Agriculture Reference
In-Depth Information
Phosphorus
Phosphorus is the other most important mineral. It's generally present in the soil, but is
often unavailable in soils with a low organic matter content and a poor soil food web. Hav-
ing active biology in the soil is probably the most important factor in getting phosphorus
into plants. Carey Reams taught that calcium and phosphorus are two of the most limiting
elements in soils.
Phosphorus is the element P on the periodic table. Fertilizers show phosphate rather than
phosphorus on their labels. Phosphate is the main form of phosphorus that plants use. Actu-
ally, fertilizers show
available
phosphate, which is the phosphate that is more readily avail-
able to plants, not tied up. Available phosphate is P
2
O
5
, two atoms of phosphorus attached
to five atoms of oxygen. In NPK fertilizers, for example, the middle number represents
available phosphate as a percentage of the weight of the bag. Multiplying that by 0.44 gives
us the amount of phosphorus. If the middle number is 10, the phosphorus is 4.4% of the
weight of the bag. Soil tests may give you the phosphorus number or the phosphate num-
ber. The nutrient testing chapter showed how to tell which is which.
Along with magnesium, phosphorus is necessary for photosynthesis. It's in every living
cell. It's the major catalyst in all living systems, which means its presence is vital for many
other reactions to take place in the plant, and for many other nutrients to get utilized. For
example, it promotes more photosynthesis and higher brix. The phosphates not only help
produce the sugar, but also bring it to the roots where it's excreted to soil microbes. Then,
the microbes make more nutrients that become available to the plant, so the plant can make
more sugar.
It also circulates throughout the plant, up and down, carrying other nutrients to where
they're needed. For example, calcium bonds with phosphorus to create phosphate of calci-
um. The phosphate brings the calcium through the plant and drops it where it belongs. It
transports all nutrients throughout the plant with the exception of nitrogen. Phosphorus is
also needed in order for carbon and nitrogen to be made into all of the things they're made
into — amino acids, proteins, enzymes, vitamins and all of the things that are the basis for
plant health and the health of every living thing on earth.
A conventional soil test doesn't tell us much about phosphorus availability, as the num-
ber given is relatively inaccurate. A Reams test gives better information on phosphorus
availability, and for this, the ideal number would be 400 pounds per acre. For either test,
the ratio of phosphate to potash would ideally be 2:1 in general, not 1:2 as many labs re-
commend. It should be more like 4:1 for grasses and leaf crops, such as lettuce and greens.
When it gets lower than this, plants will suffer and be more prone to insect and disease
damage, and there will be more broadleaf weeds such as plantain. The only way to main-
