referring to the drawings , and fig1 a in particular , there is illustrated an agricultural mulch sheet 10 in strip form . side portions 10a and 10b extend the coverage to lines which are somewhat less than halfway to the adjacent growing rows . the mulch sheet 10 is preferably formed of an imperforate material , such as aluminum foil or plastic . the latter can take the form , for example , of a polyethylene sheet material of 11 / 2 mils thickness , transparent or opaque , reflecting or non - reflecting , or a combination thereof , all as described more fully below . as best seen in fig1 b and 2a and 2b , the sheet 10 is shown centered on the ground over a growing row 11 with the side edges anchored in the earth to secure the sheet in position . placement of the sheet can be done manually or automatically . the latter can , for example , be accomplished by a tractor drawing a pair of laterally spaced shallow - set trenching blades which scrape hollow trenches to receive the edges of the plastic sheet as it is unrolled from a reel , followed by a pair of wheels to press the sheet into the trenches and by a pair of spaced pusher blades which replace the earth in the trench over the plastic . such apparatus and techniques are conventional in the art and , therefore , need not be illustrated or described more fully . if desired , seeding equipment can be mounted directly ahead of the film layer as part of the same tractor rig . in a typical 3 - foot wide sheet mulch , approximately 9 inches can be buried at each edge and 18 inches of center left exposed . a continuous , recurring slit pattern 12 defining a band width , indicated by the numeral 12a , is formed in the center of the sheet 10 . the band width is such that it straddles the growing row 11 with margins of several inches on either side . in the embodiment illustrated in fig1 a and 1b , the slit pattern is comprised of a series of substantially parallel and equally spaced slits 13 , each having a length corresponding to the band width 12 and with each adjacent pair defining a transverse ribbon 14 which straddles the growing row . a ribbon width of about one inch or 25 mm ., and length of about 5 inches or 125 mm . is preferred for crops such as corn , beans , peas and potatoes , but narrower or wider widths can be used consistent with the strength of the sheet material on one hand and the ability of the emerging plants to push their way through the sheet by ribbon displacement on the other . the transverse ribbons 14 connect to the side portions 10a and 10b to integrate the sheet laterally by affording tension or bridging members . when in place on the ground , the flat ribbons collectively form a mulching surface along the band width 12a overlying the growing row . the slits , however , afford water drainage openings through which rain or irrigation water falling on the top of the sheet can seep into the ground in concentrated amounts precisely along the growing row . they also afford controlled venting which prevents overheating which can occur particularly in the case of transparent sheets which can kill corn on certain hot sunny days . the transverse ribbons 14 are easily parted laterally so that it is possible to plant seeds through them in the event the mulch sheet is applied prior to seeding . it is preferred however , in most cases to place the mulch sheet after seeding . this can be accomplished , for example , by carrying the seeding or transplanting apparatus in the same tractor rig as the sheet - laying apparatus described above , causing the seeds to be planted in a row in advance of the sheet deposition , singly as for beans , corn and peas , or in clumps as for tomatoes . registration of the growing roll 11 substantially along the center line of the sheet is readily accomplished within the latitude afforded by the band width 12a . there is no need , however , in accordance with the invention , to establish registration of each seed planted with any given position along the longitudinal axis of the sheet because the seeds can emerge at any point without damage of being blocked by the sheet . when the seeds emerge as small plants , the pliant ribbons part along the proximate slit to allow the plants p to grow as best seen in fig1 b and 2 . during the period prior to the emergence of the plants from the ground , the undisturbed ribbons define a substantially continuous mulch which protects the ground from the elements , thereby minimizing crusting and erosion of the earh prior to plant emergence . the impact of the rain and wind is borne primarily by the plastic . in the case of plants having low , spreading growth characteristics , a few of the ribbons can readily be severed on one side of the growing row after growth has begun to form enlarged openings to accommodate the plants . in the case of &# 34 ; rooting &# 34 ; plants , such as peanuts , the later developing roots can also find their way back down to the earth . the sheet material can be made of a polyethylene plastic which degrades slowly in the presence of ultra - violet radiation from the sun or by exposure to soil chemicals , so that , at the end of the growing season , it will degrade into harmless bits . alternatively , it can be formed of a less degradable material and taken up at the end of the growing season for re - use or re - cycling . while the suggested width of the ribbons 14 is about 1 inch and the suggested length about 5 inches , it will be understood that the widths and lengths can be varied as a function of the thickness and pliability of the plastic sheet , as well as the strength of the germinating plants , i . e . the ability of the plants to part the pliant ribbons as they emerge from the ground . it will be understood also that modifications in the slit pattern can be made for assisting plant penetration . referring to fig3 for example , there is illustrated a modification of the invention in which a plastic sheet 15 includes a slit pattern 16 defining transverse ribbons 17 of slightly greater width than the ribbons 14 of fig1 and 2 . in this modification , each of the transverse slits 18 is crossed by one or more short longitudinal slits 19 , each severing for a very short distance a portion of two contiguous ribbons 17 . thus , each ribbon includes one or more small rectangular flaps along its edge and overlying the growing row . the flaps can yield upwardly under the pressure of the plants and thereby ease the growing process as the plants emerge into the free air . this arrangement has the additional advantage of exposing less of the earth because less distortion of the main body of the ribbon occurs in many cases . the arrangement of fig3 also includes two parallel slit - pattern bands , the sheet 15 being sufficiently wide to cover two parallel growing rows plus portions of the distances to the next growing rows . if desired , the longitudinal slits 19 on one side of the growing row can be extended to be very close together to form tear - out sections or bridges which part under plant pressure to form an enlarged opening to accommodate certain types of plant growth . referring to fig4 a , another modification of the slit pattern is disclosed . formed in the sheet 20 is a central slit pattern formed by transverse undulating slits 21 spaced apart by the desired ribbon width and with the slits having their undulating pattern in phase . in this fashion , small arcuate flaps are defined at the edges of each of the transverse ribbons which assist the plant in penetrating the plastic sheet with minimum distortion of the complete ribbon . each of the slits 21 thus defines laterally extending coplanar flaps on the basic continuous ribbons 21a . in fig4 b , the slits 21 &# 39 ; are longitudinal and are arranged in stacks or clusters along the length of the sheet mulch . in this fashion , there are provided longitudinal continuous ribbons 21b in the slit pattern area in addition to the transverse continuous or bridging ribbons 21a &# 39 ;. the latter are , however , reduced in number and therefore afford somewhat reduced overalll lateral strength . for planting such crops as corn and beans in spaced &# 34 ; hills &# 34 ;, the slit clusters can be correspondingly spaced . the non - linear slits of fig4 a and 4b can have a geometry which is sinuous , saw - tooth or modified saw - tooth such , for example , as having squared tips . also , supplemental minor slit patterns can be superimposed on the basic pattern in the manner , for example , as shown in fig3 . it will be understood , therefore , that a wide variety of slit patterns can be provided within the scope of the invention , using either linear or non - linear , transverse or longitudinal , or interrupted slits , or various combinations of different slit patterns . it is required basically that the pattern afford an integrated structure by providing continuous bridging ribbons with components along one or both axes of the sheet . also , in accordance with the invention , one or both sides of the sheet can include an environmentally degradable coating such , for example , as a water - soluble or ultra - violet degradable material , which will keep the slits closed for a period of time before releasing the ribbons as the plants germinate in the presence of moisture and sunlight , as described below referring to fig8 a . the mulch sheets can include a variety of other features depending on their intended use . for example , the upper surface of the sheets can include indicia 22 ( fig4 ) to serve as a guide for spacing seeds and useful in the case of hand - planting subsequent to laying of the sheets on the ground . also , seeds can be attached to the sheets or the paper tape which is put down with the sheet . in each of the embodiments of fig1 through 4b , the slit patterns , in addition to the other advantages noted , afford a means whereby the sheet mulch can be readily taken up after the growing season . in the case of conventional sheet mulches in which a single aperture is made corresponding to the placement of the seed beneath , the plant becomes an anchor for the plastic sheet which prevents it from being pulled away at the end of the growing season . a field of cornstalks , for example , makes it difficult to remove the plastic , and additional expense and effort are involved in removing the cornstalks before attempting to roll up or bunch up the plastic sheeting for disposal . plant stalks which become part of the waste plastic bundle cannot be utilized as fodder or as organic matter to be tilled into the soil . in accordance with the present invention , because of the continuous slit pattern defining transverse ribbons , it is a relatively simple matter when it is desired to remove the sheeting from the soil to slit the plastic sheet longitudinally to one side of the plant stalks but within the band width of the ribbons formed by the slit pattern . knives , shears and hot wires are examples of severing means which can be used . the two sides of the plastic sheet can then be readily rolled up without being impeded by the plant stalks because the severed ribbons enable it to be easily freed from the stalks . it is possible , therefore , to remove the sheet plastic mulch at any time before or after maturity of the crop without damage to the plants . in the arrangement of fig4 b , the longitudinal ribbons 21b &# 39 ; are pliant and thus yield to form openings to the sky under the pressure of the emergent plants regardless of where a plant might press across the entire width of a ribbon . the longitudinal ribbons 21b &# 39 ;, however , serve the dual purpose of also strengthening the sheet against deformation under tension along the longitudinal axis of the sheet . the transverse ribbons 21a &# 39 ;, not being defined by elongated side - by - side slits , are much less pliant for plant emergence but serve primarily to strengthen and stabilize the sheet against deformation under tension along the transverse axis of the sheet . in the other embodiments of the invention as described below having reference to fig5 a - 8 , the majority of the ribbons running in various directions through the slit pattern are functionally more concerned with structural integrity of the slit sheet than pliancy for plant emergence , the latter function being taken over for the most part by flap configurations . referring to fig5 a and 5b , the slit pattern takes the form of a myriad of uniformly spaced slits 23 which can be u - shaped or semi - circular , arranged in closely spaced relationship in longitudinal and transverse rows . the rows define continuous longitudinal ribbons 24 and transverse ribbons 25 which give longitudinal and transverse integrity and strength to the sheet , and the ribbons are sufficiently narrow as to be deformable by the growing plants . each slit also forms a small flap which swings upward under the force of the emergent plant . by orienting the slit pattern on the growing field , it is possible to arrange the flaps so that they shield the majority of the emergent plants from the prevailing cold winds and cold sky radiation ( the north sky in the northern hemisphere ). on steep slopes , the flaps can be faced downhill to help control erosion and leaching of fertilizer . it will be understood that the slit pattern can be made throughout the entire surface of a plastic sheet because the continuous ribbons between slits form sheet - integrating bridges . such overall pattern is particularly useful for growing grasses , grain crops , broadcast crops , radishes and turnips , which are sown uniformly over the ground as opposed , for example , to crops such as corn which are sown in discrete rows . the pattern can , however , be used in rows corresponding , for example , to those of fig1 - 4 , although certain of the advantages of the uni - directional or one - axis bridging ribbons are not achieved . referring to fig5 b , the slits are shown more closely spaced to cause nesting of the flaps and in which like parts are identified by like primed reference numerals . the transverse ribbons 27a &# 39 ;, 27b &# 39 ;, 27c &# 39 ;, etc . thus become wavy or sinuous as opposed to straight , as in fig5 a . under transverse tension , the sheet tends to deform causing the flaps to open . this effect can be utilized where greater early exposure of the soil is desired or where heating might occur under occasional unique weather conditions , such as a very hot , clear , sunny day . a tractor wheel run along the edge of the sheet , for example , can force the sheet into a shallow depression in the soil which stretches the sheet more tautly to pull open the flaps . referring to fig6 there is shown a more complex slit pattern 26 which reduces the number of longitudinal and transverse ribbons 27 and 28 respectively in relation to the flap size . the flaps are not oriented in one direction . the pattern of fig7 a is defined by an array of cross - slit configurations 29 in which the slits are on the two 45 ° diagonals and the crosses are spaced slightly apart to form longitudinal and transverse bridging ribbons 30 and 31 respectively . the pattern of fig7 b in which the 90 ° cross - patterns 29 &# 39 ; are alternately turned 45 °, yields slightly less accessing area for the emergent plants through the sheet , but it has the offsetting advantage of adding strength by avoiding direct alignment of adjacent slits , thereby lengthening the tearing path between slits . the pattern of fig8 is formed by a more complex slit pattern in which each repeating slit configuration includes a central cross 33 with right - angle slits 34 in each of the four corners or quadrants , which affords eight hinged flap sections . the sheeet has structural integrity longitudinally by virtue of the longitudinal continuous ribbons 35 ; laterally , by virtue of the transverse ribbons 36 ; and along both 45 ° diagonals , by virtue of the diagonal ribbons 37 and 38 . in this fashion , the sheet 41 has high stability against distortion , notwithstanding complete coverage of the sheet surface with slits . the patterns of fig5 - 8 are particularly adapted for use on large sheets for covering steep banks on which it is desired to establish an earth - holding crop of grasses . the sheet serves the multiple purposes of preventing erosion and preventing leaching of fertilizers while permitting the grasses to grow more or less uniformly and densely throughout the area . the sheets can be left in place until they completely deteriorate under the influence , for example , of sunlight . also , if desired , the upper surface of the sheet can include animal repellents . the undersurface of the sheets can carry agricultural chemicals in water - soluble form . herbicides , such as atrazine for corn and dymid for tomatoes , can be applied in the case of transparent sheets , and pesticides in the case of both transparent and opaque sheets . the chemicals can be applied , for example , in liquid slurry form to the sheets using a water or other liquid carrier , together with a suitable wetting agent , such as detergent , to assist in achieving a uniform deposition , together with a suitable binder or sticker , such as a milk solid derivative or synthetic material , for holding the deposition on the sheet . the chemical layer can also serve the purpose of forming an environmentally degradable coating over the sheet ( in water or sunlight ) which will hold the flaps or slits for a limited time after the sheet is placed on the ground until the coating dissolves or breaks down . as shown in fig8 a , coatings 39 and 40 on the two sides of the sheet 41 attract water particles in the form of condensate on the underside and on the upper side in the form of rain which slowly dissolves the layers both to free the flaps in the slit sheet and to allow the chemical to reach the earth . in accordance with the invention , herbicides can be applied to the sheet in different forms and different concentrations . for example , in the case of the centrally slit embodiments of fig1 - 4b , lesser concentrations of selective herbicides can be applied to the central ribbon area and stronger non - selective herbicides can be applied to the side margins or shoulders . while the invention has been described above having reference to preferred embodiments thereof , if will be understood that it can take other forms and arrangements within the scope of the invention . referring to fig1 for example , the sheet 10 can include opaque or partially opaque sides for weed control without herbicides , and a transparent center portion to admit sunlight to soil prior to germination . at a later date , the foliage of the crop will tend to preclude weed growth in that area . the opaque portions can be black or green , reflecting , partially reflecting or white ( the latter three tending to maintain the coolest soil temperature and also to reflect sunlight upward against the underside of the leaves to enhance foliage growth and to discourage aphids ). laminations can be used for different characteristics in different areas of the sheet material . the ribbon slit pattern is particularly useful for covering crop plantings in small trenches , to define protective greenhouse spaces for early season planting . as the plants mature , they can either push their way through the slit ribbons or , if necessary , ribbons can be severed to form plant access openings or additional ventilation or both . this process can be effected manually or by farm machinery , the latter using a plant sensor such as a sensing finger which actuates a switch which in turn actuates a cutter such , for example , as a heated wire . the ribbon sequence pattern enables the grower to create openings of any desired size . in the early growing season prior to emergence through the sheet , the slits will admit water to the trench and prevent accumulations of mud on the sheet over the trench , thus continuing to admit needed sunlight . the invention should not , therefore , be regarded as limited except as defined in the following claims :
there is disclosed an agricultural mulch in sheet form pierced by a recurring , closely spaced slit pattern which provides normally closed passage points between the open air and ground at any random point in the growing area for germinating plants and rain water . the slit pattern can provide structural integrity for the sheet laterally , longitudinally , diagonally or various combinations thereof , and can additionally provide flaps which can be oriented to protect the emergent plants from cold winds .