Agriculture Reference
In-Depth Information
Rock Phosphate (Mainly Phosphorus and
Calcium)
There are two main sources of phosphate we use in organic gardening. Both of them
come from the same parent material, which is ancient animal bones. They contain a lot of
both phosphorus and calcium, although the amounts can vary greatly depending on the
source. They are called hard rock phosphate and soft rock phosphate, and they cost $20 to
$40 for 50 pounds, twice as much as many of the calcium sources. They can be difficult to
source, but some farm supply stores have them. I wish the most important products were
more readily available, but I'm sorry to say that's not always the case. I still have to recom-
mend them because they're the most effective. Unfortunately, it may be a bit of work for
you to find a couple of them. It's work for me, too.
Hard rock phosphate is the more commonly available of the two, containing approxim-
ately 30% phosphorus and 30% calcium. It's mostly tricalcium phosphate and is difficult
for plants to get out of the soil because the phosphorus is bound tightly with the calcium.
The calcium will become available over time, but we don't use it to rapidly change the ratio
of calcium on the cation exchange sites.
The phosphorus is only 3% available and takes many years to become available to
plants. It's often ground to a find dust and granulated with chelators that are supposed to
make it more available to plants, but it's still not the best. I'll use it in a pinch, though, in
order to build up long-term phosphorus reserves. In my experience, it won't give as effect-
ive a result as soft rock phosphate. If you do use it, be sure you're also focusing on coaxing
together the most active, healthy soil food web you possibly can. Of course, it's a good idea
to do this anyway.
Hard rock phosphate is also treated with acids to create chemical fertilizers like super-
phosphate (0-20-0) and triple superphosphate (0-46-0). These are more available to plants,
but 85-90% of the application will be leached, wasting money and polluting the environ-
ment, not to mention the other soil problems these fertilizers can cause.
Soft rock phosphate, on the other hand, is a by-product of hard rock phosphate mining.
It's so much better than hard rock phosphate that I've driven hours just to go get a couple
hundred pounds. It can be difficult to find. Traditionally separated from the hard rock as an
impurity and trucked away to holding ponds, it's now considered superior to the hard rock
form because it's colloidal, meaning it's in a form that won't leach and doesn't tie up as
tightly as hard rock phosphate.
Like hard rock phosphate, it may only be listed as 3% available, but over a few years, it's
100% available to plants. It does combine with calcium, but not in the unbreakable bond
that hard rock creates. It's also a good source of silicon and many trace minerals.
