Compatibility and Phytotoxicity of Herbicide-Fertilizer Combinations

During the past decade herbicides and liquid fertilizers have been combined for joint application with increasing frequency. This practice has been labeled the “weed and feed” concept. It saves time and labor, reduces soil compaction, and reduces application expenses. Thus, the effect of compatability treatments on phytoxicity of mixture of 10 herbicides with 11 fertilizers were studied in the laboratory and greenhouse. The objective was to determine the feasibility of joint application of each herbicide with each liquid fertilizer selected. Herbicides were generally compatible with suspension-grade fertilizers and maintained their phytotoxicity for 24 hours with agitation as determined by greenhouse bioassays. Of the clear liquid fertilizers, 11-37-0 was the most compatible with the 10 herbicides. Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], chloramben [3-amino-2,5-dichlorobenzoic acid], and propachlor [2-chloro-N-isopropylacetanilide] were the herbicides most compatible with the liquid fertilizers. Chloramben remained dispersed for 24 hours without agitation in most of the fertilizers. In general, separation of the herbicide-fertilizer mixtures occurred within 1 hour after initial mixing, without agitation. Agitation was essential for maintaining phytotoxicity of most herbicide-fertilizer mixtures. COMPEX (alcohol sulfates) at 0.25% volume/weight (v/w) of spray solution and ATTAGEL 150 (attapulgite clay) at 0.75% weight/weight (w/w) of liquid fertilizer solution were the most effective compatibility treatments added to the herbicide-fertilizer mixtures. COMPEX retarded the formation of precipitates in fluorodifen [p-nitrophenol-α,α,α-trifluoro-2-nitro-p-tolyl ether] fertilizer mixtures. Use of a 2:1 water:herbicide (w/w) dilution aided in mixing wettable powder herbicides with liquid fertilizers. Addition of SURFACTANT WK (dodecyl ether of polyethylene glycol) increased compatibility of some wettable powder herbicide-fertilizer mixtures. However, with adequate agitation nearly all herbicide-fertilizer mixtures were compatible and herbicide phytotoxicity was maintained without the need for the addition of compatibility agents.