In such areas, accumulations amount to about 50 pounds per acre a year. In areas at a distance from industrial centers, annual accumulation is less than 5 pounds, not enough for normal crop needs.
Fortunately, many fertilizer materials used today are sulfates and supply sulfur along with the element in combination with it. Requirements of some crops for sulfur are quite high, particularly those in the mustard family, the brassicas-alyssum, stock, candytuft, nasturtium, and hesperis among flowers, and cabbage, broccoli, cauliflower, radish, turnip, and kale among vegetables. Onions, too, need sulfur to develop their tear-jerking odor.
Sulfur is important because it is a basic element in protein manufacture by the plant. If it is in short supply, older leaves are robbed to supply younger, more active leaves and growing tips. If the plant continues to "starve" for sulfur, protein synthesis stops while amino-acids, cystine and other nitrogen-bearing compounds accumulate in plant tissues; these unused building blocks of protein cannot be set in final place for lack of sulfur.
Perhaps the second most important role of sulfur is in synthesis in the plant of the so-called plant hormones or growth regulators.
Home gardens seldom lack sulfur. In addition to sulfur washed down from the atmosphere, quantities of this element are provided by humus and other organic matter in the soil, or by the fertilizers that contain some sulfates, such as sulfate of ammonia and superphosphate. Thus, while sulfur is fully as essential as nitrogen, phosphorus and potash, it can be generally taken for granted.
Although the majority of garden soils are well supplied with iron, one of the most common plant troubles is a chlorosis or yellowing of foliage, a symptom of iron deficiency. This is due either to a lack of iron (in rare cases) or to a locking up of this vital element in a soil of too high pH. Iron chlorosis often appears with dramatic suddenness following an overdose of lime. Sulfur, as noted, plays a vital role in acidifying overly sweet soil.
As might be judged from its effect on green foliage color, iron plays an important role in chlorophyll formation. Since lack of chlorophyll prevents plants from manufacturing starch needed for en-
ergy and growth, iron-starved plants become unthrifty. Lime is therefore "verboten" where broadleaved evergreens such as rhododendrons and other acid-soil, iron-dependent plants are growing. Also, there is a still-undefined but apparently unfavorable relationship between iron and such elements as copper, manganese and zinc in the soil.