Re: what minerals are found in lahar/volcanic ash?

Before I try to answer your questions, I need to give you a little 
background.  There is an important difference between the way geologists 
use the word mineral and the way it is used in ordinary language (eat your 
vegetables, they have lots of vitamins and minerals in them).  Geologists 
use the word to refer to naturally occurring crystalline solids.  These 
are almost always chemical compounds, some of which can be quite complex.  
There are more than 3000 known minerals, but only a handful are common.  
When people using ordinary language talk about minerals, as in vitamins and 
minerals, they usually mean metal ions or other ions that are found in 
foods.  These could include potassium, calcium, iron, sodium, zinc, etc.

To grow plants, you need three critical nutrients, nitrogen (N), potassium 
(K), and phosphorous (P).  At least in the US, plant fertilizer comes 
labelled with the percent N,K, and P on the bag.  Plants need some iron, to 
make chlorophyl.  Plants also need a number of trace elements, things that 
can be very scarce, but are very important to have in small quantities as 
they allow the plants to make the right protiens and other molecules 
necessary for life.  Finally, some elements found in minerals, such as 
nickel, can be toxic to plants, if present in any abundance.

Lahars and ashes typically come from volcanoes of intermediate to high 
silica content (andesite to rhyolite magmas).  In those magma types there 
is between 60 and 75% SiO2 (silica) and smaller amounts of magnesium oxide 
(MgO), iron oxide (FeO), aluminum oxide (Al2O3), potassium oxide (K2O), 
sodium oxide (Na2O), and calcium oxide (CaO).  There is also a small amount 
of phosphate (P2O5) ion and there may be small amounts of other things 
including fluorine and chlorine, sulfur, etc.  These elements organize 
themselves into common minerals as the magmas begin to crystallize.  These 
include quartz (SiO2), alkali feldspar (KAlSi3O8), plagioclase feldsapar 
(CaAl2Si2O8 to NaAlSi3O8), and hornblende and biotie (complex Fe and 
Mg-bearing silicates).  Other minerals such as apatite (calcium phosphate) 
and magnetite or ilmenite (iron to iron-titanium oxide) are usually present 
in very small amounts.  Topaz (Al2SiO4F2), fluorite (CaF2)and other 
minerals may form rarely in deposits from a magma enriched in fluoride or 
other unusual elements.

Since a volcano erupts liquid magma, by definition, some part of the magma 
has not yet crystallized, and sometimes none of it has.  As this liquid 
erupts it is cooled very rapidly.  The eruptions of these volcanoes is 
often violent and so the resulting ash is often composed largely of 
fragments of glass with the same chemical composition as the magma.  
Because it lacks a repetitive internal structure, volcanic glass is not 
considered a mineral by geologists, but it does contain the sorts of 
chemical nutrients that the rest of the world calls mineraals.

Soils that form directly from ash, or from lahars, where ash mixes with 
water and washes down the mountain, will have available the elements that 
were present in the magma.  These elements leach out relatively easily, 
especially from the ash particles, which have high surface area to volume 
ratios.  Of the major plant nutrients, K will likely be abundant, P is 
present but not so abundant and N is almost absent.  N can be obtained from 
air by plants, especially those with special bacteria living on their roots 
that take N2 from air and turn it into the nitrates that plants can then 
use (legumes, like peas, clover, or vetch are very good at this).  If 
plants can't get enough N, the nitrogen-rich fertilizers would need to be 
added, such as composted manure.  

Healthy soils also need organic matter and soil microrganisms to help 
plants grow well.  These are not going to be present in fresh ash or lahar 
deposits, but will accumulate over time.  They can be supplied by working 
composted plant material into the soil.

Volcanic soils are typically very fertile, once they have had a little time 
to weather and start to break down and release their nutrients.  This is 
one reason people often live and farm on the sloes of dormant volcanoes.  
Since you are from the Phillipines, I imagine you are asking this with 
reference to the ash from Pinatubo.  Unfortunately, I can't give you any 
specific data on the nutrient content of that ash.

I hope this helps,
Dave Smith, Geology and Environmental Science
La Salle University, Philadelphia, PA 19141