In order for crops to grow and develop, soil nutrients (chemical
elements) need to be absorbed by roots and moved into the plant.
These nutrients, coming from soil parent material or from added
fertilizers, function in structural and metabolic systems to enable
the plant to carry on processes necessary to living. To obtain
the full yield potential and produce a high?quality product, the
soil must contain enough nutrients to support vigorous plant growth
throughout the life cycle. Since the weathering of soil parent
material is a slow, long?term process, assuring proper nutrition
for crop plants generally requires the addition of fertilizers
to the soil.
When a grower decides to fertilize, he is faced with the choice
of which fertilizer to use and the task of determining how much
fertilizer to apply. No universal guidelines are available for
answering these questions as soils differ in fertility, and crops
differ in their nutritional requirements. The grower must apply
what seems to be the correct amount of fertilizers based on soil
tests, plant analyses, previous experience and advice from others.
Modifications can be made during the season or in the following
year based on how the crop is growing, and on the amount of time
remaining in the current season.
Personal beliefs often enter into the selection of a fertilizer
with many individuals choosing to use organic fertilizers rather
than manufactured products. Both types of fertilizer can be used
to supply soil nutrients to the plant, but with few exceptions,
natural or organic fertilizers contain nutrients in low concentrations
and relatively insoluble forms as compared with synthetic fertilizers.
The concentration and solubility of the nutrients govern how much
fertilizer to apply because the nutrients must be dissolved in
soil water before they can be absorbed by plants.
Nutrients in organic fertilizers become soluble through a process
termed mineralization or weathering where the complex molecular
structure is broken into smaller water?soluble mineral ions that
can be utilized by the plant. Some organic substances have a rapid
rate of mineralization and release all of their nutrients during
the first growing season with little to no residual value for
crops in subsequent years. Other organic substances have a slow
rate of mineralization and release portions of their nutrients
over several years and may be considered soil?building materials.
Materials slow to mineralize generally have limited value for
growth of plants in the year they are first applied. Some natural
rock materials that are very slow to weather may have little value
in supplying nutrients to a crop or in enhancing the fertility
of a soil.
In the fertilization of crops, growers must first be concerned
with supplying the primary macronutrients—nitrogen, phosphorus
and potassium. Since organic or natural fertilizers have variable
chemical composition, a balanced or adequate supply of all of
the primary macronutrients from one organic fertilizer is unlikely.
Therefore, more than one kind of organic fertilizer is usually
needed to provide sufficient crop nutrition in any system of organic
gardening. This situation differs from that of synthetic fertilizers
that are manufactured to contain from one to all three of the
macronutrients and can be purchased in practically any formulation.
Nitrogen, one of the most likely nutrients to be deficient in
most soils, is seldom lacking in organic fertilization programs
because of the relative availability and slow release of this
source of nutrient in organic materials. Some nitrogen accumulates
in the soil from rainfall and from nitrogen fixation (the conversion
of gaseous nitrogen in the atmosphere to organic nitrogen) by
free?living microorganisms. This accumulation rarely exceeds 10
pounds per acre per year, however, and cannot support much crop
production. A grower who relies on precipitation for sources of
nitrogen is unlikely to have much production success.
Manure, one of the oldest known fertilizers, can serve as a
source of organic nitrogen. The actual nitrogen content of manures
varies with the type of animal and feed given to the animal. Poultry
manure is considerably higher in nitrogen than the manures from
larger farm animals, and the better any livestock is fed the richer
the nutrient content of the manure. Bedding, such as straw, wood
chips, or sawdust, that is added to manure greatly reduces the
nitrogen value. The nutrients are diluted and, if the manure is
incorporated into the soil, nitrogen is immobilized and unavailable
as microorganisms "tie?up" this nutrient in the process
of decomposing the bedding. Although nitrogen content is reduced
by one-half, manures with a high proportion of bedding should
be composted before they are mixed in the soil to prevent immobilizing
Fresh manures with low bedding content should be turned into
the soil as soon as possible after spreading to prevent volatilization
and the loss of nitrogen from the manure to the atmosphere in
the form of ammonia. Manures that are left on top of the ground
for 2-4 days have only half of the nitrogen value of those that
are plowed or tilled in immediately after spreading. Manures incorporated
into the soil release approximately 50 percent of their nitrogen
content for plant growth the first year. The rest of the nitrogen
is retained in the soil and builds the soil fertility for subsequent
cropping years. The nitrogen in organic fertilizers such as dried
blood, alfalfa meal and seed meals is released almost entirely
in the first season they are applied to the soil, leaving little
residual nitrogen available for the following season unless the
application was in excess of the requirements for the crop.
The organic grower has only a limited number of phosphorus fertilizers
from which to choose. Plant residues, farm manures, and composts
are, practically, too low in phosphorus analysis to be considered
for any purpose other than maintaining soil fertility following
a build?up of this element in the soil through the use of more
concentrated materials. Residues from the bodies of animals are
excellent sources of phosphorus with bone meal being the most
significant among available animal residues. Unfortunately, bone
meal, the oldest phosphorus fertilizer, is expensive, so its use
is generally limited to garden?sized plots.
Rock phosphate, mined from deposits, has a high phosphorus analysis,
but the material is of such low solubility that special application
techniques are required to achieve any benefits in the soil. Colloidal
rock phosphate, taken from a lower grade ore than regular rock
phosphate, is claimed to release phosphorus more readily than
regular rock phosphate. Although colloidal rock phosphate costs
the same as regular rock phosphate, the phosphorus content is
only about 2/3 that of rock phosphate. The low pH of the soil
helps release the phosphorus from the phosphate source. Super-phosphates
are manufactured by treating rock phosphate with sulfuric or phosphoric
acid, simulating the action of acid soil on the rock. Unless restricted
by marketing or philosophy, organic growers may want to use the
super-phosphates during an initial build?up of phosphorus reserves
in the soil. Once a soil test indicates an adequate level of phosphorus,
organic materials can be used to maintain phosphorus fertility.
Most plant residues and farm manures can serve as sources of
fertilizers in building and maintaining the available potassium
reserves of a soil. Except for rinds and peelings, the vegetative
portions of plants are higher in potassium than are the fruits
and seeds. Hay, straw, hulls (or shells) and any other plant residues
contain from 2-9 percent potash with the exact amount depending
on the plant product and on the fertility of the soil in which
the plants were grown.
Dried manures are about 2 percent available potash, and fresh
manures with bedding have about 10 pounds of available potash
per ton. Actually, soils already contain thousands of pounds of
essentially unavailable potassium in the form of primary minerals.
Fertilization programs to provide other plant nutrients such
as calcium, magnesium, sulfur, and the minor elements (iron, zinc,
copper, manganese, boron, molybdenum, and chlorine) are usually
not necessary in an organic system. Other minor elements come
as impurities in limestone.
The application of farm manures, composts, or plant residues
to meet the nitrogen requirements of a crop will satisfy the calcium,
magnesium, sulfur and most of the minor element requirements of
the crop. Other than the reserves in the soil, this type of organic
matter is the most important source of micronutrients and sulfur.
Organic matter produced from soils deficient in minor elements,
however, will be deficient in those nutrients. In these situations,
organic matter should be brought in from outside sources to enrich
and maintain the soil or minor elements must be added.