EFFECTS OF INITIAL WATER-CONTENT ON MACROPORE MATRIX FLOW AND TRANSPORT OF SURFACE-APPLIED CHEMICALS/

Pesticides and fertilizers are often broadcast on no-till fields in th e spring when soil water content can be quite variable. Soil water con tent may influence the contribution of macropores and matrix porosity to water movement and chemical transport in subsequent rainfalls. Ther efore, we surface-applied SrBr2 . 6H(2)O, atrazine -chloro-4-ethylamin o-6-isopropylamino-s-triazine), and alachlor [2-chloro-2',6'-diethyl-N -(methoxymethyl) acetanilide] on nine, 30 by 30 by 30 cm, undisturbed soil blocks obtained from a no-till corn (Zea mays L.) field and maint ained at three initial moisture levels 1 h before a 30-mm, 0.5-h simul ated rain. To distinguish applied water from resident water and assess interaction of the rainwater with the soil matrix, RbCl was added to the simulated rain as a tracer. Sequential percolate samples were coll ected from the base of the blocks in approximate to 10-mL increments u sing a 64-cell grid lysimeter. Flow-weighted concentrations of Cl- and Rb+, respectively, were 75 and 836% higher in percolate from dry (the ta = 0.11 kg kg(-1)) than from wet blocks (theta = 0.21 kg kg(-1)), in dicating that displacement of resident water and interaction of rainwa ter with the matrix increased with initial soil mater content. As a re sult, percolate concentrations of the reactive, surface-applied, const ituents (Sr2+, atrazine, alachlor) decreased with increasing soil wate r content, High block to block variability precluded detection of sign ificant differences in percolate volume and total chemical transport a mong moisture levels. The relative contribution of macropores to chemi cal transport and water movement appears to be greatest when tire soil is dry and decreases as the soil becomes wetter.