Some questions related to farming

[Primarygun]

In a farm land, we add ammonium sulphate to lower the alkaline of the soil.

What's the reaction? Ammonia and sulphate ion and hydrogen ions?

and to reduce the acidity, we add some calcium oxide, quicklime, why not add some calcium hydroxide, slaked lime?

[budullewraagh]

ammonium suilfate dissociates, but the ammonium cations are hardly positive. the hydronium cation that is coordinate covalently bonded to the nitrogen is attracted more to the sulfate. some of the hydronium cations dissociate

 

 

calcium oxide absorbs hydronium cations, forming the hydroxide and then water and a calcium salt. calcium hydroxide absorbs less hydronium cations per molecule.

[Primarygun]

calcium oxide absorbs hydronium cations, forming the hydroxide and then water and a calcium salt. calcium hydroxide absorbs less hydronium cations per molecule.

Isn't the amount of hydronium cation absorbed the same for both?

[jdurg]

I would tend to agree with Primarygun. The only place that the hydronium ion exists is in an aqueous solution, and in an aqueous solution CaO forms Ca(OH)2. While it's not very soluble in water, as it does dissolve it reacts with the hydronium ions forming a calcium salt and water. This pushes the equillibrium further towards the right and allows more calcium hydroxide to dissolve. Therefore, there is really no difference between calcium oxide and calcium hydroxide. I believe the one place where CaO is a bit better than Ca(OH)2 is the speed with which is will raise the pH. With the CaO it will probably take a bit longer and be more gradual of a pH adjustment since the oxide has to dissolve in water first before it can take on the hydronium ions, while the hydroxide can just take on the hydronium ions immediately when it's surrounded by water. Therefore it will be effective for a slightly longer period of time.

[Tetrahedrite]

CaO (quicklime) will react with water as fast, or faster, than Ca(OH)2 (slaked lime), releasing large amounts of heat in the process. Slaked lime only has limited solubility in water, even when heated, so I would say that straight CaO is actually better at reducing the pH, and faster.

[budullewraagh]

I would tend to agree with Primarygun. The only place that the hydronium ion exists is in an aqueous solution, and in an aqueous solution CaO forms Ca(OH)2.

exactly, but it gets to the hydroxide by absorbing hydronium. it absorbs hydronium further to form water and the calcium salt

[chadn]

and to reduce the acidity, we add some calcium oxide, quicklime, why not add some calcium hydroxide, slaked lime?

 

It is used and its an excellent source of liming material. (In agriculture the term lime can refer to almost anything that is used to reduce pH). CaO has a higher Calcium Carbonate Equivalent and so is a better liming material, but slaked lime is still an excellent source with a CCE of 134. CCE or Calcium Carbonate Equivalent is a method of comparing the effectiveness of liming materials. It uses Calcium Carbonate as the standard and assigns it a value of 100. The higher the CCE the more effective the liming material and anything with a CCE of over 100 is an excellent source. If there is a reason why slaked lime is not as common it has to do with the availability and not to its effectiveness.

[Tetrahedrite]

exactly, but it gets to the hydroxide by absorbing hydronium. it absorbs hydronium further to form water and the calcium salt

I don't think absorbing is the right word. The difference here is that CaO will actively produce OH- ions, even in basic water that contains very little H3O+ ions, where as Ca(OH)2 is limited by its Ksp.

[budullewraagh]

yes, it will make OH- ions in two ways. water will form H+ and OH-

the H+ and OH- will go to the CaO and form Ca(OH)2

in an acidic solution the Ca(OH)2 formed will make water and salts

[chadn]

I don't think absorbing is the right word. The difference here is that CaO will actively produce OH- ions, even in basic water that contains very little H3O+ ions, where as Ca(OH)2 is limited by its Ksp.

 

But this isnt basic water, this is the soil solution. Soil Chemistry is a whole sub-branch of its own, much like biochemistry. You do not deal with simple, basic solutions of water and one or two chems. A soil chemist I know once told me a story about a conversation he had with a physical chemist. The guy said "we spend all this time coming up with these rules for solutions and you guys go and throw dirt into them..." Soil Chemistry is messy. With soil chem one must deal with something called reserve acidity. H+ ions in soluton do not necessarily come strait from the disassociation of water. Instead they arrive in the soil solution indirectly, with the majority being absorbed to the Cation Exchange surfaces of clay particles. Ca2+ has this little tendency to become absorbed to these surfaces and be taken out of the solution.

 

That is why Ca(OH)2 makes such an excellent liming material, it will not stay wrapped up with the hydroxide ions, but instead find its way to negatively charged clay surfaces.

[Tetrahedrite]

But this isnt basic water' date=' this is the soil solution. Soil Chemistry is a whole sub-branch of its own, much like biochemistry. You do not deal with simple, basic solutions of water and one or two chems. A soil chemist I know once told me a story about a conversation he had with a physical chemist. The guy said "we spend all this time coming up with these rules for solutions and you guys go and throw dirt into them..." Soil Chemistry is messy. With soil chem one must deal with something called reserve acidity. H+ ions in soluton do not necessarily come strait from the disassociation of water. Instead they arrive in the soil solution indirectly, with the majority being absorbed to the Cation Exchange surfaces of clay particles. Ca2+ has this little tendency to become absorbed to these surfaces and be taken out of the solution.

 

That is why Ca(OH)2 makes such an excellent liming material, it will not stay wrapped up with the hydroxide ions, but instead find its way to negatively charged clay surfaces.[/quote']

Please don't give me the rubbish about "because it is in the soil, the same chemistry does not apply". Solution chemistry in rocks and soil is my area of study and I can assure that experimental chemistry still applies, it just more complicated. If we want to get technical, the pH in soil is affected by many factors including humic and fulvic acid contents, adsorption, absorption, carbonate equilibria, silicate equilibria, bacterial activity, total ionic activity coefficients, temperature, free energy of formation for specific mineral species, etc, etc.

My point is that it doesn't matter whether the soil is basic or acidic, CaO (quicklime) is still the most effective agent for raising the pH in soils. In the end H+ + OH- >>>>> H20 and the presence of the soil does not matter

[chadn]

Please don't give me the rubbish about "because it is in the soil, the same chemistry does not apply". Solution chemistry in rocks and soil is my area of study and I can assure that experimental chemistry still applies, it just more complicated. If we want to get technical, the pH in soil is affected by many factors including humic and fulvic acid contents, adsorption, absorption, carbonate equilibria, silicate equilibria, bacterial activity, total ionic activity coefficients, temperature, free energy of formation for specific mineral species, etc, etc.

My point is that it doesn't matter whether the soil is basic or acidic, CaO (quicklime) is still the most effective agent for raising the pH in soils. In the end H+ + OH- >>>>> H20 and the presence of the soil does not matter

 

I dont think anybody has claimed that Ca(OH)2 is as good as CaO, but your posts make it sound as if Ca(OH)2 is completely worthless in neutralizing soil pH, when in fact it is quite effective.

 

And those other factors are essential. If you have a removal of Ca from the solution then more Ca(OH)2 will disassociate, increasing the OH- concentration. If we look at as simply the disassociation of the compound in basic water we do not see this and the effectiveness of the Ca(OH)2 is not fully realized.

[Tetrahedrite]

I dont think anybody has claimed that Ca(OH)2 is as good as CaO' date=' but your posts make it sound as if Ca(OH)2 is completely worthless in neutralizing soil pH, when in fact it is quite effective.

 

And those other factors are essential. If you have a removal of Ca from the solution then more Ca(OH)2 will disassociate, increasing the OH- concentration. If we look at as simply the disassociation of the compound in basic water we do not see this and the effectiveness of the Ca(OH)2 is not fully realized.[/quote']

My apologies, the main point of the above post was that soil chemistry is still just chemistry. Ca(OH)2 is indeed effective, as is calcium carbonate and bentonite clays. As for the above point, clays and other minerals can also cause the removal of OH- ions from solution, nullifying and positive dissociation effects.

[Primarygun]

calcium is commonly used to neutralize the acid in a river, right? Is it better than calcium carbonate in this case?

[budullewraagh]

pure calcium???