Evening star. [volume] (Washington, D.C.) 1854-1972, 1957-01-27
GARDENS
~ 81
GARDEN
GUIDANCE
Use Of SIudqe as Fertilizer
By W. H. YoUNeMAN

The gardeners of the Wash
ington area frequently ask ques'
tons about the use of sewage
sludee as a fertilizer. Is it SAT-
to use in the garden? DR.. MyroI
S Anderson, in Department O.
Agriculture circular, has at
tempted to answer this and othel
questions. Many of his remarks
apply to the sludge from thu
Districts sewage disposal sys'
tenn.
The Washington system is the
type known as the "digeStel
system" In it the sewage i!
settled by gravity and subsequent
anaerobic digestion of the solids
This takes from 10 to IA day:
after which the material passes
through a rotary-vacuum filtel
to remove most of the water
Standing in tanks further Solidi-
fies the mass before it is taker
away for use on lawns and fields
Digested sludges usually have
an offensive odor that persists
for some time, especially if APP
piled to the soil in cool weather
However, the odor differs greatly
from that Of raw sludge.
Comparative Values
The fertilizer value Of digested
sludge is rather low. The nitro
gen content will vary. but it is
approximately 2 per cent; phos-
phoric acid about Ia per cent;
and potassium oxide, Ola pei
cent, which is very low. OF the
secondary constituents. calcium
oxide, "a per cent; magnesium
oxide, 0.6 per cent; sulphur 0.9
per cent; and iron oxide, "A per
cent, all of which are too low
to be Of much significance.
The minor, or trace elements,
are relatively much higher. In
parts per million they are: Cop
per, 435; zinc, 1900; boron, 8;
manganese, 140: and molybde-
mum, "a
These amounts are greater
than are normally found in the
soils and probably would main
tain an adequate supply for
plants. However, little is known
of the availability of these ele-
ments to growing plants. Are
they in a form that plants can
take up?
Nominal Plant Foods
From the foregoing it would
appear that the sludge from the
Washington plant has only nom
inal primary plant foods-about
35 per cent-and that the Sec
ondary constituents are quite
moderate in amount, but of some
significance for lawn use. The
minor elements are in large
enough, quantities to be impor-
tant in those soils where leach
ing has reduced the level to Pc
low plant needs.
However, all Of these elements
are not fully available to the
plants in the form in which they
exist in the sludge.
Only about 8 per cent of the
nitrogen in the digester system
sludge is available to the plants
within q comparatively short
time. The availability of the
others has yet to be determined,
except through crop tests which
show increased yields Of varying
amounts resulting from their
application at rates similar to
the use Of animal manures.
Restriction as to Vegetables
The question as to the sanitary
condition of sewage sludge is
generally answered as being sat
isfactory except where vegetables
are to be grown. 1t is the opinion
of the Committee on Sewage
Disposal of the American Public
Health Association that all
danger is removed by action in
the soil after a period Of about
three months during growing
season.
The above summarizes DR..
Anderson's material insofar as
the value and safety of the