1 A METHOD OF TREATING SOIL INCLUDING APPLYING A SOIL WETTING AGENT The present invention relates to a method of treating soil including applying a soil wetting agent to soil in a block of land in order to improve water absorption and 5 retention in the soil. Much of Australia's agricultural land comprises non-wetting soils, soils that are hydrophobic and repel water. Sandy soils and gravelly soils, or gravels, are just two examples of non-wetting soil types. When soil has a chemical nature that repels water, rainfall runs off the top soil. Then water is not retained within soil to 10 acceptable extent for plant growth. Moisture content within such soils is below the permanent wilting point (PWP) representing the amount of soil moisture remaining when plant foliage begins to die as a result of moisture stress (see McIntyre, DJ, Water Potential and Moisture Characteristics in Loveday, J, Methods for Analysis of Irrigated Soils, Tech Communication No. 54, Commonwealth Bureau of Soils, 15 Melbourne). As a result, water is not consistently absorbed through the top soil. Such soil hydrophobicity may be caused by coating of grains of sand or gravel with organic compounds having non-polar fragments which repel water. Hydrophobic soil type may be indicated by localised "dry spots" or areas where plant growth is absent or meagre. Such soil type is not agriculturally productive without treatment; that is 20 potentially very expensive when wetting agent costs and broadacre land area are taken into account. Apart from the severe impact the conjunction of non-wetting soil and low rainfall has on productivity of agricultural land, there results very inconsistent moisture level in top soil through the season, severely limiting germination of crops 25 and pastures which require a consistent moisture level in the rootzone to encourage growth through good water supply, useful microbial growth and fertiliser mineralisation. Limited germination leads to wind erosion and loss of nutrients and organic matter which are crucial for crop and pasture germination. A vicious cycle is formed resulting in reduced crop and pasture production. 30 Soil wetting agents, or wetters, are developed in an effort to combat the problem and are typically applied to the soil after seeding or 'in-crop'.. Soil wetting 2 agents are surfactants that reduce water surface tension and improve water entry to, and retention in, soil to improve germination and also increase water release under drainage. Consistent availability of water to the root zone of crops and pasture is important to plant growth. Where water is lacking, crop or pasture may start to grow 5 but then, due to lack of water, the crop or pasture dies off. This is called a 'false break'. Such 'false breaks' are costly because, despite the investment in seed, agricultural chemicals and other items, crop/pasture yields do not cover these costs or do not cover them sufficiently. In such cases, not planting a crop may be the less costly option. 10 It is an object of the present invention to provide an improved method of treating soil, particularly the soil of agricultural land including applying a soil wetting agent which provides increased soil water capture and soil water holding capacity; better weed control; and improved crop and pasture yields. With this object in view, the present invention provides a method of treating 15 soil including applying a soil wetting agent to a broadacre block of agricultural land for increasing water capture and water holding capacity of soil in said broadacre block of agricultural land for promoting plant growth wherein said method comprises: forecasting a timing for watering of the block of land by pre-season rainfall; distributing the soil wetting agent by spraying over untreated soil of said block of land 20 and wherein said block of land is sprayed with soil wetting agent when in "bare earth" state prior to seeding of crop or pasture at a predetermined timing prior to pre-season rainfall to provide a treated wettable soil with water holding capacity above water holding capacity of the untreated soil and above-permanent wilting point for the treated soil; and wherein said soil wetting agent is distributed over said block for 25 promoting weed germination, including of weeds with a hard seed coat, and weed control by use of herbicides, preferably pre-emergent herbicides, prior to seeding of crop or pasture in preparation for a plant growing season. The untreated soil is typically a non-wetting soil in which uneven or no germination or growth of plants - particularly crops and pasture - is normal. 30 Permanent wilting point (PWP) varies with soil texture and a range of PWP has been reported for wettable soils by Bot et al., Drought Resistant Soils:optimisation of soil moisture for sustainable plant production, FAO Land and Water Bulletin, No. 11, 2005, page 50, as follows: % by weight moisture soil depth (cm) 3 Medium Sand 1.7 0.7 Fine Sand 2.3 1.0 Sandy Loam 3.4 1.5 Fine Sandy Loam 4.5 2.0 5 Loam 6.8 3.0 Silt Loam 7.9 3.5 Clay Loam 10.2 4.5 Clay 14.7 6.5 10 As PWP represents the minimum acceptable moisture content to sustain plant growth in the treated soil, higher water holding capacity is sought for the treated soil since more than plant sustenance is required. Optimal crop and pasture yields are also sought through achievement of treated soil water holding capacity substantially above: at least 3% above, preferably at least 5% and most preferably at least 10% 15 above water holding capacity for the untreated soil. The treated soil water holding capacity may further be a similar amount above PWP for the treated soil type, for example being at least 3, 5 or 10% above PWP as indicated by the above Table. Watering refers to the dominant mechanism for supplying water for plant growth on the block of land. Seasonal rainfall is usually the most important source of 20 water for a block of agricultural land such as that used for broadacre crop or pasture growing. Crops and pasture have a growing season that is well understood and defined in the agricultural field and the growing season usually generally corresponds with seasonal rainfall. However, water may also be supplied by irrigation or other means again during a growing season or phase. The wetting agent is distributed at a 25 predetermined time prior to the growing season and seeding where relevant, for example for crops. Watering of the block of land is by rainfall and the soil wetting agent is advantageously applied prior to the expected timing of opening season rains with the intention of capturing and retaining pre-season rains that would normally be subject 30 to runoff and/or evaporation, being lost as a source of water to support crop/pasture growth. The wetting agent is in spreadable liquid form, and is distributed over the block by spraying. Spraying is suitable for broadacre distribution of wetting agent in agricultural applications. A preferred spraying technique involves boom spraying. In 4 boom spraying, a vehicle fitted with a tank containing the liquid to be sprayed; and boom(s) extending away from the vehicle traverses a block of agricultural land. As the vehicle traverses the block of land, the wetting agent is supplied to the boom and distributed through spaced apertures in the boom to apply the wetting agent at a 5 controlled application rate (both in quantity and distribution) to the block. The same vehicle may be used to apply other agricultural chemicals be they pesticides, herbicides or otherwise. Application rate of wetting agent may be controlled having regard to soil type and texture (for example as above defined by Bot et al), temperature and soil 10 moisture content. Prior to seeding, a block of agricultural land may be described as being in a "bare earth" state. There is some licence in the use of the term "bare earth" as some plant growth may be evident in the block. However, such plant growth is to be distinguished from the target germination and crop growth following seeding. Apart 15 from the plant growth, residual stubble may remain from a previous growing season. The wetting agent is desirably distributed over the block of agricultural land whilst the block is in the bare earth state. Watering timing involving rainfall would typically involve rainfall forecasting generating a profile for rainfall over a period of time. Rainfall forecasting may involve 20 various models aiming to identify the time at which there is highest probability of rainfall, Various agencies are able to provide expected rainfall patterns across agricultural land. For example, in Australia, the Bureau of Meteorology and divisions of the CSIRO, such as the Division of Marine and Atmospheric Research have provided rainfall forecasts sometimes well into the future. More qualitative models 25 may also be used. That is, rainfall is typically seasonal and likely rainfall timing may be forecast by reference to the season and month of the year. For example, in Southern Australia, rainfall tends to be winter dominant and rainfall timing may be set with regard to the onset of winter. A still further model could involve use of a farmer's own rainfall records to set the optimum time for application of wetting agent. 30 The intended timing is prior to pre season rainfall (expected in late February/March (late Summer-early Autumn) in South Western Australia). This timing could vary somewhat year on year. A particularly advantageous timing for application is prior to pre-season rainfall, rainfall not usually relied on to support crop/pasture growing on a commercial scale because it occurs in Summer months. This pre-season rainfall is 5 the early 'opportunistic' rain that occurs earlier than the rainfall at the start of the growing season (mid to late April (mid-Autumn) in South Western Australia). Advantages of application prior to this timing are described below. The predetermined timing for application of the wetting agent, in agricultural 5 applications, is set prior to the forecast rainfall timing. A desirable time range is not less than 4 to 10 weeks before forecast rainfall timing, more desirably 6 to 8 weeks prior to forecast rainfall timing. The soil wetting agent retains its efficacy, enabling storage or "banking" of water within soil in readiness for plant growing season(s), for at least this period of time, which usually corresponds with relatively dry to very dry 10 conditions - particularly in the Western Australian context. Soil wetting agent efficacy may last for a considerably longer period than weeks or months, perhaps 2 to 3 growing seasons, that is, 2 to 3 years after application of wetting agent to the block. However, annual application of soil wetting agent, most particularly prior to pre season rainfall, would still typically be practised to mitigate against risk of 15 unsatisfactory soil water retention. The method may be applied with efficacious results in the case of various crops including canola and barley but benefit for other crops is not precluded. The method may also usefully be applied to enhance pasture growth. Growth of genetically modified ("GM") crops such as GM canola may also be increased by the 20 method, In another application, pasture growth may be improved following the method of the invention. The method may allow better control over weeds since the application of wetting agent at the predetermined timing, prior to 'opportunistic' rainfall such as pre season rains, promotes germination of weeds en-masse after these pre-season 25 rains. This also promotes an increase in the germination of weeds with a hard seed coat that are typically very difficult to germinate without sustained soil moisture content. This contrasts with staggered weed growth and minimal hard seed coat weed germination encountered with non-wetting soils or soils not treated in accordance with the above method. This results in more effective application of 30 knockdown herbicides such as glyphosate and pre-emergent 6 herbicides such as Metolachlor, Diuron,,Atrazine and Simazine. Seeding advantageously follows the weed control step achieving a benefit of less competition between weeds and crop/pasture. The method of the invention is now described with reference to the 5 following non-limiting examples. Example I A block of agricultural land, located near Narrogin, Western Australia, comprises very hydrophobic or non-wetting soils comprising gravels, sandy loams and mallee soil. Annual rainfall mean at Narrogin is low at 493.5mm annually 10 (Australian Bureau of Meteorology, 2011, record for Narrogin, the contents of which are hereby incorporated herein by reference) Without application of soil wetting agents, rainfall run-off is an extreme problem affecting agricultural production. The blocks were to be seeded with Jardee hybrid canola and Vlamingh 15 barley. The block was treated - on a trial basis - with the synthetic LURE H20 wetting agent, containing polyalkoxylated glycol block copolymers in admixture with anionic surfactant(s), and sourced from Sacoa Pty Ltd, Perth, Western Australia. The synthetic nature of the wetting agent assists resistance to 20 microbial degradation and longer lasting efficacy in soil. The wetting agent liquid comprising a minimum of 10 litres of LURE H20 in admixture with 100 litres of water was then applied to the block in the first quarter of 2010 about six to eight weeks ahead of pre-season rainfall timing. At this time, the soil was dry and in a bare earth state. 25 Following pre-season rains, weeds germinated in a consistent, rather than staggered manner, and were effectively controlled using a knockdown (eg, glyphosate) or pre-emergent herbicide (eg, trifluralin). Such herbicide attacks susceptible weeds at the germination stage. In-crop herbicides were not necessary at this time because seeding had not yet occurred. 30 Rainfall for the growing season was very low at 155-160mm - below target rainfall for a typical growing season (over 200mm) - and, normally, this would result in poor crop yield. Due to the ability to retain the 'opportunistic' or pre- 7 season rain that was captured outside the normal growing season, soil moisture content was maintained well above PWP and harvest yields were 1.4 tonne per hectare Jardee hybrid canola, 3.5 tonne per hectare Vlamingh barley and 3 tonne per hectare barley overall. The barley productivity of 3 tonne per hectare 5 favourably compared with an expectation of about 1 to 1.5 tonne per hectare for a year with 160 mm rainfall and no application of the LURE H20 wetting agent. Therefore, the present method also allows soils to be made more drought resistant, allowing better crop yields even in demonstrably dry seasons. Without wishing to be bound by any theory, the wetting agent and its timing 10 of application allowed the capture of pre-season or 'opportunistic' rains to support crop/pasture growth during the growing season, in contrast to current practice and moisture to be better retained in treated soil in preparation for the plant growing season. This moisture was then held at a consistent level within the top soil, and more particularly the root zone of the growing crops, allowing better crop 15 growth than had wetting agent not been applied in the above described manner. Excellent control over the growth of rye grass weeds was achieved using the pre-emergent herbicide. Such control was observed to be superior to the case where wetting agent had not been distributed over the block of agricultural land in the manner above described. 20 Example 2 Pasture was to be grown on a non-wetting gravel soil. The wetting agent of Example 1 was applied to the pasture block, 6 to 8 weeks prior to pre-season rainfall. Pasture yield for the block not treated with wetting agent was 747 kg/ha/ 25 At an application rate of 10 1/ha LURE H20, pasture yield rose to 1319 kg/ha and at an application rate of 15 1/ha, pasture yield rose still further to 1880 kg/ha. The block was safe for grazing following the treatment of the block with wetting agent. Example 3 Plant biomass measurements were made for a non-wetting gravel soil 30 block when not treated with wetting agent as used in Examples 1 and 2; and when treated with 10 I/ha and 15 1/ha wetting agent respectively. Plant biomass 8 was 250% that grown on untreated soil for the 10 1/ha wetting agent application and 390% that grown on untreated soil for the 15 1/ha wetting agent application. Example 4 Soil moisture measurements were made at 0-5cm depth in a non-wetting 5 gravel soil to assess surface water content. Saturation corresponds with 30% moisture in soil and this sets a baseline for the measurement, made at three sites. At site A, application of LURE H20 wetting agent - at the same timing as for Examples 1 and 2 - showed a surface soil water content of 13.8% for an 10 application of 10 1/ha wetting agent. This compared with surface soil water content or water holding capacity of 7.9% for untreated soil at site A. Such low water holding capacity would be either marginal or insufficient for plant growth in permanent wilting point terms. At site B, application of LURE H20 wetting agent - at the same timing as 15 for Examples 1 and 2 - showed a surface water content or water holding capacity of 18.5% for an application of 15 I/ha wetting agent. This compared with surface water content or water holding capacity of 12.1% for untreated soil at site B. At site C, application of LURE H20 wetting agent - at the same timing as for Examples 1 and 2 - showed a surface water content of 19%, very 20 substantially above PWP, for an application of 10 1/ha wetting agent and 13.1%, still substantially above PWP, for an application of 5 1/ha wetting agent. Both values compared with 8.8% water holding capacity for untreated soil. The result of the method is to capture and retain in the soil rainfall that may occur prior to the start of the growing season ('pre-season rains') that would 25 otherwise be lost. The effect of improved water capture and retention in soil continues through the crop/pasture growing season. The method of the invention has the following potential advantages: " Improved weed control: Superior weed germination is achieved or promoted allowing for more effective weed control via knockdown 30 herbicides in preparation for the growing season. * Improved crop and/or pasture germination and growth: Retained soil moisture improves plant establishment and growth.
9 * Improved herbicide and fertiliser activity: Retained soil moisture improves the activity of herbicides and fertilisers that are used in-crop during the growing season. " Improved plant survival during dry periods: The improved soil moisture 5 retention achieved reduces plant stress during dry periods. Modifications and variations to the method of applying a soil wetting agent to soil in accordance with the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within 10 the scope of the present invention. 15