298 Mineral Nutrition of Plants 



However, it is of significant importance to note that 20 tons per acre 

 are not uncommon and yield records in Pennsylvania fields have ex- 

 ceeded 30 tons per acre. The average yields since 1940 have been con- 

 siderably higher than 5 tons per acre. Since the average yield is so much 

 lower than the possible yield, it becomes a challenge to condition all 

 soils to at least raise the average yield if not to accomplish the highest 

 yield with all growers. Under most conditions the lack of plant nutrition 

 is the limiting factor, although in certain years disease is a definite fac- 

 tor. However, if a satisfactory crop is not grown, which is true in many 

 cases, the use of spray for disease control is of no avail. 



Careful analysis of many fields and plants has revealed that approxi- 

 mately 100 pounds of nitrogen, 200 of potassium oxide, 35 of phosphoric 

 acid, 93 of calcium, 15 of magnesium, 10 of sulfur, and a few ounces of 

 iron, manganese, copper, and zinc with small amounts of boron and 

 molybdenum per acre are necessary to produce for harvest 10 tons of 

 tomatoes. This is a considerable amount of nutrients and is not sup- 

 plied by the average soil without supplemental applications of fertilizer 

 materials. The amount of supplemental application of fertilizer ma- 

 terials varies according to the soil types and farming practices in the 

 various localities. Practically none of the leached soils of the eastern sea- 

 board has enough plant nutrients to grow 10 tons of tomatoes without 

 supplemental applications. 



The largest need of fertilizer occurs in the soils of the Eastern Shore 

 of Maryland and Virginia. These soils have been notoriously low in 

 average yields over a long period of years. The soil fertility problems 

 vary from farm to farm and must be handled individually, but the 

 actual deficiencies occur in approximately the following order: (a) The 

 soil reaction or pH value, for the most part, is below 5.5. Soils are ex- 

 tremely low in calcium and magnesium and high in readily soluble 

 aluminum. This makes the question of liming of prime importance 

 and the use of dolomitic limestone necessary, (b) The soils are low in 

 organic matter and total nitrogen supply, making the second problem 

 of importance the provision of the necessary amount of nitrogen, (c) 

 The available potassium content is low and the total supply is low, 

 making it necessary to apply potassium for each crop, (d) The supply 

 of boron also is low, and if lime and fertilizer are applied in suffici- 



