by Jonathan Hull

My mind was churning, neurons were firing in sync; a pattern had been recognized.  What got the idea mill running was when I learned of the astonishing relationship between dust storms in Africa and the fertility of the Amazon rainforest.

Behind this process of global nutrient cycling was a set of relationships, a pattern, that had exciting implications for fertility on a much different scale: in the garden.   Ideas began to tumble one after another that centered on just how important foliar applications could be in facilitating the growth of nutritious food and potent medicinals. And, for me, the biggest question of all:  What would the African/Amazonian relationship mean for my own garden in Northeast Ohio?

I. Introduction

Foliar applications are a broad range of techniques that attempt to boost plant health by changing the microscopic environment on its leaves.  I have had impressive results using a few of these techniques, but until recently I did not fully understand how they worked.

It might seem strange that a process occurring on a global scale could lead to a breakthrough in understanding one that happens on a much smaller scale; but I’ve found such cross scale connections to be very illuminating.  It can be a way to shake the mind out of its tendency to view things in isolation.  It is easy to get hung up on tools and techniques without considering the context in which they are being used.  As the expression goes “to a hammer, the whole world looks like a nail.” 

However, understanding the context of our actions is easier said than done.  If you are like me, then you were trained to act in a world that is thought to operate in a linear, static and superficial way.  But this is not how the natural world works.  Natural systems are complex, dynamic and adaptive.

We might look into the garden in a superficial way and see plants that are “under-performing” and grasp at the latest fad to whip it into shape.  Not only is this kind of thinking dangerous, the picture of the world it produces is dreadfully boring.  Instead we could look at the structure of a plant as a flow of energy.  This flow of energy is the expression of relationships between an innumerable set of processes.

With this picture of the world, we will see that some of these processes structure this flow of energy with the broadest strokes.  The latitude where this plant looks out toward the sun will pattern it in the most profound way.  The climatic patterns in which it grows, whether it is near an ocean or up in a mountain, will affect the flow of energy that is this plant.  Right now the structure of any plant is the expression of relationships between processes that happened eons ago.   It might be most vibrantly healthy when growing in soils containing minerals brought in by now-long-absent glaciers.

There are also those processes that fill in the details of the larger processes in which they are embedded.  A plant might be growing in a certain way because it is in a small depression in the earth that protects it from winds and extends the heat energy of the sun.  A plant might be growing well because the flow of energy through it is being facilitated by all the minerals it needs for its metabolism.  These minerals might have been pulled from a rock by a bacteria so that the plant could absorb it.  The shape of the quantum vibration of one these minerals might be structuring an enzyme that dramatically speeds up chemical reactions in the plant.                   

It can be both breathtaking and dizzying to look at world in this way.  Any picture of such a complex world is going to be  limited.  It also might seem strange to spend all this time exploring when all you want to do is grow food.   But I think it is an essential exploration if we are to participate responsibly in the world.  If we want to dance to the rhythm of life than we have to listen to its music.

This is the kind of exploration we are going to make in regards to foliar applications.  We will start on the global scale.    There is a process of global nutrient cycling that dramatically patterns the structure of the natural world.   This global process will help us understand how we can use foliar applications to cycle nutrients in our garden.   Some of what we will learn will have direct implications for our practical efforts.  Other things we might learn might not be practical but will be grist for the intuitive mill. 

Lastly, I wanted to explore this connection for more than just its utilitarian value.  I like to garden not just for food but because even the most practical work participates in a great chain of being that runs through worlds incredibly small to those incomprehensibly vast.  Researching this topic deepened my awareness of how profoundly the natural world is shaped by its interconnectivity.  This awareness of interconnectivity sparks in me a sense of awe – an awe in being part of the living tissue of organism earth.

This was all the more the case when I learned of the importance of dust.

II.  Dynamics of the Global Ecosystem

The Amazon rainforest is at odds with what I’ve learned about soil.  In most terrestrial ecosystems, nutrient rich soil is the foundation of a healthy biome.  Rainforests are burgeoning with life, but more often than not they sit atop notoriously poor soil.  How can this be?  One explanation is that in rainforest ecologies, nutrient cycling happens at breakneck speeds.  Nutrients in dead organic matter do not have time to build up in the soil because they are quickly reassembled back into living tissue.  A number of plants called litter trappers  have even evolved to capture organic matter before it makes is to the forest floor! 

These soils are also lacking in nutrients because they are weathered soils. The torrents of rain that give these forests their name comes with a price: all that water leaches minerals out of the soil profile.  The rainforest might be a prolific nutrient recycler, but with so much rain it is inevitable that soil minerals are carried away in rivers and eventually deposited in the ocean. 

Chief among these important minerals is phosphorus.  In the orchestra of life, phosphorus plays a central role in the flow of energy in all living things.  However, when it is in an inorganic form it is anything but energetic.  It is not easily induced into biochemical processes so that only a small portion of phosphorus in an ecosystem is being cycled through living systems.  The rest is much more likely to be leached away by weathering.  In soils that are heavily weathered, phosphorus is often the limiting factor in plant growth.    

Unlike its northern cousins, forests in the Amazon have not had their soil minerals renewed by the erosive power of ice age glaciers.  One would expect that without such an input, the slow and steady loss of soil minerals like phosphorus would degrade the biomass potential in a kind of ‘wet desert’ ecosystem.  Yet, where civilization has not wreaked havoc, these biomes are unmatched in their vibrant diversity.  What gives?

An explanation is found in the amazing story of how dust storms that occur in the Saharan desert in Africa fertilize the Amazon rainforest in South America.

[Fig. 1] 

The dust borne on continent-spanning winds is rich in minerals like the all-important phosphorus.  This critical input of dust has been found to perfectly balance what is leached away by rain and lost to the ocean!  How wondrous that two biomes at such complete extremes and separated by an ocean of water can be so intimately connected!   

Equally incredible is the set of finely tuned processes that allows for this instance of global nutrient cycling.  The majority of Saharan dust that falls in the Amazon comes from one small area called the Bodélé depression.  It is only .5% the size of the Amazon and only .2% of the Sahara, but it is the greatest contributor of global atmospheric dust, emitting on average over half a million tons of dust per day!

[Fig 2.]

A small area producing such an immense volume of dust is only possible because of a very specific set of geographic and meteorological circumstances.  Geographically the Bodélé depression is located at the mouth of two large magma formations.  These formations are situated in just the right way that they focus and accelerate, like breath through a straw, powerful winds that carry the dust aloft.

[Fig. 3]

These winds, referred to as low-level jets,  are unusual in themselves and only formed because of a particular set of regional conditions including the shape over northern Libya of the high altitude globe spanning jet-stream.   The directionality of this low-level jet is important because it moves the dust into the right position to be picked up by the higher altitude winds that travel to South America.

The Sahara and the Amazon are not just spatially linked, there are also processes that link them through time. The dust from the Bodélé depression isn’t any old dust.  It is uniquely mineral rich because this area was once at the bottom of a vast freshwater lake.    

Thousands of years ago the Sahara was a different environment – more closely resembling the Amazon than the desert we see today.  It was full of rivers and lakes that teemed with life.  The  Bodélé depression was the lowest point in the largest of these lakes – Paleolake Megachad.  At its peak nearly 7,000 years ago it was larger than all of the Great Lakes combined.  Global climate patterns shaped the jet-stream so that it brought the monsoon rains further north than it does today.  Lakes like Megachad were supplied with nutrients that were washed from the soils of Northern Africa by these monsoon rains.  These nutrients in combination with the steady sun shining on this equatorial latitude made for the biological equivalent of a freshwater paradise.

The ancient creatures that populated Lake Megachad complexed and concentrated these essential minerals as part of their metabolism.  Once integrated into a biological process many of these minerals cycle within the ecosystem (see below), but as we have seen some is inevitably lost to the system.  In this case, when aquatic creatures die some of their remains collect on the lake bottom.

One of the more important creatures for our story is a group of microscopic algae called diatoms.  As a function of their metabolism they secrete tiny crystalline shells made of silica.  The Bodélé depression is absolutely chock full of these tiny shells – a deposit called diatomite – so that the dust that blows from this area is predominately composed of them.   The hollow structure of these shells make them incredibly light and explains why the dust can travel such vast distances in the upper atmosphere.

[Figure 4]            

Higher forms of aquatic life, like fish and turtles, complexed and concentrated minerals in their bones and scales.  These also fell to the lake bottom and are also present in deposits found in the Bodélé depression.  The diatomite mentioned earlier erodes these bones like sand from a sandblaster.  The result is that a significant portion of the phosphorus that settles on the Amazon originated from the remains of ancient fish!

This is important because as we detailed earlier, phosphorus that is in an inorganic form is resistant to being incorporated into biologic processes.  On the other hand, the phosphorus found in fish bones is dramatically more bio-available.  In fact, all of the phosphorus that travels in the dust is altered by a process of acid leaching that occurs in the atmosphere.  This acid leaching makes all forms of phosphorus more water soluble; an important step in making it more available to biological processes.  These are critical factors in the fertility of the rainforest and will be processes that we will mimic in the practical discussion of foliar applications to follow.           

At some point in the last 5,000 years, the climate of the Sahara shifted dramatically.  The monsoons no longer tracked north to fill its lakes.  The prime driver of this shift is a change in the orientation of the earth’s axis.  It is remarkable to think that the global cycling of nutrients that is occurring between the Sahara and the Amazon is itself embedded in an even larger process occurring between the earth, sun and moon!    

Yet this might not even be the most incredible part of this story.  Although a change in earth’s axis may have been the prime mover of this shift in climate, how it shifted suggests a more complex process.  It shifted in a disturbingly rapid fashion, which the gradual change in the earth’s axis does not fully explain.  In something called the North African climate cycle, it also vacillated several times between two regimes of a wet Sahara and dry Sahara.

Recent studies have suggested that changes in global vegetation patterns may have both dampened and  amplified the effects of earth’s shifting axis, triggering what regime the climate favored in this region.  One provocative theory has even formulated that cycles of excavation and deposition of the Bodélé depression may itself be responsible for these climate cycles!  In simplified form: as the depression is emptied of its mineral rich deposits, it prompted a degradation in vegetation patterns.  This prompted a change in global climate patterns, which among other things favored a change in the track of the monsoon.  This allowed for the depression to once again be filled and for the cycle to continue.  This is global nutrient cycling  occurring on a truly grand timescale.

Although highly speculative, this theory has more going for it than just changes in vegetation patterns.  The dust from the Bodélé depression also has a huge effect on the size and type of phytoplankton blooms that happen in the Caribbean Ocean (in this case iron is the limiting nutrient of which this dust is also rich).  Supporting a number of major ecosystems means that this dust is involved in sinking vast amounts of atmospheric carbon.  In combination with a number of other effects like cloud physics and radiant heating, the dust from this area is being considered as a “tipping element,”a force that can determine the larger state of global climatic patterns. 

Clearly atmospheric dust is incredibly important to the global ecosystem.  Many other ecosystems are dependent on mineral inputs of atmospheric dust that originate from other parts of the world.  Jared Diamond, in his book Collapse, cites the varying distribution of volcanic dust as a factor that determined why some Pacific Island cultures endured while others did not.  Closer to home, the forest of Eastern North America also receive important inputs of minerals from dust.

But it was the role of dust in the Amazon that triggered the “AHA! moment” that started this exploration.  I’ve studied a lot about healthy soil and continue to work on building the best soil I can for the creatures in the garden.  When I learned about foliar applications I could never see how feeding the plant through its leaves could have near the effect as feeding it through the soil.  But when I tried them they seemed to work quite well.  How could this be? 

The inspiration came with the thought: if plants in the Amazon are growing in nutrient poor soil, perhaps they adapted the ability to trap and absorb the minerals from dust that fell on their leaves before it even reached the soil?  Might all plants have on their leaves something akin to microscopic litter trappers?

This will be the starting point for the next installment in this series, where I will present evidence that this may indeed be the case.  From the global processes we have charted here, we will take a journey into the microscopic ecosystem of the leaf–including the leaves in our Ohio gardens.  Stay tuned!

Photo Captions/Credit:

Figure 1: Conceptual image of dust from the Saharan Desert crossing the Atlantic Ocean to the Amazon rainforest in South America. Image via Conceptual Image Lab, NASA/Goddard Space Flight Center

Figure 2: NASA image courtesy the MODIS Rapid Response Team, oddard Space Flight Center

Figure 3: The surface wind focusing toward the Bodélé. Right: 3D topography of the Sahara; left: a rare shuttle image of emission from the Bodélé between the Tibesti and the Ennedi mountains. From: The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest:

Figure 4: Coloured scanning electron micrograph (SEM) of a Triceratium sp. diatom. By Steve Gschmeissner. : http://fineartamerica.com/featured/26-diatom-sem-steve-gschmeissner.html

Figure 5:  Sub-fossil skeleton of a 1.15 m long Nile Perch preserved within diatomite on the floor of palaeolake Mega Chad within the Bodélé. From Solid-phase phosphorus speciation in Saharan Bodélé Depression dusts and source sediments.