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Soils used for fruit production are generally low in organic matter which can lead to problems of surface crusting/slaking, soil compaction, poor water penetration, poor aeration and over all poor root growth. Soil conditioning is the process of improving soil structure and overcoming these problems.

Soil conditioning involves application of chemical amendments, such as gypsum, and/or organic matter supplements.

Organic supplements

Various supplements can be used to improve soil structure. Applying these products is a more expensive means of increasing soil organic matter levels than by growing a cover crop, but the process requires less management input and causes less disruption to orchard practices. Most of these products also have some fertiliser value.

The choice of material will depend on local availability, requirements of the particular fruit block and cost of the material. As these materials are bulky, distance from the source and resultant freight cost is a major consideration.

Types

Grape marc
Grape marc, compressed grape seeds and skins, is a by-product of wine making. The fresh material is usually composted prior to soil application. It must decompose before any nutrients are released. If applied for its nutrient value it is best incorporated into the soil. Typical analysis figures are 2.4% nitrogen, 0.3% phosphorus and 1.8% potassium, but these can vary between batches. Unlike many manures and fertilisers grape marc is low in sodium and chloride.
The suggested application rate is 20 tonnes per hectare, but higher rates can be used.

Poultry litter
Deep litter, a mixture of sawdust and fowl manure is the product most frequently used. Fowl manure obtained from beneath battery cages is much higher in nutrient content and can burn plants if caution is not used. Fowl manure products are best incorporated into the soil. Additional applications of zinc and manganese may be required where fowl manure has been applied.
The suggested application rate of deep litter is 6 - 9 tonnes per hectare.

Sheep manure
Sheep manure is easy to handle, but is slow to break down in the soil. The source of the manure should be checked. If the sheep have been grazing on salt bush or drinking saline bore water the manure may contain high concentrations of sodium and chloride. Sheep also consume a large number of weed seeds which remain viable and can be introduced onto the block with the manure. Sheep manure should be used with caution.
An application rate of 6 tonnes per hectare is considered adequate.

Pig manure or effluent
Pig manure which is mixture of straw and droppings is rarely available. Piggery effluent is a liquid slurry and can vary in terms of solids, salinity and nutrient value. Preliminary analysis prior to purchase is advisable.
Rates used are generally 15 - 20 tonnes per hectare.

Horse and cow manure
These products have useful levels of both organic matter and nutrients but are rarely available in sufficient quantities.
Where available application rates of up to 15 tonnes per hectare can be used.

Commercial products
A range of commercial products are available, for example Top Humus Booster. These processed materials are uniform in consistency and easy to handle and spread. Favourable results have been obtained from their use.

Application
Specialised equipment is required for spreading due to the high rates per hectare required and the physical properties of most organic supplements. Some suppliers as well as contractors offer this service.

Organic matter is usually applied soon after harvest or during the autumn/winter period. Application during dormancy is preferable as application of organic materials at the recommended rates has the potential to tie up soil nutrients. This is a temporary effect but can induce short term deficiency, particularly of nitrogen. If spreading takes place after harvest, extra nitrogen may be needed depending on the amount of growth the trees are showing at that time.

If the aim is to improve soil structure, incorporation into the soil is not recommended in most circumstances. Cultivation physically breaks up the organic matter. This along with increased soil contact makes the organic matter more susceptible to breakdown by soil organisms. Organic matter is best left on the soil surface where it is more likely to produce humus and provide a longer term beneficial effect on soil structure.

Leaving organic matter on the surface can suppress the germination of weeds. On the other hand some organic materials are a source of weed seeds, a fact that should be considered when sourcing organic supplements.

Gypsum

Gypsum, calcium sulphate, can improve soil structure and water penetration. On heavy soils gypsum improves soil aeration, facilitating root growth. Gypsum can alleviate sodic soils, that is, soils significantly downgraded by the presence of sodium salts, but only where high water tables and poor irrigation management are not the cause.

Before applying gypsum it is advisable to check that low organic matter level is not the primary cause of the soil problems being experienced. Even when tests indicate gypsum application is needed, increasing organic matter levels will enhance the benefit obtained from gypsum application.

Gypsum works in two ways:
1. Electrolyte effect - is short term (1-12 months) ceasing when all the gypsum has dissolved. Each irrigation dissolves some of the gypsum raising the electrolyte concentration of the soil water which decreases soil dispersion (a cause of soil crusting). This is the predominant process occurring when gypsum is applied to sandy loams and loams.

2. Exchange effect - is longer term (12-24 months). The addition of high concentrations of calcium to the soil displaces sodium salts attached to clay particles. Sodium salts then enter the soil solution and are leached below the rootzone by irrigation. Displacement of the sodium salts helps to prevent surface crusting and clay dispersion in the rootzone. This is the predominant process occurring when gypsum is applied to clay soils.

Types and quality

Gypsum is available in two forms. "Flour" gypsum looks like coarse flour and varies in colour from white to pink. "Seed" gypsum consists of crystals usually a few millimetres in size. "Flour" gypsum is faster acting and is used when a quick response is needed whereas "seed" gypsum is used if a long term response is desired.

Different sources of gypsum contain differing amounts of impurities. Impurities such as sand have no detrimental effect except increasing the freight cost per tonne of actual gypsum. Knowing the percentage of actual gypsum is also important for determining the application rate per hectare. The proportion of actual gypsum should preferably be above 80 percent.

Many gypsum deposits contain sodium chloride. Gypsum used for agricultural purposes should not contain more than 2 percent sodium chloride and this should be confirmed by a recent analytical report from the supplier.

Application

Rate
The amount of gypsum to apply depends on the soil type and its condition.

In Table 1, rates given for initial application refer to soils with surface crusts, poor water penetration and salinity problems. Follow up rate refer to applications applied 2 - 3 years later to ensure that crusting or water penetration problems do not recur.

Table 1: Application rate of gypsum per hectare for established orchards.

Where salinity problems are severe or where there are other underlying problems such as high water tables or poor irrigation systems a soil test and site specific advice is recommended.

Timing
Late autumn or winter is the best time to apply gypsum in established orchards. This avoids stressing the tree crop and makes use of winter rainfall for leaching. Application prior to planting can assist in establishment of cover crops.

Incorporation/irrigation
Gypsum is normally broadcast evenly between rows. It should be incorporated into the soil soon after spreading, otherwise wind loss can occur.

On sandy loam and loam soils gypsum moves readily into the soil with irrigation. However in the case of compacted or clay soils a single pass with a disc or rotary hoe then irrigation is preferable.

The amount of irrigation applied should follow normal practice for the soil type. Ideally enough irrigation should be applied to wet the full depth of the rootzone but no more.