Yield of Wheat. 



l 7 



possesses a large root system well able to supply the plant with 

 water from that stored in the subsoil. It is generally recognised 

 that wheat grows best in comparatively heavy soils, which retain 

 a considerable store of the winter's rains for the service of the 

 crop in the summer. 



The effect of high rainfall is harmful in several directions. If 

 it comes in autumn it washes nitrates out of the ground and 

 militates against the development of a full root system, the chief 

 process going on in late autumn and early winter. There Is, 

 therefore, a reduction of crop; indeed, over a period of years 

 almost a mathematical reduction. Shaw has shown that the 

 average crop in England varies above and below a certain limit 

 in inverse proportion to the rainfall of September, October, and 

 November, his formula for the Eastern Counties of England being 

 — yield = 46 bushels — 2' 2 rainfall in inches. This formula only 

 holds if the weather conditions later m the growth of the plant 

 are normal; i.e., a high yield is only possible if the autumnal 

 rainfall has been low, but a low autumnal rainfall may on occasion 

 be followed by a low yield because some factor depressing the 

 yield has intervened later. If much rain falls at or a little before 

 harvest time the corn does not ripen well, and is in any case 

 difficult to get in. It is this circumstance that limits the north- 

 ward extension of wheat in the British Isles. The limit can be 

 pushed somewhat further by the use of phosphatic manures, which 

 tend to hasten maturity and thus enable the harvest to be got in 

 a few days earlier. 



On the other hand, a good rainfall towards the end of spring 

 is beneficial, especially if the spring is early ; such a rainfall is a 

 usual feature of the good wheat seasons. It is significant also 

 that the exceptional years already referred to, in which the second 

 increment of nitrogen produces less effect than the first, are 

 generally years of low spring rainfall. 



(3) Temperature. — High temperatures are not at all necessary 

 for the production of wheat, excepting at the time of maturation. 

 In the best season it commonly happens that the summer (June 

 and July) temperature is below rather than above the average. 

 For winter-sown wheat, a mild open winter, not too wet/ is 

 desirable to bring the plant forward in early spring, but is by 

 no means essential. 



Spring Wheat.— The conditions regulating the growth of spring 

 wheat are not quite the same ; owing to the shortened period of 

 growth the yields are rarely so high, and the crop appears to be 

 more susceptible to rust and other diseases. For a good yield 

 it is essential that the soil shall contain enough moisture to ensure 

 a good start to the seed, but any excess of rainfall in the first 

 month or two of growth is prejudicial because it restricts the 



