412 CRAFTS 



merits applied before the crop is up are termed pre-emergence, those applied 

 afterward post-emergence. Those not involving crops are usually confined to 

 nontilled areas and are commonly somewhat permanent in their effects. 



Turning first to the soil treatments made in crops, that is, the pre- and 

 post-emergence applications, it should be apparent that these must be in some 

 way selective so that the weeds are killed while the crop survives. The matter 

 of selectivity is discussed in the next section; it should be mentioned here, 

 however, that selectivity of herbicides is only relative (Crafts, 1946); a 

 chemical to which a crop is sensitive must be so positioned that it will not 

 come into contact with the crop roots if they are to remain uninjured. Because 

 most weed seeds germinate in the top half inch of soil, excessive leaching is 

 detrimental. And the position of the chemical is a function of soil type and 

 precipitation. 



As was pointed out after early pre-emergence trials with 2,4-D (Crafts, 

 1948), there are four possible conditions that will determine the effects of a 

 given treatment: 



1. No rainfall following treatment 



2. Very light rainfall 



3. Moderate rainfall 



4. Excessive rainfall or flooding 



It should be apparent that of these four conditions only condition 3 will give 

 success; conditions 1 and 2 will fail because the chemical will not be moved 

 to the proper depth ; condition 4 will fail because the chemical will be leached 

 below the rooting zone of the weeds. In regions where crops are dependent 

 upon summer rainfall, soil-applied pre-emergence herbicides have been used 

 with eminent success and only low or excessive rainfall has affected the 

 treatment adversely. Soil type, however, must also be considered; treatments 

 on sandy soils often fail because of excessive downward movement of the 

 chemical. In the semi-arid west where irrigation is more prevalent, these 

 soil treatments are not widely used because many crops come up and grow 

 to considerable size, or even to maturity, without a drop of rain; subsequent 

 irrigation has not proved effective in bringing such herbicides into action. 



In spite of the above difficulties, pre-emergence herbicides are finding great 

 favor in the midwest and along the eastern seaboard, and many new, promis- 

 ing chemicals are being tested. These include the phenoxyacetic, propionic, 

 and butyric acids in their 2,4-D, 2,4, 5-T, and MCP forms, as acids, salts, 

 esters, and amides. Also included are the carbamates, chloroacetamides, di- 

 thiocarbamates, and a number of coded materials the compositions of which 

 have not been revealed. With chemicals of varying solubility and adsorption, 

 it may be possible to find materials for many widely varying weed situations. 



Permanent soil sterilants. Relatively permanent soil sterilization that will 

 completely inhibit vegetation on nonagricultural areas has been achieved for 

 many years. The chemicals used include sodium arsenite, white arsenic, sodium 



