Everglades Station Mimeo Report 58-16


 TESTING ORGANIC SOILS*

 Howard E. Ray**



 Four factors are essential to a worthwhile soil testing program. First,
acceptable soil sampling procedures must be worked out. This is a large subject
in itself and will not be discussed further at the present time. Second,
analytical procedures must be developed which will afford rapid and reasonably
accurate means for determining the amounts of plant-available nutrients in a soi7,
Results obtained from such procedures are relative, not absolute, and the numeri-
cal values obtained are important only on a relative basis. Third, using the
procedures that have been developed, minimum or "threshold" soil levels of a
given nutrient required for optimum production must be determined for each crop.
Such information must be obtained experimentally. Fourth, means for calculating
the amount of a nutrient that must be supplied through fertilizer to raise the
soil to any desired fertility level must be determined.

 Under Everglades conditions, the problem of developing a useful soil test
is somewhat simplified by the fact that the soils are all highly organic, and of
more or less the same general type. They range in organic matter content from
about 40 to 90 percent, contain as much as 3 percent nitrogen expressed on a dry
weight basis, and furnish adequate amounts of nitrogen for optimum crop produc-
tion in many instances. Due to the limestone underlying the area, these soils
are not highly acid, and are well supplied with calcium and magnesium. Otherwise,
however, they are quite low in mineral content. Phosphorus, potassium, copper,
manganese, zinc, and boron must commonly be supplied through fertilizer to permit
successful crop production.

 Methods presently used in the soil testing laboratory at the Everglades
Experiment Station have evolved principally through the work of Dr. W. T. Forsee,
now Chemist in Charge. Soil acidity is measured by means of a glass electrode
pH meter using a 1:2 soil-water ratio (volume basis). Phosphorus is determined
by standard colorimetric procedures on a water extract, and potassium is
determined on a 0.5 N acetic acid extract by means of a flame photometer
(Beckman DU). All samples are measured rather than weighed. Not only does this
increase the rapidity of the procedure, but it is believed to be more satisfactory
for organic soils in the Everglades where bulk density changes as time progresses.
Calcium and magnesium are not routinely determined as these elements are almost
invariably present in adequate amounts. Results are reported in terms of pounds
of elemental phosphorus or potassium per acre.

 Needs for phosphorus and potassium on organic soils, together with pH
 relationships, have been investigated extensively by Forsee to correlate soil
 test results with crop requirements. In this connection, experiments have been
 conducted on Everglades organic soils with celery, potatoes, beans, cabbage,
 head lettuce, escarole, endive, sweet corn, grasses, and fiber crops. As a
 result of these investigations, minimum levels of phosphorus and potassium in
 the soil which are necessary for satisfactory production have been worked out
 for a number of the crops grown in this area. These levels are&summarized in
 Table 1.

 Presented at Southern Soil Test Work Group Meeting. Gainesville, Florida,
 June 19, 1958.
 ** Assistant Soils Chemist r :
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June 20, 1958