How to manage soil for citrus
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 60 cm of topsoil. However, large areas of these soils are difficult to find in Australia. In fact, 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.
Very acid soils
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.
Very alkaline soils
Citrus soil assessment
You can get a general idea of the soil’s condition by closely observing it. If you notice any of the following signs you may need more detailed soil investigations and laboratory testing:
- a yield decrease that cannot be linked with insect, disease or climatic problems - may be due to the physical or chemical condition of the soil;
- unhealthy trees - may indicate problems with root activity or nutrition;
- obvious damage by machinery or traffic, particularly under wet conditions - soil repair is usually necessary;
- waterlogging - may be due to poor surface drainage and penetration;
- the fruit crop responding slowly to applied irrigation - often indicates structural problems in the soil;
- excessive water loss - for example, if alluvial soils overlie sand or limestone subsoils, or if supply channels leak, which wastes water and raises saline groundwater levels.
Digging the pit
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. One metre is ideal — citrus feeder roots rarely penetrate below that depth; however, 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.
What to look for
A wet layer (sometimes below the compacted or smeared zone) may indicate that:
- drainage is poor;
- the roots cannot properly extract the water.
Record the depth and thickness of the damaged layer as a future guide to how deep you should dig.
- Continuous stable macropores (large pores). These indicate that the soil is not damaged. Macropores are important for water entry and drainage. They are created by roots swelling and shrinking, and soil fauna such as earthworms and ants that connect the subsoil to the surface.
What to test for
Clod stability when wet. Test the stability of clods and aggregates by immersing them in water. You can test topsoil and subsoil in the field or at home:
- Drop three air-dry clods (2.5 mm diameter) into a dish of irrigation water. Do the same using a dish of rainwater, for comparison.
- Examine after 5 minutes, then leave undisturbed for about 2 hours.
If the clods are intact after 5 minutes, this usually means that soil conditions are excellent for plant growth. This is 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, so allowing water to drain quickly and roots to penetrate easily. And 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 microaggregates (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 microaggregates 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 may 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.
Salinity. Salinity is a serious problem and is expensive to overcome. Laboratory testing is necessary if, on a moist soil surface, there are dark greasy areas that form salty-tasting white crystals as the soil dries. Other signs of salt include unusually crumbly soil and a water table within 2 m of the surface.
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:
- 0–15 cm
- 15–30 cm
- 30–60 cm.
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:
- exchangeable sodium percentage (ESP), which is the primary way of measuring a soil’s tendency to disperse;
- exchangeable calcium:magnesium ratio (a secondary way of measuring ESP);
- electrical conductivity (a measure of salinity, which also affects soil dispersion);
- organic matter content (a way of measuring a soil’s tendency to slaking);
- effective cation exchange capacity (sum of the most common exchangeable cations) of surface soil (an index of the ability of the soil to self-regenerate by shrinking and swelling).
Principles of soil management
Soil structure is defined by the arrangement of sand, silt and clay particles and the spaces between them.
Well-structured soil has:
- a large proportion of aggregates that remain stable after being wet;
- an adequate number of large pores, called ‘macropores’, for air and water entry and drainage after heavy rain or irrigation;
- an adequate number of small pores, called ‘minipores’, to retain water for growth.
For good soil structure
There are four main ways of maintaining and improving soil structure:
- Reducing cultivation and traffic.
- Conserving and increasing organic matter.
- Adding gypsum and liming materials, as required.
- Mechanical regeneration.
Options 1 and 2 are mainly used to maintain good structure. Options 3 and 4 are used to improve poor or degraded structure.
1. Reducing cultivation and traffic
Cultivation. Citrus topsoils can be cultivated in order to maintain adequate infiltration of water. Cultivation achieves this by:
- controlling weeds, thereby conserving soil moisture for the trees;
- loosening a hard or compacted surface layer, thereby increasing the porosity of the topsoil. Unfortunately this effect is only temporary because bare soil exposed to rainfall or irrigation is vulnerable to structural breakdown.
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.
- Traffic. Traffic is one of the major causes of soil compaction in citrus orchards. This is one of the reasons why sod culture is the preferred soil management option, as it decreases the impact of orchard traffic.
2. Conserving and increasing organic matter
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:
- reduced cultivation and traffic (as discussed);
- green manuring or the growing of winter cover crops;
- the addition of organic matter, e.g. manures;
- a permanent sod.
3. Adding gypsum and liming materials
In addition to maintaining and increasing organic matter levels (see 2 above), some soils require gypsum and/or liming materials to improve structure.
Gypsum. Gypsum (calcium sulfate) is a salt. Its prime function is to improve the structure of sodic clay soils by dissolving in the soil solution. Its effects are both immediate and long term.
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 may be necessary to work it in.
Liming materials. Liming corrects soil acidity. The three main types of liming materials are:
- agricultural lime — calcium carbonate obtained by crushing limestone rock. It moves slowly in all but very sandy soils and is used mainly for treating strongly acid topsoils. The finest quality lime is preferred;
- dolomite -about 60% calcium carbonate and 40% magnesium carbonate;
- magnesite -100% magnesium carbonate.
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).
4. Mechanical regeneration
‘Mechanical regeneration’ refers to cultivation immediately below the plough layer. It inevitably cuts off old root channels and other macropores which transmit air and water to the subsoil. But 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:
- shallow subsoil cultivation;
- deep tillage; or
- soil aeration.
The soil water content at the time of the operation 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 (rainfall 400 mm)
Riverina soils vary widely, the result 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 the use of 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:
- poor water infiltration.
Murray valley (rainfall 273 mm)
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 the development of 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 runoff.
Coastal areas (rainfall 1200 mm)
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 acid. There are few frosts. A dry spring and wet summer/autumn period is common.
There is a similar situation in the Windsor district, except most of the citrus is grown on deep alluvial loams as found along the Hawkesbury and adjacent river systems. These areas flood from time to time.
Historically, orchardists have applied an annual dressing of poultry manure, which is available from the local poultry industry. 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.
Preferred management options
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.
Winter cover cropping
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 the preferred approach to management of citrus soil in New South Wales.
Permanent sod culture
This is particularly applicable to irrigated citrus. A permanent sod benefits the soil in two ways:
- It protects the surface soil from the structurally damaging effects of wind, water and traffic.
- The pasture roots help create and maintain macropores, which in turn assist infiltration and drainage.
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.
How residual herbicides affect sod
If you have used residual herbicides in the past, the sod will have some difficulty with germination and establishment. There are two ways to overcome this:
- spelling the soil for several years, allowing time for the herbicide to break down; or
- incorporating some form of activated carbon (rice hull ash, for example) which ties up the herbicide.
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.
What to sow, and sowing rates
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.
Cost of establishment
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.
Management of the sod
Irrigation is essential for the successful establishment and growth of the sod. 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. 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 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 microsprinklers make water management easier. Periodically slash or mulch, and apply a residual or knockdown strip herbicide (optional in overhead systems). Microsprinklers 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:
- run a furrow beneath the tree skirt and a second, sod furrow in the centre — this first system means the sod furrow does not need to be watered at every irrigation, an advantage when you need to bring in machinery; or
- run a single sod furrow — this second system is only used in flatter country with little or no side fall.
Recent developments in soil management
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.
- suppressed weed growth;
- maintained soil moisture in the wet zone around the drippers;
- reduced summer soil temperature in the wet zone;
- soil structure did not deteriorate;
- increased number of surface feeder roots.
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 saves 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 the preservation of 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.
- Each site was given 4 hours of drip irrigation (4 L/hour drippers). Then, 26 hours later at 2 pm, each was sampled (0–8 cm). The air temperature was 32°C. The moisture content of the bare soil was 15.4%, but was 22.6% under the weed mat.
- Several weeks later there was a second test, involving a longer period between watering and sampling. The results showed a similar trend. The saturated soil moisture content of 34% (at 0–8 cm) was first recorded 2 hours after irrigation. Almost 2 weeks later, the moisture content of the bare soil (at 0–8 cm) was only 1.9% but was significantly higher, at 8.2%, under the mat.
Because 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 days of summer.
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:
- In the top 10 cm of soil, temperatures under the weed mat were 2.4°C lower than temperatures under the bare soil.
- At 10–20 cm, the temperatures under the weed mat were 1.2°C lower than temperatures under the bare soil.
The soil under weed matting stays moist and is protected from the sun, wind, raindrop action, 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, which revealed an increase in worm activity and fine citrus feeder roots in the top 2 cm of soil.
Costs and benefits
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. But if you use traditional citrus varieties and traditional 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).
The future could see a minimal use of herbicides and insecticides if we use:
- a combination of a selected variety/clone and rootstock with high-density GTD dwarfing of 800 trees/ha;
- drip irrigation with fertigation;
- weed matting and interrow sod culture.
The benefits could include:
- targeting a profitable niche market;
- higher production (tonnes per unit area);
- a more efficient delivery of water and fertiliser;
- cost savings in water, fertiliser and herbicides;
- fewer destructive soil practices (such as repeated cultivations);
- improved levels of organic matter;
- more time available to the farmer.
The 12 ha site was originally saline, dispersive, and had a 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.
Physical and chemical soil properties
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 which 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 the 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 the very high rate of 10 t/ha. It improves the organic matter content and provides 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 may 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:
1. Correcting/minimising soil acidity
- Monitor the soil pH regularly using soil analysis. Collect samples from similar areas within the orchard at depths of 0–15 cm and 15–30 cm. For new sites also sample at 60 cm. Select several test sites as standards for monitoring long-term changes in pH.
- Apply lime to counter the acidifying effects of nitrogen fertilisers or to improve the pH of new sites. Intersperse the lime with dolomite or magnesite, depending on the exchangeable calcium:magnesium ratio (the ideal is 2:5). Application rates of 1.5–2.5 t/ha should be based on results from soil analysis.
- For new or replanted sites, apply the lime 6–8 weeks before planting. Work in the lime as deeply as possible and use sufficient to achieve a soil pH of 6.0–7.0 in the plant row.
- In established orchards, broadcast superfine lime in the plant row where nitrogen fertilisers are applied. The usual time is June/July, but any time when heavy rains are not expected is suitable. In the future it may be possible to inject lime into the subsoil, using deep-slotting/Agrowplow® implements currently being developed.
- Use the least acidic forms of nitrogen such as urea or ammonium nitrate, and reduce sulfate forms of fertiliser such as ammonium sulfate.
- To reduce leaching, apply nitrogen fertilisers in small amounts at frequent intervals. Alternatively, apply nitrogen through irrigation water (fertigation) a third of the way through the irrigation cycle.
- To avoid overwatering and leaching out of plant nutrients (which increases soil acidity), monitor the soil water, using a tensiometer.
2. Improving plant nutrient supply
- To avoid nutrient availability problems associated with the pH being too low or high, maintain soil pH at 6.0–7.0.
- For short-term control of micronutrient deficiencies such as zinc, magnesium and manganese, use foliar fertilisers as needed.
- To improve organic matter content, apply slow-release organic fertilisers (e.g. poultry manure) at 2 t/ha in late summer/autumn. Organic fertilisers are particularly important on sandy soils, increasing the organic matter and decreasing nutrient loss from leaching.
3. Maintaining and improving soil structure and drainage
- Install agricultural tile pipes to improve drainage. Locate drains above the impermeable clay layer.
- Plant trees on raised beds. Sow green manure crops and apply organic matter (for example, animal manure).
- Mulch along the tree rows.
- Sow and maintain a permanent sod between tree rows.
4. Weed control
- Mulch the tree rows, using organic matter or weed matting.
- Sow and maintain a permanent sod between rows.
- Use a registered knockdown herbicide in plant rows, as required.
5. Improving irrigation infiltration
- Make irrigation efficient by using water-scheduling aids such as tensiometers.
- Reduce orchard traffic when soil is wet.