Teaching Organic Farming and Gardening

Soil Chemistry and Fertility

Unit 2.2 | 23
Students’ Lecture Outline

figUre 4. the phOSphORUS CyCLe
(ADApteD FROm WWW.extenSIOn.Umn.eDU/DIStRIBUtIOn/CROpSyStemS/)

phosphorus is highly immobile and roots only take up what is only a fraction of an inch away,
so if the p is not close to the root, it will not be available. maintaining adequate moisture
throughout the growing season will facilitate p movement. phosphorus availability is also
affected by temperature. In cool temperatures p may show as deficient in plants even though
there is enough present for the plant needs. As temperatures warm, deficiency signs may go
away. Organic p tends to be more available than inorganic, so use of organic amendments, along
with promoting biological activity, will make p more available.

e) phosphorus in amendments

the best source of phosphorus to use in the garden is “recycled” — compost and manures.
Compost and manures are fairly low in p content but may have more available p than some
other sources. poultry manure has the highest amounts. Organic amendments should have a ph
between 6.5 and 6.8 to maximize availability. If these are not sufficient to meet needs, bone meal
(finely ground bones from slaughterhouses) or colloidal or rock phosphate are some sources that
are high in phosphorus. the negative side to bonemeal is that it is usually dependent on large-
scale confined animal operations. Colloidal and rock phosphates come from non-renewable
sources. Another drawback to bone meal and rock phosphate is that, while they are p rich, the
phosphorus is slowly available and most of what is applied serves more for crops to be planted in
following years.

Phosphate

added in fertilizer

Inorganic

available

phosphate

Uptake of

phosphate Mineralization of

organic phosphate

Fixed phosphate

Runoff to water body

Crop residue

and manure

return phosphate

to soil

Removal by crops

Cathy Genetti Reinhard