GARDENERS FACTSHEET NO. 18
SEPTEMBER, 1979
ORGANIC GARDENING
Soil Fertility
Dr. John M. Gerber
Vegetable Specialist
Many gardeners have chosen not to use pesticides and
fertilizers manufactured in laboratories and factories but they
would prefer to use those, produced by nature. Among the
reasons cited for gardening organically are energy conservation,
fear of poisons, the high cost of chemicals, environmental
concerns or simply the idea “natural is better”. Regardless of the
reason, all gardeners have the right to choose the method they
prefer. This factsheet is written to help those people who have
chosen “the organic way”.
Most practices and principles of organic gardeners are the
same as non-organic gardeners. Both are involved with choosing
varieties, seeding, transplanting, weeding, staking and harvesting.
The few differences that do exist usually involve the controversial
topic of concentrated chemicals in the environment. Most organic
gardeners prefer to substitute fertilizers such as compost and
manure for concentrated fertilizer such as 10-10-10. Although it is
possible to supply plants with adequate nutrition without using
concentrated fertilizers, a good understanding of soil fertility and
a lot of hard work is required.
SOURCES OF NUTRIENTS
Most of the nutrients required by green plants are available
in abundant quantity from the air, water and soil. A good garden
soil that receives periodic additions of organic matter will contain
all of the trace elements and most of the major elements required
for growth. (See Gardeners Factsheet No. 16 for a discussion of
essential elements.)
The elements of greatest concern are nitrogen, phosphorus
and potassium. These elements can be supplied naturally by soil
organic matter and soil minerals. However after several years of
cropping, even virgin soils will be depleted of these nutrients. Soil
organic matter and soil minerals release nutrients slowly over time,
however not in great enough quantity to supply rapidly growing
vegetables.
It is necessary to add supplemental nutrients each year in
order to equal the elements extracted by fast growing vegetables.
These supplements can be made by concentrated fertilizers such
as potassium nitrate or by organic fertilizers such as dried blood (a
by-product of beef slaughterhouses).
Nitrogen
Nitrogen is required by plants for maximum leaf production.
When nitrogen is limiting, plants appear yellow (chiorotic) and
grow slowly, often producing smaller than normal leaves.
Although soil organic matter releases nitrogen continuously over
time, the amount released cannot satisfy vegetables during their
period of maximum growth. Most vegetables produce a major
portion of their growth from three weeks through eight to ten
weeks after seeding. In order to produce good quality vegetables
they must receive adequate nutrition and water during this
period.
Additional nitrogen must be supplied from a fertilizer such
as manure, dried blood, fish emulsion or cottonseed meal. Each
of these sources can supply enough nitrogen if used in adequate
quantity (Table One). Since organic fertilizers do not release
nitrogen immediately, they should be applied at planting time in
order to be available during the peak nitrogen demand.
TABLE ONE.
Amount of Organic Fertilizer Required per
100 sq. ft. to Supply Adequate Nitrogen
Fertilizer
Amount
Fresh Manure (except poultry)
4 bushels
Poultry Manure
1 bushel
Dried Blood
5 lbs.
Fish Meal
10 lbs.
Cottonseed Meal
10 lbs.
Animal manure is one of our oldest fertilizers and is still
used today throughout the world. Although the amount of
nitrogen in manure varies according to its source, most fresh
manures other than poultry will contain ½ of one percent
nitrogen. This is 1/20th the nitrogen as in an equal amount of
10-10-10 fertilizer. Therefore, about 20 times more manure is
needed than 10-10-10 to supply the same amount of nitrogen.
Most of the available nitrogen is in the urine, so it is best to
apply fresh manure to the soil. Since tender seedlings may be
injured by concentrated urine, wait at least 2-3 weeks before
planting. Although you may plant immediately following an
application of decomposed manure, it will contain little available
nitrogen.
Poultry manure often contains 3-4 times more nitrogen
than cow manure. It is also drier and more concentrated, though
not quite as concentrated as 10-10-10 fertilizer. Less poultry
manure is used than cow manure to avoid burning plants with
too much nitrogen. Allow poultry manure to dry before applying
to the garden.
Dried blood is a by-product of slaughter-houses. As blood
is drained from recently killed animals, it is dried and ground into
a fine powder. Although blood is relatively expensive, it is a
good source of quickly available nitrogen.
Products and suppliers mentioned by name in this publication are used as examples and in no way imply endorsement or recommendation of these products
or suppliers.
Issued in furtherance of Cooperative Extension work, acts of Congress of May 8 and June 30, 1914 (as amended), in cooperation with the U.S. Department of
Agriculture, D.S. Padda, Director, College of the Virgin Islands Cooperative Extension Service. The College of the Virgin Islands Cooperative Extension
Service is an Equal Opportunity/Affirmative Action organization, providing educational services in the field of agriculture, home economics, rural
development, 4-H youth development and related subjects to all persons regardless of race, color, religion, sex or national origin.
Fish meal or fish wastes contain some nitrogen which will
become available to plants. However it is slowly available and
plant growth may suffer. Fish emulsion is a different product that
contains up to 5% nitrogen that is readily available. However it
can be quite expensive to supply plants with adequate nitrogen
solely from fish emulsion.
Cottonseed meal is a by-product of the cotton industry
which contains some nitrogen. It is not rapidly available and
should not be relied upon as the sole source of nitrogen for
vegetables. It also can be quite expensive.
Phosphorus
Most Virgin Islands soils contain some native phosphorus.
However the amount may be inadequate to support quick growing
vegetables. Also, phosphorus is rapidly tied up in an insoluble
form when the soil pH is above 7.5 (alkaline soils). Soils that
contain caliche (limestone) will require additions of phosphorus.
The concentrated fertilizer superphosphate is made from
rock phosphate treated with sulfuric acid in factories. This can be
done to some extent in the soil by adding elemental sulfur to
organic sources of phosphorus. The sulfur will lower the pH of
the soil immediately near the phosphorus and make it more
available.
TABLE TWO.
Amount of Organic Fertilizer and Sulfur
Required per 100 sq. ft. to Supply Adequate
Phosphorus
Fertilizer
Amount
Amount of 
Sulfur
Steamed Bone Meal
5 lbs.
1 lb.
Finely Ground Rock Phosphate
50 lbs.
10 lbs.
Fresh Manure (except poultry)
8 bushels
-
Poultry Manure
1 bushel
-
Steamed Bone Meal is a by-product of slaughter houses. It
is a relatively good source of phosphorus, however it can be
expensive.
Rock phosphate is ground rock that releases phosphorus
very slowly. Although laboratory tests indicate that rock
phosphate contains as much as 30% phosphorus, it is simply not
in an available form. Additions of rock phosphate will do little
good, especially in alkaline soils.
Potassium
Potassium is found in abundant supply in many Virgin
Islands soils. It is usually found in the same soils as caliche,
although it is not a direct result of caliche deposits. Before
carrying large quantities of organic potassium to your garden,
have the soil tested. If potassium levels are above 300 lbs/acre, no
additional potassium is necessary. If it is below 300 lbs/acre add
one of the organic potassium sources listed in Table Three.
TABLE THREE. Amount of Organic Fertilizer Required per 100
sq. ft. to Supply Adequate Potassium
Fertilizer
Amount/100 sq. ft.
Finely Ground Granite Dust
50 lbs.
Greensand
50 lbs.
Wood Ash
10 lbs.
Fresh Manure (except poultry)
4 bushels
Poultry Manure
1 bushel
Granite Dust contains about 5% potassium, little of which
will be available during this century. Finely ground rocks simply
will not supply elements rapidly enough for vegetables.
Greensand is mined from sedimentary deposits of rock
containing about 6% potassium. It is very slowly available.
Wood ash is an excellent source of potassium which has
been used for many years. It is rapidly available, yet is not likely
to burn plants. Do not use charcoal ash as it may contain toxic
amounts of sulfur.
Trace Elements
Trace elements are those elements required in very small
amounts. (See Gardeners Factsheet No. 16 for a discussion of
trace elements). Soils which contain adequate organic matter and
have pH range from 6-7 will provide all the trace elements
required by vegetables. Since many soils in the Virgin Islands are
quite alkaline (pH above 7), vegetables may appear yellow due to
deficiencies of iron, manganese or zinc.
Additions of elemental sulfur can lower the pH to below 7
and release many trace elements as well as phosphorus. Since
sulfur is either mined from the ground or produced as a by-
product, it is usually accepted by organic gardeners. The amount
of sulfur required to lower the soil p1-I to 6.5 depends on the
initial pH and the soil type. Table Four shows how much sulfur is
required per 100 sq. ft. of garden to lower the pH to 6.5.
TABLE FOUR.
Sulfur Required per 100 sq. ft. to Adjust 
pH to 6.5
Soil Type
Initial pH
Sandy
Loamy
Clay
8.5
4.5 lbs.
5.5 lbs.
7.0 lbs.
8.0
2.5 lbs.
3.5 lbs.
4.5 lbs.
7.5
1.0 lbs.
2.0 lbs.
2.5 lbs.
Soil Building
Organic fertilizers will substitute for concentrated fertilizers
such as 10-10-10 but will not alleviate poor soil building
practices. All gardeners should supply generous amounts of
compost to maintain soil organic matter at a level that promotes
good plant growth. Organic matter improves the tilth and
friability of soil. It increases the drainage in heavy soils and
helps sandy soils hold more water. Organic matter is truly the
gardeners best friend.