according to this invention it has been unexpectedly found that addition of soluble divalent metal cations , customarily abbreviated m 2 + , to either an ethyl metribuzin or metribuzin / trifluralin tank mix will inhibit metribuzin or ethyl metribuzin particle growth . the preferred m 2 + cation is calcium , but magnesium , zinc and others are also effective . typically , they may be added as the following salts , chloride , nitrate , sulfate , ligninsulfonate , alkylarylsulfonate , or dodecylbenzenesulfonate . furthermore the amount of m 2 + needed to inhibit particle growth is proportional to the amount of monovalent cations ( m 1 + ) introduced into solution as part of the formulation ( s ). examples of m 1 + are sodium , potassium , lithium and ammonium . it is common to formulate metribuzin products with anionic surfactants containing m 1 + counter ions . typical counter ions are ligninsulfonate , alkyarylsulfonates , alkylsulfosuccinates and methyl taurates . particularly prevalent are sodium derivatives . thus as a consequence of dispersing the metribuzin product in the spray tank solution , m 1 + is introduced from the dissolution of the anionic surfactant ( s ). as currently practiced , the amount of m 2 + is present in commercial formulations of metribuzin is negligible . metribuzin particle growth proceeds rapidly when trifluralin is admixed . by introducing m 2 + into a metribuzin - trifluralin mixture so as to adjust the mole ratio of m 1 + / m 2 + in solution to less than 2 . 5 , metribuzin particle growth is inhibited . the m 1 + / m 2 + ratio can be adjusted in a number of ways as described . it is required that m 1 + and m 2 + are soluble species . it is possible to inhibit particle growth by adding the required amount of m 2 + , as a water - soluble salt , to the solution prior to the addition of metribuzin and trifluralin . although effective , this procedure has the disadvantage of adding an extra step during the preparation of the spray tank mixture . it is preferable to build the proper m 1 + / m 2 + ratio into the product formulation so no extra handling is required . since m 1 + is introduced almost entirely as part of the metribuzin formulation ingredients , it is appropriate to adjust the m 1 + / m 2 + ratio in that product . alternatively , m 2 + could be added to the trifluraline so that , when combined with metribuzin , the desired m 1 + / m 2 + ratio is obtained . one method to adjust m 1 + / m 2 + in the metribuzin product is to incorporate the required amount of water - soluble m 2 + salt into the current formulation . for this purpose caso 4 is particularly useful , znso 4 and mgso 4 are also acceptable . the anion portion of the salt is unrestricted , so long as it is agriculturally acceptable and renders the salt water - soluble . acceptable anions include halides , sulfates or nitrates . another method for adjusting the m 1 + / m 2 + ratio in the metribuzin product is to replace m 1 + derivatives of anionic surfactants with m 2 + derivatives . for example , it is common to use a sodium ligninsulfonate , and this could be replaced by a calcium ligninsulfonate . not all of the m 1 + must be replaced with m 2 + , only enough to meet m 1 + / m 2 + requirement . although the use of m 2 + surfactants in metribuzin formulations has been recognized in a general way in a prior patent , u . s . pat . no . 4 , 036 , 632 , the relationship of m 1 + / m 2 + and its effect on metribuzin particle growth was not taught nor was the acceptable ratio . a method for adjusting m 1 + / m 2 + through alteration of the trifluralin product is to add a soluble m 2 + species to the concentrate . a particularly useful material for this purpose is calcium dodecylbenzenesulfonate . an example of a representative solid metribuzin formulation is given in composition i . ______________________________________composition iingredient wt . % ______________________________________metribuzin 75 . 0technical impurities 5 . 6reax ® 45t 7 . 0petro ® dispersant 98 2 . 0kaolin clay 10 . 4______________________________________ the components are blended together and pulverized until substantially all the particles are less than 50 microns . the resulting powder disperses readily in water . the surfactants used in this example , reax ® 45t and petro ® dispersant 98 , are sodium derivatives of anionic surfactants of a type commonly used in solid agricultural formulations . reax ® 45t ( westvaaco ) is a blend of anionic wetting agents combined with a ligninsulfonate dispersant . petro ® dispersant 98 ( petrochemicals co .) is a wetting / dispersing agent derived from alkylnaphthalenesulfonate . this example demonstrates one method for adjusting m 1 + / m 2 + to the proper ratio in a formulation which uses m 1 + surfactants , like composition i . thus to composition i , the m 2 + salt is added directly , replacing some of the inert clay . the result is composition ii shown below . ______________________________________composition iiingredient wt . % ______________________________________metribuzin 75 . 0technical impurities 5 . 6reax ® 45t 7 . 0petro ® dispersant 98 2 . 0kaolin clay 8 . 4caso . sub . 4 . 2h . sub . 2 o 2 . 0______________________________________ the components are blended and pulverized in the same way as example 1 this example illustrates the removal of sodium surfactant and replacement with a calcium derivative . thus using i as the standard , the reax ® 45t is replaced with daxad 21 ( w . r . grace ), a calcium salt of polymerized alkyl arylsulfonic acids . ______________________________________composition iiiingredient wt . % ______________________________________metribuzin 75 . 0technical impurities 5 . 6daxad ® 21 7 . 0petro ® dispersant 98 2 . 0kaolin clay 10 . 4______________________________________ the components are blended and pulverized in the same way as example 1 . this example illustrates the addition of a soluble m 2 + species to the trifluralin formulation so that in combination with a metribuzin formulation like i the required m 1 + / m 2 + is obtained . in this case calcium dodecylbenzenesulfonate ( ninate 401 , stepan chemical co .) is added to a commercial trifluralin product , treflan ® ( elanco ). the utility of the invention is demonstrated by comparing the performance of i , ii and iii in combination with treflan ®, a standard commercial trifluralin formulation , and by the combination of i plus iv . the test is designed to simulate conditions that might be found in actual commercial operation . since most spraying apparatus are equipped with filter screens having apertures of 50 mesh ( 300 microns ) and larger , metribuzin particles must be kept smaller than this in order to be applied without plugging the screens . therefore of interest in these tests is determining the amount of material retained by a 50 mesh screen after a specified period of time . the tests are run by dispersing the metribuzin containing powder in dionized water . treflan ® ec is then mixed in , and the material is allowed to stand for 6 hours . after this time the entire mixture is washed through an astm 50 mesh sieve , and any material retained is collected , dried and weighed . the material collected is essentially all metribuzin , so the weight of the retains is divided by the total weight of metribuzin in the mixture to find the percent of the metribuzin greater than 50 mesh . the results are as follows . ______________________________________ after 6 hoursingredients in % metribuzin + mole m . sup . 1 + / mole m . sup . 2 + 250 ml h . sub . 2 o 50 mesh in solution______________________________________ ( test 1 ) 1 . 5 g i + 18 . 0 4 . 14 . 7 ml treflan ® ec ( test 2 ) 1 . 5 g ii + 0 . 15 2 . 14 . 7 ml treflan ® ec ( test 3 ) 1 . 5 g iii + & lt ; 0 . 1 0 . 64 . 7 ml treflan ® ec ( test 4 ) 1 . 5 g i + & lt ; 0 . 1 1 . 74 . 7 ml iv______________________________________ the m 1 + / m 2 + mole ratio is determined in the following manner . each sample , i - iv and treflan ® ec , is analyzed separately for soluble m 1 + and m 2 + , in this case sodium and calcium . the analysis is performed by : dispersing 1 . 0 gram of sample in 100 ml of dionized water , filtering the solution , and measuring the cation content in the aqueous phase by standard atomic absorption techniques . from the m 1 + , m 2 + content of each component , the total m 1 + , m 2 + content of the mixture is calculated and the result is expressed in terms of the mole ratio m 1 + / m 2 + . the benefits of the current invention are evident in tests 1 - 4 . in test 1 , where m 1 + / m 2 + is greater than 2 . 5 , a relatively major amount of metribuzin is larger than 50 mesh after six hours . in tests 2 - 4 , where m 1 + / m 2 + l is in the desired range less than 2 . 5 , the amount of metribuzin larger than 50 mesh after six hours is relatively small . it is obvious that use of metribuzin - trifluralin tank mixtures under conditions of tests 2 - 4 will result in fewer application problems . the herbicidal mixtures can include additional pesticides . one herbicide which may be included to the mixture is ethyl 2 -[[( 4 - chloro - 6 - methoxypyrimidin - 2 - yl ) aminocarbonyl ] aminosulfonyl ] benzoate .
asymmetrical triazines form stable tank mixtures with trifluralin in the presence of an effective amount of a suitable divalent metal salt .