Only 16 elements are known to be required for
plant growth. These nutrient elements can be divided into four
- Macronutrients - carbon dioxide and water
- Carbon (C)
- Hydrogen (H)
- Oxygen (O)
- Primary nutrients from the soil
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- Secondary nutrient from the soil
- Calcium (Ca)
- Magnesium (Mg)
- Sulfur (S)
- Micronutrients from the soil
- Boron (B)
- Zinc (Zn)
- Manganese (Mn)
- Coper (Cu)
- Molybdenum (Mo)
- Iron (Fe)
- Chloride (Cl)
The nutrient elements C, H, and O are obtained by plants from
air (CO2) and water (H2O) in sufficient
amounts to support maximum growth. There is no need for concern
about supplying these elements in fertilizers.
• Primary Nutrients
The elements N, P, and K constitute about 3 to 5% of the dry weight
of most plants and are the most frequently deficient nutrients
in Alabama soils. Accordingly, these are the nutrients in fertilizers
most frequently and abundantly applied.
Nitrogen is the nutrient that is most frequently limiting
crop production and is needed in greates quantities for most nonlegume
crops. Soil tests are not reliable for determining the nitrogen
supplying capacity of individual soils in the Southeast. There
are several reasons for this. First, nitrogen is stored in the
soil's organic matter. The rate of N release for crop use is affected
by organic matter content, temperature, moisture, length of growing
season, and other factors which make it impractical to predict
the amount that will be supplied by the soil for a growing crop.
Second, Alabama soils are low in organic matter and do not vary
much in their capacity to supply nitrogen. Therefore, nitrogen
recommendations are based primarily on the crop to be grown.
The most economical rates of application have been determined
in numerous field experiments. The amounts recommended should be
adjusted by growers based on experience with rates used previously.
Nitrogen supply is the dominant fertility factor in determining
rate and amount of growth of most crops. Legume crops get most
of the N required for their growth from the air, which contains
about 80 percent N.
Phosphorus content of practically all Alabama soils originally
was low. Fertilizers used in the early days of fertilization were
much higher in P than in N or K. Fortunately, P does not leach
through soils but forms compounds with other elements in the soil
and is released slowly. However, P is lost from fields where the
surface soil is allowed to erode. Also phosphorus in the plow layer
may become diluted when turning the land deeper than normal, resulting
in possible phosphorus deficiencies.
Crops require much smaller quantities of P than of N and K. Usually
plants contain about .2 to .3 percent of P. Therefore, under continuous
fertilization, soil content of P has increased on many soils to
High levels. About 50 percent of all samples received by the Soil
Testing Laboratory in recent years have been High in P, and crops
grown on those soils would not be expected to respond to P applications.
Experiments at several of the substations and experiment fields
have shown that where P has been built up to High or Very High
levels, they may go for 40 years or more without reducing yields
of most crops from lack of P. Therefore, on soil test reports for
field and forage crops, none is recommended High levels, but growers
should to sample each year where none is applied to avoid any loss
in yield should soil P drop back into the Medium range, where some
response could be anticipated.
Potassium. As yields have been increased by higher N and
P fertilization, the need for K on some soils has increased. Most
of the sandy soils of Alabama are deficient in K, while the clays
and clay loams are less likely to be deficient. Response to K has
been determined in numerous experiments throughout the State. Excessive
use of K may cause Mg deficiency, especially on sandy soils. Recent
residual studies have shown that K may accumulate in most soils
where recommended rates are applied. When soil tests indicate that
this accumulation has reached the High level, growers may reduce
or cease applications until the soil level drops back into the
• Secondary Nutrients
The elements calcium (Ca), magnesium (Mg), and sulfur (S) are
classified as secondary nutrient for plant nutrition. Some Alabama
soils and crops should be supplied with these nutrients.
Calcium (Ca) is supplied in both calcitic and dolomitic
lime. Where soil pH is maintained in the proper range, calcium
deficiency is not likely to occur. Peanuts, tomatoes, and peppers
are the only crops which have been found to suffer from lack of
calcium in Alabama fields. This deficiency may result in unfilled
pods (pops) in peanuts and blossom end rot in tomatoes and peppers.
All samples are analyzed for calcium in the Soil Testing Laboratory.
When the tests indicate that deficiencies may occur on these crops,
gypsum may be recommended if lime is not needed.
Magnesium (Mg) content of all soil samples is also determined.
Some sandy soils in Alabama are deficient in Mg and these soils
are usually low in pH. The most practical way to prevent Mg deficiency
is by using dolomitic lime when soil tests indicate that Mg is
Low. On soils where Mg is not found to be deficient, calcitic or
dolomitic lime is satisfactory.
Sulfur (S) deficiency has increased as high analysis fertilizers
made from ammonium phosphates have replaced fertilizers made from
superphosphate, which contains about 12% S. Sulfur added to soils
in rain has decreased in areas where effluvium from industrial
plants has been scrubbed to reduce air pollution. Sandy soils of
the coastal plains and sandstone plateaus are most likely to be
deficient in S, because this element may be leached from sandy
surface soils. There is no practical soil test procedure to use
in predicting S deficiency. It frequently accumulates in the subsoil
and plants may recover from the deficiency when roots reach the
subsoil. It is recommended that all crops receive an average of
about 10 pounds of S per acre per year, applied in fertilizer or
in pesticide applications. Crops most likely to respond to S fertilization
on sandy soils are wheat, corn, cotton, and vegetable crops. Farmers
should always check when buying mixed fertilizer to be certain
that it contains enough S to supply this amount. When fertilizers
are not applied each year, fertilizers should contain enough to
last until the next application.
Although the seven micronutrients are as important in plant nutrition
as the primary and secondary nutrient, they are needed in much
smaller quantities, and most Alabama soils contain adequate amounts
for most crops. The Department of Agronomy and Soils of the Agricultural
Experiment Station has conducted field, greenhouse, and laboratory
research continuously since about 1930 on the response of crops
to micronutrient elements. Field experiments with boron, zinc,
manganese, copper, iron, and molybdenum have been conducted with
various crops on the substations, experiment fields, and on farmers'
fields throughout the State. Whereas some crops may use between
20 and 200 pounds per acre of N, P, K, Ca, Mg and S, they use less
than 1 pound per acre of the micronutrients. Most Alabama soils
have an abundance of minerals containing micronutrients. In some
cases, over application of micronutrients (e.g. Zn, Cu, B) could
lead to a toxicity. Metals such as Zn and Cu build up in the soil.
These may also be found in certain pesticides. Soil test for Zn
and Cu may be more valuable for avoiding toxic buildup rather than
for predicting deficiencies.
Research with field and forage crops has shown that most deficiencies
of micronutrients are limited to boron and zinc for a few crops
on certain soils. The most practical recommendation for these nutrients
is to apply them to specific crops in all cases or on soils that
may be deficient. This eliminates the need for soil tests that
are, in most cases, expensive and not always dependable enough
to serve as a basis for recommendations. Although some soil testing
laboratories determine micronutrient levels in the soil and make
specific recommendations, their interpretations are not based on
research for Alabama soils. A chemical analysis or soil test is
of little or no value unless calibrated against crop responses
to applications of the nutrient in question. Such information can
only be obtained from fertility experiments conducted under a wide
range of soil and climatic conditions. The following is a brief
description of the Auburn Soil Testing Laboratory's recommendations
for the micronutrient elements.
Boron (B) is recommended for cotton, peanuts, clovers grown
for seed, alfalfa, cauliflower, broccoli, root crops, apples, pears,
and plums. Analysis for B is expensive and the cost of applying
recommended rates is quite low; therefore, recommendations based
on needs of specific crops are more practical than routine soil
Zinc (Zn) is recommended for corn on sandy soils where
the pH is above 6.0 or for the first year after applying lime.
It is also recommended for peaches, pecans, apples, and pears.
These are the only crops that have responded to Zn on Alabama soils.
Zinc deficiency in corn seedlings is likely to occur in cool, wet
seasons. Corn plants usually recover when warm weather arrives
but yield may be decreased by the early deficiency. Routine analysis
for Zn is not necessary in most cases, but soil and plant analysis
for Zn may be helpful in diagnosing suspected cases of deficiency
or toxicity. Simultaneous applications of excessive amounts of
both lime and phosphorus can induce Zn deficiency on almost any
crop. Soil tests showing pH values above 7.0 along with Very High
or Extremely High P indicate a probability that Zn deficiency may
occur on some soils.
Zinc toxicities could occur on sensitive crops such as peanuts
where excessive Zn application have caused high soil Zn levels
(> 10 mg/kg extractable Zn) on sandy soils. Maintaining a soil
pH above 6.0 may help to reduce Zn toxicity symptoms.
Broiler litter, certain pesticide, and some industrial by-products
used as soil amendments may contain high concentration of Zn.
Iron (Fe) is a common deficiency for only a few crops (e.g.
soybeans) on the high pH soils of the Black Belt and for some specialty
plants (e.g. azaleas, centipedegrass, and blueberries) where lime
or phosphorus is excessive. . This deficiency cannot be corrected
by application of Fe to the soil, but can be corrected on ornamental
and fruit crops by spraying with a dilute iron solution. Soil analysis
for Fe is worthless. Some soybean varieties are less susceptible
than others to Fe deficiency. Varieties rated for susceptibility
to Fe deficiency can be found in soybean variety reports.
Molybdenum (Mo) application to soybeans as a foliar or
seed treatment at planting is recommended for all soils of North
Alabama and for Black Belt soils. Deficiency of Mo on soybeans
on acid soils can usually be prevented by liming. Because Mo is
needed in such small quantities Soil Testing is not recommended.
Manganese (Mn) is high in almost all Alabama soils and
is not recommended for any crop. Soybeans grown on sands with poor
internal drainage, high organic content, and a pH above 6.0 may
show Mn deficiency. Symptoms of cyst nematode damage are very similar
to those for Mn deficiency on soybeans.
Coppper (Cu) and Chlorine (Cl) have not been found
to be deficient for any crop on Alabama soils. There is no need
to supply these elements in fertilizers in Alabama. Excessive Ca
may be applied to soil in broiler litter and certain Cu-containing
Present Policy on Micronutrients
Due to the points listed above, the laboratory does not analyze
soil samples for S, Mn, Cu, Cl, Fe, or Mo. If and when research
shows that analysis for one or more of these elements is justified,
this service will be provided.
Under certain conditions, a soil test for B and Zn may be helpful
in diagnosing problems. Therefore, samples will be analyzed on
request for B and for Zn at an extra charge. Soil Test reports
may have to be delayed for several days when these elements are