parasite may be brought about has been ex- 
tensively studied. The pioneering work of 
Millard and Taylor (54) on utilization of amend- 
ments to control potato scab was followed by 
great attempts to control plant diseases and 
nematodes by amendments. The literature has 
been exhaustively reviewed in recent articles 
(16, 30, 31, 39, 77, 78, 85, 91). Since this type 
of preferential disturbance of established 
microbial equilibria by use of amendments is 
of extraordinary significance in soilborne 
plant pathogens and nematodes, afew examples 
will be cited only from this particular group 
of plant parasites, 
Linford and coinvestigators (43, 45) obtained 
control of the root-knot nematode of pineapple 
by heavy applications of organic matter. 
Chopped-up pineapple plants were used as the 
organic supplement and cowpeas as the test 
plant for estimating populations of Heterodera 
marioni in soil after treatment, Linford et al, 
(45) suggested that ''decomposition results in 
a greatly increased population of total nema- 
todes in the soil, and that these, in turn, sup- 
port the building-up of large populations of 
plant and animal forms destructive to nema- 
todes, including nema-capturing fungi, non- 
trapping, fungal parasites, predacious 
nematodes, and predacious mites.'' Johnson 
(35) found that several mature dry crop 
residues, when added to nematode-infested 
soil, appreciably reduced root knot of tomatoes. 
The effectiveness of the amendments depended 
on the incubation temperature (36). Some 
attempts to control nematode populations by 
adding organic matter, with or without supple- 
mental nematode-trapping fungi, were un- 
successful (26, 50). Details of the work of 
Linford and coworkers and of other similar 
research were summarized recently (5, 26, 
74). 
Elegant in their inception and execution 
were the experiments by Garrett (29, 30) on 
the use of amendments to control Ophiobolus 
graminis, the "take-all'' disease of wheat. 
O. graminis spreads in soil along roots of 
its host plant, but it can also existin a resting 
stage in dead host tissue (29). The rate of 
growth of the pathogen along roots decreases 
with the increase of carbon dioxide in the 
surrounding atmosphere. Microbial activity, 
stimulated by the addition of organic matter 

85 
and by cultivation aimed at close packing of 
soil around roots, undoubtedly results in an 
increase of CO 2 in the immediate vicinity of 
the pathogen. This kind of indirect control of 
of actively growing mycelium along roots is 
distinct from the biological control of resting 
mycelium of this pathogen. Garrett (30) showed 
that addition to infested soil of organic matter, 
poor or lacking nitrogen, resulted in consid- 
erable loss of viability of the colonizing resting 
mycelium, The effectiveness of organic matter 
depended on its nitrogen content. He (30) 
suggested that supply of organic matter rela- 
tively poor in nitrogen resulted in great 
increase of the numbers of saprophytes, which 
assimilated the mgaqcelium as a source of 
nitrogen. 
Soil organic amendments may be expected to 
be ineffectual for biological control of a large 
and important group of soil-inhabiting fungi, 
such as Rhizoctonia, which has marked ability 
to colonize competitively organic matter lying 
in or on the ground. Considerable conflicting 
evidence exists concerning the effectiveness of 
various amendments and supplemental nitrogen 
for the suppression of R. solani, an gmnivorous 
and economically important pathogen. A given 
amendment was found to be effective in one 
location at one time and ineffective in a 
different location or at a different time. Also, 
supplemental soil nitrogen either increased or 
decreased the Rhizoctonia disease of several 
crops, depending on conditions, time, crops 
used, amendment used, and the like. 
At Beltsville, Md., we found that some 
organic amendments suppressed Rhizoctonia in 
soil and others were ineffective, yet some in- 
creased the competitive saprophytic activity 
of the pathogen and the amount of disease 
incited (17, 18, 65). Saprophytic behavior 
and therefore survival of Rhizoctonia was af- 
fected considerably by the carbon-to-nitrogen 
balance of the colonizable substrates (20, 66), 
In fact, in experiments where carbon/nitrogen 
ratios of substrates had been modified by the 
addition of glucose or ammonium nitrate, to 
produce a range of carbon/nitrogen ratios, 
the carbon-to-nitrogen balance determined the 
extent of competitive colonization and survival 
of R. solani (66), A decrease of substrate 
carbon/nitrogen ratio, by the addition of in- 
creasing concentrations of nitrogen, produced 

