PASTURE CULTURE 35 



holding capacity of the soil was increased, particularly on mineral soils naturally 

 low in organic matter; the air content was over 50 percent greater; and the organic 

 matter content was appreciably lower. In another series of similar experiments, '26 

 some of these same measurements were taken over a period of years and the results 

 correlated with yields of hay. The hay yields on the plowed and reseeded areas 

 fell off progressively as the values for water-holding capacity, soil structure, and 

 content of soil air of the reseeded areas approached those for natural meadow soils. 

 It is difficult to say just which factor was the most important in raising the 

 production of the reseeded plots because it is difficult to separate the effect of 

 improved nutrient relationships and water-holding capacity in the newly seeded 

 areas from the effects of better aeration or some other changed relationship. 

 It is likely that in the case of light, mineral soils, greater supplies of plant nutrients 

 and increased water-holding capacity were important factors, but improved soil 

 aeration may also have been very important. Recent investigations on the 

 importance of oxygen in plant nutrition definitely suggest this possibility. Hoag- 

 land and Arnon believe that, "the factor of oxygen supply and of carbon dioxide 

 removal from around the roots may sometimes limit nutrient absorption by 

 plants, even when availability of nutrients, in a chemical sense, is not limiting."'"''^ 

 Similar views have recently been expressed by Shive.^^s 



In the second series of German experiments, efforts were made to increase the 

 yield of the natural meadowland by the use of various fertilizer top-dressing ma- 

 terials. There w'as some increase in yield from several of the treatments, but in 

 no instance did the yield even approach that obtained from the reseeded plots. 

 The fact that factors other than a lack of plant food nutrients were important in 

 limiting yields, explains why simply supplying these nutrients did not restore 

 hay yields. Perhaps lack of an adequate supply of available oxygen and an 

 excess of carbon dioxide were important. 



Further light is thrown on the subject of tillage by results from Russian ex- 

 periments which have been reviewed by Russell. 129 The Russians have found 

 that the structure of a soil is improved not b\' the presence of raw organic matter 

 in the soil but by its decomposition, and that the greater the rate of decomposition 

 the greater will be the improvement in soil structure. This explains why, although 

 a large amount of organic material accumulates in a soil under a continuous sod 

 cover, soil structure actually deteriorates together with other factors influencing 

 plant growth, such as moisture-holding capacity and soil aeration. It also shows 

 why a well-developed sod, when plowed under, may produce so many desirable 

 effects. As Lindsay-Robb writes — 



The age-old implement the plough is still the most efficient cultivator 

 on the farm. ... It is the only implement that enables a farmer to 

 release and cash the 'frozen' fertility that has accumulated since the 

 pasture was established. The effect of cultivation on arable land — which 

 incidentally commences with the plough — is well known, and it is equally 

 true of established grassland. i^" 



Fertility Improvement and Maintenance in Permanent Pastures 



Although one might conclude from the foregoing discussion that it is practically 

 impossible to maintain satisfactory fertility levels in a grass sod for more than 

 a few years, such an interpretation is not altogether correct. By following suitable 

 practices, fair yields of moderately good feed can be produced in permanent 



126pflanzenbau, XIV (1938), 241-264. 



127Soil Science, LI (1941), 431-444. 



128lbid., pp. 445-458. 



"^Imperial Bureau of Soil Science, Tech. Communication No. 37, (1938). 



'^University of Pretoria (South Africa), Series No. 1. BuK 36, p. 29. 



