WEED control: applied botany 413 



chlorate, boron compounds, and such waste materials as bittern, waste oils, 

 acid sludge, etc. More recently the borate-chlorate mixtures have been ex- 

 tensively employed, and today the substituted ureas of du Pont, urea-borate 

 mixtures, 2,4-D-borate mixtures, and other combinations involving TCA, 

 Dalapon, trichlorobenzoic acid with chlorates and borates are being extensively 

 used. Erbon (Swezey, 1955) of the Dow Company has been announced re- 

 cently, and undoubtedly other new materials are under test. Studies on the 

 substituted ureas have shown that clay content and organic matter are two 

 soil constituents that determine toxicity. More recent studies indicate that 

 not only clay content but the nature of the clay and the base exchange 

 capacity of a soil may be involved in both toxicity and breakdown. 



For example, Hill (1955), reporting on CMU fixation in soils, found that 

 on a bentonite clay of a high exchange capacity, around 150 p.p.m. was re- 

 quired on the clay particles to give 1 p.p.m. in the soil solution, whereas on a 

 kaolinitic clay with a low exchange capacity, less than 1 p.p.m. was required 

 on the clay particles to give 1 p.p.m. in the soil solution. And when organic 

 matter was removed from a sandy soil, adsorption was reduced by approxi- 

 mately 85 per cent. These few figures give an idea of the magnitude of the 

 soil factors that may be involved when one attempts to evaluate a given 

 chemical compound as a soil sterilant. 



Precipitation (rainfall) has been mentioned as the predominant factor in 

 the movement of soil-borne herbicides. Probably more important than total 

 precipitation is the rainfall pattern. As early as the summer of 1933, observa- 

 tion of plots treated at Davis with sodium chlorate in 1930 indicated that the 

 chemical was still present at toxic concentrations, in spite of the fact that 

 three winters had elapsed, with a total precipitation of over 40 inches. When 

 the area was diked and leached with 36 surface inches of water, all trace of 

 the chlorate disappeared. Apparently the intermittent nature of rainfall has 

 much to do with the leaching that it accomplishes. A recent report from 

 Tunisia, where a 2 pound per acre application of MCP to morning glory on 

 fallow land in the fall of 1952 resulted in crop injury to wheat in 1955, em- 

 phasizes this fact. In this case around 50 inches of rain had fallen during the 

 three winters involved, but this had not leached the chemical below the root 

 zone of the 1955 crop. In fact, the evidence indicated that the injury occurred 

 to the wheat in the young seedling stages possibly from chemical plowed to 

 the surface during preparation of the seedbed. 



Selectivity of herbicides. One of the most interesting aspects of the use of 

 herbicides is the selectivity they show. History tells us that selectivity of 

 copper solutions on plants was first noticed by a French vineyardist who was 

 spraying his vines for control of fungus. Noticing that the broad-leafed 

 mustards were killed whereas cereals were unhurt, he tried the material in his 

 grain field and found that he could control the mustard selectively (Robbins 

 et al., 1952). Similar results were soon obtained in Germany and in the United 



