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.
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):
[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!
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!
Undaunted, let’s move on to Part 6.