CALIFORNIA AGRICULTURAL EXTENSION SERVICE 
 Circular 137 July^ 1947 
 
 GENERAL-CONTACT 
 WEED KILLERS 
 
 A. S. CRAFTS 
 
 
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 Spraying Diesel oil oh roddsldfe 
 growth to form a fire-control strip 
 
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 A G R I C U L T U RE 
 RNIA • BERKELEY 
 
GENERAL-CONTACT WEED KILLERS 
 
 . . . are not selective. They destroy all kinds of vegetation — both 
 weeds and crop plants. 
 
 • These herbicides are used to control unwanted plant growth 
 along highways, rights-of-way, and fence lines, and around farm 
 and industrial buildings. Sodium arsenite has been the chemical 
 most commonly used for such purposes, but it is dangerous to 
 humans and livestock. It is now being replaced by new sprays 
 which are nonpoisonous, cheap, and easily applied. 
 
 • There are three types of general-contact herbicides: 
 
 1. Water-soluble materials, including common salts and cor- 
 rosive chemicals, sodium chlorate, and salts of the phenol 
 compounds. 
 
 2. Emulsions, made by combining water and oil. Additional 
 toxic chemicals may be incorporated in either the oil or 
 water base, or both. 
 
 3. Oils, including Diesel fuel, smudge-pot oil, stove oil, kero- 
 sene distillates, low-grade oils, proprietary weed-killer oils, 
 and oils fortified by addition of phenol compounds or sulfur. 
 
 • There are many conditions under which general-contact 
 sprays are effective, and a variety of spray materials which are 
 suitable. For this reason, the present circular is concerned with 
 giving an over-all picture of these different sprays and suggest- 
 ing possible uses, without making hard and fast recommenda- 
 tions. 
 
 • Each grower will have to decide for himself which type of 
 spray is best for his purpose, after he has studied the materials 
 available and their advantages and disadvantages. 
 
OBHBRH'COHUa Wiid KIUBRS 
 
 A. S. CRAFTS^ 
 
 Heavy weed growth on roadsides and ditchbanks encourages crop pests and 
 diseases and is a fire hazard. Weed growth can be dangerous on highways where 
 it cuts off the motorist's view, and can also block off certain areas so that they 
 are hard to reach. Mowing or disking is often used to remove such weeds, but 
 there are many places where these fairly cheap methods are not satisfactory. 
 In such cases, general-contact herbicides (weed killers) are commonly used. 
 General-contact weed killers are chemicals which kill all kinds of weeds and 
 other plants. They are used to kill all unwanted growth in and around areas 
 such as the following: 
 
 Roadsides Parking areas Farm buildings 
 
 Ditchbanks Landing fields Warehouses 
 
 Railroad rights-of-way Firebreaks Lumber yards 
 
 Fence lines Highways Irrigation structures 
 
 Dry yards Pole lines Industrial plants 
 
 Billboards 
 
 General-contact sprays are not selective. They should never be tised on crop 
 plants. 
 
 For many years sodium arsenite solution has been the standard general- 
 contact spray. The railroads have used millions of gallons of this solution. The 
 results in general have been good, but many livestock have been killed. New 
 weed killers have been found which will work as well as arsenic but which are 
 not poisonous to stock. These are taking the place of the poisonous chemicals. 
 
 The types of general-contact herbicides are: 
 
 1. Water-soluble chemicals, including common salts and corrosive chemi- 
 cals (strong acids and alkalies), and salts of the phenol compounds. 
 
 2. Emulsions of water and oil in which additional oil- or water-soluble 
 toxicants may be included. 
 
 3. Oils, including straight oils, and oils fortified with phenol compounds 
 or sulfur. 
 
 This circular describes a number of these weed-killing materials and tells 
 how to mix them. It also compares them so that the grower may judge which 
 one will be best for his need. 
 
 SPRAY SOLUTIONS MUST WET PLANTS THOROUGHLY 
 
 In order to kill weeds, a contact spray must cover the plant surfaces and wet 
 them thoroughly. Where grasses are part of the weed growth, the spray must 
 creep down the stems and kill the plant crowns. 
 
 ^ Professor of Botany and Botanist in the Experiment Station. 
 
 [3] 
 
4 California Agricultural Extension Circular 137 
 
 Spray chemicals may be mixed with either water or oil. Oil is a better spray 
 base than water, for killing grasses, because it spreads and wets plant surfaces; 
 water gathers into drops and rolls off. If a water spray solution is used, how- 
 ever, its wetting power can be increased by the addition of a wetting agent 
 such as Dreft, Vel, Vatsol, Triton B1956, other Triton wetting agents, Oronite 
 sodium sulfonates, and so forth. These materials are generally used in con- 
 centrations of about 0.1 per cent by weight. They are also useful in mixing 
 
 emulsions. 
 
 APPLICATION METHODS 
 
 Several requirements must be met in the killing of weeds by sprays. 
 
 1. Sufficient toxicant must be applied to kill the bulk of vegetation and 
 allow considerable excess for unevenness of application and differences 
 in plant thickness. 
 
 2. The spray solution must be distributed so that the growing parts of the 
 plants are completely covered. 
 
 3. Where vigorous grasses are present, the solution must creep down into 
 the crowns to prevent renewal of growth by tillers. 
 
 The first two requirements may be met fairly readily by water solutions; the 
 third requires oil or a solution having the properties of oils. Although wetting 
 agents will greatly increase the spreading and wetting properties of sprays 
 mixed only with water, or of emulsions, the high spreading and creeping 
 qualities of oils can never be completely duplicated. 
 
 One way to increase the killing power of such sprays and emulsions is to 
 increase concentration or volume, or both, until the necessary killing power 
 is obtained. In some cases, this may be done rather easily; in others, the increase 
 in dosage and hence in cost is such that use of oil alone is more economical. 
 Only experiments under his own conditions can help the grower decide which 
 of these two types of contact herbicide is more useful to him. 
 
 For satisfactory application of emulsions and sprays mixed with water alone, 
 pressure at the nozzles should be 100 pounds or more. The nozzles should be 
 spaced so that the spray fans meet at approximately the height of the vegeta- 
 tion. The boom should not be more than 18 inches above the vegetation; the 
 full driving force of the spray is needed to wet the plants thoroughly. Most 
 satisfactory results are obtained by using double coverage from different 
 angles. Where application is made by hand, spraying should be slow and 
 methodical so that all vegetation is thoroughly wet. If these precautions are 
 observed, weeds should be controlled almost as effectively as where straight 
 oil sprays are used. 
 
 The application methods given here apply to the water-soluble chemicals 
 and to emulsions, since these mixtures involve special problems which do not 
 apply to straight oil sprays. 
 
 (For detailed information on booms, nozzles, and spray equipment, wTite 
 the Botany Division, Davis, for the mimeographed circular describing equip- 
 ment for spraying weeds.) 
 
General-contact Weed Killers 5 
 
 WATER-SOLUBLE CHEMICALS 
 
 There are two types of chemicals used in general-contact weed control that 
 dissolve readily in water, and may be used in water solution as sprays. The first 
 are well known, common chemicals; the second are the new phenol com- 
 pounds. The sodium or ammonium salts of these phenol compounds may be 
 used directly in water solutions. 
 
 In selecting a water-soluble chemical for weed control, several factors should 
 be considered: (i) availability; (2) ease of use; (3) toxicity, as it relates to cost, 
 dosage, and effectiveness; (4) hazards (poison, fire, and corrosion of equip- 
 ment). 
 
 Common chemicals. These include: sodium chloride, calcium chloride, 
 sodium chlorate, sodium arsenite, sodium hydroxide, sodium borate, sulfuric 
 acid, and many others less commonly used. 
 
 Although these chemicals have been widely used in general-contact sprays, 
 they are not generally recommended because many of them are extremely 
 dangerous to both humans and animals. 
 
 Advantage: 
 
 Some of these chemicals are manufacturers' by-products. If the grower is 
 near the place of production, he may buy them at very low cost. For example, 
 one such material is bittern. 
 
 Disadvantages: 
 
 Sodium arsenite: Livestock are attracted to this chemical and, as a result, 
 many are killed annually. It is also dangerous to the spray operator, to pets, 
 and to humans who may accidentally come into contact with it. It should 
 only be used when such risks can be avoided. 
 
 Sodium chlorate: If spray containing this chemical dries on clothing, dead 
 plants, wood, or other surfaces, it becomes highly inflammable, ignites by 
 friction, and burns like gunpowder. It is hazardous to use as a spray. 
 Strong acids and alkalies are corrosive to machinery and dangerous to use. 
 
 New chemicals of high toxicity. Several new chemicals, the phenol com- 
 pounds, have been recently introduced. They are extremely toxic to plants, 
 and are now being used in place of the dangerous, more common chemicals, 
 for general-contact sprays. Among them are: pentachlorophenol, dinitro- 
 cresol, dinitro secondary butyl phenol, dinitro secondary amyl phenol, and 
 their salts. Most of these compounds are sold under trade names. Their content 
 in such commercial products is shown on the labels. Examples of these products 
 are: Sinox General and Dow General weed killers. 
 
 Sodium pentachlorophenate or sodium dinitro cresylate may be used as 
 general-contact sprays in many situations if their toxicity is increased by acti- 
 
6 California Agricultural Extension Circular 137 
 
 vating them with an acid salt such as ammonium sulfate, aluminum sulfate, 
 or sodium bisulfate. These salts, at concentrations of i to 2 per cent (or even 
 less when no grasses are present) will kill weeds that have been protected from 
 full effects of sun and wind and consequently do not have thick leaf surfaces. 
 Such protected areas are found, for example, in deep drainage ditches, shady 
 orchards, and lathhouses. 
 
 The ammonium salt of dinitro secondary butyl phenol is so toxic that it will 
 kill even grasses in their seedling stages. When used at concentrations of 0.5 
 per cent and over, with a good wetting agent, this herbicide has proved effective 
 in killing mixed weeds on roadsides, ditchbanks, etc. On state highways in 
 central California, fire strips sprayed with this material in April could be 
 burned sufficiently within two to three weeks to provide adequate fire protec- 
 tion for adjoining crops and pastures. 
 
 Advantages: 
 
 1. The salts of the phenol compounds are soluble in water. 
 
 2. They are highly toxic; so little chemical is required that hauling is re- 
 duced to a minimum. 
 
 3. If used in large quantities, they can be bought at prices that compare 
 favorably with those of fuel oils. 
 
 4. They have no serious poison hazard. 
 
 5. They form true solutions and require no agitation once they are dissolved. 
 
 6. They kill certain oil-resistant weeds, such as sweet fennel, yellow star 
 thistle, mayweed, pineapple weed, and the like. 
 
 Disadvantages: 
 
 They are not economical for use on coarse, vigorous grasses, such as wild 
 oats, foxtail, ripgut, and the like. On these weeds, an oil spray is necessary, 
 because it will creep down the grasses and penetrate the crowns, where a 
 water spray, unless applied in very large amounts, will run off. 
 
 All water-soluble herbicides require thorough application. Formulas for 
 mixing them will be found on the labels. 
 
 OIL SPRAYS 
 
 A description of oil fractions and their toxic effects on weeds will be found 
 on pages 12 to 16. Growers who are not familiar with the properties of oils 
 should considt this section before reading about oil sprays. 
 
 In contrast to sprays in water solution, oils wet plant surfaces readily and 
 tend to spread as thin films and run down the stems. They penetrate the crowns 
 of grasses where growing tissues that form new shoots are located. If an oil 
 spray wets the tops of grasses thoroughly, the film may creep from 4 to 6 inches 
 down the stems and kill all tissue from which new shoots might grow. This 
 accounts for the satisfactory results usually obtained with Diesel and smudge- 
 pot oil sprays, both of which are standard materials for weed killing. 
 
General-contact Weed Killers 7 
 
 The content of aromatic compounds in general-contact weed oils should be 
 above 25 per cent and may be as high as 50 per cent or more. 
 
 Diesel oil. To be used for making firebreaks along California highways, 
 Diesel oil must meet the following specifications: 
 
 Specific gravity at 60° F, not less than 27° A.P.I. 
 
 Flash point (Pensky-Martens closed cup), not less than 1 50° F. 
 
 Viscosity (Saybolt Universal) at 100° F, not over 50 seconds. 
 
 Distillation (90 per cent point), not over 610° F. 
 
 Water and sediment, not over a trace. 
 
 Diesel meeting these specifications is readily available from oil dealers, and 
 should be bought under the name of Diesel fuel oil. 
 
 Advantages: 
 
 Diesel combines a low acute toxicity with a medium chronic toxicity. Be- 
 cause it clings to the waxy plant surface, it wets plants easily and covers them 
 well. It is very effective against mixed weeds, including grasses, and will kill 
 completely even where growth is so dense that spray cannot be forced down 
 into the bases of the plants. Being low in cost (in many areas), noncorrosive, 
 and safe to handle, Diesel oil is an excellent herbicide for general weed 
 control. 
 
 Disadvantages: 
 
 Because Diesel is relatively low in toxicity, a large volume per acre is re- 
 quired. This makes transportation costs relatively high. It also makes Diesel 
 unfit for low-volume application by airplane unless fortified. Plants which 
 are resistant to oil require too large an application to make use of Diesel 
 economical. Because of its value as a fuel, widespread use of Diesel as a^weed 
 killer seems unwise. 
 
 Smudge-pot oil. This may be used in place of Diesel because its properties 
 are similar. It is often less refined and hence may be more toxic. 
 
 Stove oil. Although this oil may be used as a contact herbicide, it is largely 
 limited to preemergence spraying in cropped areas. 
 
 Advantages: 
 
 It may be used as a preemergence spray in row crops even after crop plants 
 have started breaking through the soil if the crops are tolerant of the oil. 
 Some tolerant plants are carrots, celery, parsnips, and parsley. (For use of 
 stove oil on carrot crops, see Extension Circular 1 36.) 
 
 Disadvantages: 
 
 It is lighter and less concentrated in chronic toxicants than Diesel oil, and 
 is therefore less effective for general weed spraying, especially where there 
 are large grasses. 
 
California Agricultural Extension Circular 137 
 
 Fig. 1. — Unsprayed alfalfa field in which foxtail is beginning to spread. 
 
 Kerosene distillate. This is useful, if obtainable, as are other petroleum 
 distillates in the same general boiling range. The gravity of such oils should 
 range between 27° and 38° A.P.I. Below 27° they are heavy and may be diffi- 
 cult to apply in cold weather; above 38° they lack the persistent action of the 
 chronic toxicants of heavier oil fractions. They are somewhat selective and 
 are therefore ineffective on many oil-tolerant plants. 
 
 Low-grade oils. In certain oil-producing regions, low quality oils that are 
 perfectly satisfactory for killing weeds may be bought from local refineries. 
 Usually such oils are high in sulfur or aromatic compounds and may be un- 
 suitable for use as lubricants or fuels. The properties that make them unfit for 
 these uses may make them especially good weed killers from the standpoint of 
 toxicity. Where such oils are available, they prove very useful in weed control. 
 
 Oil extracts. In the refining of medicinal oil, kerosenes, lubricating and 
 other finer oils, solvent extraction is used to remove the more reactive unsatu- 
 rates. Lighter grades of these by-products are recovered from the extract and 
 used as paint thinners, cleaning fluids, and solvents. When these materials 
 have been removed there may still remain a dark, foul-smelling substance of 
 little commercial value. This residue is high in aromatic compounds and is 
 extremely toxic to all plants. It is sold in limited quantities as a weed killer. 
 (Avon Weed Killer, of the Tidewater Associated Oil Company, is an example.) 
 
General-contact Weed Killers 9 
 
 Such oils are very useful as herbicides, but only a limited amount is available. 
 They may be used to fortify Diesel oil when it has been so highly refined that 
 it does not have enough killing action. 
 
 As demands for gasoline increase, stove, Diesel, and smudge-pot oils will be 
 used for its manufacture, and new types of oils will be made available in their 
 place. These new oils will be largely fractions resulting from the cracking 
 operations used in gasoline manufacture and, if unrefined, may be higher in 
 unsaturates than the present types. This should increase their toxicity. Non- 
 tillage programs in orchards, and an increased use of oil in weed control are 
 creating a great demand for weed-killing oils. It seems likely that the large 
 refiners will offer special products combining maximum spreading and wetting 
 properties with a balanced toxicity sufficient to kill all weeds without wasting 
 toxic compounds. Two such oils (Standard Weedkiller No. 2 and Shell Weed- 
 killer No. 20) are now available. 
 
 Fortified oils. Oils of low toxicity may have their killing power increased by 
 the addition of certain phenol compounds, or sulfur. They are then known as 
 fortified oils. In addition to increasing the killing power, fortification provides 
 extra toxicity where an oil is being used in emulsion or in low-volume applica- 
 tion by airplane. It also makes oils more toxic to certain oil-tolerant weeds, 
 such as sweet fennel, wild carrot, yellow star thistle, and mayweed. Most of 
 these compounds are sold under trade names, with their content in the product 
 shown on the labels. 
 
 Fig. 2, — Alfalfa field which was sprayed with fortified oil to remove annual weeds. 
 
10 California Agricultural Extension Circular 137 
 
 Pentachlorophenol and dinitro-cresol are about equally toxic in oil solution. 
 For most purposes in the field, a i/g P^r cent solution— roughly, 4 pounds per 
 100 gallons o£ oil— is enough; the same dosage rate applies to sulfur. Dinitro 
 secondary butyl phenol (active ingredient in Dow General) is about three 
 times as toxic, and only about ly^ pounds per 100 gallons is required. Dinitro 
 secondary amyl phenol (active ingredient in Sinox General) has similar prop- 
 erties. Where oil-tolerant weeds, such as sweet fennel, yellow star thistle, and 
 mayweed, predominate, the fortifying agent may be increased. (These special 
 weed types, while resistant to oils, are very susceptible to 2,4-D, and dusts or 
 sprays of this chemical may be used as separate treatments, preferably before 
 the oil spray. The 2,4-D should not be combined with a toxic oil because the 
 oil will defeat the purpose for which the 2,4-D is used. For further information 
 on use of 2,4-D, see Extension Circular 133.) 
 
 In addition to the fuel oils, several less toxic oils are useful in weed control 
 if they are fortified. Examples are wash and slop oils, reclaimed solvent and 
 lubricating oils, old crankcase oil, and gas drip oil. In fact, the wetting and 
 creeping properties of any oil may be combined with the high toxicity of a 
 fortifying agent to produce a weed killer. 
 
 Viscosity, as it affects flow through nozzles and the spread of the oil on 
 plants, is the most important factor in specifications of such oils. The viscosity 
 of any herbicidal oil, as determined by the Saybolt Universal test at 100° F, 
 should not be over 50 seconds. 
 
 Advantages: 
 
 Fortified oils are more toxic than straight fuel oils, and the volume used may 
 be cut to the smallest amount which will give thorough coverage. In one 
 case, where 150 gallons of straight Diesel oil per mile was required to spray 
 ditches in the Sacramento River delta, 75 gallons of Diesel containing 3 
 pounds of dinitro-cresol or 1 pound of dinitro secondary butyl phenol pro- 
 duced better results. In another instance, 14 gallons of fortified oil per acre 
 was applied by airplane to kill small weeds in a field where onion bulbs had 
 been planted for seed production. The field was too wet for application by 
 ground sprayer, or for cultivation or hoeing. A similar application has 
 proved effective for eliminating small annual weeds in alfalfa. (For infor- 
 mation on weed control in alfalfa, apply to the Botany Division, Davis, for 
 the mimeographed circular describing the methods.) 
 
 Fortified oils are more rapid in their action than straight Diesel or heavier 
 fuel oils. In spraying grass along the water line of irrigation ditches, for 
 controlling the breeding of mosquitoes, only 24 hours were required before 
 the ditch was restored to normal use. By killing quickly, fortified oils pro- 
 vide a longer period for burning. This makes them especially useful in 
 killing weeds on highways where it is necessary to burn while the surround- 
 ing vegetation is still green. 
 
 Fortified oils will kill oil-tolerant weeds. They are useful for spot treat- 
 
General-contact Weed Killers 1 1 
 
 ments on clumps of sweet fennel, poison hemlock, artichoke thistle, St. 
 John's wort (Klamath weed), or other shallow-rooted perennials. They kill 
 all weeds, including grasses, that come up through cracks in sidewalks, pave- 
 ment, and concrete ditch lining. They are useful for treating Bermuda grass 
 and similar pests in gravel walks and railroad ballast, and for treating 
 individual weeds, such as dandelions, plantains, and chicory in lawns and 
 permanent pastures. 
 
 Emulsions. The extreme toxicity of the recommended fortifying agents 
 makes it possible to prepare fortified oils having much greater killing power 
 than they have spreading ability. Such solutions may be applied in low volume 
 from airplanes. Similar applications can be made by ground rigs if high- 
 pressure, small-orifice nozzles are used. Low-volume applications can also be 
 made by vapo-dusting equipment, or by means of aerosol, smoke, or steam. 
 
 Although low-volume application is effective on small weed growth, more 
 volume is required when plants have grown to the stage where they cover the 
 soil and protect their own crowns. When this point has been reached, the 
 fortified oil must be applied in emulsion. In fortified oil emulsions it is usually 
 necessary to use wetting agents. Agitation in the spray tank is usually required 
 because fuel-oil emulsions are not stable enough to hold throughout the period 
 required for application. 
 
 Emulsion formulas. The formulas given below should be mixed on a weight 
 basis. One hundred gallons of emulsion weighs roughly 800 pounds; 1 per cent 
 would represent 8 pounds per 100 gallons; i/^ per cent, 4 pounds; i/^ per cent, 
 2 pounds; and % per cent, 1^ pounds. Since Diesel oil weighs about 8 pounds 
 per gallon, a satisfactory approximation can be made by adding the oil on a 
 ^olume basis— 2 per cent would be 2 gallons of oil and 98 gallons of water; 6 
 per cent, 6 gallons of oil and 94 gallons of water. 
 
 A satisfactory emulsion may be made using: 
 
 1 per cent sodium pentachlorophenate (8 pounds) 
 
 per cent aluminum sulfate, as an activator (4 pounds) 
 
 Ho per cent wetting agent (% pound) 
 2 per cent Diesel oil (2 gallons) 
 98 gallons of water. 
 
 This formula has proved effective against broad-leaved annuals and such 
 grasses as annual bluegrass and soft chess. 
 
 A better formula from the standpoint of cost includes: 
 
 1/2 per cent pentachlorophenol (4 pounds) 
 6 per cent Diesel oil (6 gallons) 
 %o per cent wetting agent (^5 pound) 
 94 gallons of water. 
 
 A similar emulsion can be made by using % per cent dinitro secondary butyl 
 phenol in place of the I/2 per cent pentachlorophenol. 
 
12 California Agricultural Extension Circular 137 
 
 These formulas are satisfactory for broad-leaved weeds and grasses grown 
 in sheltered locations. Where more vigorous grasses are present, the percentage 
 of oil should be increased. Mixtures up to 25 per cent oil have been used 
 against large, rapidly-growing wild oats, barley, and foxtail. Experience will 
 determine the oil requirement and indicate the correct adjustment for the 
 height and vigor of the vegetation. Such matters as the density of growth and 
 the maturity with respect to possible tillering and late germination of addi- 
 tional seed must also be considered. 
 
 Advantages: 
 
 Emulsions kill oil-resistant weeds at a minimum cost. Chickweed, pineapple 
 weed, and yellow star thistle have been killed in experimental tests in an 
 onion crop where selective herbicides failed and where straight oil would 
 have seriously harmed the crop. Although the emulsion injured the onions, 
 they recovered rapidly. Yellow star thistle and mallow, both of which are re- 
 sistant to Diesel, have been killed on roadsides by applications of emulsions. 
 
 The composition of emulsions may be adjusted to meet almost any con- 
 dition. The oil content may be held to 1 or 2 per cent on tender weeds in 
 orchards or in ditches, or may be increased to 1 5, 20, or 25 per cent in fighting 
 heavy, vigorous infestations of grasses. Within this wide range there may be 
 an appreciable saving of oil. And in all cases, oil-tolerant weeds are killed. 
 As the oil content is raised above a level of about 25 per cent, a point of 
 diminishing return is reached where the relative costs of oil, fortifying agent, 
 and wetting agent must be balanced with the inconvenience of mixing and 
 effectiveness of treatment. 
 
 Under most conditions, emulsions containing fortified oil are cheaper 
 than straight oil because the toxicants are so much more effective than those 
 present in oil alone. For example, the second formula on page 1 1 can be 
 prepared for about one half the cost of a straight oil spray. 
 
 Use of emulsions cuts the cost of long-distance hauling of fuel oils. This 
 becomes a real factor in regions far removed from oil refineries. 
 
 Disadvantages: 
 
 In using emulsions, the spray tank must be equipped with a mechanical 
 agitator. Also, where large, vigorous grasses are present, results may never 
 be quite so good as with oil because of the difference in creeping ability. 
 
 PROPERTIES OF OILS^ 
 
 To use oil sprays in weed control, the grower must know something about 
 oils and their effects on plants. In this way he will be able to choose the best 
 oil for his own needs. 
 
 In the oil trade, all oils are described by sets of specifications. These are 
 
 2 The four drawings are based on illustrations supplied through the courtesy of the Shell 
 Oil Company, Incorporated, to whom acknowledgement is also made for editorial advice in 
 connection with the preparation of material used in this section. 
 
General-contact Weed Killers 
 
 13 
 
 =:l 400 F 
 
 either required by law or used by the manufacturer as a standard of quality 
 for his own products. Every product must meet the specifications which its 
 manufacturer has set up for it. These standards are intended to show a 
 product's ability to do the job for which it was made. Certain oils now being 
 tested as weed-killing sprays were not really intended for this use. Thus speci- 
 fications listed for these oils do not neces- 
 sarily show how well they will act as weed 
 killers. There are no specifications for zveed- 
 killing oils. The only sure way to find out if 
 an oil is useful as a weed killer is by tests in 
 the field. Hence, in buying oils for weed kill- 
 ing, the grower will have to rely on the 
 ability of the oil dealer to supply a satisfac- 
 tory product. 
 
 This circular lists some of the words com- 
 monly used to describe oils and oil sprays, 
 and tells what they mean. It also gives a sim- 
 ple account of the way in which oil is refined. 
 The list should be helpful to the grower 
 when choosing his oil sprays. 
 
 RESIDUE 
 
 Fig. 3. — As the vapor passes up 
 the tower, the part with the high- 
 est boiling point condenses and 
 drops back. The fractions with the 
 lowest boiling points are run ofif at 
 the top. 
 
 Refining. Two main processes are involved 
 in oil refining: distillation and separation. 
 An example of simple distillation is the boil- 
 ing of a teakettle. The water in the kettle is 
 heated to boiling temperature. At this point 
 it turns to steam or vapor. If the steam 
 
 touches a cold surface, it condenses. This condensed steam is a distillate. Since 
 water has only one boiling point, the condensed steam is the only product 
 resulting from the boiling of water. 
 
 Unrefined (crude) oil, on the other hand, is made up of many parts (frac- 
 tions) which have different boiling points. The oil is put into a container, or 
 tower, and heated. The fractions which have the lowest boiling point (gaso- 
 line) vaporize first and the vapors rise to the top where they condense. These 
 fractions which evaporate quickly are the light (more volatile) ones. Other, 
 less volatile fractions, such as stove oil and Diesel fuel, have higher boiling 
 points, and condense later. Thus each batch of crude oil contains some gaso- 
 line, some stove oil, and some Diesel fuel, etc., and each fraction may be re- 
 moved within its own boiling range. (See fig. 3.) 
 
 Distillates. The liquid oil fractions resulting from distillation are called 
 distillates. Not all the materials in crude oil can be distilled; tars and asphalt 
 are left. The distillates are not pure compounds but contain a mixture of all 
 compounds which turn to vapor during distilling. Some of these are called 
 
14 
 
 California Agricultural Extension Circular 137 
 
 unsaturated compounds. There are more of these unsaturated compounds 
 in an oil such as Diesel fuel, that has not been highly refined, than there are 
 in kerosene or spray oils that have received more treatment. When a refined 
 oil is wanted, some or all of the unsaturated compounds may be separated 
 from the oil by use of chemicals. Sulfur dioxide is one commonly used for 
 this purpose. 
 
 It is the unsaturated compounds which are important in weed-killing oils. 
 They determine, in part, how well the oil will kill plants. (See "Toxicity.") 
 
 
 \ DEGREE 
 MARKINGS 
 ON TUBE 
 
 /'\ 
 
 y 
 
 BULB 
 
 Gravity. The gravity, or density of an oil has to do with 
 its weight. It is expressed in degrees A.P.I, because the grav- 
 ity of oils in the United States is determined by tests set up 
 by the American Petroleum Institute. The gravity of an 
 oil is found by use of an instrument called a hydrometer, 
 lliis is a glass tube with degree markings on the side and 
 a bulb at one end. (See fig. 4) The bulb floats in the oil 
 sample to be tested, and the depth to which it sinks, as indi- 
 cated by the marks on the tube, is a measure of the gravity 
 of the oil. The bulb does not sink so far in heavy oils as in 
 light ones. The degrees are marked on the tube in such a 
 way that gravity readings of heavy oils are lower than those 
 of light ones. 
 
 Gravity is important in choosing a weed-killing oil. 
 Heavy oils, which fall below 38° A. P. I., will kill crop plants 
 as well as weeds. For use as a selective herbicide on crop 
 plants, therefore, a weed-killing oil should not fall much 
 l)elow 38° A.P.I. 
 
 Fig. 4. — The hy- 
 drometer floats in 
 oil. Gravity meas- 
 urements are indi- 
 cated on the tube. 
 
 Flash point. Flash point is a measure of the inflamma- 
 bility of an oil. One of the means for testing the flash point 
 is the Pensky-Martens closed cup test. The oil is heated in 
 a closed container, or cup. (See fig. 5.) A slide covers a small 
 opening in the cup. This is opened at definite intervals, and a flame is passed 
 over the oil. The temperature at which the oil ignites is its flash point. 
 
 Highly volatile oils ignite at fairly low temperatures. All gasolines flash at 
 ordinary temperatures (for instance, 70° F); in fact, they will flash at freezing- 
 temperature for water. From the standpoint of safety for the operator, it 
 is dangerous to use gasoline for weed spraying. Only less volatile fractions, 
 such as some of the thinners and solvents, are fairly safe to use. However, the 
 spray operator must remember that all these products are inflammable. The 
 spray mist and surrounding air may ignite and burn with great heat. 
 
 Viscosity. This relates to the flowing quality of an oil. To find the viscosity, 
 60 cc (about 2 ounces) of oil are put into an instrument called the Saybolt 
 
General-contact Weed Killers 
 
 15 
 
 Universal viscosimeter. The oil is heated to 100° F. It is then timed as it runs 
 through a small opening in the instrument. (See fig. 6.) 
 
 The viscosity of an oil to be used as a spray determines somewhat the amount 
 of pressure needed, and the size of the spray orifices. The heavier oils will not 
 break up into drops easily, nor flow as fast as will the lighter ones. Viscosity 
 is also a factor in determining how much 
 of the oil soaks into the plant surfaces. A 
 heavy oil will stay on the plant longer 
 than will a lighter, more volatile one. 
 Thus it may soak in in larger amounts 
 and be more toxic. For use as a weed spray, 
 an oil's viscosity should be about 50 sec- 
 onds or less. 
 
 THERMOMETER 
 
 VENT COVER 
 
 KEROSENE OR 
 RJELOaS 
 
 FLAME 
 
 APPUED 
 
 HERE 
 
 
 CLOSED CUP 
 
 VAPORS CONFINED 
 
 Fig. 5. — The closed cup test for 
 determining the flash point. 
 
 Toxicity. An oil's toxic eflect on plants 
 depends in part on how volatile it is and 
 on the amount of unsaturated compounds 
 it contains. Oils vary in their toxicity. 
 Some kill all plants; some are selective, 
 and kill only weeds, leaving certain crop 
 
 plants undamaged. Light unsaturated compounds cause a rapid burning of 
 leaves called acute toxicity. Heavy unsaturated compounds injure the growing 
 parts and cause a chlorosis (yellowing of leaves). This injury comes on much 
 more slowly and is called chronic toxicity. Very light unsaturated compounds, 
 such as those from gasoline stock, cause burning of the leaves. Injury is not 
 complete, however, if the spray incompletely saturates the plant, because 
 these oils may evaporate before all tissues are killed. 
 
 Unsaturated compounds of medium weight are very toxic to grasses and 
 most weeds. They do not kill plants of the carrot family, except at high con- 
 centrations. They are usually found in unrefined petroleum distillates, such 
 as stove oil, at concentrations between 20 and 30 per cent. Heavy unsatu- 
 rated compounds, such as those in Diesel and other heavy fuels, kill plants 
 slowly by chronic toxicity. Crop plants as well as weeds are killed by such oils. 
 
 Fig. 6. 
 
 The viscosimeter operates on the principle 
 shown below. 
 
 STARTED WITH 
 
 APPARATUS 
 
 FILLED WITH 
 
 1 GALLON OF 
 
 LIGHT (THIN) OIL 
 
 AFTER ONE MINUTE 
 OIL DRAINED 
 
 SAME 
 
 SIZED 
 
 OPENINGS 
 
 STARTED WITH 
 APPARATUS 
 
 FILLED WITH 
 1 GAL. HEAVY 
 (VISCOUS) OIL 
 
 AFTER ONE MINUTE 
 SMALL AMOUNT DRAINED 
 
 6 
 
16 California Agricultural Extension Circular 137 
 
 OILS FOR WEED CONTROL 
 
 Selective oil sprays are ones which do not damage certain oil-tolerant crop 
 plants but kill many common weeds. 
 
 General-contact oil sprays kill all kinds of plants, both weeds and crops. 
 They should contain enough toxicants to kill all plants sprayed. 
 
 Emulsions are physical mixtures of oils and water. Most emulsions have a 
 water base in which droplets of oil are suspended. 
 
 Invert emulsions, however, have an oil base in which droplets of water are 
 suspended. Weed-killing emulsions may have additional chemicals dissolved 
 in either the oil or water, or both. 
 
 Fortified oils are oils which have had their toxicity increased. This is done 
 by adding chemicals (fortifying agents) such as phenol compounds or sulfur. 
 It usually costs less for these fortifying agents, per unit of toxicity, than for 
 an amount of unfortified oil of equal toxicity. Fortified oils are used as general- 
 contact sprays. The oil itself acts mainly as a carrier for the toxic fortifying 
 agent. 
 
 Oils serve two purposes in chemical weed control. They act as toxicants 
 (killers) and as carriers. In selective weed oils, the toxicants (medium-weight 
 unsaturated compounds) should be present at concentrations that kill weeds 
 without killing crops. For carrots, this concentration may be fairly high 
 (around 25 per cent). To spray safely in flax, it should be considerably lower, 
 and for onions, still lower. The oil should be so light that all toxicity is of the 
 acute type. This means that it should have a gravity above 38° A.P.I. (Shell 
 Weedkiller No. 10, for example, has a gravity of about 43° A.P.I.) This guaran- 
 tees that the toxicity is acute. At the same time, such oils evaporate fast, so 
 that the oil flavor soon leaves the crops. 
 
 The nontoxic part of the selective oil acts as a carrier for the toxic parts. 
 It cannot be replaced by water because the selective toxicants are oils and will 
 not act in emulsion. Also, the wetting properties of straight oil are required 
 to kill grasses in the crop. 
 
 Oil is a better carrier than water because it has a low surface tension and 
 high wetting ability. This means that instead of running off the plants as 
 water does, the oil creeps over them. In doing this, it soaks into the growing 
 parts and kills the tissues. This wetting ability is especially important where 
 the oil is to be used on weed growth containing rank, vigorous grasses. 
 
 Cooperative Extension work in Agriculture and Home Economics, College of Agriculture. 
 
 University of California, and United States Department of Agriculture cooperating. 
 
 Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. 
 
 B. H. Crocheron, Director, California Agricultural Extension Service. 
 
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