum, P: Placenta, PS: Placental septum, TS: True septum.

Flowers are nearly identical between species, and have not been used as a morphological tool in speciation. Each plant produces only one flower each year. These flowers have an ovary with as few as 4 cavities, and as many as 8, with the number of septals matching the number of cavities. The ovaries cannot self-fertilize, and last most commonly between 3-10 days. Flowers have yellow petals, coloured by alkaloids (betaxanthin), or white flowers with yellow distal edges, or simply as white flowers. Exceptionally Lithops varruculosa has many other colours due to another pigment (betaxanthin and betacyanin) in the petals. Ovaries are not well studied in Lithops and are not currently used for taxonomy.

Pollen is most commonly longer on the polar axis, however in some species the pollen is near spherical, or even longer on the equatorial axis. All pollen is microverrucate, or slightly textured. Apertures are oblong, and in some species no perforations of the subsurface exine layer are seen, and more commonly microperforations are present in the subsurface of the pollen viewed through apertures.

Seed capsules retain seed until moistened, and shaped in such a way that raindrops will carry seeds down and away from the parent plant. Studies have shown seeds will drop within 20 cm of the parent 75% of the time, but have been observed to carry up to 75 cm from the parent plant. Seeds do not float, but populations have been seen in linear formations down gentle slope suggesting water dispersal during heavy rain events. As the Lithops grow in specialised environments long-distance dispersal would be less likely to result in germination than local dispersal where a population already exists. Three species are known to only one locality (i.e. L. werneri, L. viridis [200 m² only], L. helmutii) and critically endangered. As for the very wide geographic distribution of several species (i.e. L. lesliei, L. verruculosa) are not well explained, and speculated to be carried on avian feet, or on the hairs of herbavores muzzles or feet.

Lithops seed cross-section (Wallace, 1988)

RP: Rostral papillum, Ro: Rostrum, Ra: Radicle, Em: Embryo, CP: Cotyledon pair (first leaves), FR: Funicular remnant (umbilical), SC: Seed coat, En: Endosperm (energy store).

Lithops lesliei moistened seed © Atreyu 2018. 

Seed coat has torn mid-seed/endosperm as seed swells with moisture. Slight swelling noticeable to the rostrum.

Seeds range from 0.35 mm (i.e. L. verruculosa, L. marmorata) to 1.50 mm (Lithops lesliei), and germinate rapidly, with best results anecdotally reported after at least one year from production. The tearing of the seed coat is not used for taxonomy, however it is reported that some species carry the seed coat on the edge of the cotyledon (i.e. L. lesliei, L. pseudotruncalis).

Seedlings grow as a fused cotyledons, with a cleft in the centre of the fusion, or across the entire surface of the plant. Primary leaves most commonly take 6 - 12 months to emerge from the cotyledon, though in my own experience Lithops karasmontana primary leaves started to emerge after 12 - 14 weeks from germination.

Lithops karasmontana primary leaves emerging. © Atreyu 2018

Note: cotyledon appears slightly pink/red compared to emerging primary leaves.

After flowering is initiated at the apical meristem, leaves will cycle annually from growth initiated at the axilliary meristem, with one pair of leaves. Two pairs of leaves can grow if each axilliary meristem initiate. Some species will have only one "head" form with one axilliary meristem remaining inactive each year (i.e. monocephalic species, cf. Lithops gracilidelineata). As the old leaves age, water is transferred to the developing leaves, and has been documented in fluoroscopic studies. Old dead leaves form a protective film on the body of the new leaves, and act as a heat shield from incident sunlight, and to protect from abrasion to the rough soils.

Lithops Genetics

As summarized in a genetic analysis (Kellner et al. 2011), the Lithops subfamily evolved over a 3-8 million year period. Adaptations included cellular features such as wider water conducting cells in the xylem without perforations, and seed capsules that would only open when moist.

Similar to most mesebryanthemums, Lithops are diploid with 9 chromosomes (2n=18), with only one exception in Lithops dinteri v brevis (2n=17). Additional genetic analysis is needed, as current analysis have been limited to enzyme electrophoresis, or just one or two plant or chloroplast genes.

Lithops show high inter-fertility between all the yellow flowered species, and separately between all the white flowered species. However, white flowered variants of yellow flowered species are common, suggesting the biochemical pathway for colouration is controlled by few genes. This may also be the case with plant colouration due the very broad plant colouration, even in single species.

Coming soon Part II: Growth Experiments 4 Months Deep.