Date: 17 Dec 2004 Authors: Lou Revelant, Sandra Hardy, Graeme Sanderson
Citrus is a major industry in NSW. The major production areas are widely scattered and each has its own soil type and management problems. We cannot overemphasise the importance of soil management for sustainable citrus production.
Good management ensures the maintenance or improvement of soil structure and provides an ideal environment for healthy root systems, which are the basis of good tree health and sustained high production.
Citrus prefer a soil pH of 6.0–7.0.* An ideal citrus soil is well-structured, with good drainage and a minimum of 60 cm of topsoil. However, large areas of these soils are difficult to find in Australia. Most Australian citrus is successfully grown on marginal soil types. Success depends on using the right rootstock, and good management of soil and irrigation.
*The acidity of the soil is measured as pH, usually in a 1 part soil to 5 parts 0.01M calcium chloride (CaCl2) suspension, or a 1 part soil to 5 parts water suspension. This publication quotes pH values measured by the CaCl2 method. The CaCl2 suspension method gives pH values about 0.5–0.8 units lower than the water suspension method, but with less variation.
Citrus does not like very acid soils (pH below 5.0). Very acid soils are deficient in some essential plant nutrients such as calcium and magnesium and are oversupplied with others such as aluminium and/or manganese.
Citrus does not like very alkaline soils (pH above 8.0). Calcareous soils that are high in free lime are also unsuitable, as they cause lime-induced chlorosis, resulting in excessive yellowing of the tree. Chlorosis is particularly evident on some rootstocks, such as Poncirus trifoliata. In severe cases, it can affect yield and tree health.
Observing the soil can give you a general idea of its condition. More detailed soil investigations and laboratory testing might be required if you notice any of the following signs:
Soil pits are usually dug in representative parts of a planting, using a backhoe or spade.
Dig pits about 4 m long at right angles to the direction of water flow and wheel traffic, and as close as possible to the trees.
Dig pits about 1 m deep, citrus feeder roots rarely penetrate below that depth and the most important inspection area is the first 50 cm.
Trim the pit walls with a flat-edged implement to remove smeared soil left by the backhoe bucket and to expose undisturbed soil. Examine the sides of the pit for compacted layers and tree roots.
Record the depth and thickness of the damaged layer as a future guide to how deep you should dig.
If the clods are intact after 5 minutes, this usually means that soil conditions are excellent for plant growth because there is enough organic matter to act as a glue between particles. This, in turn, means that the channels between aggregates and clods are likely to remain open after wetting, allowing water to drain quickly and roots to penetrate easily. When the soil dries, there are no hard crusts or flakes.
If the clods fall apart in water, there is not enough organic matter to stabilise micro-aggregates (about 0.25 mm diameter). This collapse is known as slaking. Most irrigated soils in Australia behave this way.
Although slaking is generally undesirable, you can still grow a good crop in such soils, particularly when there is enough swelling clay to create a loose, self-mulching surface after wetting and drying.
If the clods slake when tested in water, leave them for a further 2 hours. If the water then looks milky, this means that the micro-aggregates have dispersed into individual particles of sand, silt and clay. The clay particles block soil pores so that wet soil is waterlogged and dry soil is very hard.
In the field, dispersion shows on the soil surface after heavy rain and subsequent drying, with light-coloured sand separating from the clay. Dispersion is most obvious in dark clay soils. The main cause of dispersion is an excess of sodium and (possibly) magnesium on the clay particles. It indicates that gypsum might be required.
This test for slaking and dispersion gives an early indication of the soil’s structural stability. However, you should also collect soil samples for laboratory testing, as chemical analysis provides detailed advice about gypsum requirements.
Do not confuse salt (mainly sodium chloride) with calcium carbonate (lime). A simple test is to apply a low-strength solution of hydrochloric acid to the deposits. If it effervesces or fizzes, the deposit is lime.
Sometimes you need to have field observations verified by chemical and physical laboratory analysis.
Important soil information, for example texture, sodicity and salinity, can sometimes be obtained from existing soil surveys. However, if good quality survey information is not available, collect soil samples from pits or holes for analysis.
Three sampling depths are recommended at citrus sites:
Five subsamples from across the pit or from five spade holes throughout each planting should be bulked to make up each sample. Soil testing laboratories can provide information on how to take soil samples.
Send the samples to a NATA-accredited laboratory (National Association of Testing Authorities for a full analysis, including:
Soil structure is defined by the arrangement of sand, silt and clay particles and the spaces between them.
Well-structured soil has:
There are four main ways of maintaining and improving soil structure:
Options 1 and 2 are mainly used to maintain good structure. Options 3 and 4 are used to improve poor or degraded structure.
Topsoil cultivation also cuts off old root and earthworm channels to the subsoil and often results in the development of a plough-pan or hardpan; repeated cultivations virtually eliminate earthworm activity. Perhaps most importantly, repeated cultivations lower the level of organic matter because fresh surfaces are continually being exposed to microbial attack.
Organic matter, particularly the relatively stable humus fraction, plays a key role in the prevention of structural degradation in topsoils. Any factor that contributes to increased humus content will improve soil structure. These include:
In addition to maintaining and increasing organic matter levels (see 2 above), some soils require gypsum and/or liming materials to improve structure.
Its immediate effect, after dissolving in rain or irrigation water, is to increase the salinity of the water, thereby reducing swelling and dispersion of clay particles. This, in turn, increases infiltration and drainage of water, particularly in sodic clay soils. This effect is short-term, lasting only as long as there is undissolved gypsum.
The longer-term benefit is for the calcium content of the gypsum to displace sodium ions from the soil. This reduces the swelling and dispersion of the clay particles.
Gypsum is not very soluble, but irrigation speeds up its absorption into the soil. If treating subsoils, it might be necessary to work it in.
Other liming materials include burnt lime (calcium oxide), hydrated lime (calcium hydroxide) and waste materials such as kiln dusts, sewage residues and blast furnace slags.
Lime or a mixture of lime and gypsum can also be used to improve soil structure, but this is not recommended for alkaline soils (pH over 7).
‘Mechanical regeneration’ refers to cultivation immediately below the plough layer. It inevitably cuts off old root channels and other macropores that transmit air and water to the subsoil. If there are not many of these anyway, and a compacted layer (plough-pan) has developed, then cultivation is likely to help, at least in the short term.
This type of cultivation is also known as:
The soil water content when doing this type of cultivation is important, particularly in clay soils. If the soil is too wet, structural damage is likely.
Avoid deeper cultivation in mature citrus, as extensive root damage can occur, leading to a decline in tree health and possible sudden death in future years.
Soil management trends vary across regions according to soil type, slope and rainfall.
Riverina soils vary widely because of wind action over the original surface of the plain. Citrus is planted according to soil type and mostly in areas with the best surface drainage and the deepest topsoils. Underground tile drainage has now been installed in many of these areas to improve the subsoil drainage.
Soil management has turned full circle in this region. In the early years, using winter cover cropping and summer cultivation was common practice. Then, with the development of residual herbicides during the 1960s and 1970s, growers moved to zero tillage and blanket application of herbicides.
Today, some growers continue this practice but most growers have seen the adverse effects of long-term use of residual herbicides including:
In the Murray valley, citrus is mainly planted on slightly elevated windblown sandy ridges. The soils are alkaline (a surface soil pH of 7.0–8.0); at a depth of 1.5 m the pH can be as high as 9.0.
Before residual herbicides, growers used a variety of soil management practices. The most common was green manure cropping in autumn/winter followed by spring/summer cultivation. The use of frequent cultivation destroys soil structure, leading to compaction, reduced permeability and increased run-off.
In the Gosford area, citrus growing is now confined to the mountain plateau west of Gosford. Most plantings are on flat to gently sloping sites. Most soils in the area are derived from Hawkesbury sandstone and are naturally acidic. There are few frosts. A dry spring and wet summer/autumn period is common.
The Windsor district is similar, except most of the citrus is grown on deep alluvial loams as found along the Hawkesbury and adjacent river systems. These areas flood occasionally.
Historically, orchardists have applied an annual dressing of poultry manure. After World War II, annual dressings of agricultural lime became normal practice.
However, during the late 1970s and 1980s lime was not applied regularly, resulting in widespread problems with soil acidity (low soil pH). Increased use of soil analysis in the 1990s resulted in regular applications of agricultural lime being reintroduced.
The majority of citrus orchards use a permanent sod between tree rows, and weeds within tree rows are controlled by herbicides.
This adds organic matter to the soil. In late summer or early autumn, sow a winter crop of lupins, field peas or oats. In the spring, work it into the soil.
Note: The benefits of green manuring can be quickly lost through repeated summer cultivations. Most of the organic matter is quickly broken down, resulting in little long-term benefit to soil structure.
Cover cropping protects the soil surface from water and wind erosion. Sow lupins, field peas or beans in autumn; in winter, the crop is usually mown to reduce the risk of frost damage in susceptible areas.
This is particularly applicable to irrigated citrus. A permanent sod benefits the soil in two ways:
There is evidence that soil structure is enhanced by the considerable microbial activity around fibrous grass roots. Grass also makes for easier traffic access during wet periods.
Until recently, permanent sod culture has not been widely practised in the main inland citrus-growing areas; however, its use is widespread in coastal areas.
Now it is becoming popular with inland growers. The majority of growers combine the use of permanent sod with strip application of residual or knockdown herbicides along the tree rows.
One reason why permanent sods are now more popular is that the latest varieties of clovers and grasses are easier to manage and maintain.
Permanent sods are best sown in autumn when seasonal orchard operations have finished. They are mown periodically, with the cut material left on the surface to break down and build up organic matter or thrown onto the tree line to provide a surface mulch.
If residual herbicides have been used, the sod will have some difficulty with germination and establishment. There are two ways to overcome this:
Trials carried out by NSW Agriculture show that rice hull ash is very effective in deactivating residual herbicides. As a guide, aim to evenly cover the soil with 2.4 cm of ash before working it in.
Sod cultures are best sown in autumn (March to May) when seasonal orchard operations have finished. Sowing rates vary from 5–20 kg/ha, depending on the varieties used. In the Riverina, a popular sod is a mixture of Victorian perennial ryegrass and strawberry clover. Clovers should be inoculated before sowing to ensure the presence of the correct nitrogen-fixing bacteria.
Most orchards have adequate fertiliser in the soil to support the sod. However, you can ensure quick establishment with a dressing of single superphosphate (200 kg/ha).
For all sowing methods, a good seedbed is important for best results. A small combine or a sod seeder are the most effective for sowing.
Other methods are hand broadcasting or using fertiliser spreaders. Whichever method is used, do not sow the seed too deeply, 10 mm is sufficient, as these seeds are very small. A good system is to drop the seed on the surface and work it in using very light harrows or a piece of mesh dragged through the rows.
Sod cultures are relatively cheap to establish, with the major cost being soil preparation. Ryegrass seed is cheap, while clover seed is more expensive. This cost will vary according to the variety and the season. Commercial mixes are also available.
Irrigation is essential for the sod to successfully establish and grow. The sod growth should be sufficient to provide organic matter but it should not compete with the trees or pose a frost risk.
Irrigation management is more critical in orchards with sod culture. The sod uses more water but irrigation intervals are not necessarily increased, thanks to improved irrigation efficiency.
A tensiometer is an excellent aid in the orchard, particularly those under the sod. It helps take the guesswork out of irrigation timing. For further information on tensiometers, contact your local irrigation officer or horticulturist.
Practical soil management often requires compromise to fit in with other aspects of orchard management.
Overhead sprinklers or micro-sprinklers make water management easier. Periodically slash or mulch and apply a residual or knockdown strip herbicide (optional in overhead systems). Micro-sprinklers can cause water distribution problems if the sod is uncontrolled; clovers and medics particularly cause problems because of their ‘creeping’ habits.
Furrow irrigation makes sod management much more difficult. It is best to combine sod culture and strip herbicide. You can either:
Since the mid 1980s, a range of early-maturing and late-hanging navel oranges have emerged from the main citrus-growing areas in southern Australia. There are now large-scale plantings of these selections.
The rapid commercialisation of these varieties has encouraged some growers to use soil management techniques not previously used in citrus production. These techniques include weed matting and soil mounding, which are applicable to all new citrus plantings.
The benefits of weed matting were shown in a 12.5 ha late navel planting in the Curlwaa district (near Wentworth). The benefits were the result of an integrated management system that combined weed matting, drip irrigation and an interrow lucerne sod. The main reason for installing the weed mat system was to control weed growth around the young citrus, but the benefits proved more far-reaching:
The weed mat proved impervious to weed growth, including Johnson grass. Weed suppression around young trees has direct economic benefits because it saves time and expenditure on chemicals and reduces the chances of herbicide damage to young trees.
Weed growth was confined to the mat surface or around the tree butt, where accumulated soil or plant material had allowed wind-blown seed to germinate.
The weed mat has proved extremely durable. It has not deteriorated since its establishment in 1988.
Another major benefit of weed matting is preserving soil moisture. The weed mat mulches the soil surface and reduces moisture losses from evaporation, while still allowing rainfall to penetrate the mat.
The case study compared the moisture retention rates with and without weed matting. Two sites, both clay loam, were selected. The first site had bare soil; the second site had weed matting.
Since weed matting helps hold moisture in the soil, it also helps lower soil temperature. A slightly cooler soil environment helps maintain root activity during the hottest summer days.
The case study compared soil temperatures under the weed mat with (irrigated) bare soil temperatures. They were compared at three depths (5, 10 and 20 cm) between November 1989 and February 1990, during the warmest part of the day:
The soil under weed matting stays moist and is protected from the sun, wind, rain and cultivation. This lack of disturbance and the proliferation of surface feeder roots stabilises the soil.
In the case study, a section of the matting was lifted, revealing increased worm activity and fine citrus feeder roots in the top 2 cm of soil.
Weigh up the benefits of weed matting against the initial cost of the material and its installation. Early and late navels grown with weed matting should bring high returns and, therefore, a quick recovery of costs. However, using traditional citrus varieties and planting densities under this system, it will take much longer to break even.
Our experience is that weed matting should work best with high-density orange plantings, with the tree size controlled by graft-transmissible dwarfing (GTD).
Herbicide and insecticide use could be minimalised if we use:
The benefits could include:
Mounding is being successfully used by a citrus grower in the Boeill Creek area (near Buronga) to reclaim poor agricultural soil and redevelop it for late navels.
The 12 ha site was originally saline, dispersive, and had very shallow topsoil. The natural vegetation was only capable of feeding 1 sheep per 4 ha. In late 1989, the site was cleared of the low-growing salt-tolerant vegetation and the topsoil was graded into broad, raised beds (mounds).
The beds are approximately 45–50 cm high, 2.4 m wide and 7.7 m apart. Similar raised-bed management in conjunction with drip irrigation has been implemented in marginal soil on this property over the past 15 years to increase the depth of topsoil and improve the rooting zone for citrus.
The topsoil (raised beds) and subsoil were analysed in February 1990. The results indicated the need for soil improvement before planting in the spring.
The raised beds were sandy clay loam soil, strongly alkaline and low in nitrogen, phosphorus and organic matter. The exposed subsoil between the rows was a highly saline clay that was not dispersive. The raised beds are sitting on this well-structured, very saline soil, and citrus root activity will concentrate in the modified hilled soil.
The results of the soil test suggested that the soil structure of the raised beds would be improved if gypsum is broadcast at a very high rate of 20 t/ha. Accordingly, gypsum was deep-ripped through the raised beds, to a depth of 1 m.
Humus booster was also added to the raised bed, at a very high rate of 10 t/ha. This improved the organic matter content and provided a base level of fertility before planting the trees.
The site is drip-irrigated and a fertigation system is used to provide the nutritional requirements. Drip irrigation is the dominant form of citrus irrigation on the property; this provided a good irrigation model.
Modified forms of mounding might also assist the establishment of new citrus plantings in shallow soil. Mounding can also improve surface drainage.
Today, soil management in citrus plantings is predominantly aimed at the following areas:
Details within the Citrus soil assessment section were cited from the Agfact P5.3.6 (1992)Soil management for irrigated cotton by David McKenzie, David Hall, Ian Daniells, Adam Kay, John Sykes and Dr Terry Abbott. In preparing this publication, the authors acknowledge the assistance of Dr Terry Abbott, Program Leader (Soil Management) and John Forsyth, former Program Leader (Citrus).