Necrosis soon develops as small areas that rapidly enlarge to involve the veins or the entire leaf. The leaves are thick, internodes are short, and the corolla appears to be shortened.

IT IS NOT POSSIBLE to distinguish clearly between the disorders that are due to a deficiency as such and the disorders that are due to too much of another element. A deficient supply of one element implies an excess of other elements. A mass-action effect may arise as when too much of one element may interfere with the solubility, absorption, and utilization of another element to the extent of developing acute deficiency effects. The effects often may result from the acidity or alkalinity of the culture medium.

Many of the nutrient elements in excess may cause symptoms of toxicity. For example, boron in any considerable amounts results in a marginal necrosis of the older leaves followed often by stunting and death. Those symptoms often have been seen following wing the use of irrigation water that carried toxic amounts of boron. The use of potash salts in fertilizers containing excessive amounts of boron has caused serious losses.

Calcium, if present in amounts that cause alkalinity of the soils when the levels of iron, manganese, boron, or zinc are low, often results in deficiency symptoms previously described as typical for the deficiency of each of those elements.

An excess of copper causes necrosis, wilting, reduced growth, and death of plants.

Too much iron may induce a deficiency of phosphorus or manganese.

Magnesium present in large amounts may accentuate potassium deficiency if the potassium supply is low. An excess of magnesium may operate in much the same manner for calcium and show the calcium-deficiency symptoms.

Manganese when present in excess may bring about iron deficiency. Manganese is often present in acid soils in amounts sufficient to reduce plant growth. Low calcium is often associated with this condition so that plant growth will be improved by liming to neutralize soil acidity.

Excess amounts of nitrogen stimulate excessive growth and frequently may cause a deficiency of another element that is present in small amount. Often the other element is potassium; then the plant is commonly more susceptible to rusts (in the case of the small grains and cotton) and to leaf spots (tobacco). Sulfur often is added to acidify alkali soil to improve plant growth. Sometimes the continued use of ammonium sulfate and other sulfates brings about a low pH, which often is unfavorable to plant growth. Excess sulfates bring into solution the extra manganese and aluminum that may be present in the soil and thus injure plant growth.

Actually, most effects associated with soil reaction are related to solubility of nutrient or toxic ions rather than injuries associated with the hydrogen ion. Since the acidity or alkalinity of soils in humid areas may range between pH 4.0 and 8.0, most plants can grow successfully at that range if there are no complications in regard to availability and toxicity of the ions present. The so-called alkali soils show a much higher pH and present a different problem.

SALINITY is a serious problem in many arid areas and in places where ocean spray or salt water floods agricultural soils. Excessive fertilization with soluble salts sometimes causes much the same kind of injury. The effect may vary according to the plants in question and the. salts and concentrations that are involved. Sometimes the effect is merely one of concentration of soluble salts, but again, if alkali salts are present in excess, the soils are said to be alkali. The actual salts present may vary, but the three common ones are sodium chloride (table salt), sodium sulfate (Glauber's salt), and sodium carbonate (sal soda). Various other salts of sodium, calcium, magnesium, and potassium may also be present.

Most of our common crop plants are sensitive to salts. Seed germination may be retarded or prevented. Young seedlings may die. When the plants do survive, the growth rate commonly is slow, the plant wilts, and the leaves burn at the tips and margins. Fruit or shade trees may survive for a time on saline soils and show chlorosis, possibly caused by induced shortages of iron brought about by alkaline soil conditions. Their growth is reduced, leaves drop, and eventually the trees die. If conditions are not too severe, the more resistant types of plants may have drought-resistant characteristics. The leaves are small and have a thicker cuticle. Waxy coatings are more developed and the breathing pores are sunken below the outer surface, so that evaporation or transpiration are reduced.

Among the crop plants, sugar beets, Rhodes grass, and Bermuda-grass have the strongest tolerance to saline conditions. Those with medium-strong tolerance to saline conditions are alfalfa, cotton, kale, barley as a hay crop, rape, and sorgo. The crops with medium tolerance include onions, squash, flax, Ladino clover, sunflowers, rice, and rye as a grain crop. Those with the weakest tolerance include red clover, snap beans, navy beans, vetch, and wheat as a grain crop.

J. E. MCMURTREY, JR., is principal physiologist and project leader of investigations of production, breeding, disease, and quality of tobacco in the Bureau of Plant Industry, Soils, and Agricultural Engineering. He is stationed at Beltsville, Md. Dr. McMurtrey has degrees from the University of Kentucky and the University of Maryland.