36 MASS. EXPERIMENT STATION BULLETIN 380 



pastures, provided natural soil conditions are favorable. Following is a critical 

 discussion of certain widely publicized "systems" for bringing about permanent 

 pasture improvement. 



Factors Influencing the Growth of Natural White Clover. — Since the success of 

 several methods for treating permanent pastures depends upon the successful 

 growth of natural white clover, it would be well to consider some of the important 

 factors which influence its growth. 



Moisture. — Without question, moisture is the most important factor in- 

 fluencing the prevalence of natural white clover in permanent pasture sods. 

 "Indeed, it depends so much upon a general distribution of rains through the 

 season," wrote Flint in 1867, "that when they are sufficiently abundant it comes 

 in profusely even where it was not observed in other years, and hence such seasons 

 pass under the term of 'clover years.' "^^^ An adequate supply of moisture is so 

 important for this very aggressive but shallow-rooted plant that any permanent 

 pasture-improvement program which depends largely on the growth of natural 

 white clover for its success, should be confined only to those pasture areas which 

 have satisfactory moisture relationships throughout most of a normal season. 



Mineral Nutrients. — An adequate supply of mineral nutrients is just as im- 

 portant as soil moisture in maintaining a vigorous growth of white clover. At 

 the present time most permanent pasture soils in Massachusetts which have not 

 been limed and fertilized are too deficient in a number of the necessary plant food 

 elements, particularly calcium and potash, to support a good growth of white 

 clover. If adequate quantities of these elements are supplied through top- 

 dressed lime and fertilizers to soils with good moisture relationships, an abundant 

 growth of natural white clover will invariably result. 



Competition from Associated Grasses. — A third important factor influencing 

 the prevalence of natural white clover is the vigorous growth of the accompanying 

 grasses. 1*2 This competition frequently exists in pastures which have been im- 

 properly grazed or which have been heavily fertilized with nitrogen fertilizers 

 and then improperly grazed. If grass is not subdued in pastures of moderately 

 low levels of fertility, the white clover will seriously suff'er because under such 

 conditions only the low-growing forms will thrive and such forms offer poor com- 

 petition for tall-growing grasses. 



Competition from grasses is always serious following periods of drought. The 

 effects of dry weather are always much more severe on the clover than on the 

 grass. The succulent leaves and stems of the clover usually dry up completely 

 and the plant dies, whereas the grasses usually remain dormant throughout the 

 dry period and then begin growth again as soon as growing conditions are again 

 favorable. These grasses, stimulated by the nitrogen released from the dead 

 clover plants, then make very rapid growth which in turn makes survival difficult 

 for any clover plant which may have escaped the drought. It is now generally 

 believed that the effects of nitrogen fertilizers on clovers are indirect. The nitrogen 

 stimulates the grasses and the grasses in turn, unless closely grazed, tend to crowd 

 out the clovers. 



"Hard" Seeds. — The "spontaneous" appearance of natural white clover 

 wherever and whenever soil conditions are favorable, is probably explained by 

 the abundant production of "hard" seeds. Early observations have shown that 

 some of these "hard" seeds will germinate after remaining many years in vials 



l*lGras3 and Forage Plants, pp. 188.189. 



132b. a. Brown, Journal American Society of Agronomy, XXXI (1939), 326-332. 



