Good soil management requires the addition from time to time of fertiliser in the form of an organic component such as manure or compost and an inorganic component such as various trace elements and minerals. Fertilisers which contain both an organic component and an inorganic component are known in the art. One such fertiliser is described in South African Patent No. 91/2431.

The fertiliser described in this patent comprises an organic carrier in particulate form and at least one inorganic fertilising component in intimate contact with the organic carrier particles, so that the composition is in particulate form. A disadvantage with this fertiliser is that fine powder is generated in storage of the fertiliser and in its application to a locus. The fine powder creates a hazard for the person applying the fertiliser to a locus and is wasteful.

There is a need for a fertiliser which contains both an organic component and a high inorganic component and which can be applied to a locus in an effective and efficient manner.

SUMMARY OF THE INVENTION According to the present invention, a fertiliser contains both an organic component and an inorganic component and has the following characteristics: i) it is in granule form; and ii) the mass ratio of organic component to inorganic component is in the range 20: 80 to 50: 50, preferably 20: 80 to 35: 65.

According to another aspect of the invention, a method of producing a fertiliser as described above includes the steps of producing a mix of organic component and inorganic component having a consistency suitable for granulation and granulating the mix.

The water content of the mix, to provide that mix with the consistency suitable for granulation, will generally be in the range 15 to 30% by mass, preferably in the range 20 to 30% by mass, and more preferably in the range 22 to 24% by mass.

In one form of the method, the mix of organic and inorganic components prior to the granulation step, may contain less than 12% by mass water and the water content of the mix is increased to a consistency suitable for granulation.

In another form of the method, the organic component in the mix may contain at least 60% by mass, typically at least 80% by mass, water, and dry solids are added to the mix of organic and inorganic components to bring it to a consistency suitable for granulation, prior to the granulation step. The dry solids added are preferably a mixture of dry organic and inorganic components, and particularly granular fertiliser of the invention which is re- cycled.

In another form of the invention, the organic component in the mix may contain at least 60% by mass, typically at least 80% by mass, water, the inorganic component is added to the organic component to produce a mix and the mix is fluidised in a fluidised bed drier to produce a fertiliser in granule form.

DESCRIPTION OF EMBODIMENTS The fertiliser of the invention is characterised by containing both an organic component and an inorganic component with the content of the inorganic component being relatively high. The fertiliser is in granule form. The granules are preferably essentially spherical in shape and preferably have a crushing strength of at least 2,5 kg on 4 mm in diameter granules when measured by the standard compression test method such as that described in PP291 of the Fertilizer Manual of the International Fertilizer Development Centre first published 1967. Such a granule is free flowing in a planter, for example, and does not crush during handling or in transport. The components of the granules do not segregate and do not create any significant dust when being applied to a locus, such as an agricultural field. Each granule will have a relatively uniform composition and does not have an unpleasant odour. The water content of each granule is generally less than 5 percent by mass. The diameter of each essentially spherical granule is typically 1 to 5 mm, preferably 2 to 4 mm.

In one typical example of the invention, the fertiliser contains 30 percent by mass of an organic component and 70 percent by mass of an inorganic component.

The inorganic component will preferably contain nitrogen (N), phosphate (P) and potassium (K), and optionally one or more trace elements. Suitable inorganic compounds for the inorganic component are potassium chloride, potassium sulphate, monoammonium phosphate, superphosphate, gypsum, magnesium sulphate, a boron source such as boric acid, a nitrogen source such as ammonium sulphate or urea and mixtures thereof.

The inorganic component will typically contain 20 to 25 percent by mass of an aggregate of nitrogen, phosphate and potassium.

The inorganic component may include an effluent, particularly a waste effluent.

For example, the inorganic component may be the raffinate from an industrial phosphoric acid manufacture. Such a raffinate will typically contain about 10% by mass sulphuric acid and 25% by mass phosphoric acid. This raffinate may be neutralised with ammonia and thereafter used as, or as part of, the inorganic component.

The inorganic component may also be precipitated phosphate from stored fertiliser phosphoric acid, waste ammonia from coke production or waste pickle liquor, containing nitrates and calcium, from stainless steel manufacture.

The organic component will typically be a manure such as chicken manure, pig manure or cattle manure or sewage sludge. In the case of a manure, the water content is generally fairly low in comparison with sewage sludge. The organic component may also be an organic rich liquid effluent from fermentations such as yeast, citric acid and antibiotic production or a protein rich effluent such as a fish factory wash liquor.

The granular fertiliser of the invention may be produced by a method which includes the steps of producing a mix of the organic component and inorganic component having a consistency suitable for granulation and thereafter granulating the mix. The granulation preferably takes place in any known granulator such as a fluidised bed granulator or a rotary kiln granulator.

The mix will typically have a water content in the range 15 to 30%, preferably 20 to 30%, by mass to render it suitable for granulation. Depending on the water content of the organic component, different methods may be employed to produce a mix suitable for granulation.

In one form of the invention, the organic component is a manure containing a relatively low water content. A mix produced from such a manure will typically have less than 12% by mass water. In this case, the water content of the mix is increased prior to granulation.

When the organic component contains a much higher content of water, as in the case of sewage sludge, then dry solids may be added to the mix of organic and inorganic components prior to granulation. The dry solids which are added are preferably a mixture of organic component and inorganic components which has a water content of less than 5% by mass.

The water which is added may be water itself, water containing an organic component such as molasses, or a liquid effluent.

Examples of the invention will now be described. In a first example, a manure which is relatively dry, for example, a chicken manure, screened clean and containing less than 20% by mass water, is fed together with the inorganic component to a blender, such as a batch ribbon blender, to mix the organic and inorganic components. The mix is then delivered to a feed hopper. The relatively dry mix is fed to a mixer, typically a paddle mixer, where water is added to create a mix having a consistency suitable for granulation. The mix is then fed to a rotary kiln granulator in which granules having the desired characteristics as described above are produced. The organic and inorganic components which may be used in this example are: Organic Component (30% by mass) Chicken manure containing 18% by mass water.

Inorganic Component (70% by mass) Potassium chloride 9 % Monoammonium phosphate 6 % Recovered phosphate 7 % Urea 10% Ammonium sulphate 30% Magnesium sulphate 3 % Gypsum 4 % Trace elements e. g. boron, iron and manganese 1 % It was found that an organic/inorganic mix having the composition described above and a water content of 22 to 24% by mass produced particularly effective granules. Such granules were essentially spherical in shape and had a crushing strength exceeding 2,5 kg for 4 mm diameter granules.

In a further example of the invention, granules which were essentially spherical in shape and had a crushing strength exceeding 2,5 kg for 4 mm diameter granules were produced as described above, save that the recovered phosphate was replaced by a raffinate from an industrial phosphoric acid manufacture neutralised with ammonia. In a further example of the invention, a sewage sludge having a water content of 80% by mass was mixed with an inorganic component of the type described above. The blend or mix was wet and found to have a water content exceeding 50% by mass. The wet mix or blend was then introduced into a fluidised bed drier and fluidised with the addition of heat. Produced in the drier was a mass of essentially spherical granules, each having a crushing strength exceeding 2,5 kg for 4 mm diameter granules.

The granular fertiliser of the invention may be applied prior to planting, at planting or post-emergence as a top dressing. The granules are free-flowing and do not crush during handling or in transport. Further, the granules do not segregate as a blend does and are dust free when applied to a locus. Further, the granules contain no waxes or other binder agents reducing the cost and complexity of manufacture.