Sources of Nitrogen – and of Potassium – -What’s Best

Q.   I understand that urea is not Ammonium nitrate as required in the pre-plant fertilizer and the weekly feed. I would like to know what is Ammonium Nitrate?  Can you tell by the formulation?  Here in Trinidad, West Indies an agronomist at an agricultural supplies store has been selling me urea for Ammonium Nitrate.

Well I have been ‘happily’ using same and recently thought that my patchoi
(bokchoy) and lettuce needed more nitrogen, since they did not look green enough.  So, Mr. Agronomist recommended 30:10:10, which worked fine, though I was  bothered.  I knew it was not the Mittleider way.

 Also what is the difference between Ammonium Nitrate and Ammonium Sulphate?  I thought previously it was just the % nitrogen in each (i.e. 46% and 21%).

Another agronomist at the farmer’s market has advised me to use the Sulphate of Potash instead of the Muriate of Potash, as it was less acidic.  Again what is the difference between the above two and Potassium Chloride?  Well you may have guessed, Mr. Agronomist at the agricultural supplies store sold me Red Potash for Potassium Chloride.

This I know, that no one needs to guess that the labelling on fertilizers and
lack of respect by some for the Mittleider Method of Grow Box Production is causing failures untold by Grow Box Farmers here in Trinidad.

Your kind assistance is sought please.

A.  These questions are IMPORTANT to understand, so please pay close attention to my answers, and I’ll try to remove the mysteries.

Ammonium Nitrate is NH4 NO3.  It has an ammonia (with hydrogen) form of nitrogen and an “ate” or oxide form of nitrogen.  the nitrate is immediately available to plants, while the NH4 requires a step before it’s available.  That step happens quickly in warm weather, but in cold weather your plants will suffer if all they have is the NH4 type.

Ammonium Sulfate is NH4 SO4.  So it has ONLY the ammonia form of nitrogen, with the minor problems as discussed above.  It also has sulfur in the “ate” or oxide form.  The sulfur is readily available, and is an essential plant nutrient, so that’s good.  However, sulfur also LOWERS soil pH.  So, if you receive more than 20 inches of rainfall per year your pH is already BELOW 7 (neutral). Therefore it’s acid, and you probably don’t want any extra sulfur.

Urea is CH4N2O, or CO(NH2)2.  The nitrogen is bound up with both carbon and hydrogen, and it takes several chemical changes for the nitrogen to become available for plant use.  While those chemical changes take place the nitrogen is constantly tempted to volitalize and go back into the air, where it came from. 

Very often when urea is used the plants end up getting much LESS than the 46% nitrogen the fertilizer started with.  A not-too-funny sidenote:  Urea is the only form of the above three that is considered “organic”.  Urine is high in urea.

Find out what the chemical formula is for the 30-10-10.  And then get more phosphate and potash, so you end up with something close to 13-8-13.

The answer to your next question – about your potassium source – is also
important for everyone to understand.  Muriate of Potash is potassium chloride. 

Both potassium and chloride are essential plant nutrients.  According to one encyclopedia, 95% of all potash fertilizer used in the world is muriate of potash.

Potassium sulfate also contains two essential plant nutrients.  However, the
sulfur lowers soil pH, so UNLESS YOU RECEIVE LESS THAN 20″ of rainfall per year, you probably should be using potassium chloride.  You CERTAINLY get more than 20″ of rain per year in Trinidad, so you already have acidic soil, and the potassium sulfate makes it MORE acidic.  Go tell the guy he’s got it backwards.  Isn’t it interesting that anyone can call themselves an agronomist!

As to whether you should use the red potash instead of white potash, let’s
consider the differences.  Red potash contains a small percentage of iron ( an
essential plant nutrient) so might be considered better.  However, it also
contains about 4% sodium chloride, or table salt, whereas white potash only
contains about 1% table salt.  In my experience, I’ve found white potash to be a little higher grade, and therefore a better product.

Good luck with your red potash.  If you see any evidence of salinity (growth
stops, unnatural wilting), flood your beds with clean water three times to wash
out the salts.

Spread the word to those who are not yet converted to the Mittleider Method in Trinidad!  You all might as well be exporting to those countries in South
America that are so close to you.