If Rainforest Soils Are Infertile, Does This Mean Desert Soils Are Fertile?
How Rainfall Shapes Soil Nutrients in the Planet’s Wettest and Driest Biomes

Desert Soil Fertility vs. Rainforest Soils: The Role of Rainfall | March 3, 2025

Introduction

It may come as a surprise that the lush tropical rainforests sit atop some of the most nutrient-poor soils on Earth, while barren deserts often hide reservoirs of fertility beneath their surface. The key differentiator is rainfall. Excessive rainfall in rainforests causes intense leaching of soil nutrients, leaving behind acidic, thin soils^[1][2]. By contrast, the scarcity of rain in deserts means that minerals and nutrients are not washed away – in fact, many desert soils retain abundant nutrients and can be quite fertile if water becomes available​^[3][2]. This article explores desert soil fertility as a counterpoint to rainforest soils, focusing on rainfall as the primary variable while controlling for other factors like soil type, geology, climate (temperature), and topography. We compare analogous desert and rainforest settings – those with similar soil types, latitudes, elevations, and geologic contexts – to isolate how rainfall alone impacts soil fertility. Through these comparisons, we examine whether deserts accumulate the nutrients that rainforests lose, and how different desert types worldwide (subtropical, coastal, interior, cold, and endorheic) stack up against their wetter rainforest counterparts. The goal is an engaging, clear look at an ecological paradox: can the driest of lands actually hold more soil nutrients than the wettest? And what does that tell us about the delicate balance between climate and soil fertility?

Desert Soil Formation and Composition

Desert soils form under conditions of minimal moisture, which fundamentally shapes their composition and structure. In the absence of frequent rain, chemical weathering is slow – rocks and minerals do break down over time, but much more gradually than in humid climates. Instead, deserts often see more physical weathering (thermal cracking of rocks, wind abrasion) producing sand and dust. Crucially, the lack of water means that once minerals do weather out of rocks, they tend to stay in place. There is little leaching (downward washing) of soluble nutrients. As a result, desert soils often have a higher content of weatherable minerals and soluble salts than rainforest soils, which have been heavily stripped of such components​^[2]. For example, calcium carbonate, gypsum, and other salts that would normally be dissolved and carried away by rain in wetter climates instead accumulate in arid soils​^[2][4]. It’s common to find caliche horizons (hard layers of calcium carbonate) or pockets of salts in desert subsoils – basically, minerals precipitated out of the small amounts of water that evaporated, cementing the soil. As one soil science guide succinctly puts it, “the lack of soil moisture keeps minerals from leaching out of the soils and can even create cement-like horizons near the surface”​^[3].

Another hallmark of desert soil formation is the low organic matter content. Because plant growth is sparse in arid lands, there is limited input of leaf litter or roots to form humus. The topsoil of many deserts is pale or grayish, reflecting this dearth of organic material​^[4]. Yet, despite scant organic matter, desert surfaces are far from lifeless – they often host biological soil crusts (communities of cyanobacteria, lichens, and mosses) that bind the soil and even fix nitrogen from the atmosphere. These living crusts, along with periodic dust deposition, contribute essential nutrients to desert soils. In fact, wind-blown dust is a major source of fertility in many deserts: studies have found that up to 75% of important plant nutrients (like nitrogen, phosphorus, potassium, magnesium, and iron) in some desert soils come from aeolian (wind-driven) dust inputs​^[5]. Because the soil surface in deserts is often not protected by dense vegetation, this dust can settle and be held in place by the sticky bio-crusts​^[5]. Over thousands of years, such processes can actually enrich the soil with nutrients – provided they aren’t lost to leaching. And in deserts, they’re not: any rain that does fall is usually short-lived, and the dry spells between allow soils to retain the nutrients.

In short, desert soil profiles tend to show accumulation rather than loss. Soluble materials like salts, carbonates, and even clays accumulate in the lower horizons, whereas in a humid climate those would be flushed out​^[2]. The trade-off is that these soils are often high in minerals but water-limited. Without moisture, plants struggle to access the bounty of nutrients. This is why deserts bloom only after rains – the seeds and plants are poised to exploit nutrients that have been there all along, locked in a dry matrix. It’s also why humans can transform certain deserts into productive farmland with irrigation: the raw fertility is there (high base cations, etc.), waiting to be unlocked by water​^[3]. For instance, parts of California’s Central Valley, which has a semi-arid climate, were historically desert-like grasslands; with added water, these mineral-rich soils now produce hundreds of types of fruits and vegetables​^[3]. Of course, irrigation in arid soil must be managed carefully, as it can dissolve the accumulated salts and cause them to rise to the surface (salinization) when the water evaporates​^[3]. But the overarching point stands: desert soils often have a substantial “bank” of nutrients, a legacy of minimal leaching and long-term dust and mineral accumulation.

Comparison of Similar Deserts and Rainforests

To isolate the impact of rainfall on soil fertility, it’s instructive to compare pairs of environments that are alike in many respects except precipitation. Below, we examine several such comparisons – matching deserts and rainforests (or related wet ecosystems) that share similar soil types, geologic settings, latitudes, or other factors. By holding those variables constant as much as possible, we can see how the difference in rainfall alone drives divergent soil properties.

Tropical Volcanic Landscapes: Wet vs. Dry Sides

Volcanic regions provide a natural laboratory to compare soil fertility under contrasting rainfall. Consider the Hawaiian Islands, or similar tropical islands with high mountains (e.g. in the Caribbean or Pacific). These islands often have rainforests on their wet, windward slopes and semi-arid to desert conditions on leeward slopes, all on the same basaltic lava bedrock. For example, on Maui (Hawaii), the northeast side receives abundant rain and hosts lush vegetation, whereas the southwest side lies in a rain shadow and is relatively dry. The soils on both sides developed from the same volcanic parent material (basalt and ash) and at similar elevations, meaning their mineralogical origin is equivalent. The only major difference is the rainfall. And the soil differences are striking: the wet-side soils are classic tropical Ultisols/Oxisols – deeply weathered, red clays that are acidic and low in available nutrients – while the dry-side soils are Aridisols with much higher base nutrient content. In the wet rainforest zone, decades or centuries of heavy rain have leached out bases like calcium, magnesium, potassium, and sodium, leaving an acidic soil that actually requires lime and fertilization to support agriculture​^[4][6]. Farmers in Hawaii recognized this historically; intensive traditional agriculture (like dryland taro farming) was practiced only up to a certain rainfall threshold, beyond which the soils were too leached to be naturally fertile. Studies on the Big Island of Hawaii found that once mean annual rainfall exceeded roughly 1.5–2 meters, the topsoil’s base saturation (a measure of nutrient cation availability) dropped below ~30%, a point at which even hardy crops struggle​^[7]. In other words, past a certain rainfall, the soil’s “bank” of nutrients becomes so depleted that it can no longer support sustained plant growth without external nutrient inputs.

On the leeward (dry) side of these same islands, however, the soils tell the opposite story. With perhaps only 500 mm or less of rain per year, they have not been stripped of their minerals. A prime example is the Lahaina soil series on Maui’s drier lowlands – classified as an Oxisol by formation (it developed under a wetter ancient climate long ago), yet it occurs in a present-day semi-arid zone. Uniquely, this tropical soil retained an unusually high level of base nutrients and can support crops if irrigated​^[4]. It is still somewhat acidic (a legacy of its past weathering), but compared to a wetter-climate Oxisol, it has more available nutrients (earning a sub-classification of “Eutrustox”, meaning a relatively nutrient-rich tropical soil)​^[4]. Essentially, the dry climate arrested further leaching and allowed that soil to remain more fertile than its rainforest counterparts. Similarly, on the Big Island’s leeward Kona coast, soils on young lava flows that receive moderate rainfall (enough to weather the rock but not enough to cause heavy leaching) accumulated nutrients over centuries and were famously productive for crops like sweet potato in pre-contact times. Just a short distance upslope into the cloud forests where rainfall increases, those otherwise-identical lava soils become boggy and nutrient-poor, unsuitable for cultivation. This contrast within the same landscape vividly demonstrates rainfall’s effect: the more it rains, the more nutrients are washed out, turning a potentially fertile volcanic soil into a relatively infertile one​^[6]. Conversely, with less rain, volcanic soils can store a larger share of their weathered nutrients – so long as enough weathering occurs to release nutrients from rock in the first place. (Volcanic parent material is typically rich in minerals to begin with, which is why moderately wet volcanic areas often have famously fertile Andisols. It’s at the extremes of very wet or very dry that we see the polar opposites of leached rainforests vs. nutrient-rich deserts.)

Sandy Substrates: Desert Dunes vs. Rainforest Sands

Not all deserts are rocky or full of soluble salts; some are dominated by sand, which is often quartz-rich and low in inherent nutrients. To fairly compare a sandy desert with a sandy-soil rainforest, we can look at places where ancient sand deposits underlie both ecosystems. One example is the Amazon’s white-sand rainforests versus the sandy deserts of places like the Kalahari in Africa or parts of Australia. The Amazon rainforests are mostly on clay-rich soils, but in certain areas (such as parts of central Amazonia and the Guianas) there are extensive deep sand deposits. On these sands, tropical rainforests do grow, but they are notably stunted and nutrient-starved – the vegetation is more sparse, with specialized, slower-growing trees adapted to nutrient poverty. The soil itself is almost pure silica sand in some cases, with extremely low nutrient-holding capacity (very low clay or organic matter). What little nutrients exist in leaf litter are rapidly recycled by the ecosystem. Heavy rainfall percolating through these sandy soils leaches nutrients almost immediately, so the soil solution is nearly devoid of N, P, or K at any given time. Essentially, the plants survive on a tight internal recycling loop and some nutrient inputs from rainfall or dust, but the soil is an inert substrate.

Now compare this to a true sandy desert like the Kalahari Desert, which sits on ages-old sand as well. The Kalahari’s sand was also highly weathered and poor in nutrients to start (it’s often called an “ols sand sea” geologically), not unlike white-sand rainforest soil in mineralogy. However, the big difference is that in the desert, there is virtually no leaching. What little organic matter accumulates (from grasses or shrubs during better rainfall years) can create microsites of fertility that persist, because nutrients released from that organic matter stay in the vicinity. Over long periods, even scant biological activity plus dust can build up pockets of nutrients in the topsoil. Also, the Kalahari has a seasonally dry tropical climate (it’s technically a subtropical desert) – meaning it does get some rain in wet season and then long dry periods. During the dry spells, any nutrients mineralized in the soil are not going anywhere. In contrast, the Amazon white-sand forests with year-round rainfall never really get a break from leaching; nutrients can flush out with each rain event. Interestingly, ecologists note that many desert sands actually possess a seed bank and nutrient store that can support burst of plant growth after rain (witness the desert bloom phenomenon). In terms of plant-usable nutrients, a sandy desert soil may still be poorer than many other soil types, but compared to a rainforest on similar sand, the desert may hold slightly more inorganic nutrients simply because they haven’t been continually washed away. Another contrast is in soil acidity: rainforests on sand tend to develop very acidic conditions (pH can be 4–5) because of leaching of bases and organic acid buildup, whereas sandy deserts often have neutral to slightly alkaline sands (pH ~7–8) if there are any carbonates or salts around​^[6]. This higher pH in deserts can make certain nutrients (like phosphorus) a bit more available than in the acidic sands of a rainforest, where phosphorus might get tightly bound to iron or aluminum.

In summary, when comparing nutrient-poor sandy terrains, the rainforest climate makes the soil even more infertile (from a farming perspective) than an equivalent desert climate would. The rainforest on sand is limited by nutrients leaching out, while the desert on sand is limited by water – but given water (and perhaps some organic matter), the desert sand could support more growth than the rainforest sand, because its nutrients, however few, are still mostly present in the soil profile rather than drained away.

Temperate Examples

The rainfall-driven fertility contrast isn’t confined to the tropics. We see a similar pattern in temperate regions. Take the Pacific Northwest of North America as an example: the coastal areas of Oregon, Washington, and British Columbia receive high rainfall (and even are home to temperate rainforests with giant conifers), whereas just inland east of the mountain ranges, the climate turns arid (sagebrush steppe and deserts). Geologically, much of this region shares a volcanic heritage (basalt flows, mountain uplift) and similar elevations. On the wet western slopes of the Cascades, soils are often deep, dark, and support massive biomass – but they are actually not richly endowed with nutrients in the soil itself. High rainfall and a history of glaciation mean that many coastal forest soils are slightly acidic and have leached layers (for instance, Spodosols with leached ash-gray subsurface horizons under a surface organic layer). These temperate rainforest soils have moderate fertility (certainly higher than tropical Oxisols, since glacial processes and less intense weathering left more minerals intact), but even so, farmers usually must lime and fertilize them for agriculture because rain has removed many bases. Meanwhile, in the Columbia Plateau or Great Basin just to the east, the climate is semi-arid to arid. The soils there, often Mollisols or Aridisols, have accumulated calcium carbonate in the subsoil and have a naturally higher base saturation. They also got a boost from volcanic ash blowing east and loess (windblown silt) during the ice ages, which settled and stayed put in the absence of heavy rain. The result is that many interior Northwest soils are quite fertile (some of the USA’s prime wheat-growing areas are these loessal soils in Eastern Washington and Idaho) as long as water is supplied. Historically, the Palouse region was a grassland (not a desert, but fairly dry); its soil retained nutrients thanks to moderate rainfall, and today with irrigation parts of it are extremely productive. In the truly arid basins (like parts of Nevada or Utah), where rainfall is <200 mm/year, soils accumulate not just carbonates but also more problematic salts (sodium, etc.). Those salts in a temperate desert can render soils alkaline and even toxic to many plants (forming salt flats, where only halophytic plants survive)​^[2]. There is really no analog to a salt-crusted soil in rainy areas – any salts that tried to accumulate in a wet climate would be promptly dissolved and carried away. Indeed, materials like gypsum or halite that accumulate in deserts are completely absent from rainforest soil profiles​^[2]. Instead, rainforests accumulate iron and aluminum oxides (rusty red and yellow clays) which are insoluble and thus remain after everything else has been leached. Temperate rainforests like those in coastal Canada also often build up a thick layer of organic matter on the surface (a forest floor layer of decomposing needles and logs), because the cooler temperatures slow decomposition a bit. This surface organic layer can hold nutrients (released slowly as it decomposes), somewhat counteracting the leaching below. By contrast, in a temperate desert, organic layers are very thin or absent. Nutrients there are held in the mineral soil or in ephemeral organic matter that appears only in good moisture years.

This temperate comparison underscores the same principle: with more rainfall, soils tend to be more leached and acidic; with less, they retain more nutrients and salts​^[6]. The exact fertility also depends on the geologic history (e.g., glacial deposits can refresh soil minerals in temperate zones), but when controlling for that, climate is decisive. The Pacific Northwest example also shows an interesting twist: while tropical rainforests rely on extremely rapid nutrient recycling to maintain growth on poor soils, temperate rainforests recycle nutrients a bit more slowly (due to cooler climate) and can accumulate a nutrient-rich organic topsoil over time. In a way, this makes temperate rainforest soils somewhat more fertile than tropical rainforest soils – but still, if you clear-cut a temperate rainforest and repeatedly crop the land without inputs, the inherent soil fertility will decline as rains carry nutrients off. In the desert, if you clear shrubland and add water for crops, you might initially get a surprising boom (as the stored nutrients are unlocked), but then have to manage salinity and possibly add organic matter to keep it productive.

Endorheic Basins: The Most Extreme Case

One extreme case of rainfall’s impact is seen in endorheic deserts – areas with internal drainage where water has no outlet to the sea. In such deserts (for example, Death Valley in California, the Caspian Basin, or Lake Eyre Basin in Australia), any rain that falls or water that flows in evaporates on the spot, leaving all its dissolved substances behind. Over geologic time, this leads to extraordinary accumulation of salts and minerals in the soil and especially at low points (playas, salt lakes). Endorheic desert soils can have salt contents so high that only specialized salt-tolerant plants (if any) can grow there​​^[2]. These soils are a far cry from anything we see in rainforests – even a mangrove swamp or a rainforest bog has an outlet or at least enough flushing rain to avoid salt buildup. True rainforests by definition occur in climates where water is surplus and drains away; it’s virtually impossible to have a rainforest in an endorheic setting because the heavy rain would fill the basin and ultimately create an overflow (turning it into an exorheic system). The closest analogous ecosystems in wet climates might be closed-basin wetlands or bogs. For instance, consider the Okavango Delta in Botswana: it’s a well-watered oasis in a dry region, technically endorheic (water flows in from rivers but doesn’t reach the ocean). The Okavango is not a rainforest (it’s more of a seasonal marsh and woodland), but it illustrates what happens when you have lots of water in an interior basin – nutrients brought in by water tend to deposit in the delta and fuel plant growth. However, because the water eventually evaporates, some salts accumulate in certain areas, though much is also taken up by the biomass or deposited in sediments. In a pure tropical rainforest with constant outflow, any slight accumulation of minerals in soil is continuously diluted and carried away by the high rainfall. Thus, endorheic rainforests don’t really exist in nature; if they did, perhaps we’d see something like a rainforest with its soil enriched by self-contained nutrient cycling and mineral accumulation – but in reality, the water cycle prevents it.

So, in endorheic desert basins we get the pinnacle of nutrient accumulation (often too much of a good thing, i.e. toxic salt flats), whereas in rainy climates water’s connectivity ensures nutrients are always on the move, usually out to sea. Interestingly, those desert salt flats become sources of dust during dry times, which then blows to other regions. This connects back to rainforests in a fascinating way: dust from the Sahara’s endorheic basins (like the Bodélé Depression, a dried ancient lake in Chad) travels across the Atlantic and deposits on the Amazon rainforest. That dust is rich in phosphorus from ancient lakebed minerals and is effectively fertilizing the Amazon each year​^[8]! Scientists have quantified about 22,000 tons of Saharan phosphorus deposited in Amazonia annually, which roughly compensates for the phosphorus the Amazon basin loses to rivers and leaching​^[8]. In other words, the nutrients that heavy rains wash out of the rainforest are partly replaced by nutrients from deserts where no rain fell – a global-scale demonstration of how excess rainfall and lack of rainfall create complementary effects in the earth system. The desert’s nutrient surplus aids the rainforest’s nutrient deficit. Without that dust, even the rapid internal recycling in rainforests might not be enough to maintain long-term fertility, especially for elements like phosphorus which have no gaseous cycle and must come from rock weathering (something that in the Amazon’s highly leached soils is very slow).

Through these comparisons – volcanic islands, sandy terrains, temperate zones, and endorheic basins – a clear pattern emerges. When you control for factors like parent material and time, rainfall is a master variable for soil fertility. Deserts (dry climates) hold onto what they have; rainforests (wet climates) relentlessly lose nutrients to leaching, so they depend on biological turnover and external inputs. The next section synthesizes these insights by looking specifically at how key nutrients are distributed in deserts versus rainforests, and whether arid regions indeed stockpile elements that lush forests are starved of.

Nutrient Distribution in Deserts vs. Rainforests

Do deserts accumulate the specific nutrients that rainforests lack? In many cases, yes – the chemical inventory of a desert soil often contrasts sharply with that of a rainforest soil. Consider base cations (the basic nutrients calcium, magnesium, potassium, and sodium). In rainforests, these are typically in short supply in the soil because rainwater percolating through carries them away. Over time, tropical soils become dominated by hydrogen, aluminum, and iron ions (which is why they are acidic and rich in oxide clays), whereas calcium and other bases end up in rivers and ultimately the ocean​^[6].. In deserts, by contrast, those base cations remain in the soil and even build up to high concentrations. Arid soils often have a high base saturation (most of the cation exchange sites are filled by Ca²⁺, Mg²⁺, etc., rather than H⁺)​^[6] This explains why measurements show desert soils are commonly neutral to alkaline in pH, whereas humid tropical soils are acidic​^[6].. For instance, a West African savanna soil (somewhat drier climate) might have pH 6.5–7 and plenty of exchangeable calcium, while a nearby rainforest soil on similar geology might be pH 4.5 with negligible calcium. The excessive rainfall in the latter has leached out the calcium carbonate and other base sources, sometimes even silica, leaving a residue of quartz sand and iron/aluminum compounds. In fact, an extreme rainforest soil type (Oxisol) has “extremely low native fertility” specifically because of very low nutrient reserves and high phosphorus retention, with most nutrients locked up in the biomass instead of the soil​^[2]. Farmers or ecologists dealing with such soils know that adding lime (to raise pH and provide Ca/Mg) and fertilizer is necessary for sustained crop growth – or alternatively, working with the natural nutrient cycle by leaving a lot of organic matter to decompose.

Now look at nitrogen. In a rainforest, nitrogen is often cycled tightly; trees and microbes quickly take up any ammonium (NH₄⁺) or nitrate (NO₃^-) that forms, and heavy rain can cause denitrification or leaching of nitrate into streams if the system is disturbed. Tropical soils can be surprisingly low in organic nitrogen given the dense vegetation – the nitrogen is in the vegetation and litter, not so much in the soil, because decomposition and uptake are so rapid. In a desert, total nitrogen in soil is also low (due to sparse vegetation), but interestingly, certain forms of nitrogen can accumulate in inorganic forms. The most dramatic example is the Atacama Desert in Chile. The Atacama is so arid that virtually no rain fell for thousands of years in its core; there, nitrate (NO₃⁻) from atmospheric deposition and maybe ancient guano just accumulated in the soil, forming rich nitrate deposits that were mined as fertilizer (saltpeter) in the 19th century. This nitrate hoard existed precisely because there was no rainfall to dissolve and carry it away – the Atacama became a natural chemical reservoir​^[6]. Researchers have described the Atacama as having a “large near-surface soil nitrate reservoir due to the lack of rainfall leaching for millennia”​^[6]. In a humid climate, such a deposit could never form; biology and leaching would cycle the nitrogen away. Similarly, deserts can accumulate carbonates (CO₃²⁻) in soils – something rainforests lack. Many arid soils have calcic horizons (solid calcium carbonate layers) or even surface caliche. These are essentially calcium that might have come from weathered minerals or dust, combined with carbon dioxide from sparse plant respiration, precipitated because evaporation concentrated it. Rainforests, on the other hand, often have no carbonates at all in their soil profile (any calcium that remains is usually in plant biomass or in minerals deep in the subsoil that haven’t weathered yet).

Phosphorus is another telling nutrient. In old rainforest soils, phosphorus is highly depleted and often cited as the ultimate limiting nutrient. Why? In part because once phosphate leaches beyond root zones, it gets bound to iron and aluminum oxides in forms plants can’t access, or it washes into watercourses. In contrast, many desert soils still contain the phosphorus that was originally in the parent material or added via dust. They may even have phosphate-rich deposits if the environment allowed (for example, some arid regions with ancient lake beds or marine sediments have mineable phosphorite). Even when not concentrated, arid soils tend to hold onto phosphorus better; the challenge is making it available to plants, since in very dry, alkaline conditions phosphate can form insoluble calcium-phosphate minerals. But at least it hasn’t all been swept into the ocean. Here again, the global dust connection plays a role: the Amazon rainforest depends on Saharan dust for phosphorus to compensate for what’s been lost from its soil​^[8]. So one could say deserts have become a repository for phosphorus (in their soil and dust) that eventually helps sustain rainforests which lack it.

One must note, however, that having nutrients “in the soil” is not the same as being fertile for plant growth. Deserts demonstrate a case of “potential fertility” versus actual productivity. All the nitrogen, phosphorus, and potassium sitting in a dry soil do little for plants if there isn’t water to mobilize them and organisms to cycle them into usable forms. Rainforests, although their soils are low in stored nutrients, have an intensely active ecosystem that quickly recycles nutrients from dead material back into living biomass​^[3]. Virtually the moment a leaf falls or an animal dies on a rainforest floor, decomposers get to work (helped by warm, moist conditions) and plant roots reabsorb the released nutrients. This tight nutrient cycling loop is how rainforests can sustain high productivity on poor soils​^[3]. In deserts, nutrient cycling is slow – litter breaks down much more slowly due to dryness, and there are fewer roots around – so the nutrients may sit longer in the soil, but they’re not being turned over or taken up until a rain event happens. When rain does come, there is often a flush of biological activity (microbes wake up, plants germinate), and nutrients are suddenly in motion – but if the rain is too heavy or followed by too much water, some nutrients could actually leach in a desert too (for instance, a torrential downpour might cause temporary leaching in sand). Generally though, deserts lose nutrients mostly by wind erosion (dust storms) or during rare runoff events, rather than by deep leaching. Rainforests lose nutrients by perennial leaching into groundwater and rivers.

Another difference is in organic matter distribution. Tropical rainforests have relatively low soil organic matter (because it decomposes so fast), but they do maintain a thin but crucial layer of nutrient-rich humus/topsoil and a thick litter layer that is constantly decomposing. Deserts have sparse litter and often a fair amount of their organic carbon is in forms like microbial crust or roots that can persist. In cold deserts (e.g., Arctic or alpine dry areas), organic matter might accumulate a bit more because decomposition is slow and there’s not enough rain to leach the soluble by-products – this can create pockets of peat in surprising places, though cold deserts are somewhat beyond our main scope (they verge into polar climate issues).

In summary, desert soils often contain a larger stock of inorganic nutrients (especially base cations, and in some cases nitrate or phosphate) compared to rainforest soils on similar substrate. Rainforest ecosystems, however, hold most of their nutrients in the living biomass and litter, with the soil acting more like a transient medium. It’s a tale of two strategies: the desert soil is a bank vault of nutrients under lock and key (dry, sometimes inaccessible forms), whereas the rainforest soil is like a highway through which nutrients quickly pass, constantly on the move either upward into plants or downward out of the system.

The table below provides a concise side-by-side comparison of the key characteristics of desert vs. rainforest soils, assuming we are comparing areas of similar geology and terrain, differing mainly in rainfall.

Aspect	Desert Soils (Arid Climate)	Rainforest Soils (Wet Tropical Climate)
Climate & Rainfall	Very low rainfall (arid); minimal leaching of soil profile.	Very high rainfall (often >2000 mm/yr); intense leaching of soil.
Dominant Soil Types	Aridisols (dry soils); also Entisols in dunes, some Mollisols in semi-arid grasslands. Profiles often have calcic or salic horizons (accumulated CaCO₃, salts)​[2].	Oxisols and Ultisols (highly weathered tropical soils)​[3]; also some Inceptisols/Andisols in younger terrains. Profiles have oxic or clay-rich horizons, with iron/aluminum accumulations (laterite).
pH and Soil Chemistry	Neutral to alkaline pH is common (due to retained bases)​[6]. High base saturation; calcium, magnesium, potassium, and sodium remain in soil (often as carbonates or salts). May have localized high salinity in closed basins.	Acidic pH (often 4–6)​[6] due to leaching of basic cations. Low base saturation; Ca, Mg, K largely removed from topsoil​[6]. Soils rich in iron and aluminum oxides, which give red/yellow color. Soluble minerals are mostly gone.
Nutrient Reserves	High mineral nutrient reserves in soil: weatherable minerals and nutrients have accumulated or remained. For example, CaCO₃, gypsum, and other soluble salts not leached out​[4]. Nitrates and phosphates can accumulate in absence of leaching (e.g. Atacama nitrate deposits)​[6]. However, low nitrogen and organic carbon overall due to sparse vegetation.	Very low nutrient reserves in soil itself: most nutrients have been leached or are locked in unavailable forms​[2]. Phosphorus is often tightly bound or depleted; bases are scant. Available nutrients are mostly in the biomass and rapid topsoil recycling, not stored long-term in soil​[3]. Total soil N and organic C can be low to moderate, but turn over quicklyinum oxides, which give red/yellow color. Soluble minerals are mostly gone.
Organic Matter	Low organic matter in soil (little plant litter input, except seasonal pulses). Topsoil often thin and pale, with minimal humus. What organic matter exists decomposes slowly (due to dryness) or forms a protective biocrust. Cold deserts may accumulate some organic peat in isolated spots, but generally OM is scarce.	Low to moderate organic matter in soil – though rainforests are lush, litter decomposes extremely fast in warm, wet conditions​[3]. A thin humus layer is present but rapidly consumed. Most carbon is in living biomass rather than soil. (In cooler/cloudy tropical forests, a thicker litter layer can build up, but still fast turnover relative to temperate forests.)
Biological Activity	Pulsed and limited by moisture. Dormant seeds and soil biota spring to life after rain, briefly making nutrients available to plants. Nitrogen fixation by soil crusts or legumes is important for N input. Decomposers are less active until moisture arrives. Overall, nutrient cycling is slow and episodic.	Year-round high biological activity (in true rainforests) with decomposers constantly breaking down litter​[3]. Nutrient uptake by roots is immediate, creating a tight recycling loop. Little accumulation of nutrients in soil – they move from dead matter to living plants rapidly. Microbes, fungi, and fauna are highly active (unless limited by periodic drought in seasonal rainforests).hin humus layer is present but rapidly consumed. Most carbon is in living biomass rather than soil. (In cooler/cloudy tropical forests, a thicker litter layer can build up, but still fast turnover relative to temperate forests.)
Limiting Factor for Plants	Water is the primary limiting factor. If water is added, many desert soils reveal high productivity potential because nutrients are present​[3]. Secondary limitation can be excessive salt or poor structure in some deserts.	Nutrients (and sometimes rootable soil depth) are the limiting factors. Even with abundant rain, plants struggle without a constant supply of recycled or externally supplied nutrients​[2]. Soil may also be acidic or toxic (Al³⁺) to some plants. Water is plentiful but washes nutrients away, so fertility depends on rapid nutrient turnover.
Typical Soil Horizons	Well-developed lower horizons of accumulation: e.g. calcic horizon (caliche hardpan of CaCO₃), gypsum layers, or salic horizon (salt crust in very dry basins). Surface often has desert pavement or biological crust. Minimal leached E horizon (because not much leaching).	Thin organic layer (litter/humus) on top, often a leached light-colored E horizon beneath (in some tropical soils), and a thick subsurface B horizon of clay and oxides (lateritic layer). No caliche or salt layers (bases leached). In older soils, a deep weathered zone can reach many meters, with virtually no unweathered minerals in upper profile.
Example Regions	Sahara Desert (North Africa) – old sand and rock with high salt in places; Sonoran Desert (N. America) – caliche-rich soils; Central Australia – red desert soils holding iron oxides and some nutrients in situ; Gobi Desert (Asia) – continental dry soils with basic pH. All characterized by nutrient accumulation relative to any wetter analogs.ent or biological crust. Minimal leached E horizon (because not much leaching).	Amazon Basin (South America) – deeply weathered red Oxisols low in fertility​[2]; Congo Basin (Africa) – leached clay soils under rainforest; Southeast Asia (Borneo, etc.) – acid Ultisols supporting rainforest; Queensland, Australia (Wet Tropics) – ancient leached soils under coastal rainforest. In all, most nutrients are in biomass, and soils are inherently poor without that ecological nutrient cycling.

Conclusion

By comparing deserts and rainforests side by side, controlling for factors like soil type and geology, we see clearly how rainfall governs the fate of soil nutrients. In climates of excessive rainfall – tropical rainforests being the prime example – soils are highly weathered and leached. Nutrients are continually washed out of the root zone, leading to acidic, mineral-depleted soils that paradoxically support exuberant plant growth only because of efficient biological recycling and external inputs (like dust or periodic floods)​^[3][8]. The lush forest is essentially a hydroponic system on a thin nutrient film, with the “bank account” of nutrients stored in the biomass rather than the soil. In stark contrast, deserts illustrate what happens when rainfall is absent or infrequent: the soil becomes a storage warehouse for nutrients that would have otherwise leached away. Desert soils often contain plentiful soluble minerals and weatherable material – to the point that if you add water, they can explode with fertility (at least initially)​^[3]. Nutrients like calcium, potassium, and even nitrate linger in the soil, sometimes building up to concentrations that create their own challenges (salinity).

Our exploration showed that when deserts and rainforests share similar underlying conditions (be it volcanic rock, sandy parent material, or latitude), the desert soil typically holds more plant-essential nutrients chemically, whereas the rainforest soil has less in reserve but more in rapid circulation. There are of course nuances: not every desert is a nutrient haven (some extremely old deserts have soils so weathered and flushed by ancient climate oscillations that they lack certain nutrients), and not every rainforest soil is utterly devoid of fertility (young volcanic areas or river floodplains in the tropics can have richer soils even under high rainfall). But broadly, the trend is that water availability inversely correlates with nutrient retention in soils​^[6]. High rainfall leaches nutrients and forces ecosystems to evolve tight nutrient cycling strategies (or rely on geological renewal), whereas low rainfall conserves nutrients but limits their biological utilization.

One might ask: which is “better” for fertility, a desert or a rainforest soil? The answer depends on perspective. If you’re a plant, a rainforest soil provides ample water but scant nutrients – you must partner with fungi, rapidly recycle litter, and extend roots wide to capture the ephemeral nutrients. A desert soil provides nutrients aplenty but little water – you must wait for rain and then quickly capitalize on the brief moist window. Humans have historically favored the middle ground: semi-arid or seasonally wet-dry regions (like savannas, prairies, and deciduous forests) often have the best balance of nutrient-rich soils and enough moisture, which is why many agricultural civilizations thrived in those intermediate climates. Indeed, indigenous Hawaiian agriculture flourished in intermediate rainfall zones, not the wettest or driest extremes​^[7], and the world’s grain belts are in moderate climates, not in rainforests or true deserts.

In closing, examining desert versus rainforest soils through the lens of rainfall illuminates a central ecological principle: Water is the great driver of chemical redistribution on land. Too much water, and the land is bled of its life-giving elements; too little, and life cannot access those elements. Fertility is thus a balancing act between retaining nutrients and having them in a usable form. The desert and the rainforest stand as opposing endpoints of this spectrum – one with a hidden wealth of nutrients locked in dry earth, the other with nutrients unlocked but constantly swept away by water. Far from being mere trivia, this understanding has practical implications. It helps explain why tropical agro-forestry must emulate natural nutrient cycles, why irrigating dry lands can be both boon and bane, and how dust from one continent can sustain forests in another. Above all, it highlights the intricate connections between climate, soil, and life: even as verdant jungles and barren deserts seem worlds apart, they are two chapters of the same story, each illustrating how earth and sky (soil and rainfall) collaborate to shape the fertility of ecosystems.

So, rainforests have infertile soils and deserts have fertile soils. What about plants that don't grow in soil like the water lettuce which floats on the surface of water? Where does it get its nutrients from? And if they're not rooted in the river bed, how to they end up upstream and not just all float downstream? If you want to know the answers to these questions about water lettuce, read the blog below. Or if you still have questions on the fertility of desert soils, go to the FAQ section below.

FAQ

References

1. Internet Geography. (n.d.). The nutrient cycle in the rainforest. Retrieved from https://www.internetgeography.net/topics/the-nutrient-cycle-in-the-rainforest/
2. University of Idaho. (n.d.). Oxisols. Retrieved from https://www.uidaho.edu/cals/soil-orders/oxisols
3. Soils 4 Teachers. (n.d.). Desert soils. Retrieved from https://www.soils4teachers.org/deserts
4. College of Tropical Agriculture and Human Resources, University of Hawai'i at Mānoa. (n.d.). Aridisols. Retrieved from https://www.ctahr.hawaii.edu/mauisoil/b_aridisol.aspx
5. Belnap, J., Phillips, S., Duniway, M., & Reynolds, R. L. (2003). Soil fertility in deserts: A review on the influence of biological soil crusts and the effect of soil surface disturbance on nutrient inputs and losses. In Desertification in the third millennium: Proceedings of an international conference (pp. 245–252). Retrieved from https://pubs.usgs.gov/publication/70178436
6. Zhang, Y.-Y., Wu, W., & Liu, H. (2019). Factors affecting variations of soil pH in different horizons in hilly regions. PLoS ONE, 14(6), e0218563. https://doi.org/10.1371/journal.pone.0218563
7. Lincoln, N., Chadwick, O., & Vitousek, P. (2014). Indicators of soil fertility and opportunities for precontact agriculture in Kona, Hawai'i. Ecosphere, 5(4), 1–20. Retrieved from https://ouci.dntb.gov.ua/en/works/9Q2eO8g7/
8. Sci.News. (2015, February 24). Phosphorus-rich dust from Sahara Desert keeps Amazon soils fertile. Retrieved from https://www.sci.news/othersciences/geophysics/science-phosphorus-rich-dust-sahara-desert-amazon-soils-02533.html.