MODERN SOIL SCIENCE—KELLOGG 235 
Pounds per acre 
Element Hardwood Evergreen 
Calcium es eee 2 ere ee ee ee ee ee 2653.6 26. 5 
INitrogene eset Seenye). epi 2 LU OE eels Se S16H6 23. 6 
Potassium 2 eee Serre rise scree Beats 2 a a 1S 6. 5 
Magnesium. ---_--_- Se ee ee as Moy seeps 9. 2 4.5 
PhOsphOnuseee sesamiae Alc sae Bees 3.3 1.8 
These figures are broad averages and conceal significant differences 
among the specific organic-mineral cycles of individual soil types. Then 
too, the cycle varies in speed. In tropical areas it is very rapid: A small 
amount of a plant nutrient goes a long way since it is used over and 
over again in its rapid cycle from the soil to a plant, to the soil, and 
back to another plant again. 
Thus it is clear that there is a continual cycle of nutrients out of the 
soil into the plants and back into the soil again. We are concerned 
both with the amounts involved and also with the conditions that affect 
them. In fact, the amounts by themselves can be misleading. Take 
calcium, for example: In the arid to subhumid regions the rainfall is 
insufficient to remove the calcium and it accumulates in a special 
horizon called the ‘lime zone’ in the lower part of the soil profile. 
Calcium would not need to be added to this type of soil even after thou- 
sands of harvested crops. Yetin humid forested regions the leaching of 
calcium completely out of the soil is so great, in relationship to that 
brought up by the plants, that it must be returned to the soil in 
agricultural practice, either through the addition of organic matter 
or mineral amendments. 
In the famous Kentucky bluegrass region the underlying rock is 
very rich in phosphorus. The soil contains supplies for thousands 
of crops. Yet in most parts of the world this critical element must be 
added by the farmer to supplement the natural mineral-organic cycle. 
The kind of vegetation in the natural landscape has much to do with 
these vitally important cycles of elements. On the whole, trees do not 
require large amounts of phosphorus, and they do not preserve it in the 
soil to the extent that the grasses do. Thus when the vegetation is 
removed and replaced by the plant crops the soil is often found to be 
deficient in one or more of the plant nutrients. What man is doing is 
establishing a new landscape, an unnatural one, if you please. He must 
discover the appropriate mineral-organic cycle through trial and re- 
search, and then introduce practices to compensate for the change. 
This gets at the heart of the fundamental problem to which soil science 
addresses itself in its practical application to agriculture. 
Soil fertilization, for example, is not simply a matter of replacing the 
nutrients removed by harvested crops. Some soils may be able to 
supply all or certain nutrients indefinitely; others may need heavy 
fertilization, far beyond what the plants take out, for long initial 
periods. Nor are we concerned simply with the effects on just one 
