GB2268929A - Plant growing medium - Google Patents

Abstract

A plant growing medium for hydroponic culture comprises an expanded porous granular mineral such as perlite, together with about 10% vermiculite enriched with an aqueous plant nutrient solution. The porous mineral may be impregnated with 5% by volume of a concentrated nutrient solution of conductivity about 10,000 mS. A calcium supplement and trace element supplement may be included. The medium has good handling properties and can be used for cultivation by the simple addition of water.

Description

PLANT GROWING MEDIUM The present invention relates to a plant growing medium, which may be used for hydroponic plant culture, or as a peat substitute in conventional horticulture.

Hydroponics is the science of growing plants without the use of soil. The plants are nourished by means of a nutrient solution containing all the necessary minerals, such as calcium, potassium, ammonium, phosphate, nitrate and trace elements, to ensure healthy vigorous growth. In one conventional hydroponic system, plants are supported above a trough through which is passed a film of nutrient solution. The plant roots grow in the film of solution.

Alternatively, plants are grown on small rockwool cubes onto which nutrient solution is dripped or trickled. As the plants grow bigger the cube is placed into a hole in a larger cube so as to give the roots more room. Eventually the plants are bedded in a large bolster where they grow to maturity.

As an alternative to rockwool, perlite has been used as a growth medium for hydroponic culture. A system employing perlite growth medium is disclosed in the inventor's prior publication GB2230170. Perlite is an expanded volcanic mineral whose use in horticulture is well established. However, perlite is a light friable material whose handling poses some difficulties. As produced commercially, perlite is present in a wide range of particle sizes from dust up to granules of 1 to 2mum in diameter. Although the dust is not a health hazard as such, it can prove to be an irritant to workers handling the material, and is generally inconvenient.

Within the horticultural community, peat is widely used as a growing medium, either by itself or in combination with soil, gravel etc. It is beneficial in that the peat holds water and allows oxygen access to the plant roots. Peat can also act as a useful reservoir for fertiliser. However, peat is a natural product and its supply is limited. For environmental reasons, alternatives are being actively sought to replace peat, and thus preserve the natural peat bogs which form a scarce wildlife habitat. Various alternatives to peat have been canvassed, particularly coconut coir, but as yet none has been generally accepted.

It is an object of the present invention to mitigate these problems.

Thus, the present invention provides a plant growing medium which comprises; an expanded porous granular mineral material which is enriched with an aqueous plant nutrient solution.

Usually, the plant nutrient solution will be in concentrated form, that is to say more concentrated than would be required (or could be tolerated) by the growing plant. Typically, the nutrient solution will be from 2 to 10 times as concentrated as the solution which is to feed the plant, both for use in hydroponics and as a peat substitute.

Typically, the nutrient solution will contain a conventional N,P,K fertiliser, containing nitrogen, phosphorus and potassium together with magnesium and other trace elements. Preferably an enhanced level of phosphate is employed. In soft water areas, calcium may also be supplemented. Compositions available under the trademark Osmacote Plus Hydro or Hydrosol (Peters) are typical suitable nutrient compositions. Typically, the solution in contact with the plant has a conductivity of around 1,000 microSiemens/cm depending on the growth requirements of the plant. Thus, the conductivity of the plant nutrient solution in the growing medium will generally be up to 15,000 mS, for example in the range 2,000 to 10,000, typically 5,000 to 7,000 micro Siemens. The pH will usually be in the region 6.5 to 7.5.

The plant growing medium is intended to be contacted with water prior to use, which dilutes the concentrated nutrient solution down to the level required by the plant.

The expanded porous granular mineral material is one of those materials known in the art, such as perlite or vermiculite. Perlite is preferred and industrial grade perlite generally has a density of 9.7 kilograms per 100 litres and comprises a wide particle size from dust up to granules of 2 to 3 mm diameter. Perlite can contain up to 0.5 litres of water per litre of perlite. However, according to the present invention it is preferred to add 2 to 10% by volume, preferably 3 to 7% by volume aqueous nutrient solution to the expanded porous granular mineral material. The nutrient solution is usually applied by spraying. The plant growing medium of the present invention still flows easily and does not become sticky and thus complicate handling and packaging.The presence of the aqueous plant nutrient solution, however, has the benefit of improving handling properties in that there is very much less irritant dust, and also the weight of the material is increased so that it does not blow about in the wind. Preferably, the perlite medium includes 5 to 20% by volume of a vermiculite such as Carolina vermiculite.

The plant growing medium itself may also be provided pre-packaged, such as in sacks, containers etc.

The plant growing medium of the present invention has been found to be excellent for growing seeds, seedlings, cuttings and mature plants. The nutrient enrichment in the expanded mineral material provides an excellent source of nutrients to the plants when initially sown or planted, and delivery is found to be better in the initial stages of growth than employing a conventional dilute hydroponic nutrient solution. Thus, the plant growing medium of the present invention provides improved results in the initial stages of growth, and also better overall growth.

The invention also relates to a growing container comprising a layer of the growing medium, which is above a capillary mat which enables water to be provided below the surface of the medium.

Another aspect of the invention provides a plant growing container which comprises the enriched plant growing medium and a solid controlled release fertiliser.

The slow release fertiliser may be any of those pelleted or granular products currently known on the market, such as Osmacote pellets. Osmacote is a resin-coated pelleted controlled release fertiliser whose release properties are temperature dependant, the release rate increasing with increase in temperature. These pellets are conveniently either intermixed with the plant growing medium (for immediate use) or contained in a permeable sachet. The porous sachets may be arranged to be added to the medium just prior to use (to avoid premature release of fertiliser) or at a chosen point in the growth of the seedling (e.g. 2-3cm high). The permeable sachet allows infusion of surrounding aqueous solution and dissolution and release of fertiliser. The controlled release fertiliser concentration builds up around the plant roots as the nutrient solution becomes depleted from the plant growing medium itself. Further doses may be added later if necessary. Thus, the combination of the plant growing medium of the present invention which provides an immediate dose of fertiliser, together with the long term fertilisation provided by the controlled release fertiliser provides a complete plant nutrition system which maximises plant growth. Depending on the plant growth requirements, each container may comprise from 1 to 25 grams of controlled release fertiliser.

Typically, these are provided in amounts of 2 gms, 4 gms, or 10 to 25 gms depending on the plant requirements.

In this way, it is unnecessary to feed the hydroponically grown plants with nutrient solution, all that is required is to provide water. The entire plant nutrition may be provided either from the nutrient present in the growing medium, or the nutrient provided from the solid slow release fertiliser. This considerably simplifies the management of hydroponic systems, since it is not necessary to monitor the nutrient content (e.g. the conductivity) of the recirculating nutrient solution. It is particularly advantageous in domestic circumstances, where a growing plant need only be provided with water and thus can be left for long periods without attention.

The plant growing medium is also most suitable as a peat substitute in general horticulture, since it provides the necessary degree of aeration and water retention to support healthy root structures. It also has good capillary properties, is sterile and re-usable. The inclusion of aqueous nutrient solution according to the present invention considerably assists handling properties.

The pre-enrichment with nutrient improves growth in the early stages. Moreover, the plant growing medium of the present invention has a better quality control and is more reliable than the naturally occurring peat, whilst'still providing the necessary balance of air and water supply.

Embodiments of the invention will now be described by way of example only.

EXAMPLE 1 A general purpose mixture for sowing and cuttings was made up from the following ingredients: 1000 litres expanded perlite (SILVAPERL 3-2-1, OR CELLITE 2-2-1), 100 litres Carolina vermiculite (C3 or C4 grade), lkg ground magnesian limestone, sieved to remove rogue particles, 375 grams Micromix controlled-release fritted trace elements, and 50 litres nutrient solution at a conductivity of 10,000 mS (containing ingredients listed below) The nutrient solution is made by adding equal quantities of two concentrated stock solutions, known as "A" and "B", to 50 litres of water until the above conductivity is reached.

"A" consists of 5.2 kg of calcium nitrate, glasshouse quality, dissolved in water to make 50 litres of stock.

"B" consists of 10 kg of Hydrosol, which is a proprietary blend of NPK (5-11-26) with trace elements (and optionally additional phosphate); and 400 grams of ammonium nitrate (Nitram).

These are dissolved in water and made up to 50 litres.

Seaweed extract (Maxicrop natural organic seaweed) is added to the final solution which is then thoroughly stirred. This component is not essential to the mix.

The formula is blended as follows in order to achieve a homogenous product.

The dry perlite is poured into a Universal Blender (designed and manufactured by Universal Fabrications Ltd., Sandy, Bedfordshire, for the mixing of composts; its action imparts a fairly gentle but thorough intermixing action.) The Carolina Vermiculite is strewn across the mound of perlite, followed by the ground magnesian limestone and the fritted trace elements. A pump is switched on and when the nutrient solution starts to jet from the spray bar under the rim of the blender, the blender itself is activated and run for not more than 45 seconds. To this end, the pump must be able to deliver its 50 litres of solution in this time, or less. The shorter the blending time, the less the perlite is abraded and the higher the yield. Ideally, 30 seconds is the target.

After blending, the machine is stopped and the elevator is made ready. Then both blender and elevator are started simultaneously and the material is transported by belt or auger to the hopper. This is achieved in under a minute, but nevertheless involves further mixing and consequent abrasion. Alternatively a better material is produced if the blender is raised high enough to allow its contents to flow directly into the hopper by gravity.

The particle sizes of the perlite and the Carolina vermiculite are important. Also, the origin of volcanic perlite has a bearing on its quality after expansion.

Some perlites are softer than others and abrade more readily. Also, there are variations in the quality produced by different manufacturers using identical ore.

The reasons for choosing vermiculite from Carolina rather than from any other source are as follows.

Firstly, it has a significantly lower and more dependable pH than other vermiculites. This is conductive to improve control of pH in the growing medium where it acts as a buffer.

Secondly it has a more advantageous trace element specification than most other vermiculites.

Thirdly, it gives a natural brown colouration to the mixture which is more acceptable than the stark white of perlite alone.

EXAMPLE 2 A nutrient analysis of a typical characteristic enriched perlite and Carolina vermiculite mixture according to the invention is as follows.

pH 6.9 Conductivity 1414 micro Siemens/cm Extractable nutrients (mg/L) total extractable N 164 phosphorus (P) 45 potassium (K) 128 calcium (Ca) 50 magnesium (Mg) 37 sodium (Na) 80 iron (Fe) 3.46 manganese (Mn) 0.75 boron (B) 0.46 copper (Cu) 0.31 zinc (Zn) 0.18 Figure 1 shows a schematic cross-section of a seed tray containing the plant growing medium, together with an arrangement for providing water from below the surface of the growing medium.

It has been found preferable to water from below when the growing medium of the invention is employed. This minimises washing out of the nutrients. The figure shows a seed tray 2 of size about 35cm x 20cm x 5cm (without holes in the base or walls) which contains a layer 4 of enriched perlite growing medium (about 2L). In order to provide water to the growing medium, a strip of capillary mat 6 about 4cm wide is disposed in the base of the tray and its ends 8, 10 protrude upwardly.

Water is fed to the seed tray via a further strip of capillary mat 12, one end of which dips into a tub 14 containing water and the other end of which tucks under the end 8 of the capillary mat 6 so that water is drawn from the tub and moistens the mat 6.

Generally, the water requirement is about 0.5L water to 1L of perlite growing medium. The wetted growing medium could be planted with seeds, and the tray watered via the capillary mat as described above.

Claims (13)

1. A plant growing medium which comprises an expanded porous granular mineral material which is enriched with an aqueous plant nutrient solution.

2. A medium according to claim 1 wherein the expanded mineral material is perlite.

3. A medium according to any preceding claim which further comprises vermiculite.

4. A medium according to claim 3 wherein the vermiculite is Carolina vermiculite.

5. A medium according to claim 3 or 4 wherein the vermiculite is 5 to 20% by volume of the expanded mineral material.

6. A medium according to any preceding claim wherein the aqueous plant nutrient solution is present in an amount of 2 to 10% by volume of the expanded mineral material.

7. A medium according to any preceding claim wherein the plant nutrient solution is at least twice as concentrated as required by a growing plant.

8. A medium according to any preceding claim wherein the plant nutrient solution has a conductivity at least 2000 mS.

9. A medium according to claim 8 wherein the conductivity is 5000-15000 mS.

10. A medium according to any preceding claim which further comprises a source of calcium sufficient to supplement the calcium content in soft water areas.

11. A medium according to any preceding claim which further comprises a controlled-release fritted trace element material.

i2. A plant growing medium substantially as disclosed in the Example.

13. A growing container comprising a layer of plant growing medium as described in any preceding claim and a capillary mat disposed beneath the growing medium for supplying water-thereto.