> Can anyone help me compare the following two methods of applying
>rock phosphate:
>
>1) Using micronized rock phosphate fed into irrigation lines at 25-100 lbs
>per acre. ($30 for 25 lbs)
>
>vs.
>
>2) Field broadcasting of one ton/acre of granulated rock phosphate ($140/ton)
>
>
>A local farmer has asked me what makes more sense, and I'm not sure how to
>compare them. Any ideas???
Golly, I just can't get away from this, can I ?
Actually I don't think a lot of either method. Micronized rock
phsophate is still *suspended* in water, not dissolved, and it seems
ready made for plugging something somewhere. Irritation systems (excuse
me, irrigation systems) are enough trouble without giving them another
excuse to act up. $30-120/ acre is an outrageous cost for 5 - 20 lbs
of *total* phosphate, of which only 10-20% will be available the first
year.
Field broadcasting is a better option, but still not a good one. If
it's Idaho phosphate the granulation is usually pretty good, but if
it's from Florida what they call "granules" are really small clumps
that didn't fully break up when they dried and ground the old
slime-pond residues. Your biggest challenge in New Mexico --- or most
anywhere west of about 98-W longitude --- is that the soils are usually
alkaline. The stable form of phosphate in those soils is calcium
phosphate, which is the same stuff you'd be adding and expecting to
have released. Doesn't work very well.
Phosphate rock needs an acid environment (preferably below pH 6.3) to
break down.
Your best bet out there is to mix phosphate with manure, compost it,
and apply the compost to the land. The organic acids in the manure will
release much of the phosphate, and the phosphate will stabilise much of
the manure nitrogen against loss during the composting process. What's
going on chemically is that much of the nitrogen in manure is in the
form of ammonium carbonate (unstable). That reacts with the calcium
phosphate to form calcium carbonate (limestone) and mono-ammonium
phosphate (stable). This is the same MAP sold as 12-50-0 at the local
fertilizer plant, but if you 'make' it this way it's allowed for
organic production, while the stuff in a bag will get you busted. Go
figure.
You can add about 50 lb of phosphate rock per cubic yard of manure, and
if moderately composted and applied at about 5 tons/acre it will add
about 200 lb/ac of phosphate to the system, including phosphate from
the manure. Most of it will be reasonably available, so your client
should get a crop growing quickly to transfer the phosphate to the
living fraction of the system. Otherwise it will eventually react with
the soil and revert to calcium phosphate, which gets you back where you
started.
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