MANAGING ACID SOILS FOR PRODUCTION OF BURLEY TOBACCO
J.L. Sims and K.L. Wells
Department of Agronomy
Excessive soil acidity is considered
the single greatest problem influencing yields of burley tobacco grown
in Kentucky. Based on UK Soil Test Laboratory summaries, half to two-thirds
of tobacco fields need to be limed each year to decrease soil acidity to
recommended levels. Although strongly acid soil conditions affect the availability
of other plant nutrients, the most important effects on tobacco are that
solubility of manganese is increased to toxic levels and solubility of
molybdenum is decreased to deficient levels. Molybdenum availability in
soils and molybdenum nutrition of burley tobacco has been addressed in
Cooperative Extension publication AGR-82. The following material deals
primarily with manganese: the effect of excessive levels of manganese on
growth of burley tobacco and management practices to alleviate manganese
Functions of Manganese in Plants
Manganese is an essential plant nutrient.
When present in adequate but not excessive amounts in the plant, it functions
in photosynthesis and activates enzymes for plant growth. However, when
plants contain excessive levels of manganese, the concentrations of iron,
chlorophyll synthesis, rates of photosynthesis, enzyme activity, and subsequent
plant growth are lowered. The latter condition, a disease commonly called
manganese toxicity, is estimated to cost Kentucky tobacco growers $30 to
$40 million annually due to lowered yields and quality.
Relationship of Plant Manganese and Plant Growth
Generally, manganese toxicity occurs
early in the growing season soon after plants are transplanted and established
in the field. Severely affected leaves (Fig. 1) develop a light green or
yellowish color (chlorosis) between veins while the midrib and lateral
veins remain dark green. These symptoms are very similar to those for iron
deficiency. Plants grow slowly and the chlorotic spots on the older leaves
turn brown and eventually die and fall from the leaf. Such leaves become
less sound, weigh less at harvest, and are trashy and of low quality after
The specific toxicity symptoms exhibited
and their location on the plant depend on the concentrations of plant manganese.
In field-grown plants, visual leaf symptoms (chlorosis) usually occur when
manganese concentration exceeds 400 to 500 parts/ million (ppm)in the tissue.
However, reduced growth can occur at concentrations below 400 ppm (Fig.
2) even though visual symptoms are absent. Under moderate conditions, plants
recover from the disease about midseason and only leaves on the plants'
lower portions are affected. However, the disease may be so severe under
some conditions that plants become badly stunted or die. The disease is
less severe in years with above normal temperatures.
Plant Root Symptoms
Not only does manganese toxicity affect
above ground plant parts; plant roots are also affected. In general, affected
roots are brown and, under severe conditions, the root tips are pruned
causing increased branching of roots behind the tips. The overall volume
of roots is lessened, thus decreasing the uptake of nutrients and water.
Additionally, excessive levels of manganese may decrease the uptake of
other nutrient cations such as iron, calcium and magnesium because manganese
competes with other cations for uptake sites on plant roots and replaces
calcium in cell membranes. When calcium is replaced, the disrupted membranes
lose their stability and the cell contents, such as organic solutes or
inorganic ions, may move out of the plant cells.
Soil and Fertilizer Factors Affecting Plant Available Manganese
Manganese toxicity is greatly affected
by soil acidity. As soil becomes more acid, greater amounts of manganese
are available, and tobacco plants take up more manganese than they need
for normal growth. Soil pH of 5.0-5.5 is the critical pH range for manganese
availability. When the soil pH is 5.5 or higher, available manganese is
oxidized to unavailable forms and toxicity seldom occurs. At pH 5.0 or
lower, soil manganese is solubilized and toxicity in tobacco is likely.
Between pH 5.0 and 5.5, toxicity may or may not occur in any given year.
Another soil factor affecting the solubility
of manganese is the oxidation-reduction potential. The oxidation-reduction
potential is a measure of the relative oxidizing or reducing power of soil
and is related to the oxygen levels in soil. In well-drained, well-aerated
soil at pH above 6.0 much of the manganese exists as manganese oxides.
However, when soils are waterlogged for 2 or 3 days, the oxygen is lost
from the soil, and soil microorganisms use the chemically combined oxygen
in manganese and iron oxides for their respiratory needs. This process
releases manganese from non-available to available forms and increases
the pool of available soil manganese. Thus, excessive moisture and a source
of readily available organic matter (green manure crops) lead to high levels
of available soil manganese by reduction of manganese oxides. These conditions
commonly exist in tobacco fields in Kentucky each spring and early summer
during the early growth stage of transplants.
The dominant factor influencing soil
acidity and manganese availability in tobacco fields in Kentucky is the
large amount of fertilizer used in tobacco production. Growers are applying
an average of 2200 lb/acre of mixed fertilizer and an additional 700 lb
of nitrogen fertilizers. These quantities generate large amounts of soil
acids so that soil pH at midseason often is 0.6 to 1.0 pH unit below what
it was before N-P-K fertilization (Fig. 3). Most of the acidity results
from the conversion of soil and fertilizer ammonium to nitrate nitrogen
by soil microorganisms. Because of this situation, growers need to lime
tobacco soils to pH 6.4 to 6.6 so that soil pH will remain above 5.5 throughout
the growing season. When soil pH drops below 5.5, one can expect not only
manganese toxicity but lowered availability of other nutrients such as
molybdenum, phosphorus, magnesium and calcium.
Cultural Practices to Control Manganese Toxicity In Tobacco
Liming Acid Soils
The best way to control manganese toxicity
is to lime soils used for tobacco to pH 6.4 to 6.6. Limestone will correct
soil acidity more effectively if applied and thoroughly mixed with the
soil one to three years ahead of the crop. Summer and fall are good times
to apply lime since the soil is dry, dealers in agricultural lime are not
as busy and the lime applied can react over the winter. Greater effort
should be given to insure proper mixing with soil when lime is applied
close to the time of transplanting the tobacco crop. If applications are
made two or more years ahead of the crop, the entire application may be
applied to the soil surface. When liming needs are not determined until
the fall or early spring before transplanting on strongly acid soil (water
pH 5.5 or below), half the lime should be plowed down and the other half
disked in after plowing. Since soil acidity must be corrected in a relatively
short period under these conditions, the kind and quality of the liming
material, particularly its fineness, is of great importance. While liming
soils before transplanting is best, sometimes manganese toxicity can be
alleviated by liming after transplanting. A fast-dissolving source of limestone
such as finely ground or pelleted lime should be used. A broadcast application
of about 1000 lb/acre over the top of plants followed by cultivation to
work it in the soil is the method most likely to be successful.
Liming acid soils is essential, but
excessive liming should be avoided. When soils become neutral, or basic
(pH 7.0 and higher), phosphorus will revert to insoluble forms and some
minor elements will be less available for plant growth.
Nitrate Nitrogen and Molybdenum
Use of nitrate sources of nitrogen
and molybdenum aid the control of manganese toxicity. Table 1 shows the
effects of these two N sources when they are applied to an acid soil deficient
in molybdenum and shows how they affect plant manganese concentration of
burley plants 50 days after transplanting. Average plant manganese concentrations
for the calcium nitrate source were about half those for urea. Adding molybdenum
in the presence of urea cut manganese concentration in half. The lowered
manganese concentrations in this study likely resulted from increased plant
growth due to improved molybdenum availability in the presence of molybdenum
fertilizer and calcium. Also the calcium from calcium nitrate may have
had a small neutralizing effect on soil acids, thereby lowering manganese
solubility. Figure 4 shows that a plant molybdenum
concentration of about 0.4 ppm appears to be associated with minimum levels
of plant manganese.
Table 1. Effect of molybdenum and source of nitrogen on concentration
in field grown burley tobacco approximately 50 days after transplanting.
Rate of sodium molybdate, lb/acre
---Concentration of manganese, ppm---
Use of Sod Crops
Use of sod crops in rotation with tobacco
is thought to lessen the incidence of manganese toxicity. Plowing under
a good sod will enhance the development of granular soil structure that
tobacco roots can readily penetrate. The continuous production of tobacco
in one location often leads to deteriorated soil structure, less soil aeration,
increased danger of manganese toxicity and increased risk for incidence
of such diseases as black root rot and black shank. Ideally, a good plan
is a crop rotation of 2 years tobacco, with a winter cover crop between
crops of tobacco.