Importance and Symptoms
Manganese toxicity is a major disease of tobacco in Kentucky. This disease causes plants to grow slowly following setting, turn light green or yellowish between the larger veins, and gradually develop numerous dead spots, particularly in the older leaves. This disease is often so severe that the plants are badly stunted or killed, thus reducing yield and quality. Occasionally, however, plants recover and develop a normal green color in the younger leaves later in the season. Manganese toxicity can be easily prevented, but once it occurs in the field, little can be done for that crop.

Cause
Manganese toxicity and soil acidity go hand in hand. As the soil becomes more acid, greater amounts of manganese are available, and the tobacco plants take up more than they need for normal growth. Soil pH 5.0 - 5.5 is the critical level for manganese availability. When the soil pH is 5.5 or higher, trouble from manganese toxicity seldom occurs, but at 5.0 or lower, toxicity in tobacco is very likely. Between pH 5.0 - 5.5, toxicity may or may not cause trouble in any particular year.
When a soil becomes acid, not only does the amount of available manganese increase with resulting manganese toxicity, but the availability of other important nutrients such as phosphorus, nitrogen, and potassium is reduced. Figure 2 shows that a soil with pH above 5.8 gives the greatest availability of major plant nutrients.

Figure 2

With losses of such nutrients as calcium or magnesium, the soil becomes more acid; however, a main cause of soil acidity is the use of nitrogen fertilizers such as ammonium nitrate, ammonium phosphate, ammonium sulfate, urea, and anhydrous ammonia. The degree that ammonium nitrate changes soil pH in a silt loam soil and the persistence of such changes are shown in Table 1. The same amounts of phosphorus and potassium were applied in all cases.
Thus, Table 1 shows that heavy applications of a nitrogen fertilizer can increase soil acidity so much that many nutrients are less available to plants. Organic matter (manure, cover crops, tobacco stalks, etc.) also increases soil acidity upon rotting, but to a lesser degree than ammonium fertilizers.

Table 1 -Degree of pH Change by Applying Ammonium Nitrate

Pounds Ammonium Nitrate per acre	Date of Soil Sampling
	April 25*	May 7	June 6	July 3	Aug. 2
	pH
0	5.75	5.75	5.85	5.65	5.40
375	5.90	5.45	5.35	5.25	5.10
750	5.40	5.25	4.80	4.75	4.70
1500	5.50	5.05	4.85	4.50	4.70

*Before fertilizer was applied on same date.
CAUTION: These are experimental plots and the high rates of nitrogen are not recommended for crop production.

Prevention
Prevention of soil acidity is the best control of manganese toxicity. Your best approach is to follow a planned rotation. A grass or a grass-legume sod is usually best in a tobacco rotation. Ideally, you should not grow tobacco in the same field more than 2 years in a row. After each year following tobacco, take a representative soil sample. If the pH of this soil sample is below 5.8, apply limestone based on your soil test.

Control
If manganese toxicity has become a problem and enough land is available, you will want to lime the soil after the tobacco crop and seed the field to a grass or grass-legume mixture for at least 3 years before returning to tobacco. In the meantime, the lime will have reduced the available manganese, and the sod will have improved the structure of the soil.
If manganese toxicity becomes a severe problem and you don't have enough land to follow a planned rotation, you can apply part of the needed limestone following tobacco harvest; disk it in to insure thorough mixing with the soil. Then the following spring after the field is plowed, apply the rest of the limestone and disk in prior to setting the tobacco. This limestone, properly mixed with the soil, should prevent the toxicity for several years in all but very severe cases.

Don't Overlime
Sometimes growers get such excellent responses to liming that they begin to apply too much limestone. Excessive liming will reduce the availability of both phosphorus and potassium and most of the trace elements. Apply only enough limestone necessary for the legumes in a tobacco-sod rotation. You can take soil tests once or twice during each rotation to see just how much limestone is needed.
If the soil test shows the soil to be moderately to strongly acid, you may have to apply 2 or more tons of limestone per acre for the good stand of grass and legumes needed to improve the soil for tobacco.
For land you have in continuous culture, determine lime and fertilizer needs by taking a good representative soil sample every year as soon as the tobacco is harvested.