Substratum for soil-free cultivation

Substrata for soil-free cultivation are characterized by a relatively low by-volume mass. They are also constituted of fine fibers. The substrata present the advantage of a high degree of water retention, even for small thicknesses.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to fibrous substrata for soil-free or hydroponic 
cultivation. 
2. Discussion of the Prior Art 
The constantly developing practice of soil-free cultivation has led to the 
utilization of substrata of various natures, especially vegetable-derived 
fibrous materials, natural mineral products such as gravels and 
pozzolanas, or else processed mineral products such as expanded perlites 
or rock wools. 
The choice of substratum depends at once on its characteristics which 
facilitate cultivation, good solution retention, good aeration, geometric 
and chemical stability, etc . . ., and economic data such as: cost of the 
substratum, replacement frequency, as well as the necessary investment, 
depending on the type of cultivation under consideration, which, can be 
related to the type of substratum utilized. 
Among the processed mineral materials, rock wools present advantageous 
properties. They offer a very high degree of porosity, of about 95%, good 
water retention and good aeration. The material is also easy to handle due 
to its lightness. On the other hand, the cost of these substrata normally 
leads to multiple uses, for obvious economic reasons. These uses require 
disinfection, thus handling, which becomes increasingly difficult after 
successive cultivations because the structure of the material 
deteriorates. The deterioration of the material also causes a loss of 
porosity and sinking, which change the cultivation conditions. 
Among the advantageous characteristics of the rock wool substrata, the 
"available" quantity of water retained presents a particular interest. 
This characteristic determines the safety margin which is available to 
maintain the satisfactory moisture conditions. The greater this available 
water value is for a given volume of material, the greater the degree of 
safety. If the material presents a large quantity of available water, the 
feed of liquid to the substratum during cultivation can be effected at 
less frequent intervals. Still better, the volume of substratum which is 
necessary for cultivation can be decreased when the available water 
quantity per unit of volume increases. 
The latter characteristic is of great practical interest. A smaller volume 
of substratum, and more precisely, a smaller amount of fibrous materials, 
leads to a less costly material. When this decrease in cost is sufficient, 
it can be accompanied by other advantages, in particular, below a certain 
threshold, the one-time use of substratum can be envisioned, which allows 
the elimination of the operations required for the sterilization of the 
substratum between successive cultivations. 
SUMMARY OF THE INVENTION 
The purpose of the invention is to provide new mineral fibrous substrata 
for soil-free cultivation. These mineral materials in accordance with the 
invention present characteristics such that the amount of available water 
is appreciably increased. 
The inventors have highlighted the existing relations between water 
availability for fibrous substrata and the structural characteristics of 
these substrata. The inventors have especially been able to establish the 
conditions of by-volume mass and fiber fineness which are most 
satisfactory to produce a good available water reserve. 
The substrata according to the invention present a by-volume mass which is 
under 50 kg/m.sup.3 and preferably under 40 kg/m.sup.3 with fibers whose 
average diameter is under 8 micrometers and preferably under 6 
micrometers. 
On an indicative basis, for rock wools which are traditionally utilized as 
substrata, the by-volume mass is ordinarily higher, and is in the range of 
70-80 kg/m.sup.3 or more. The substrata according to the invention are 
thus quite appreciably lighter than the traditional substrate. 
This lightness does not have profound effects on porosity. Indeed, even the 
conventional substrata offer a high degree of porosity, of about 95%. In 
other words, in the substratum, the fibers occupy only 5% of the volume, 
with the rest corresponding to the space which can be occupied by water. 
Thus, a decrease in the by-volume mass does not substantially increase the 
available space for the solution. But the decrease in by-volume mass with 
a decrease in the average diameter of the fibers (and the multiplication 
of these fibers) seems to promote the development of the capillary actions 
which can explain the improvement in the substratum's increased water 
availability. 
It must be emphasized that a reduction in the by-volume mass does not 
always necessarily imply a decrease in the diameter of the useful fibers, 
i.e., those which participate in the formation of the capillary network 
which retains water. Especially for rock wools, the production method 
causes the presence of a relatively significant proportion of 
"non-fibrous" particles. The term "non-fibrous" indicates particles having 
a diameter which is much greater than that of the fibers properly 
speaking, and which is set arbitrarily, for example, at over 40 
micrometers. The proportion of non-fibrous materials often reaches, or 
even exceeds 30% of the total mass of the substratum. As mentioned above, 
considering the mass of these large particles, they contribute very little 
to the formation of the capillary network and thus to the solution 
retention properties. It is thus desirable to utilize products as much as 
possible which are free of non-fibrous materials, or which have very low 
contents thereof. 
In practice, it is possible during the production process to modify the 
by-volume mass of fiber felts by compressing them essentially at the time 
of thermal treatment, which normally establishes their configuration. 
Thus, substrata are produced, having fibers of the same size, and which 
differ only by the by-volume mass (and less significantly, the overall 
porosity) thereof. 
For substrata made from larger fibers, namely, those having an average 
diameter greater than 6 micrometers, it is noted that the available water 
increases as the by-volume mass increases. This explains why, in the case 
of rock wools, whose fibers in the most recent techniques and especially 
those utilized for the production of substrata, have a diameter of about 6 
micrometers, the tendency is to implement heavy products. 
With finer fibers, for example, 4.5 to 3 micrometers or less, which are 
also considered in accordance with the invention, the influence of the 
by-volume mass is much less appreciable. Thus, it is highly advantageous, 
in addition to what will be seen below with respect to the buckling of the 
substratum under the weight of the solution, to utilize very light felts 
having fine fibers. 
This difference in behavior, here again, is probably explained by the 
constitution of a much larger capillary network with fine fibers. 
If it appears to be advantageous to reduce the diameter of the fibers 
comprising the substratum, it is often difficult to pass below a certain 
limit for various reasons. A first reason is that the production of very 
fine fibers, for example, those under 1 micrometer, requires techniques at 
a cost which is unacceptable for cultivation substrata. 
Another reason is, for example, that the felts formed from very fine fibers 
(and having very low by-volume masses) present a small degree of 
resistance to mechanical stress. They can especially sink under the weight 
of the solution which is absorbed. 
For these reasons, it is advantageous according to the invention to utilize 
substrata whose fibers are between 1 and 8 micrometers and preferably 
between 2 and 6 micrometers. 
The partial sinking of the saturated material mentioned above does not 
necessarily constitute an obstacle for use. For the lightest materials, a 
certain settling during wetting can be envisioned. In this case, it 
suffices to adjust the dry thickness of the substratum so that, in the 
damp state, the volume provided for the solution remains sufficient. As 
such, very light materials, whose by-volume mass can be as low as 15 
kg/m.sup.3 can be utilized in a satisfactory manner as substrata according 
to the invention. 
Such products, even reduced, for example, to half of their initial volume 
under the mass of the solution with which they are saturated, still 
correspond to very low by-volume masses as compared to that of the 
conventional substrata. 
The buckling which occurs in these materials does not change the cohesion 
thereof, and is reversible. As soon as the pressure caused by the presence 
of the solution is relieved, the substratum recovers its volume. 
In addition to the advantage of production cost and quality with respect to 
water retention, the light substrata allow improved packaging and storage. 
Indeed, it appears that the conventional substrata are relatively rigid 
products, precisely because of their by-volume mass. They are especially 
incompressible and cannot be folded or rolled up. Conversely, light fiber 
felts are known for their good compressibility and further, for their 
ability to recover their thickness when the pressure is removed. In other 
terms, the light substrata in accordance with the invention can be 
compressed, rolled up into a small volume to facilitate the transportation 
and storage thereof. This capacity increases as the by-volume mass 
decreases. 
Above, we mentioned the great importance of the available water retained by 
the substrata for the cultivation process was noted. If the capacity for 
root aeration is also an important factor, in practice, this aeration does 
not require a substantial fraction of the volume of the substratum to be 
occupied by air. Indeed, aeration occurs also by means of the oxygen which 
is dissolved in the nutritive solution, and this aeration is better 
ensured as the frequency of the replacement of the solution in contact 
with the roots increases. For this reason, although the substratum plays a 
role in aeration, the irrigation aspects takes priority. 
More so than the quantity of water which is retained by the substratum, it 
is the available water which is important. Indeed, the water penetrating 
the substratum is essentially retrained by said substratum. If the water 
is bound too strongly to the substratum, it can no longer be utilized by 
the plant. Conversely, the substratum must exert a certain degree of 
retention, without which the irrigation solution would be immediately 
drained. 
To characterize the retention of the substratum, the water content of 
samples is determined by subjecting it to pull forces. Thus, for a 
depression expressed as a function of the logarithm of the height of the 
water column (in cm), also called pF, the percentage of the volume of the 
substratum which is occupied by the aqueous phase is defined. Two values 
for pF are particularly important in characterizing the substratum: a low 
pF corresponding practically to the conditions of maximum retention and 
which is arbitrarily established at equal to 1 (or 10 cm on the water 
column) and a pF equal to 2, which in practice corresponds to the highest 
degree of pull which can be exerted, for example, by garden plants, and 
thus constitutes the lower dampness limit above which the substratum must 
be maintained on a constant basis. 
The greater the proportion of water extracted between these two pF values, 
available water, the better the substratum. 
Various methods for determining water retention, which can produce slightly 
different results, have been proposed. The method adopted by the inventors 
is explained in detail in the examples for embodiment. 
Experiments have shown that, for all of the mineral fiber substrata that 
retention is high at pF1 and very low at pF2, in comparison with the other 
types of natural or artificial substrata. However, differences can appear 
among these mineral fibrous substrata, especially for the values at pF1. 
The substrata according to the invention have a high degree of retention at 
pF1 and thus a large available reserve. This available reserve is not 
under 40% and most frequently is greater than 50%. 
For substrata comprised of extremely fine fibers and having very low 
by-volume masses, the retention capacity is determined using a dampened 
substratum, to take into account the substratum's propensity to buckle 
under the weight of the liquid impregnating it. 
Due especially to this large quantity of available water, the substrata 
according to the invention comprised of very fine fibers can be utilized 
in smaller thicknesses than those traditionally used for rock wool-based 
substrata. 
In practice, rock wool substrata proposed for soil-free cultivation are 
relatively thick, with said thickness normally exceeding 70 mm. Indeed, it 
seemed preferable, especially for reasons of durability and cost, but also 
undoubtedly for reasons related to the methods used in the cultivation 
process, to utilize relatively voluminous substrata. 
Research conducted by the inventors has shown that soil-free cultivation 
could be effected advantageously on appreciably thinner mineral wool 
substrata. These substrata have a lower initial cost, which allows the 
conditions for implementation to be improved, especially through use for 
fewer cultivations and preferably for a single cultivation. Moreover, each 
cultivation can be conducted under more constant conditions. 
The substrata for soil-free cultivation according to the invention are 
advantageously comprised of mineral wool felts, the thickness of which is 
not greater than 40 mm, and preferably is not greater than 30 mm. During 
tests conducted, it was discovered that such thicknesses, which are much 
smaller than those previously utilized, are perfectly compatible with good 
cultivation yields and without stifling growth. Indeed, it appears that 
the volume of the substratum offered to the plants is sufficient for 
satisfactory root development, without modifying the surface density of 
the plants. This volume is also sufficient to maintain a good feed of 
nutritive solution to the plants. 
This small thickness of the substrata compared to prior substrata of the 
same type also allows for a better control of the nutritive solution which 
they are saturated. Indeed, solution consumption is practically identical 
whether a thick or thin substratum is used. The quantity of solution 
retained is smaller with the thin substratum and, with the supply of new 
solution relative to the mass of liquid being greater, the composition of 
the solution which is retained is constantly closer to that of the initial 
solution. 
If it appears advantageous from the economic point of view to utilize thin 
substrata, in practice, said substrata must nonetheless provide a certain 
volume for solution retention and root development. Techniques exist in 
which growing is done without a substratum. In these techniques, the roots 
grow in the same container in wich the nutritive solution circulates on a 
constant basis. This cultivation method requires a highly specialized 
installation and large investments. For these reasons, many users prefer 
cultivation methods in which the substratum is retained. 
To maintain a sufficient quantity of solution and provide the roots with 
the volume necessary for their growth and still, without changing the 
surface density of the plants, the thickness of the substratum according 
to the invention is not less than 10 mm. 
For most current cultivations, the mineral wool substratum according to the 
invention has a thickness of about 15 to 30 mm. The thickness which is 
chosen, in addition to the water retention capacity of the substratum, 
depends on the plants, the density thereof and the frequency of irrigation 
which is used. This thickness can possibly also depend on use for more 
than one cultivation, but, in this case, the use of these substrata does 
not provide all of the aforementioned advantages. It is specially 
necessary to envision a sterilization between the successive plantings. 
The fibers comprising these felts can be produced from a variety of 
materials and using various techniques. 
Up to the present, only "rock" mineral wools have been utilized to serve as 
substrata for soil-free cultivation. These rock wools are in fact made 
from inexpensive materials: basaltic rock, blast furnace cinders and 
similar materials. 
These materials are traditionally processed according to techniques which 
produce felts containing a high proportion of non-fibrous materials. In 
use as cultivation substrata, the presence of these non-fibrous materials 
is of little consequence, but, as we have seen, makes the product heavy 
without improving the properties thereof. The essential for this 
production method is that it is relatively economical, which, combined 
with the low cost of raw materials, allows the production of substrata at 
prices which are comparable to substrata of different types. 
On the whole, these substrata also possess a good level of chemical 
inertness. 
The invention also envisions the use of glass wool felts. These felts, 
contrary to the former, normally present a great degree of homogeneity due 
to the method utilized for the production thereof. This pertains 
essentially to fibers which are formed by passing a melted material 
through a centrifuge drawing device. The absence of non-fibrous materials 
normally leads to felts which are much lighter and have similar mechanical 
resistance properties. In other words, the by-volume mass thereof is 
normally lower. This allows at least a partial compensation for the fact 
that their production is generally slightly more costly than that of rock 
wools. 
Production techniques for glass fibers also present the advantage of the 
ability to produce fibers which are both very fine and very homogeneous, a 
fineness and homogeneity which cannot be obtained with rock wools. 
It is thus possible to produce glass wool substrata having fibers with an 
average diameter of less than 3 micrometers, and which can be less than or 
equal to 1 micrometer, as mentioned above. 
In the case of the thin substrata according to the invention, the 
structural properties must be still better ensured, and, as a general 
rule, glass wool felts present advantageous properties from this point of 
view, because of the both the fineness of the fibers and the homogeneity 
thereof. 
Moreover, the reduction in the volume of the substratum envisioned 
according to the invention tends to limit the relative share of the cost 
of the fibers in the final cost of the product, such that the differences 
on this point between rock wools and glass wools are less appreciable. 
Prior to the invention, the possibility of utilizing glass wool as a 
cultivation substratum raised objections, especially because of its 
assumed lack of chemical inertness. Indeed, it was feared that the glass 
fibers in contact with the nutritive solution would release a large 
quantity of sodium ions. Cultivation tests conducted with glass wool 
materials according to the invention have shown that these substrata 
yielded results fully comparable to those obtained with rock wool 
cultivation. In fact, a slightly higher sodium ion content is generally 
noted during the first irrigation. But this content, which is acceptable, 
subsequently decreases very quickly, settling at values which are similar 
to those obtained with rock wools. These results are all the more 
interesting that, due to the use of very fine fibers, exchanges with the 
solution are greater. For the intended use, the inertness of glass fibers 
in current use can thus be considered as completely satisfactory. 
Quite obviously, the glass compounds chosen do not contain elements which 
are toxic for plants. 
Conversely, it is possible to consider the utilization of fibers which are 
not systematically inert. Fibers can serve, for example, as a source of 
trace elements which diffuse slowly in contact with the nutritive solution 
or can contain phytopathological compounds. 
Most often, however, it is preferable to make the fibrous substratum 
perfectly inert and to reserve the role of supplying the necessary 
elements for growth to the nutritive solution. 
Mineral fiber felts are normally bound using organic bonding materials such 
as phenolic plastic resins. These resins have no appreciable influence on 
cultivation at the levels at which they are normally used, namely about 2 
to 3% by weight of substratum. 
The proportion of bonding materials can vary according to the nature of the 
fibers, thus, for very fine fibers and low by-volume mass felts, the 
gravimetric proportion of the bonding material can be slightly higher, 
normally without exceeding 10%. 
It must be noted that, if the mineral fibrous substrata are normally 
derived from products utilized for insulation, the composition of the 
bonding materials can be appreciably different. Indeed, it is common to 
add compounds intended to change the properties of the felts to the resin. 
The composition of the bonding materials can especially include substances 
which give insulation felts improved resistance to humidity. This 
pertains, for example, to silicone-based products. Bonding agents which do 
not contain these hydrophobic products are utilized in the production of 
the substrata according to the invention. 
In addition, even if non-hydrophobic bonding materials are utilized, it is 
noted that the traditional substrata made of rock wool are very difficult 
to moisten if they are not impregnated with a certain quantity of an 
appropriate surface-active agent. 
The introduction of the surface-active agent can be done, for example, in 
the manner which is described in the French Patent Application published 
under No. 2,589,917. 
The surface-active agent is chosen so that it has no harmful effects on 
cultivation. It can especially pertain to non-ionic agents, such as the 
product which is marketed under the name of "Dobanol 91-6". 
The utilization of very fine fibers according to the invention, by 
modifying the capillarity of the substratum, can make the use of a 
surface-active agent unnecessary. This is noted especially with glass wool 
substrata, whose fibers have an average diameter equal to 4.5 micrometers, 
but the hydrophilic nature of the fibers changes in a progressive manner. 
For each degree of hydrophilicity, it is possible to associate a maximum 
fiber size which allows this degree to be attained. 
The substrata according to the invention are also distinguished, if needed, 
by the manner in which they are implemented. Indeed, if the general 
process of cultivation is maintained, when the volume of the substratum 
which is used is decreased, in other words, when the substratum is thin, 
the conditions for irrigation to meet the requirements of nutritive 
solution on a constant basis are different. 
Generally, the substratum is utilized with either one-cycle or recycled 
solution irrigation. In the first case, the substratum is fed either by 
percolation or sub-irrigation, so as to keep the solution content within 
acceptable limits. The purpose of the essentially discontinuous supply is 
to compensate losses of solution due to absorption by the plants and 
evaporation. In the second case, the substratum is fed in a constant 
manner, and the excess solution which is not retained is recycled after it 
is supplemented and the content of its various constituents is readjusted. 
With the "reserve" of solution offered by the thin substrate being smaller, 
when the irrigation is discontinuous, the latter is replaced more 
frequently, but with smaller quantities of solution. This greater 
frequency, as we mentioned above, allows a better adjustment of the 
composition of the nutritive solution near the roots. 
The modification of the irrigation frequency does not constitute a problem 
to the extent that this operation is normally conducted in an entirely 
automatic manner following a pre-established schedule, and the execution 
of which is ensured by a complex of measurement, dosage and distribution 
equipment without the intervention of the operator. 
The invention is described in detail below, in reference to the drawings, 
in which:

DETAILED DESCRIPTION OF THE EMBODIMENTS 
The plants which are utilized for cultivation can be prepared on soil or on 
an inert substratum, of the same type which is utilized for cultivation or 
otherwise. Finally, they are separated from each other with a form 1 which 
is intended to be placed on the growing substratum 2. 
For cultivation, the substratum is placed on a waterproof container 3 which 
prevents loss of the nutritive solution. The container is normally 
comprised of an inert, relatively rigid polymer sheet which is held in the 
form of a trough or box by regularly placed stakes. The latter are not 
shown in the drawing. 
This arrangement is normally supplemented by the presence of a water-proof 
sheet covering the substratum, with the exception of the areas where the 
forms are placed, having the function of reducing evaporation of the 
nutritive solution held in the substratum through contact with the 
surrounding atmosphere. This sheet is not shown in FIG. 1 for clarity. 
The nutritive solution in the method which is shown in distributed by 
percolation, through capillary tubes 4, directly on the forms 1. The 
capillary tubes are fed by a distribution conduit 5. 
The container 3 can be placed on the base, or, in the traditional manner, 
on an insulating sheet made, for example, of polystyrene. 
The complex can also include heating equipment, located especially directly 
above the containers. 
The nutritive solution can be distributed in a continuous manner, 
especially when recycling is planned. In this case, the base is placed so 
that the excess solution which exudes from the substratum can flow and be 
collected on the side or at an end of the container to be sent to the feed 
equipment. It can also be distributed in a discontinuous manner, either at 
predetermined intervals and quantities which are known to provide the 
appropriate dampness level for the substratum, or as a function of a 
constant measurement of the dampness rate which allows the feed to be 
activated when this dampness falls below a certain level. 
FIG. 2 shows a mode of embodiment for a substratum according to the 
invention, in which the mineral fiber felt comprising the substratum is 
covered with a flexible water-proof sheet 6 to prevent evaporation. The 
substrata according to the invention can be made with the upper surface of 
the substratum alone covered with this sheet. Said sheet can also 
completely surround the felt. 
FIG. 3 shows the device utilized to determine the water retention in the 
substrata for different pF levels. 
For said determination, these samples 7 of material comprising the 
substrata are all 7.5 cm high and are cut into 10 cm side squares. 
These samples are immersed completely for 1 hour, then placed on a porous 
material 8 lining the bottom of a box 9. The porous material, a bed of 
sand, for example, is initially saturated with water. 
The bottom of the box 9 is connected by a flexible conduit 10 to a vessel 
11, the level of which is fixed (by an overflow system). The position of 
the vessel 11 on a vertical support can be adjusted as desired. 
The measurement of the depression d is done systematically by referring to 
the midpoint of the sample. Various levels are successively determined, 
corresponding to the pFs being studied. The measurements are made after 
the samples have been maintained up to the obtention of an equilibrium in 
each new condition of level change. 
At equilibrium, the sample is removed, weighed, dried and weighed again 
after drying. The difference yields the mass of water retained and 
subsequently the proportion of water and air for each pull condition 
established. 
The retention curves as a function of pF for various materials allow their 
ability to ensure a good irrigation for cultivation to be compared. 
These curves, for mineral fibrous materials, are shaped as shown in FIG. 4. 
For these curves, the abscissa shows the logarithms for the pulls in water 
column centimeters; the ordinate shows the percentages of the volume of 
the substratum occupied by water, air and fiber. The latter, in a constant 
manner in the example shown, occupies about 5% of the total. These 
percentages define the areas respectively labeled A, B and C on the 
diagram. 
The differences in percentage between pF1 and pF2 for the portion occupied 
by water determines the available quantity of water. 
For mineral fibrous substrata, pF2 is always very low, with the main 
differences noted between the various materials for available water thus 
stemming from pF1. Curves I and II illustrate this type of differences. 
They correspond respectively to a traditional rock wool-based substratum 
and a substratum according to the invention, having very fine fibers, for 
a same by-volume mass. The actual reserve R.sub.2 is appreciably greater 
in the second case. 
The conditions under which the measurements are taken (thickness 7.5 cm of 
sample) correspond to the traditional substrata. If these conditions allow 
the products to be compared, they do not reveal the advantages peculiar to 
the thin substrata proposed according to the invention. 
The study of the distribution in the height of the sample in fact shows a 
very high degree of non-homogeneity. The upper part holds very little 
water and a great deal of air, and the opposite applies for the lower 
part. 
Systematic measurements were thus taken for different products according to 
the invention and others which do not have the accepted characteristics, 
on a comparative basis. These measurements cover products having different 
by-volume masses, fiber fineness and thicknesses, but which are made of 
the same glass and with the same quantity of wetting agent, of about 300 
g/m.sup.3 of felt. 
The retention measurements at pF1 taken for different fiber diameters, two 
series of by-volume mass and two thicknesses are as follows: 
______________________________________ 
Thickness Diameter micrometers 
in mm kg/m.sup.3 
8 6 4.5 
______________________________________ 
80-85 80 61 86 95 
80-85 40 46 57 81.5 
20 40 25 54 86 
______________________________________ 
In all cases, these results show an increase in retention for a decrease in 
the average fiber diameter. This increase becomes greater as the by-volume 
mass and thickness decrease. 
By choosing small thicknesses and a low by-volume mass, a great degree of 
retention can be obtained when the fibers are sufficiently fine. 
The measurements done on a same felt and for different thicknesses evidence 
a great degree of stability in retention for felts comprised of very fine 
fibers. 
At the different thicknesses studied, the felt comprised of fibers having 
an average diameter of 4.5 micrometers and of 40 kg/m.sup.3 present the 
following retentions: 
______________________________________ 
thickness (mm): 
20 35-40 55-60 80-85 
retention 86 85 87 81.5 
at pF1: 
______________________________________ 
Taking into account the error inherent in this type of measurement, the 
differences found are insignificant. 
According to the measurements taken, it seem that in the case of felts 
having high by-volume masses and especially comprised of fibers with 
larger average diameters, the thickness influences retention, with said 
retention being appreciably lower then the thickness decreases. 
Moreover, it must further be emphasized that only fine fiber felts are 
suitable for use without a moistening agent. 
Taking these results into account, it thus appears totally advantageous to 
utilize thinner substrata with fine fibers. 
Two types of substrata were utilized: the first is comprised mineral wool 
panels made from blast furnace cinders, the second of glass wool panels. 
The respective composition of the fibers in these substrata is as follows: 
______________________________________ 
Casting cinder fibers 
Glass fibers 
______________________________________ 
SiO.sub.2 42.8% SiO.sub.2 
66.9% 
Al.sub.2 O.sub.3 11.9% Al.sub.2 O.sub.3 
3.35% 
CaO 38.7% Na.sup.2 O 
14.7% 
MgO 3.6% K.sub.2 O 
1% 
Fe.sub.2 O.sub.3 1.2% CaO 7.95% 
MnO, B.sub.2 O.sub.3 MgO 0.30% 
0.4 
TiO.sub.2, P.sub.2 O.sub.5 MnO 0.035% 
SO.sub.3 0.3% Fe.sub.2 O.sub.3 
0.49% 
Misc. 1.1% SO.sub.3 
0.26% 
B.sub.2 O.sub.3 
4.9% 
______________________________________ 
The substratum panels are bound with a formophenolic resin in a proportion 
of about 2.5% by weight of the entire product. In the case of rock wools, 
the substratum also contains about 1% surface-active agent. 
The panels are cut to the size of 1000.times.200 and have a thickness of 50 
mm for rock wool and 25 mm for glass fiber. The average diameter of the 
fibers is 5 micrometers for the rock wool (non-fibrous materials not 
counted and 4 micrometers for the glass wool. 
The respective by-volume masses of the rock wool substratum is 40 
kg/m.sup.3 that of the glass fiber substratum is only 25 kg/m.sup.3 
corresponding to respective porosities of 95 and 98%. 
Water retention at pF1 for these substrata is in both cases approximately 
equal to 70%. Consequently, in both cases, a good water/air equilibrium 
occurs, which promotes growth. 
The growing of Montfavet type tomatoes is effected in a greenhouse 
according to the methods indicated below. 
The sewing is done on 70.times.75.times.60 mm blocks of rock wool of the 
same type as the substratum mentioned above. Placement on the substratum 
is effected when the first leaves appear. 
As a comparison, cultivation is also done on a traditional rock wool 
substratum, having a thickness of 75 mm and a by-volume mass of 70 
kg/m.sup.3 with average fiber diameters therein being 6 micrometers. 
For the three types of substratum, the growing process is the same. The 
plants are 30 cm apart in the direction along the length of the 
substratum, which corresponds to a planting of 2.5 plants per square meter 
of the cultivating device. The feed installation is the type described 
above in relation to FIG. 1. 
Irrigation is done with a Coic-Lesaint type solution containing 12.2 
milliequivalents per liter of nitric nitrogen, 2.2 milliequivalents per 
liter of ammonia nitrogen and 2.2 milliequivalents of phosphate. The pH is 
controlled at around 6. 
The plants are fed in a discontinuous manner as a function of the 
conductivity measurement in the solution contained in the substratum. The 
feed maintains a conductivity above the threshold corresponding to a 
content which is not less than 12 milliequivalents of nitrogen per liter. 
About 24 weeks pass from the planting until the end of the harvest. 
The yield in all cases was about 6.5 kg per foot. Especially, there was no 
marked difference noted between the cultivation conducted on the thick or 
thin rock wool substratum. There was also no appreciable difference noted 
with respect to the yield, between the cultivation on thin rock wool and 
glass wool substrata. 
A better structural tolerance was noted in the glass wool substrata, 
despite their relatively smaller by-volume mass. This is probably due to 
the presence of longer fibers, which reinforce the cohesion of the felt. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.