Every Spring, since we’ve been here, we’ve had a Robin’s nest near the house. That’s why we call the place Robin Hill – plus it has a little bit of Robin Hood in it.
Year One (2008) they chose the rafters of the carport and Year Two they chose the nook next to Year One’s nest. We never understood why they do this, as the carport is a relatively busy place. Each time we would walk in or past, the Robin on duty would take off with a great flutter of wings to perch on a nearby tree branch from which to scold us until we left.
I know that Robins will return ever year but will never re-use a nest, and now it seems that they won’t even use the same space. In anticipation of their return I had moved the two old nests so they could go there again, as they seemed to like it so much. Instead they chose to move into the Japanese Andromeda that is right next to the mudroom entrance and the guest room window. An even busier place!
We now use our other (main) door – which leads straight into the living room – as often as we can, and try to tiptoe around, but it is difficult not to disturb them. The frantic escape from the dense bush is even more alarming what with all the leaves flying off as well. Still, it makes for great observation. Maybe we will install that webcam.
So far there are three beautiful blue eggs in the nest (Robins lay one egg a day and usually stop at four) — ah, that was based on my quick peek yesterday: today there are four!
And one wary Momma Robin (it’s usually the females who incubate the eggs).
There must be a bird’s nest in our shed as well. Each time we walk in there is a loud chirping, but we haven’t located it yet, so I can’t say what it is. Maybe the wrens, who always hang out in that shed.
The other night my birdwatching neighbor came over to tell me there are is Barred Owl (Strix Varia) nesting in the trees behind our property and that I should listen for its calls. That evening, there it was, that typical “Who Cooks For You” call. By the time we got the mike out there, the call had changed to:
(We are thinking of placing a mike on top of our roof, and whenever we hear something – the fisher cat, or the owls – we plug it into a laptop and record it. Yet another scheme here on our Hill!)
As we listened that evening I said to DH how wild it was, how I love how wild this place is (I wrote about the contrast with Europe here). DH remarked that surely an owl is not that wild – maybe he had jaguars in mind, and grizzly bears.
I replied an owl is pretty wild. What do I mean by wild, or wilderness? It took me not a second to answer it: Wild is Old.
That owl up there, high up in the tree, in the wind and the total darkness, is calling for a mate as it has been calling, with that exact same call, for millions of years.
Compare this with us, humans, our many, many languages, our many more ways of wooing, of saying “I want you” and “here I am”. And we’re changing those every thousand years, every generation, every day. We are constantly adapting, transforming, cultivating, culturing.
The owls, the fisher cats, the bees, they don’t change. They stay wild. Their wild ways work for them as they did millions of years ago. That is wild. Wild is Old.
I heard the sound again and this time we ran out to record it. It was further away and it sounded a little different from last time – less catlike – but though the “words” are different, the voice seems the same (to the one in my memory). In any case, if you can tell us what it is, if not a fisher cat, let us know!
You guessed it: it’s time for another episode in the Calcium in the Soil and Plant series! Take heart: we’re getting close to the end (maybe only one more part to go?). Actually, it took me so long to post on this again because this one took me a long time to figure out. If you want to brush up on the previous parts, check out this page.
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Part 8. Selective Nutrient (and Water) Uptake by Roots
Nutrients arrive at the root surface in three ways:
The first of these is root interception. As roots grow, they make direct contact with nutrients. This mechanism is less important because roots come into direct contact with only 1-3 percent of the soil volume exploited by the root mass. Mycorrhizae – fungi that form a symbiotic association with plant roots – can increase the surface area that roots can extract nutrients from. Calcium and magnesium, because they are so abundant, are often intercepted by root contact.
The second mechanism is mass flow, wherein plants, sucking up water (through the various pumps and pulls discussed in the previous part), also move the nutrients that are dissolved in it. Especially mobile (free) nutrients are “attracted” in this manner: nitrate-nitrogen, chloride and sulfur, which are never absorbed by the colloid and thus always exist in solution, and calcium and magnesium, which are held only loosely to the colloid. The drier the soil, the less mass flow.
The third mechanism is diffusion, by which ions in the soil spontaneously move from a point of higher concentration to a point of lower concentration (like in osmosis). Diffusion happens in the soil because the immediate root area, once it is depleted, has a lower concentration of the nutrient ions. Immobile nutrients like phosphorus and potassium, which have a low solubility, are strongly held by the colloid, and are only present in small concentrations, reach the root through this mechanism. The soil porosity is important here: smaller pores will block diffusion.
The last two mechanisms are the more significant mechanisms of nutrient uptake. Which one is predominant depends on the nutrients, the amount of water in the soil and the physical conditions (e.g., crumb structure) of the soil which dictates the movement of water through it.
Nutrients (especially immobile ones) then need to be wrested from the colloid by an ion exchange – the cation exchange capacity (CEC) talked about on a soil test. As we saw, the positively charged nutrient cations are held to the negatively charged colloid by a small electro-magnetic bond. When the root hairs release hydrogen ions (H+) and these come into contact with the colloid, they take their places on the colloid, breaking or weakening the colloidal-nutrient bond. The nutrients are knocked free and this makes them more available to be taken up by the root hairs.
Once the nutrient has arrived at the plant root surface and has been made available, the root needs to take it in: the nutrient-ion needs to travel from the root’s exterior to its interior.
As we saw in Part 7, the membranes of the cells making up the epidermis and the endodermis of roots are semi-permeable. This means several things. First, roots allow movement in, but not out, which allows osmosis to take place, by which water is taken up by the plant roots (cf. Part 7). Second, they allow only small solutes in, so they are impermeable to the large molecules of organic solutes (more about that in the next part). Third, some small solutes are allowed in, but others are not: plant roots are selective about their food.
It is the last aspect that interests us here. The uptake of the nutrients (as well as sugars and amino acids) by the roots is selective because of two main features:
First, the root membrane has channels that are ion-selective: one type of channel will let through only phosophorus ions, another fits only calcium ions, or potassium or nitrate, etc. Think of the toddler’s toy: the box with the star and pentagon and circular shaped holes into which only the star and pentagon and circular blocks fit. The root too is constructed like that.
The actual ferrying through these channels is done by ion-selectivecarriers: so-called coupling proteins that are embedded in the membrane of the root cells and that only react with specific ions, passing them on. Different plants require different amounts of nutrients, and so they will have different types and densities of ion carriers on the surface of their cells. These ion carriers are also most numerous on the surface of root hairs and root tips, which shows that roots are the main conduit for nutrient uptake in plants.
That explains the root’s selection of particular nutrients. Now, how does it select their quantity? How does it say, that’s enough?
As for water, its protein carrier is the aquaporin. Aquaporins are embedded in the cell membrane, forming transmembrane pores that conduct just water molecules. They prevent the passage of ions and other solutes by a filter (the ar/R filter) of amino acids that bind only water molecules and let them in (single file), while excluding all other molecules. When there is a lack or an excess of water, a gating mechanism changes the shape of the aquaporin so that it blocks the pore and stops the water flow. These gates can fail and an excessive amount of water can break the gates, as it were, and “drown” a plant.
Nutrients like calcium ions are taken up by different transmembrane protein carriers, which actively transport them, that is, they require energy to do so, because they have to pull in ions against their concentration gradient. For instance, there’s a good chance the root cells already have a higher concentration of calcium than the soil in the root area, but it might still need more. The energy required comes from a part of the cell (called the ATP, a nucleotide). If the plant has enough of a nutrient, it can simply stop drawing on the energy source. Also this mechanism can fail, and an excess of nutrients can lead to a toxic overdose and kill the plant.
So, however well-equipped roots are to select what the plant is in need of, it is still up to us, gardeners, to know how much of what a certain plant in our care needs and how much of it is present in our soil.
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Next up, nutrients not in mineral but in organic form, and how those can make it into the plants. Yes, the egg shells. Finally!
Friends are coming to visit for a couple of days, and I doubt I will have the time, or the inclination, to interrupt the fun we always have to post here. But before I go, a few notes:
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Turns out that the bobcat I heard a couple of weeks ago was most likely a fisher cat. My neighbor saw one crossing the street this morning and immediately fired off an email to let me know. And it clicked, because Suldog had raised this possibility in his comment to my post. Apparently fishers make that haunting sound during mating season, though they’re also know to make it when they’re trapped or attacking. No bobcat, then, but pretty wild anyway!
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I’ve signed two petitions in the last two days.
One for allowing chickens in Cambridge, Mass. (there’s a blog article about it here, and the petition is here).
The other for allowing the sale of raw milk by a dairy farm in Framingham, Mass (about raw milk in Massachusetts, click here, and here, and to sign the petition, click here).
There’s a Food Revolution and I’m on it!
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I totally missed the “Focus on Feeders,” which the Mass Audubon Society organized this year on 6 and 7 February. But I am thinking about sending one or two photos to their amateur photo contest. Here’s a selection (click for larger):
They are: Black-Capped Chickadee, Tufted Titmouse, Rose-Breasted Grosbeak, Downy and Hairy Woodpecker side-by-side, female Northern Cardinal and Red-Breasted Woodpecker (photo taken yesterday).
I like the first two because of their wintry atmosphere: the birds seem cloaked in the snow-laden sky. The Grosbeak was such an exception at my feeders, and I love the color of his breast. The Hairy Woodpecker (the large one) is so darn ugly; even his eye looks scruffy! But it was great to see the two kinds side by side. The female Cardinal gives us such a stern look, and look at the soft colors of her belly. And that last picture is just so vivid.
This is already the seventh part in a series on how calcium and other nutrients get into the soil and then into plants. Here we finally meet the plant roots, and investigate how they take up water. Click to read part 1, part 2, part 3 and part 4, part 5 and part 6.
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7. Water uptake through osmosis, turgidity, root pressure and transpirational pull
I’ve given so much attention to the solubility of calcium in water because plant roots can take up nutrients only if they are dissolved in water. Let’s stick with the water for now and add nutrients in part 8. How do plants take up water?
The surface of root tips are made up of epidermal cells (epidermis = outer skin) and their extensions, called root hairs, which form a fuzzy band and increase the absorptive surface area and thus the rate of water uptake several hundredfold. These surface cells and hairs draw water from the soil by osmosis (Greek osmos = a push).
Osmosis happens when there is an unequal concentration of solutes in the water on either side of the epidermis. On the outside, the soil water consists mostly of water with a small amount of salts (any dissolved ions – see part 5). On the inside, the epidermal cells contain water with a much larger concentration of salts, sugars and other substances. The water in the soil seeks to dilute the water inside the epidermal cells, and it pushes, through the epidermis, into the root. The epidermal cell membranes allow this free movement, but only in this direction. If outward movement were allowed, this system would not work, and the root would lose its precious salts and sugars.
The water is stored in the vacuole of the cell, making it turgid or swollen. When the vacuole is fully inflated, the water uptake will slow down, because the internal pressure or turgor inside the cell will squeeze the water out to the next cell, and so up into the rest of the plant, to where it’s needed. You see this effect when you water a wilted plant: slowly all its deflated cells are filled with water through turgidity.
This explains what the problem is with excessively saline soils (see part 5). Even if there is enough water in the soil, it is not diluted enough, and so the inequality between it and the water in the root cells is not large enough, to achieve strong osmosis. Even worse, there might be less water content in the soil water than in the root cells, which reverses the direction of the osmotic flow. Deflated of their torgur pressure, the plant will wilt and, if this continues, die.
Cross section of a plant root
(image from Capon, Brian, Botany for Gardeners, Timber Press, 1990, p.141)
Water constantly circulates into, through and out of the plant. This happens through two specialized vascular tissue systems that run up and down through the entire plant. One is the xylem tissue, which carries water and solutes, and the other is the phloem tissue, which carries mainly organic nutrients, like sucrose. We’re interested in the xylem, which is situated at the center of the root.
To get the water from the epidermis (outer skin) to the xylem, it has to cross another boundary, the endodermis (= inner skin). The endodermis is a second osmotic pump, adding to the pressure (but I’ll return to this one in part 8, because it has to do with how nutrients are taken up). The epidermal and the endodermal osmotic pumps together create root pressure, which moves water (and nutrients) from the root tips to the tips of the leaves, through the xylem.
But just root pressure is not be sufficient to pump water all the way up into the branches of high trees. A second system is necessary for this, called transpirational pull. As the terms suggest, root pressure is a pushing (up) force, from roots to leaves, whereas transpirational pull is a pulling (up) force, from leaves to roots.
Very simply, transpirational pull works like this. Water molecules cohere together, forming an unbroken string or column of water in the xylem, all the way from root tip to leaf tip. When one water molecule is lost at the surface of the leaf through transpiration, or evaporation, the next water molecule is pulled up, along with the whole string of molecules. At the bottom, the roots get to suck fresh water from the soil.
And not just water, of course, but also the nutrients that are dissolved in it. Plants are, however, selective in what nutrients they will allow in: they won’t take up what they don’t need. That in the next part.
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I foresee one more part. Maybe two. But I keep an option on three.
We’ve arrived at Part 6 if this extraordinary saga of how calcium arrives and behaves in the soil (if I’ve occasionally typed “soul” instead of “soil”, is it really a typo?). Click to catch up on part 1, part 2, part 3 and part 4 and part 5.
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6. Soil base saturation and soil pH
The term “soil acidity” expresses the quantity (expressed in meq/100g) of the acidic cations (cf. part 3) that the soil can hold on to. The percent base saturation – another important term on your soil test results – is the percentage of the soil’s cation exchange capacity (CEC) occupied by the basic cations.
This is from our soil test:
This means that calcium occupies 50.6% of the total exchange sites. In other words, in 100g of my soil, 15.6 meq can hold on to cations, both basic and acidic. Of that, 7.9 meq is occupied, or saturated, by calcium, 1.65 meq by magnesium, 0.64 meq by potassium. So, as far as I can learn from the test results (*), 10.19 meq/100g of soil, or 65.3% of the CEC, is saturated by bases. That leaves 35.3% of the CEC (*) for the acidic cations (hydrogen and aluminum).
(*) Sodium (also a base cation) is not listed on my test results, which means its levels are low, so I don’t have a sodic soil (cf. part 5).
Not surprisingly, the greater the percent base saturation, the higher the soil pH. Because calcium is normally the major cation, by virtue of its abundance taking up about half the CEC (as in our soil), we can say that there is less calcium in acid soils and more in alkaline soils.
But if the soil is very alkaline (pH > 7.0), the high levels of calcium may have negative effects. For one, more calcium taking up the CEC very simply means that there is less room on the colloid for everything else. Secondly, an excess of calcium can no longer be adsorbed onto the colloid. This “free” or unadsorbed calcium begins to accumulate in the soil water and goes on to react with what other nutrients are present.
For instance, the free calcium will readily attract soluble boron (B-), which is an an-ion (a negatively charged ion), and form a nearly insoluble compound with it, thus making the boron less available to plants.
Excess calcium will also tie up, or immobilize into insoluble compounds, cations like iron (Fe++), phosphorus (P+++) aluminum (Al+++), zinc (Zn2+), copper (Cu2+), cobalt (Co2+), and manganese (Mn2+), as well as magnesium (Mg ++) and potassium (K+).
Lastly, calcium also increases the pore space in the soil by flocculation, which, as we saw in part 5, is desirable. But when pore space exceeds 50% of the total soil volume, the soil can dry out much easier, like sand.
In short, too much calcium in your soil and many nutrients become insoluble and thus unavailable to plant roots, and the soil structure is damaged to boot.
But, on the other hand, if the soil is very acidic, and thus if there is not enough calcium, many of the other cations can become excessive and thus toxic. Then calcium applications with limestone are called for. The aim when attempting to adjust soil acidity is never so much to neutralize the pH as to replace lost cation nutrients, particularly calcium.
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Next time, I promise, we’ll finally meet the plants, and discover by what magical means they get the calcium out of the soul soil.
This is part 5 of a series on how nutrients, mainly calcium, get into our soil and vegetables (click for part 1, part 2, part 3 and part 4). It is the longest and most difficult part of my expose, and the least “popular” one, judging by the fact that the issues discussed will not show up on the average soil test. Still, I include it because it gives us something to think about when we irrigate our garden and – I admit it – because it introduces that most enchanting of words in soil science. Flocculation. Come on, say it, out loud, taste it! Now you have to find out what it means.
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5. Soil structure, flocculation, salinity and sodicity
As we saw, it’s good to have some amount of clay, as clay particles are negatively charged and thus able to attract and hold on to nutrients, which are positively charged. Now clay particles can either be unattached and dispersed, or clumped together, “flocculated” into aggregates (flocs = flakes).
Flocculation happens because opposites attract and like repels like. Thus one negatively charged clay particle will repel another negatively charged clay particle. But the positively charged cations create bonds between them, shaping them into clumps or flakes.
Because of their varying charge, certain cations are good flocculants, like calcium (Ca++) and magnesium (Mg++), whereas others are poor flocculants, like sodium (Na+) and potassium (K+). Add water in the mix, and the cations’ flocculating power diminishes, because the cations will also spend some of their positive charge on attracting the hydrogen ions (H-).
Flocculation is a good thing. Unattached, dispersed single particles sit together in a dense cement that allows no air pockets, called pores. Clumpy aggregates, on the other hand, will not fit together so perfectly and create pores. It is in these pockets that the rapid exchange of air, water and colloidal cations with plant roots can take place. It is also in and through these spaces that roots grow.
But in such a lively realm as soil, flocculation is a transitory thing. It is best if the aggregates are stable, which stability depends on (1) the amount of soluble salts in the soil, and (2) the balance between calcium and magnesium (the more powerful flocculators) and sodium (the weak one).
As for (1), had I known about it, I would have shelled out the extra $5 for a soluble salt test to be done on my sample. Soluble salts are any dissolved ions, be it calcium, sodium or potassium. Ions in solution conduct electricity. The extra test would have given me the electrical conductivity (EC) of my soil, which would have given me another indicator of its nutrient richness.
As for (2), that extra test would have enlightened me about the balance between calcium and magnesium on the one hand, and sodium on the other, as it would have given me the Sodium Adsorption Radius (SAR):
[Na+]
————–
[Ca++] + [Mg++]
How do EC and SAR matter?
Well, flocculation or aggregate stability occurs (1) if the amount of soluble salts (calcium, magnesium as well as sodium) in the soil is increased: more positive ions means more electrical conductivity (EC), which means more binding of clay particles into clumps. Conversely, soil particle dispersion occurs when the amount of soluble soils and thus the EC is decreased.
Soil particles also flocculate (2) when concentrations of Ca and Mg are increased relative to the concentration of Na ions (that is, when the SAR is decreased), because Ca and Mg are much stronger flocculants. Conversely, soil particles will disperse when the SAR is increased. (I recommend this powerpoint presentation for a more visual explanation of these interactions.)
As we saw, hydrogen anions (H-) diminish the soil’s cations’ flocculating power, so irrigating with “pure” water – water that has low amounts of soluble salts and is thus a very poor conductor of electric current (EC) – can destabilize soil aggregates.
If you irrigate with so-called saline water – water with a high EC, or high amount of soluble salts – then that soil will have a good structure. However, as can be expected, if there is an excess of salts in the root zone, it will hinder plant roots from withdrawing water from the soil (this will be further explained in part 7).
Another word of caution: if you have sodic irrigation water, that is, if it contains a high amount of sodium (Na), it could damage your soil structure, making life difficult for plant roots and causing problems with irrigation.
That is because Na ions are larger than Ca and Mg ions. When too many large sodium ions (with their low flocculating values) come in between the clay particles, they act like wedges, separating the particles, breaking up their aggregation. This soil dispersion causes the clay particles to plug the soil pores and create cement.
If you soil cracks when it is dried up, you have a sodic soil. One of the solutions is to decrease the SAR by introducing calcium (mostly in the form of gypsum), which will compete with the same spaces on the colloids as the sodium, and flush them out.
Something to think about, when we water our garden!
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I really did enjoy that – no kidding. I used to study metaphysics in grad school and this reminds me of it, a bit. Let me know what it did for you!
We’ve reached part 4 of this riveting story of how calcium and other nutrients make it into into the soil and thence into our vegetables and thence into our own bodies (and into chicken eggs). We’ve had some cliffhangers already, so be sure to check out parts one, two and three.
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4. Solubility, carbonation and chemical weathering
It is the reaction of calcium and calcium compounds with water (see last part) that makes them soluble. Solubility or dissolution is the process by which a “solute” forms a homogeneous mixture with a “solvent” (here water).
This happens as the solute molecule breaks down and its ions dissociate. The positive ions attract the partially-negative oxygen in H2O and the negative ions attract the partially-positive hydrogen in H2O. The ions thus get spread out and become surrounded by the water molecules. The dissolution is complete, or in equilibrium, once it’s all been spread around. (source)
One calcium compound is more soluble than another. Calcium carbonate (our eggshell) has a very poor solubility (47 mg/L at normal atmospheric CO2 partial pressure and 25 degrees C). As we shall see, this is important for gardeners who plan to enrich their soil with eggshell calcium, but I will come to that later.
However, if carbon dioxide is also present, that carbon dioxide will react with the water to form carbonic acid (H2CO3), which is a weak acid – it’s the bubbly in our soft drinks. This carbonic acid will in turn react with the calcium carbonate to form calcium bicarbonate (or calcium hydrogen carbonate). So
CaCO3 + CO2 + H2O → Ca(HCO3)2
Calcium bicarbonate is five times more soluble in water than calcium carbonate—in fact, it exists only in solution.
This is the main process by which carbonate rocks of the Earth’s crust are weathered. As we saw, if water is saturated with carbon dioxide, it produces a mild carbonic acid. This is what happens with (unpolluted) rainwater (water plus atmospheric CO2), which has a pH of around 5.6 (polluted, “acid” rain has a pH of as low as 3.0), and with water in aquifers underground, where it can be exposed to CO2 levels much higher than the ones in the atmosphere.
In a process called carbonation, this water’s carbonic acid reacts with the solid calcium carbonate in rocks like limestone or chalk, forming calcium bicarbonate and dissolving it. This solution of water and mineralized calcium is then borne off into the soil, where it is deposited on the colloid, and where it waits to be again dissolved in water and made available to plant roots.
This is the third article in a series on how calcium and other nutrients end up inside our vegetables, and on how to interpret certain soil test results. It is preceded by part 1 and part 2.
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3. Water and pH
Let’s investigate the water in the soil. For one, water brings the minerals to the colloid, and it can take them away again (but so do the soil critters). Also, for reasons that will become clear later, calcium is available to plants only in dissolved form, that is, as part of a solution in water. Thirdly, this watery context heavily impacts the lives of soil critters. The most important factor in all these matters is the water’s pH or acidity or alkalinity.
As we saw, a molecule of water is composed of one oxygen atom and two hydrogen atoms: H2O. In a vat of pure water, most water molecules remain intact, but a very small amount of them react with each other in the following manner:
H2O + H2O ===> H3O+ + OH–
Water + Water ===> hydronium ion+ (an acidic cation) + hydroxyl ion– (a base)
The hydronium ion ( H3O+) is the chemical unit that accounts for the acidic properties of a solution, and the hydroxyl ion (OH–) is the chemical that accounts for the basic or alkaline properties of a solution. How?
Well, in pure water, the amounts of H3O+ and of OH– are equal, so the acid and the base cancel each other out, so pure water is said to be neutral, with a pH close to 7.0. Also, in pure water the concentration of H3O+ and OH– are in balance, so that an increase in the concentration of H3O+ causes a proportional decrease in the concentration of OH–.
This means that, if you add an acid like hydrochloric acid (HCl) to water, it reacts with some of the water molecules like this:
HCl + H2O ====> H3O+ + Cl–
And this increases the H3O+ or the acid concentration, throwing off the balance and lowering the solution’s pH to below 7, making it acidic. But if you add a strong base, such as calcium, to the water, it ionizes as follows:
Ca + H2O ====> Ca(OH-)2 + H2
Thus, the addition of calcium to water increases the OH- or alkali concentration of the resulting solutions, making the solution alkaline.
The cations (positively charged ions) we’re interested are either bases or acids:
Basic cations: calcium (Ca++), potassium (K+), magnesium (Mg ++) sodium (Na +) Acidic cations: aluminum (Al+++) and hydrogen (H+)
The pH of the water that saturates the soil (see 4) regulates the solubility of minerals in that soil (see 4), thus their availability to plant roots (see 5), as well as the activity of soil bacteria (see 6).
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Mm, on to that pesky problem of solubility, which took me a while to understand.For now let me add that I forgot all the chemistry I learned in secondary school (way back), and that this excursion has proved to be a fantastic rediscovery of all that magic.
