Toxicity of three 2,4-D formulations in California soils

Abstract does not appear. First page follows.

Introduction

One of the most widely publicized herbicides in recent years is 2,4-diehlorophenoxy acetic acid, commonly known as 2,4-D. In 1949 the acreage of crops successfully sprayed with this chemical ran into the millions. Because of its selectivity, it has been used primarily to control broad-leaved weeds in cereal crops. It has been used to a lesser degree to control perennial weeds, brush, and trees where no crop was involved. The chemical is usually applied in solution, emulsion, or suspension as a spray to the leafy vegetation of plants.

(Blackman’s (1945))3 report that 2,4-D may act as a selective temporary soil sterilant suggested a new use for this chemical in the field of preëmergence weed control.

(De Rose (1946)) confirmed the toxicity of 2,4-D in soils. (Crafts (1949)), using three indicator plants, studied toxicity, rate of decomposition, and displacement in eight California soils, showing clearly the selectivity between monocots and dicots even in soil applications. The fact that 2,4-D acid is only slightly water soluble has led to the manufacture of various water and oil soluble formulations, the use of which is preferred or recommended in different cases. Since with 2,4-D it is possible to obtain herbicidal action by treatment of the soil as well as by foliage application, it was important to determine the toxicity and rate of inactivation of different formulations when in contact with the soil. It was also important to know whether the effect on subsequent crops depended upon formulation or was related to some particular soil characteristic.

Materials and Methods

Air dried and screened samples of three California soils were used: Yolo fine sandy loam, Yolo adobe clay, and Hanford fine sandy loam. All three soils are of neutral reaction. Both series are of a recent alluvial nature; the Yolo is of sedimentary origin and the Hanford of an acid igneous parent material. Three indicator plants were used: Kanota oats, sunflowers, and vetch. Because of the greater resistance of monocots, the 2,4-D concentrations used for oats are ten times greater than those added to the dicot cultures. For the oats, the series was 0.0, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6, 51.2, 102.4, and 204.8 p.p.m. 2,4-D acid equivalent, air dry soil basis.

Three formulations were studied: the water soluble sodium and triethylamine salts, and the oil soluble butyl ester.

The methods followed were described in detail by (Crafts (1949)). The necessary amounts of 2,4-D were measured from a stock solution or emulsion, diluted to a total volume calculated to bring the soil to field capacity, and