PASTURE CULTURE 27 



by the term is desirable. Unfortunateh* no simple, concise definition can be given 

 because soil fertility is "a complicated and multi-dimensional problem." Soil 

 fertility is not simpl)' a matter of abundant supplies of nitrogen, calcium, potas- 

 sium, and phosphorus, but includes many other chemical, physical, and biological 

 relationships of the soil as well. Soil fertility really consists of a large number 

 of factors operating favorably toward the production of a crop. Some of these 

 factors are known, others as yet little understood, and probably many others 

 still unknown. Perhaps the best definition of soil fertility which can be given 

 was formulated many years ago by one Nathan Bowen in a letter to Jared Eliot 

 in 1761, when he wrote, "The grand secret of the planters Art, seems to be to 

 Supply the plants with a Sufficient & proper Food, water and air in due propor- 

 tion & Time. This Art being well understood and practiced by him, will give the 

 fruit of his Labour to his Sattisfaction if he have any Reason at all."92 A high 

 level of soil fertility implies a capacity on the part of the soil to "satisfy simul- 

 taneously and continually during the whole time of the growth of a plant its 

 maximum demand in water and food." A low level of soil fertility or infertility, 

 of course, implies the opposite condition. 



Soil Fertility and Yields 



To point out the relationship of soil fertility and crop yield may seem super- 

 fluous since such a relationship should be perfectly obvious. Within certain 

 limits this has been generally recognized with most other field crops but not with 

 pastures. The fact that pasture yields are extremely difficult to measure and 

 the fact that pastures will withstand a great deal of abuse in their cultural manage- 

 ment and still produce something have partially obscured the direct relationship 

 which exists between soil fertility and herbage yields. Some appear to go so far 

 as to assume that, although such a relationship exists with tilled crops, with 

 pastures it is relatively unimportant and the most economical way, if not the best 

 way, to manage a pasture is to starve it. Those who wish to fit the plant to the 

 soil and not the soil to the plant and who are searching for a high-yielding, nu- 

 tritious, palatable pasture species which will thrive on "run-out"soils fall in this 

 group. 



Soil fertility influences pasture yields in two important ways. The first is 

 through the direct relationship which exists between the level of soil fertility 

 in a pasture and the productivity of whatever pasture species may be present. 

 Any pasture species, irrespective of its particular growth habit or soil adaptation, 

 is much more productive at high than at low fertility levels. 



The second way is through the close correlation which exists between the soil 

 fertility level and the botanical composition of the pasture vegetation. This is 

 more noticeable in semi-permanent pasture than it is in permanent pastures. 

 In the former, highly productive species, such as alfalfa or Ladino clover, will 

 maintain themselves and remain productive as long as soil conditions are favorable; 

 but when soil conditions become less favorable, they are supplanted by less produc- 

 tive species and disappear altogether. In permanent pastures, a similar suc- 

 cession of lower fertility species supplanting higher fertility ones is easily observed 

 as soil fertility levels fall. As permanent pastures become exhausted, bent grass 

 and low-growing white clover supplant blue grass and tall-growing white clover; 

 poverty grass supplants both bent grass and white clover; while mosses and 

 woody shrubs eventually succeeded by forest trees complete the whole succession. 



It has long been observed that many of the more productive pasture species 

 demand a relativeh' high level of fertility before they will grow satisfactorily. 

 John Worlidge observed in 1675 that "Land too rich for Corn, cannot be too rich 



92Field Husbandry, p. 208. 



