Soil pH and Nutrient Availability
Soil acidity (pH) and its relation to nutrient availability can be a complex subject. With many constantly changing variables converging dynamically to determine a soils ongoing acidity levels ; and each soils optimum desirable pH measurement being dependent on what crop is being grown and the growers target objectives.
Since each individual cultivar has its own specific nutrient requirements and irrigation demand, plus, distinct biological interactions with its underlying rhizosphere[1]; there is no one-size-fits-all solution for any grower, regardless of where they are or what they are growing.
With various soil types[2] offering unique structures and compositions(each harboring their own distinct physical and chemical attributes), greatly impacting the way that minerals interact with each other; sometimes, making the most suitable farming decision about fertilization amendments or irrigation scheduling can turn into a rather complicated endeavor.
Mineral Availability
As many of you probably already know, various nutrients have different solubility and varying mobility depending on a soils acidity level. With some nutrients being more available for absorption at lower pH levels (in more acidic soils), while others are more available at slightly higher pH levels(closer to alkaline).
According to the common classical view,[3] the primary macro-minerals such as calcium, magnesium, phosphorus, and potassium are said to be most efficiently absorbed at pH levels close to neutral [4]( around pH7), with either slightly alkaline or slightly acidic conditions allowing for the best absorption. This is commonly taught as standard knowledge among many educational organizations, portrayed in various university textbooks and governmental institutions world-wide.
Although, there are a considerable number of credible scientific researchers who have produced conflicting results that do not align with the more classical belief of optimal soil pHs. With certain research groups releasing contradictory data (5) and some institutions recently publishing new field test results [6] which clearly indicate that a more acidic pH range(closer to 6) can actually be the more optimal range for maximum mineral absorption regarding the primary macronutrients.
And much of the reasoning and logic behind some of these disagreements within the scientific community can be found in various research papers featuring the on-going debate between soil scientist NJ barrows (views on phosphorus availability[7] and Penn & Camberatos (opposing views on phosphorus/acidity relationship).[8]
But despite the fact that the general consensus is split amongst experts regarding the optimal pH range for primary macronutrient availability; the vast majority can agree unanimously that the most important micronutrient minerals (such as zinc,[9] manganese,[10] or iron,[11] generally become much more available to plant roots at relatively lower (more acidic) pH levels.
So, consequently, if the soil pH is either too high or too low, plants can become sick or damaged as a result of an overabundance of certain minerals accumulating to toxic concentrations or they can become sick from minerals becoming virtually unavailable for absorption from soil solutions, creating deficiencies within the plants.
Some common examples of these types of ailments can be observed in tropical environments where manganese[12] or aluminum[13] can become excessively available to the point of being toxic to plants due to the very high soil acidity levels. While other minerals such as calcium[14] or magnesium[15] can simultaneously become less available to the plants at those same excessively acidic conditions.
And conversely, in overly alkaline soils, phosphorus [16] and most of the micronutrients [17] can potentially become less available for plant roots as well.
Although, with the recent increasing variability of genetic traits[18] that have arisen from the modern advancements in hybrid breeding techniques, it is gradually becoming more and more difficult to accurately estimate the optimal thresholds[19] for various nutrient amendment applications in accordance with new and evolving genetic variations.
And it has also become more and more difficult to effectively estimate what a particular crops susceptibility to Manganese [20] or Aluminum[21] toxicity might be given the increased genetic variability in mineral stress tolerances.
Plus,, these types of absorption tendencies and thresholds are heavily impacted by the specific structural formations of primary mineralogical compounds that actually compose the majority of the soils granules of sand and clay. Which ultimately determine how acidity can impact the availability of nutrients [22] at different pH levels as well.
For example; various iron or aluminum based mineral-compounds[23], which are prone to having variable surface charges (such as gibbsite or hematite), retain distinct crystalline structural formations which interact with nutrients in soil solution differently compared to some of the other more permanently charged silica based mineral-compounds (such as montmorillonite or vermiculite[24]). So, because of these difference in crystalline structures and molecular formation, they exhibit distinguished and contrasting behavior in their tendencies to bond with certain positively charged cation(mineral partiicles) or negatively charges anion(mineral particles) at different acidity levels
This is partially due to the fact that certain mineralogical compounds retain structures which primarily encourage the inner-sphere bonding[25] of positively charged cation(mineral particles) while other mineralogical compound typically retain structures that predominately facilitate the outer-sphere bonding of these cations. So each individually specific mineral structure can react to changing acidity differently despite the fact that they might even be composed of the same chemical elements
This variation in mineral response to changes in soil pH can significantly impact the cation exchange capacity [26] of certain types of mineral specimens while having little to no impact on others types; so the specific composition of a soils petrological composition is a major determining factor[27] in how soil acidity can impact nutrient availability.
And, due to the differing chemical properties and changing electromagnetic bond attributes occurring between various combinations of elements; each specific mineral compounds may release charged mineral ions(particles) at different rates [28] of speed at different pH levels according to their individual sensitivity and reactivity.
Because minerals naturally exist in so many various forms(29), it is important to maintain an intuitive perspective in order to understand the different ways that pH and nutrient availability can be impacted by changing environmental factors
And unfortunately, the majority of soil testing laboratories do not have the ability to provide the type of mineralogical data necessary for performing a proper strategic mineralogical soil analysis during their standard soil testing operations.
So, it is important to keep in mind that the majority of mainstream data available to-date has been analyzed without calculations for the potential impact of fluctuations in redox potential[30](oxidation tendency) of a soils minerals due to any possible changing environmental conditions(such as heat, light, oxygen, ect).
Which is a critical factor for certain minerals, since the oxidation of their elements can lead to drastic changes in their solubility and availability for plants uptake. For instance, take sulfur for example[31], which has proven to become much more bioavailable to plants when oxidized.
And the impact of mineral oxidation on nutrient availability is not universally consistent across every type of mineralogical compound for each element, of course.
While the oxidation of some nutrients can lead to an increased bioavailability, the oxidation of other nutrients can actually result in reduced bioavailability for plant roots. For example, the oxidation of iron can lead to the formation of insoluble iron oxides, which can decrease iron availability to plants[32], which is a phenomenon that is especially common in higher soil pH ranges.
Moreover, a soils pH balance can have a drastic impact on microbial activity [33] and the proliferation and composition of their communities; which can obviously have a significant impacts on mineral/nutrient solubility and availability as well,
And depending on the plants species and the composition of their root exudates, the plants may also have mechanisms for actually altering soil pH to better suit their nutritional needs [34] along with the nutritional needs of the microbes that benefit them [35].Which can ultimately create long term synergistic effects and chain reactions between rhizobacteria communities and the surrounding general soil microbial populations as well.
So, since much of the scientific data and literature does not incorporate the impact of widely diverse plant physiologies [36] and their unique absorption tendencies and abilities[37], along with the wide variety of possible biological interference within the rhizosphere from plant root exudates and the surrounding microorganisms, it is important to digest the scientific literature with an understanding that laboratory results from research papers and field studies do not always necessarily reflect the best or most practical real world applications for growers due to the exceptionally complex and multifaceted nature of the subject.
Therefore, if possible, each grower should have some type of a plant-health focused monitoring system in place in order to control and maintain a stable balance in their crops. While implementing a carefully formulated list of mitigation procedures, pre-arranged, to effectively counteract any unwanted fluctuations in pH (acidity).
Furthermore, in regards to soil amendments, certain nutrient applications, such as nitrogen fertilizers[38], or elemental sulfur[39], can potentially contribute to increasing acidity in soil. While other amendments such as crushed limestone[40], wood ash (biochar[41]), or animal manure [42] can be used to decrease excessive acidity levels and raise soil pH.
Although, it is important to keep in mind that each mineral element can have a unique impact on a soil ph according to its specific form. Such as when elemental sulfur is added to soil as an amendment, it can reliably decreases pH [43] by creating sulfuric acid.
However, SO4- sulfate in the form of gypsum amendments, do not have such a straight forward impact. Because 1.) the sulfate form of sulfur is more stable in regards to redox potential since it is already in its oxidized form, and 2.) the calcium within its structure has a counterbalancing effect. So gypsums impact on soil pH balance can often be inconsistent. With certain field studies showing an increase in soil acidity [44] levels resulting from gypsum applications; while other studies show a decrease in soil acidity [45] as a result of gypsum application. And some research studies have shown gympsum to have no impact on pH at all. Most likely due to a range of variable mineralogical and environmental factors.
Which makes gypsum a great example of why minerology and redox potential can be important factors to be considered when predicting nutrient availability and making decisions about potential soil amendments in relation to soil acidity and consequent plant health,
Moreover, it is important for growers to recognize the fact that each 1 point decrease on the pH scale actually equates to a ten fold increase in acidity[46] So numerically small fluctuation of just 0.2 or 0.3 on the pH scale can actually signify a doubled or tripled acidity level, often time leading to significant impacts on nutrient availability and plant health.
For example, certain micronutrients such as Zinc (Zn), Copper and Manganese can potentially increase as much as 100 fold in absorption availability with every one unit decease in pH balance(47). And an inverse decrease of 100x is true with every one unit increase in pH as well.
Although, these micronutrients are not lost at higher pH levels, but they become bound by increased reactions with soil surfaces which leave them trapped in unavailable compounds that are insoluble and unavailable for plant root absorption.
Similarly, at higher alkalinity pH values, greater than pH 7.5 for example, certain macro-minerals, such as phosphate, tend to react quickly with others minerals such as calcium or magnesium to bind together and form less soluble compounds[48]. Also, similar issues can be observed at very acidic pH values, with phosphate ions potentially reacting with other minerals such as aluminum or iron[49], forming new less soluble compounds.
So, generally speaking, phosphate and its availability for absorption can be particularly sensitive to precipitation at both extremes of the pH spectrum due to its inherent reactivity to changing acidity levels.
Nitrogen Uptake
As far as the subject of nitrogen absorption goes and its relationship with soil acidity, the science behind it is actually rather intricate and multifaceted, although we will attempt to briefly summarize here as best as possible.
With certain plant species showing preference[50]to nitrogen supplied in its ammonium form and certain species showing preference to nitrogen supplied in its nitrate form, the pH level of a soil can potentially have an impact on the performance and health[51] of various cultivars in relation to the prevalence of one form of nitrogen or the other.[52] For example, consider how blueberries prefer the ammonium form[53] of nitrogen for instance and they are more commonly grown in soils with very low pH ranges. Or how cabbages or melons prefer a higher ratio of the nitrate form[54] of nitrogen, more commonly grown in soils with comparatively higher pH ranges. Although, this preference can sometimes change during certain periods of a plants lifecycle[55]. So this issue can be a significant factors to consider for farmers who are focused on implementing precision agriculture strategies in unforgiving, high competition markets.
Nitrate
In regards to the oxidized form of nitrogen; Nitrate is generally naturally less prevalent in highly acidic soils[56](with low pH), due to higher rates of leaching losses[57] and due to less microbial activity/less proliferation[58] of various bacteria which commonly promote the formation of nitrates through nitrification and mineralization[59].
This trend can be observed [60] from the increased nitrogen mineralization which is commonly seen after the liming of acidic soils.[61](when growers increase soil pH with crushed limestone/calcium carbonate amendments),
While at near neutral pH levels(closer to 7pH), larger amounts of nitrates are established by fixation[62] of atmospheric nitrogen from soil microbes and nitrates are less mobile due to better sorption to soil surfaces[63], because nitrate can be more than 50 times more mobile[64] than the ammonium form of nitrogen; which ultimately results in higher ratios of nitrates in near-neutral pH soils comparatively to ammonium due to less nitrates being lost to drainage waters and sub-surface layer
Although, interestingly enough, despite the lower prevalence in low ph soils, most plants root absorption mechanisms for nitrate are actually more efficient at lower pH ranges.[65](possibly as an evolutionary adaption to the naturally low environmental concentrations of nitrates in higher acidity soil layers).
So it is important to distinguish the difference between a minerals presence/prevalence (availability) vs actual absorbability. And to find a moderate range in the middle which can accommodate each.
And additionally, another factor to consider is that high alkalinity conditions can cause nitrates to undergo chemical reactions which convert them directly into gaseous nitrogen, a process known as denitrification. So, excessively alkaline pH levels in soil can result in nitrate losses [66] due to the resulting nitrogen emissions released into the atmosphere as well. Which makes it crucial to realize that nitrate fertilizers can sometimes have an alkalizing impact under certain soil conditions, so growers operating in high alkalinity soils should consider that factor when making decisions about potential soil amendments
. So reasons like these are why maintaining balance and equilibrium is so important and why growers should plan long term strategies to ensure that soil pH remains in a moderate and controlled range. As acidity that is either too high or too low can result in a decreased availability of nitrates for plant absorption. Since there is a constant balance at play, with different chemical reactions being predominant at different pH levels.
Ammonium
As far as the ammonium (NH4-) form of nitrogen is concerned, its prevalence is naturally higher in more acidic[67] soils in comparison to nitrate. And the addition of ammonium fertilizers can increase acidity in soil [68] to even lower levels, potentially multiplying this effect. Although, similar to nitrate, it seems like plants roots have adapted to absorb ammonium most efficiently where it is less prevalent [69](at higher pH ranges near neutral).
Although, when considering ammonium availability, growers should consider that excessively alkaline pH balances can often result in less total ammonium in the soil, because with increasing soil pH the total balance of ions shifts, leading to increased nitrogen losses from the soil from ammonia volatilization[70]. But the degree of nitrogen loss is heavily dependent on various soils factors such as the cation exchange capacity, organic matter content, and overall moisture levels
So, growers should understand, that if their soil pH is either too high or too low, for whatever reason, that the overall ammonium availability for plant roots might ultimately decrease. So balance is key in this regard.
And; It’s crucial to realize that a great portion of a soil acidity’s impact on nitrogen availability is a direct result of microbial population disruptions.
Because soil pH can determine the rate of proliferation of various bacteria populations [71]which are responsible for converting nitrogen into plant absorbable forms. With certain species making nitrogen either more or less available.
For example, certain types of bacterial (such as various strains of Nitrospira) convert ammonium to nitrate more efficiently at near neutral pH ranges[72]. Which is important since certain crop species under certain soil conditions require higher ratiox of nitrates compared to ammonium and can suffer from an unbalanced ratio with too much ammonium and not enough nitrates.
While other types of bacteria (such as Paracoccus) can release larger amounts of soil nitrogen into the atmosphere at near neutral pH ranges[73]. Leading to decreases in nitrogen content due to the gaseous nitrogen emissions resulting from their metabolic processes. o depending on the dominant species most prevalent in the soil, the pH range of a soil can have a significant impact on the amount of nitrogen that these bacteria are able to release into the atmosphere.
Also, certain beneficial atmospheric nitrogen fixing bacteria (such as Azotobacter) typically prefer slightly acidic to slightly alkaline soils[74], depending on the specific species, with each having its own optimal pH range. So population of these types of nitrogen fixing microbes are also very relevant and can be sensitive to pH changes as well.
So, pH balance can ultimately have major impacts on the diversity and abundance of various microbial communities, and the overall nutrient cycling of a crop lands soil.
With each specific bacteria species impacting nitrogen differently depending on their metabolic processes and the surrounding soil conditions, it can be quite beneficial for a grower to understand how soil acidity changes in the soil can impact microbial populations, and the resulting impacts of nitrogen availability.
Which means that, ideally, it would be beneficial for a grower to remain informed and aware of individual plant growth parameters, specific pH requirements of their distinct cultivar variety, the status of their soils nitrogen composition, and the microbial populations that are prevalent in their soil.
Organic Matter
Organic matter is another important factor[75] in regards to soil pH and nutrient availability because of the fact that organic matter typically acts as a sort of pH buffer[76] allowing soil to stabilize and resist pH changes[77] through the increase of its cation exchange capacity.
Because organic matter typically contains both acidic and alkaline functional groups, it can either release or absorb hydrogen ions, (depending on the environmental conditions and the chemical composition or origin of the organic matter), so organic ic matter can be beneficial for maintaining a stable soil pH balance. Since organic matter decomposes gradually over time, various mineral compounds and organic acids, are slowly released.
Since most organic matter has a relatively high cation exchange capacity, it is able to bind with and accumulate additional beneficial nutrients on its surfaces, (such as calcium, magnesium, potassium, and ammonium) providing a sort of reservoir of stored nutrients that can gradually be released to plant roots over time as decomposition progresses. Which typically means that higher levels of organic matter[78] results in reduced nutrient leaching, allowing soil to retain higher concentrations of positively charges cations, leading to reduced acidification.
Additionally, under the correct soil conditions, an increased soil pH has the potential to enhance the solubility of soil organic matter[79] by increasing the dissociation of acid functional groups. Making dissolved organic carbon more available for microorganism and subsequently making the other nutrients contained in the organic matter more available for uptake plants as a result.
Soil Structure
Ideally, growers should always attempt to make their best effort to gather as much detailed information as possible regarding their soils distinct physical properties and chemical characteristics; allowing them to make comprehensively informed and knowledgeable decisions to best maximize yields though precise and strategic farming practices.
Among the most important soil attributes to understand about a farmland is the soil type and the soil structure, partially because a soils physical characteristics are known to have an important impact on a croplands pH levels and nutrient availability.
For example, smaller particle sizes, such as clay, can often result in higher cation exchange capacities,[80] in soil (compared to larger particle sizes such as sand). So higher clay content in soil can essential lead to a more stable soils pH(with higher buffering capacity) generally speaking. Because of the larger amount of surface area, they are often more capable of maintaining higher concentration of nutrients such as positively charged cations like magnesium or calcium or potassium. While soils with larger particle sizes such as sand, have less total surface area to hold nutrients, so these larger particles are more likely to leach nutrients and are much more prone to changes in pH balance and acidification or alkalization; fluctuating at a much faster rate from environmental or chemical influence.
But this trend does not always hold true in all soil types because the sizes of the soil particles are ultimately less important than the actual elemental composition and crystalline structure [81] of the minerals which form the particles ( since elemental properties and mineral structure are what determine the sorption capacity and surface charge of the soil particles). Which is why we can observe some of the very acidic soils with high clay content and low cation exchange capacities in South America[82] and Africa. Because their clay elements have been weathered down to a point where they are no longer conducive for retaining bonds with positively charges nutrient ions such as magnesium, calcium, ammonium, ions.
Which means that, theoretically, if you were to compare two mineralogically identical soils with each other; (one with larger particle sizes, and one with smaller particle sizes) the soil with smaller particle sizes would have the higher cation exchange and higher water holding capacity. Just as a rule of thumb. Although its important to remember that no two soils will ever be mineralogically identical.
So,these relationships can can be helpful for professionals to understand because the types of interactions which take place within a soil can be better predicted by knowing the specific geological formation[83] that the soil was originally formed from. Since the original parent material that the soils was initially derived from can determine various characteristics[84](about the soils functionality and can greatly affect the soils tendencies and performance. Ultimately determining its capability to retain nutrients and rebound from environmental changes which can impact pH.
For instance, Limestone derived soils have their own unique chemical properties compared to granite derived soils, with each category of parent materials offering their own blend of minerals(85) that react to environmental and chemical influences differently.
For examples, calcium carbonate based(limestone derived) soils, are often more alkaline with higher cation exchange capacity, compared to silica based(granite derived) soils and this can lead to a more stable pH balance and a wide range of implications regarding nutrient interactions and availability.
And although having a stable pH balance is most often considered a positive feature to have; sometimes a soil that is resistant to change can result in an unwanted situation that is difficult to correct. Because, excessively alkaline soil are a commonly dealt with problem among many farmers around the world, and it’s not a pleasant problem to have a very stable soil pH, retaining a high buffering capacity, when you are attempting to alter the soil pH in order to achieve more suitable growth conditions regarding a certain crops pH requirements.
This can become a long standing problem in some soils with high clay content due to the fact that clay soils with more surface area have much higher buffering capacity than silt or sand, so they can absorb a greater number of reactive molecules
Also conversely, many soils types have the problem of weathering excessively over time, with various minerals in the soils undergoing chemical reactions that release or consume certain ions, including hydrogen ions (H+). The release of hydrogen ions during weathering processes can potentially increase the acidity of the soil, lowering the pH. A common process known as acidification.[86]
And in regards to long term trends, the pH level of a soil can be significantly influenced by the rate of weathering and the balance of acidifying and neutralizing processes resulting from weathering of various mineral compounds within the soil.
So, factors such as rainfall, temperature, organic matter decomposition, and the mineral composition of parent material can all impact the rate of weathering and subsequent pH changes.
For example, if the parent material that a soil was derived from contains a large percentage of highly reactive minerals that release a significant amount of positively charged hydrogen ions during weathering, or if there is limited neutralization capacity due to inadequate mineral dissolution or plant uptake, then the soil may end up becoming more acidic over time..
On the other hand, if the parent material that the soil was derived from contains a high percentage of stable minerals with a high amount buffering capacity, or if there are sufficient inputs of alkaline substances (e.g., through organic matter decomposition), then pH may remain relatively stable or become more alkaline
Location
Two good examples of contrasting soil types on opposite sides of the spectrum are 1.) the highly acidic and nutrient depleted Oxisol type soils of the jungles of South America vs 2.) the moderately alkaline and nutrient rich fluvial silty-clay loam soils of the Nile valley in Egypt.
Oxisols
First off, the very acidic and infertile Oxisol type soils commonly found in tropical and subtropical regions(such as South America, West Africa, & East Asia), are characterized by excessively high acidity levels, porous and permeable soil structure (poor water holding capacity), very high iron and aluminum content(very low nutrient content profile, and a low capacity to retain agriculturally-relevant minerals.
Oxisol type soils[87] are often derived from parent materials harboring low nutrient content, such as sandstones, granites, or quartzite. Typically containing high percentages of particles which have been heavily weathered down from sedimentary rocks high in neutrally charged minerals (with low cation exchange capacity), such as silicates or feldspars. Also Oxisols contain relatively low amounts of agriculturally significant minerals such as calcium, potassium, phosphorus, or magnesium.
Oxisols soils are typically found in areas of high rainfall where intense weathering has occurred over thousands of years, leading to the depletion of nutrients and the accumulation of highly weathered soil material. These soils are generally not very suitable from crop establishment and are infamous for being problematic for use in agricultural production (typically requiring the utilization of extremely high inputs of fertilizer and alkalizing soil amendments
Nile Fluvial Loam
On the other side of the spectrum, we have the very fertile Nile Valley fluvial silty-clay loam soils which are formed from a diverse array of sediments deposited by the Nile river over time, from various water sources across Africa, containing a nutrient rich mix of clay, silt, organic matter, calcium carbonate, phosphate rock, and many other organic and inorganic materials derived from a variety of different alluvial deposits. All coming together to form a unique mixture of desirable soils attributes with good water holding capacity and nutrient retaining properties.
The Nile Valley silty-clay loam soils [88] are found in an arid, low rainfall climate, alongside the nile river, nutritionally enhanced by the rich deposition of minerals sediments deposited from the nile river(abundant in all of the agriculturally significant minerals important to plant growth , very high in organic matter content ,and flourishing with beneficial microorganisms). Perfect for growing a plethora of diverse high quality agricultural crops, from fruits to vegetables, to legumes, to seeds, to grains, to cotton, to herbs, and more.
The soils structure, the mineral composition, the climate/weather, the ph balance, the availability of nutrients , and the overall accommodating conditions for soil microorganism growth and organic matter retention, all play a major role in making the major difference between the fertility of these two very different soil types on opposite sides of the spectrum.
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