CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application Ser. No. 61/697,987 filed on Sep. 7, 2012 which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an improved fertilizer applicator knife that is operable for receiving a flexible fertilizer tube attached to a fertilizer implement. 
     BACKGROUND OF THE INVENTION 
     Fertilizer knives are employed by farmers to deliver nitrogen enriching fertilizers such as anhydrous ammonia or liquid fertilizers to a targeted zone beneath the surface of the soil of a crop field while minimizing disruption to the structure of the upper layer of the topsoil. Typically, a fertilizer implement carries a pattern of fertilizer knives spaced for simultaneously delivering fertilizer to a plurality of parallel furrows. 
     Typical prior art fertilizer knives are cast as one piece or are more commonly a combination of a tough steel knife blank and cast iron knife point that is welded to the knife blank. The cast iron knife point has a wedge-shaped leading edge for cutting through soil. With a typical prior art fertilizer knife, a steel fertilizer tube is welded to the trailing edge of the knife blank. For most fertilizer knives, the steel fertilizer tube is welded to the back surface of the knife blank and the fertilizer tube terminates at the base of the knife. Because the fertilizer knife is usually fashioned from a combination of steel and cast iron, the fertilizer tube is fashioned from steel so it can be welded to the trailing portion of the fertilizer knife. The use of a steel fertilizer knife tube presents two disadvantages. First, the skilled reader may recall that the knife shank is bolted to an implement shank with two bolts—an upper bolt and a lower bolt. The lower bolt is designed to function as a structural fuse. If the fertilizer knife encounters an obstacle during use, the lower bolt fails and allows the fertilizer knife to swing back without damaging the fertilizer implement. However, when the lower attachment bolt fails, and, as the fertilizer knife swings back, the steel fertilizer knife tube is often damaged beyond repair. While this action has protected the implement from damage, a damaged fertilizer knife tube makes it necessary to replace the knife. Although an operator will often have spare shank bolts on hand, an operator will usually not have a replacement fertilizer knife available. Accordingly, the damaged fertilizer tube often causes a delay in operations. Second, in the case of anhydrous ammonia, NH3, which boils at approximately −30° F., it is highly advantageous to deliver NH3 to the soil in liquid form. This is because the liquid NH3 absorbs large amounts of heat when it boils. Further, vaporization of NH3 in the highly thermally conductive steel fertilizer knife tube causes great amounts of heat to be transferred into the fertilizer knife tube from the steel and cast iron knife. This is because the heat of vaporization of liquid NH3 is far greater than its specific heat. In relatively cool soil conditions, this can cause ice to accumulate at the lower end of the fertilizer knife which greatly reduces the utility and function of the fertilizer knife. What is needed is a fertilizer knife that eliminates the above noted disadvantages inherent in a fertilizer knife having a steel fertilizer tube. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above described needs are addressed by an improved fertilizer knife having a knife point that presents a leading edge and rear tube receiving portion. The rear tube receiving portion is precision cast to frictionally receive the distal end of the flexible polymer fertilizer tube that is connected, at least indirectly, to an implement fertilizer tank. The rear tube receiving portion also has at least one opening at its lower end for allowing fertilizer to escape from the flexible polymer tube into the surrounding soil during a fertilizing operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of the tubeless fertilizer knife with a flexible fertilizer tube shown in phantom. 
         FIG. 2  is a perspective view of the fertilizer knife of  FIG. 1  with a flexible fertilizer tube shown in phantom. 
         FIG. 3  is a cross-section perspective view of the fertilizer knife of  FIG. 2  taken from plane A-A of  FIG. 2 . 
         FIG. 4  is a perspective view of a tube receiving portion of a tubeless fertilizer knife. 
         FIG. 5  is a side view of the tube receiving portion of a tubeless fertilizer knife. 
         FIG. 6  is a top view of the tube receiving portion of a tubeless fertilizer knife. 
         FIG. 7  is an end view of the tube receiving portion of a tubeless fertilizer knife. 
         FIG. 8  is a perspective view of a second embodiment of a tubeless fertilizer knife. 
         FIG. 9  is a side view of the second embodiment of the tubeless fertilizer knife. 
         FIG. 9A  is a top view of the second embodiment of the tubeless fertilizer knife. 
         FIG. 10  is a front view of the second embodiment of the tubeless fertilizer knife. 
         FIG. 10A  is a partial cross-section side view of the second embodiment of the tubeless fertilizer knife taken from plane A-A of  FIG. 10 . 
         FIG. 11  is a perspective cross-section view of the second embodiment of the tubeless fertilizer knife taken from plane A-A of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings,  FIG. 1  shows a first embodiment for an improved fertilizer knife  10 . Fertilizer knife  10  is suitable for bolting with bolts  6 A and  6 B to a fertilizer implement shank  6 . The skilled reader should refer to U.S. Pat. Nos. 8,161,894 and 7,568,439, which are incorporated herein by reference for background concerning the general arrangement for securing a fertilizer knife to a fertilizer implement and the general configuration and use of such a fertilizer knife. As can be seen in  FIG. 1 , the fertilizer knife  10  includes a knife blank  12 , a point portion  50  and a tube receiving portion  70 . Knife blank  12  includes an upper portion  14  suitable for mounting to an implement shank as shown in  FIG. 1  and a lower portion  20 . Lower portion  20  includes a leading edge  20 A and a trailing edge  20 B. Point portion  50  is welded to leading edge  20 A. Tube receiving portion  70  is welded to trailing edge  20 B. As can be best seen in  FIG. 3 , upper portion  14  has an upper bolt hole  14 A and a lower bolt slot  14 B. Upper bolt hole  14 A and lower bolt slot  14 B are arranged generally vertically on upper portion  14 . An elongated slot is used for lower bolt slot  14 B to accommodate any varying bolt positions that might occur for various fertilizer implements. Bolts  6 A and  6 B are used to attach knife  10  to an implement shank  6  as shown in  FIG. 1 . As noted above, lower bolt  6 B is designed to shear off when knife  10  encounters an obstacle prior to any significant damage being inflicted on the fertilizer implement. 
     Typically, knife blank  12  is fashioned from tough steel capable of transferring bending loads, while point portion  50  and tube receiving portion  70  are typically fashioned from hard, but brittle cast iron suitable for withstanding the abrasion that occurs when fertilizer knife  10  is drafted through soil. 
     As can be seen in  FIG. 1 , point portion  50  is welded to the lower end of knife blank  12  and generally has a configuration well known to those skilled in the art, with the exception that point portion  50  must be designed so it does not interfere with tube receiving portion  70 . Tube receiving portion  70  is welded to trailing edge  20 B of the lower end of knife blank  12  as shown in  FIGS. 1 and 2 . Tube receiving portion  70  is a precision cast iron part. A tube channel  72  extends from the upper end of tube receiving portion  70  to at least one opening  74  at the lower end of tube receiving portion  70 . Tube receiving portion  70  is oriented so that tube channel  72  is partially upright and angled forward as shown in  FIG. 2 . In this example, tube channel  72  is preferably tubular and has internal dimensions that are adapted for frictionally receiving a standard, flexible polymer anhydrous ammonia fertilizer implement tube  5  as shown in  FIG. 3 . In this example, tube  5  is fashioned from ethylene-vinyl acetate (EVA) as is well known to those skilled in the art. Although channel  72  is cylindrical in this example, it does not have to be cylindrical. Those skilled in the art know that EVA fertilizer tubes are flexible, and for most applications, have a standard outside diameter, which in this example is 0.59 inch. Thus, in this example, the inside diameter of the channel  72  is generally 0.61 inch. Preferably the diameter of channel  72  should be held within a narrow tolerance and should not fall below the diameter of tube  5 . It is important that it is always possible (or at least with only very rare exceptions) for an operator to insert tube  5  into channel  72 . Those skilled in the art also know that ethylene-vinyl acetate is a polymer that is a thermal insulator (especially when compared to steel). For reasons noted above, this is a useful property for a tube for conveying liquid NH3 to a release point. 
     In this example embodiment, flexible EVA fertilizer tube  5  is received by channel  72  of tube receiving portion  70 . Tube  5  terminates at the lower end of channel  72 . A series of openings  74  are defined in the rear wall of tube receiving portion  70  which communicate with the lower end of channel  72 . Openings  74  are intended to accommodate various configurations that might be used with tube  5  where various openings may be defined in tube  5 . A single opening  74  at the bottom end of channel  72  may be sufficient for many applications. 
     The skilled reader will appreciate from the above description that with this first embodiment fertilizer knife  10 , NH3 is conveyed to a release point at the lower end of channel  72  of tube receiving portion  70  through EVA fertilizer tube  5  which has a relatively low thermal conductivity and not through a thermally conductive steel tube as is the case with prior art fertilizer knives. Thus, liquid NH3 is more likely to be released from the lower end of the knife as a liquid and is less likely to boil and absorb great amounts of heat as it reaches the release point. The skilled reader will also appreciate that since tube  5  (shown in  FIG. 3 ) is flexible, a failure of a lower attachment bolt  6 B (shown in  FIG. 1 ) will result in no damage to any tube that is needed to convey fertilizer to the release point. Merely replacing the sheared lower bolt  6 B is all that would be needed to bring fertilizer knife  10  back into operation. 
     Referring to the drawings,  FIG. 8  shows a second embodiment for an improved fertilizer knife  110  which is suitable for bolting to a fertilizer implement shank (not shown) as described above. As can be seen in  FIG. 11 , fertilizer knife  110  includes knife body  112  which has an upper attachment portion  114  and a lower blade portion  116 . As with knife blank  12  described above, upper portion  114  of knife body  112  has an upper bolt hole  114 A and a lower bolt slot  114 B. Upper and lower bolt hole  114 A and lower bolt slot  114 B are also arranged generally vertically on upper portion  114 . As described above, an elongated slot is used for lower bolt slot  114 B to accommodate any varying bolt positions that might occur for various fertilizer implements. As can be seen in  FIG. 11 , a generally upright but forwardly angled tube channel  170  is defined for receiving a flexible fertilizer tube  105  (shown in phantom in  FIG. 11 ). Tube channel  170  slopes mostly down and partially forward as shown in  FIG. 11 . Knife  110  is preferably a precision cast iron part. 
     Tube channel  170  extends between an upper opening  170 A and a lower opening  170 B. In this example, tube channel  170  has a specific shape which is adapted to receive a typical rubber tube used for delivering liquid fertilizer. As can be seen by inspecting  FIGS. 9 and 9A , in this example, tube channel  170  has an oval shape that is approximately 15% to 25% wider in the longitudinal direction than in the transverse direction. In this example, tube channel  170  at its upper end is approximately 19% wider in the longitudinal direction than in the transverse direction. Further, as can be best seen by referring to  FIG. 10A , channel  170  tapers between a location that is generally halfway between its upper opening and its lower opening and lower opening  170 B so that channel  170  is substantially narrower at lower opening  170 B. 
     As can be best understood by consulting  FIG. 11 , a flexible fertilizer tube  105  is inserted as far as possible into channel  170 . Preferably, a zip tie  172  (shown as if cut away in  FIG. 11 ) or a standard hose clamp (not shown) is secured by using a clamp opening  114 C in order to firmly clamp fertilizer tube  105  to the back surface of lower portion  116  of knife body  112 . With this type of fertilizer knife, a liquid fertilizer, which includes various fertilizing components dissolved in water, is delivered through tube  105 . The boiling of a liquid fertilizer and the resulting accumulation of ice is not a concern in this case for second embodiment fertilizer knife  110 . However, as noted above, the welded steel fertilizer tube has been eliminated from the fertilizer knife so that should knife  110  pivot back as a result of encountering an obstacle, flexible tube  105  is very unlikely to break or be damaged. Thus, by using second embodiment fertilizer knife  110 , an operator can resume operations should the lower attach bolt fail by merely pivoting knife  110  back into position and replacing the sheared lower bolt. 
     It is to be understood that while certain forms of this invention have been illustrated and described, it is not limited thereto, except in so far as such limitations are included in the following claims and allowable equivalents thereof.
An improved fertilizer knife presents a leading edge and rear tube receiving portion. The rear tube receiving portion is precision cast to frictionally receive the distal end of a flexible polymer fertilizer tube which is connected, at least indirectly, to an implement fertilizer tank. The rear tube receiving portion also has at least one opening at the lower end thereof for allowing fertilizer to escape from the flexible polymer fertilizer tube into the surrounding soil during a fertilizing operation.