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Basic(ish) inorganic nutrient chemistry

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You don't need a degree in chemistry to understand the basics needed in creating your own fertiliser mixes.  In this post I'll try and run through everything you need to know to create specific nutrient mixes, and I'll do my best to KISS.


Inorganic salts (nutrients) are a combination of (positive) cations, and (negative) anions.


Cations have a positive charge, and are nutrients such as Potassium (P+), Ammonium (NH4+), Calcium (Ca2+), Magnesium (Mg2+) and Iron (Fe2+).


Anions have a negative charge, and are nutrients such as Nitrate (NO3-), Sulfate (SO42-), Phosphate (PO42-) and Chloride (Cl-).


In life, things must maintain a neutral charge, meaning that you can't have some solution of just Potassium (P+), or just Nitrate (NO3-).  Instead, these two elements can bond to form a salt (solid) as KNO3 (Potassium Nitrate).  So that Potassium Nitrate is a salt of potassium and nitrate, and if this potassium nitrate is dissolved in a solution (water), the bond would break leaving free Potassium cations (P+) and free Nitrate anions (NO3-) in the solution, with the solution remaining as a neutral charge (equal positive and negative ions).


Note that things like Potassium (P+) have a single positive charge, with Sulfate (SO42-) having a double negative charge.  In order to have a salt of Potassium and Sulfate (Dipotassium sulfate (K2SO4)) there needs to be two Potassium (P+) ions, that only contain a single positive charge each, to bond with the double negative charge of Sulfate (SO42-).  This is probably only really useful if you want to check the accuracy of your sample analysis, since we know that the sample must contain a neutral charge and an analysis of a sample that contains an imbalance of positive and negative charges would indicate measurement error.  But anyway, I'll move on.


What it does show is that we can't just add Nitrogen, or just Potassium, or just Sulfate to our growing medium, since there is always another ion attached.  So if we want to increase the Nitrogen concentration in the medium, we must also add Potassium (KNO3) or Sulfate ((NH4)2SO4)) or Calcium (Ca(NO3)2) for example.


If we want to add some nutrient to our growing medium, we can determine the concentration of the salt added as simply the amount of salt added in grams, times 1000, divided by the number of liters of water the salt is added to ((amount added in grams * 1000) / liters of water).  So lets say we add 10 grams of an inorganic nutrient to 100 liters of water.


10 * 1000 = 10000 (amount added in grams * 1000)

10000 / 100 = 100 (result of above / liters of water)


So adding 10 grams of this inorganic nutrient to 100 liters of water would result in a nutrient concentration of 100 ppm (parts per million).  If we want to determine how much of this nutrient concentration is a specific nutrient we can use a molar mass calculator.


Plugging in KNO3 (Potassium Nitrate) to the molar mass calculator gives the following percentages.


Potassium = 38.6717%

Nitrate = 61.3284%


That is to say, that KNO3 contains 38.67% by mass of Potassium, and 61.32% by mass of Nitrate.  So if we take the nutrient concentration we determined above (100 ppm) and times that by the percentage of the specific nutrient we can find the concentration of that specific nutrient.  In this case, 100 * 38.67% = 38.67 ppm of potassium and 100 * 61.32% = 61.32 ppm of nitrate.  That is to say that if 10 grams of KNO3 was added to 100 liters of water, there would be 100 ppm of the nutrient solution, of which the Potassium concentration would be 38.67 ppm and the Nitrate concentration would be 61.32 ppm.  We can extend this further to find just the Nitrogen concentration which is 13.85% by mass in KNO3, and thus would equal 13.85 ppm.


A dosing solution is also relatively easy to determine.  Lets say you have a one liter container, and you want each ten milliliter dose to add 10 ppm of Nitrate to 100 liters of water.  Lets use KNO3 as the nutrient source.  First lets work out some solution maths.  1000 ml / 10ml = 100, or in other words there are 100 doses of 10 ml solution in each liter of solution.


To add 10 ppm of Nitrate from KNO3 to 100 liters of water, we have to do some of the above maths in reverse to make it easier in the long run.  We can't just add the Nitrate, we have to add all the other stuff that comes with KNO3 also, so lets work this out.  Nitrate is 61.32% by mass of KNO3, and we have to find the inverse.


100% / 61.32% = 1.63


So lets do a bunch of maths in reverse.


10 ppm (Nitrate) * 1.63 (Inverse percentage of Nitrate in KNO3) = 16.3 ppm (total concentration of KNO3 needed to be added to water)

16.3 ppm * 100 (liters of water) = 1630.

1630 / 1000 = 1.63.


Or in other words, adding 1.63 grams of KNO3 to 100 liters of water will raise Nitrate concentration by 10 ppm.  For our solution we simply multiply the grams by the number of doses in the dosing solution.


1.63 * 100 = 163 grams.


Or in other words, adding 163 grams of KNO3 to one liter of water, will give a 10 ppm dose of Nitrate to 100 liters of water with a 10 ml dose.  Pretty easy right............


Worse case scenario you should be able to purchase KNO3 for $25 per kg.  1000 grams / 163 grams = 6.13.  $25 / 6.13 = $4.08.  So this dosing solution would deliver 100 doses for $4.08.  You might pay $30 or so for something like this with a pretty label.  Best case scenario you purchase KNO3 for $1 per kg.  $1 / 6.13 = $0.16.  So this dosing solution would deliver 100 doses for 16 cents.  Companies purchase tons of nutrients at a time for an even better cost per kg, so it becomes quite clear that when you purchase nutrients in liquid form, basically you pay top dollar for water, a bottle, a label, laziness and a couple of grams of the stuff that actually does something for the plants.


Doing the maths long hand like this all the time is good for practice, but boring and repetitive.  I suggest to add the maths to a spreadsheet, I have a spreadsheet for aquarium use that is rather complicated, I plan to prepare a spreadsheet for this specific purpose with better ease of use at some later date.


If you're ready to go like a bull out of a gate, be mindful that inorganic nutrients have solubility limits.  In the case of KNO3 this is 316 grams per liter at 20 degrees Celsius.  If you try and add more it just will not dissolve, .  Also, some of the nutrients do not play nice with each other.  I'll discuss this and some other topics as time permits.



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Are you a chemist audionut ?


Only as far as I have taught myself.


I got a fish tank and all the advice was to just throw them in tap water, since they'll be fine.   Pfffft.....  I take a sick pleasure in knowing that the fish I have, are in water exactly to the parameters that I desire.  If you're going to do something, you might as well do it properly.


When we started getting 5 gram quarts (sad days when even your local drug dealers are shonky bastards), it was the final nail in the coffin to convince the other half that we needed to start growing our own.  I've only relied on the soil + low nutrient concentration fish water (90 ppm) for the garden veggies and herbs, but these weed plants are going to get everything I've learned, and what I will continue to learn.  I like learning new things, which is just as well, because I have the attention span of a monkey.


I like sharing what I've learned, since I'm likely to help at least one person, and it helps me to refine my knowledge of the subject by discussing it.  So please, ask questions if you have them.

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what do think about constant use of inorganic ferts on soil , farmers seem to every year have to put

more on & more on & more on & moron & moron & moron , sorry Dr Elaine Ingham joke 


but that is a serious question cause even if in pots , do you flush the soil & reuse it , where do you

put the waste from that flushing , where do you put the used soil if not using again & how long do the salts

hang round in the environment 


Just an organic grower here if you didn't already guess 


That's a shit load of work you put in there audionut , must like your aquariums by the pic in other post ,

looks very clean would you be using PNSB purple non-sulphur bacteria , prob called something else in

aquarium land , you would use it to keep the tank clear of algae build up from memory 


What's audionut mean , you just like music , your a muso , work in the audio industry , DJ

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Well my knowledge of soil interactions isn't where I would like it to be, but here's what I understand.


Soils have a Cation Exchange Capacity that describes how well the soil can retain cations (positive charge nutrients).  Soil has a negative charge which allows it to retain these cations, but I'm not entirely sure what specific negative charges these are.  We have things like plain silca sand which has virtually no negative charge and no ability to retain cations, and things like humus (organic matter has the best capacity) which has a very large capacity to retain cations.


For me, everything that comes out of the soil that isn't eaten is return to the soil.  The plants suck up and store nutrients, and as they break down they will release these nutrients.  Then there's crop rotation and a bunch of other things you can do to account for the fact that plants take up more of some nutrients then others.  But in agriculture, stuff is constantly taken from the soil and virtually never returned.  Since agriculture is all about economy of scale, it's significantly easier and cheaper just to top it up with inorganic nutrients.


Cations with a stronger charge like Calcium (Ca2+) and Magnesium (Mg2+) have a greater attraction to the negative charge of soil, then those cations with a weaker charge such as Potassium (P+).  So if you over lime soil, basically what you do is push all of the weak charged nutrients off of the negatively charged soil and replace those with Calcium.  As the water drains away, so do all these nutrients that have been pushed from the soil.  With a sandy soil that has a low CEC, the same sort of principles apply, whereby there is a lack of negatively charged sites in the soil, these limited sites are filled with stronger charged nutrients, and weaker charged nutrients such as potassium are simply washed away.


Then there's just sheer strength in numbers, where you can dislodge stronger charged nutrients with weaker charged nutrients just with quantity.  H+ (acid) will perform this function as acidity (H+ concentration) increases.


You can never take something without giving something in return, or maintaining charge balance.  As plant roots expand through the soil, they search for more and more nutrients (especially for things like Phosphorus, which is often in excess quantities required for plant growth in the soil, but mostly in forms that are unavailable for plant uptake, and so the plant must spread roots in an effort to find available forms of P), and as they find these nutrients one of the methods they use for uptake is to release a H+ ion (which helps to free the wanted cation (I'll delve into available and unavailable forms of nutrients and how H+ affects this process at a later date)), and thus replaces the uptaken cation (such as Ca2+, Mg2+, P+) with a H+ ion.  Clearly, without a resupply of these cations they will eventually deplete.  The good thing about H+ ions is that they are weak and are easily dislodged, allowing for easy exchange with other (wanted) cations (nutrients).


Ratios between nutrients only really shows us that we need more of some nutrients then others.  There is no one specific ratio of nutrients that provides optimal growth.  The goal is simply to ensure adequate supply of all of the nutrients.  If the plant has too little of a nutrient then it struggles to perform certain processes required for growth and begins to show deficiency symptoms, with decreasing supply leading to decreasing ability to grow, up to the point of death of the plant.  At some concentration, there will be an excess of a nutrient to the point where it also begins to effect plant growth, and again increasing concentration can lead to plant death.  But there's actually quite a wide band of nutrient concentrations with which the plant will grow without distress.


But of course, when growing weed we generally don't just want a healthy plant without signs of distress, we want those fuckers growing at the fastest rate possible.  Almost always Nitrogen is the limiting nutrient on growth rate.  So we can have some concentration of nitrogen that does not meet the demands required by the plant for adequate growth without distress, some concentration that supplies adequate levels required for healthy growth, some concentration that increases growth rate of the plant, and some (even higher) concentration that begins to have negative affects on the plant.  I believe it's important to understand these concepts, because if you have a healthy plant and you're looking to increase the growth rate, then an oversupply of nitrogen probably isn't the best way to go about it.  If the plant is healthy, then you're already above the concentration required for adequate growth, and you should probably be looking at only small increases in nitrogen concentration to ensure you don't reach the point of toxicity.


So anyway, I'll try and get back on topic.  Everything works in a cycle.  Soil has N, plant uptakes N, plant breaks down releasing N back to soil (some of the N in weed gets released to the atmosphere when we burn the plant).  That nitrogen atom doesn't cease to exist, it doesn't loose effectiveness over time, it simply cycles through life.  When people describe old soil, what they're probably really describing is soil that has lost (some of) it's CEC, soil that has all of the CEC filled with an imbalance of nutrients (excess of some, and not enough of others), and/or soil that's left to dry and contains no microbe life forms.  Sodium (Na+) for instance is an issue, since it tends to repel water and thus is not easily leached from soil.  If you use water with an excess of sodium (some tap water), gradually and eventually you will fill the soil with sodium ions rendering it very poor for plant growth without significant effort to reclaim.  So you should be very careful with "flushing" soil, since you're likely to reach the goal of removing excess amounts of nutrients added during a growth cycle, by rendering the soil inert after some time.


edit:  If you have a problem with salinity, you should really look into water filtration such as reverse osmosis.


I'm not sure if all of that really answers your question(s).  I guess what I am trying to emphasize is to always maintain soil balance.  A good soil, especially one that is constantly topped up with nutrients will always have way in excess of nutrients then what the plants need for healthy growth.  If you've added bulk Nitrogen to the soil and you want to flush this excess out, ideally you should put the waste on things like tomato plants that are nitrogen hogs, and will appreciate the added nitrogen.  Otherwise, just dump the waste water where it will not leach into rivers and creeks, as excess nutrients leached into these waters has environmental effects.  It's not the inorganic nutrients in and of themselves that are the problem, it's simply there affect on the concentrations of the available nutrients.  Organic nutrients take time to break down and become available, inorganic nutrients are primed and ready to go.  The soil could just go into the garden, it will eventually break down.  As I touched on above, the salts never dissipate to nothing, they cycle.


With my aquarium, I try and mimic nature.  That old girl has been running this planet for some time, so she probably knows what she is doing.  I control algae with predominantly three methods, fresh water, consistency and life.  Lots of water changes, where I currently do 15% fresh water daily by simply turning a ball valve on and off, and knowing what I'm like, I'll eventually replace the manual ball valve for some electronic one, meaning one less manual procedure to perform.  Consistency.  Life adapts, and so by maintaining consistency, and by this I mean not allowing things to become excess or deficient, the life adapts to the parameters of the water.  Healthy plants will discourage algae, and so by maintaining consistent parameters that support the life of the plants, they spend more time growing and discouraging algae, then attempting to constantly adapt to changing conditions.  And finally, algae eating life such as catfish, algae eaters, snails, etc.  I'm actually at a point where I spend a little effort trying to promote algae growth to maintain a free food source for this life.


I've been making a massive push towards self sufficiency in all aspects of my life recently.


The username, ha.  I really like music.  I never got into production or engineering (lazy fucker basically), but I really enjoy sitting down and listening to some tunes.  If you're going to do something, you might as well do it properly, yeah.......  So I tend to do a lot of things to reach this goal that make people think I'm a nutcase (hence the last three letters of my username).  25 years ago I was hanging mums nice thick blankets up near walls because I was just smart enough to understand reflections.  These days thanks to technology I can do things much more efficiently, but money is still the same issue it was 25 years ago, and so I still do things that are extremely focused on function, with virtually no effort expended on, "wow that looks really pretty".  If it has improved the sound hitting my ears, job done!  This mindset extends to most things really, my fish tank has water that serves a specific function, but the amount of crap I have around the tank to support this little patch of water, I'm sure most people would call crazy.  Efficiency and function.


I hope that answered at least some of your questions.  Cheers.

Edited by Audionut
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