are bulky and may contain a high percentage of water—
Table 18.2
so considerable work is needed to apply them per unit
Products Used by Organic Growers to Supply Nutrients
of nutrients. The timing of nutrient release is uncertain,
%N
%P2O5
%K2O
because it depends both on the type of organic materials
Alfalfa pellets
2.7
0.5
2.8
used and on the action of soil organisms. Their activi-
Blood meal
13.0
2.0
—
ties change with temperature and rainfall. Finally, the
Bone meal
3.0
20.0
0.5
relative nutrient concentrations for a particular manure
Cocoa shells
1.0
1.0
3.0
used may not match soil needs. For example, manures
Colloidal phosphate
—
18.0
—
may contain high amounts of both N and P when your
Compost
1.0
0.4
3.0
soil already has high P levels.
Cottonseed meal
6.0
2.0
2.0
Fish scraps, dried & ground
9.0
7.0
—
Selection of Commercial Fertilizer Sources
Granite dust
—
—
5.0
It is recommended to include organic fertilizer sources
Greensand
—
—
7.0
as part of a nutrient management program to sustain
Hoof & horn meal
11.0
2.0
—
soil health, but on many farms additional commercial
Linseed meal
5.0
2.0
1.0
fertilizers are still needed to achieve good yields. On the
Rock phosphate
—
30.0
—
global scale, until better practices (use of cover crops,
Seaweed, ground
1.0
0.2
2.0
better rotations, decreased tillage, and integrating ani-
Soybean meal
6.0
1.4
4.0
mal and plant agriculture, etc.) are used on farms, com-
Tankage
6.5
14.5
—
mercial fertilizers are still needed to meet the demands
Notes:
of our growing population. There are numerous forms of
1. Values of P2O5 and K2O represent total nutrients present. For fertilizers listed in commercial fertilizers, many given in table 18.1. When
table 18.1, the numbers are the amount that are readily available.
2. Organic growers also use potassium–magnesium sulfate (“sul-po-mag” or
you buy fertilizers in large quantities, you usually choose
“K-mag”), wood ashes, limestone, and gypsum (listed in table 18.1). Although
the cheapest source. When you buy bulk blended fertil-
some use only manure that has been composted, others will use aged manures
(see chapter 12). There are also a number of commercial organic products with a
izer, you usually don’t know what sources were used
variety of trade names.
unless you ask. All you know is that it’s a 10-20-20 or a
Source: R. Parnes (1990).
20-10-10 (both referring to the percent of available N,
P2O5, and K2O) or another blend. However, below are a
• Although muriate of potash (potassium chloride) is
number of examples of situations in which you might
the cheapest K source, it may not be the best choice
not want to apply the cheapest source:
under certain circumstances. If you also need mag-
• Although the cheapest N form is anhydrous am-
nesium and don’t need to lime the field, potassium–
monia, the problems with injecting it into a soil with
magnesium sulfate would be a better choice.
many large stones or the losses that might occur if
you inject it into very moist clay may call for other N
Method and Timing of Application
sources to be used instead.
The timing of fertilizer application is frequently related
• If both N and P are needed, diammonium phosphate
to the application method chosen, so in this section we’ll
(DAP) is a good choice because it has approximately
go over both practices together.
the same cost and P content as concentrated super-
Broadcast application, in which fertilizer is
phosphate and also contains 18% N.
evenly distributed over the whole field and then usually
210
Building SoilS for Better CropS: SuStainaBle Soil ManageMent