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Management of the orchard floor undoubtedly influences orchard productivity. Undertree management is difficult in practice because of the need to compromise between various objectives. No system can optimise all of these aspects.

Objectives of undertree management are:

• Preserve soil structure
• Maintain adequate soil organic matter levels
• Control weeds
• Allow efficient and effective irrigation of the tree crop
• Allow efficient nutrition of the tree crop
• Allow access for orchard traffic
• Minimise frost risk

There are three options for undertree management.

1. Permanent sod culture offers the most benefits, but is difficult to sustain in a stone fruit orchard situation.
2. The second option is cover cropping in autumn/ winter, leaving the ground slashed or bare during the tree crops growing season. This is the most practical system for many orchards.
3. Keeping the soil bare all year round is the final option, but is not recommended.

Permanent sod culture

Successful maintenance of permanent sod in stone fruit orchards can be difficult due to shading of the sod species during the fruit trees growing season. Large trees and close planted orchards are particularly restrictive to permanent sod culture. Low vigour of the sod species over the summer months is advantageous as it means the sod is less competitive with the tree crop for water and nutrients, but it must be able to rejuvenate quickly in autumn.

Permanent sod culture is more easily achieved in orchards irrigated with full cover sprinklers. Drip, mini and micro sprinklers that do not irrigate the entire inter-row area result in droughting and death of the sod species in the unwatered zones. Flood irrigation is inefficient for applying the light irrigations that a shallow rooted sod species requires over the winter months.

Where permanent sod has been successfully established it allows access of machinery much sooner after irrigation/ rainfall and lessens soil compaction. The amount of dust in the orchard is also reduced, which is particularly important at harvest. Soil erosion caused by water run-off is reduced in sloping blocks.

Establishment

Before establishing permanent sod, soil structure and organic matter levels should be adequate. Although the sods root system will aid in developing soil structure and permeability, as well as adding organic matter, this is a slow process. Where a problem exists, soil amelioration by application of organic matter and/ or gypsum should be carried out prior to planting. Permanent sod established without adequate pre-plant soil preparation will have a short life span as the resultant poor sod growth will allow soil compaction from orchard traffic and invasion of weeds.

Once soil problems have been dealt with the sod species can be established in the same manner as a cover crop. Perennial ryegrass, white or strawberry clover or mixtures of these are suitable. Prostrate lucerne, such as Jindera, is another alternative. Autumn is the best time to plant, as traffic in the orchard must be kept to a minimum until the sod is well established.

Maintenance
To maximise organic matter production, growth of the sod needs to be promoted through the winter months. This is the period that the sod is working to maintain the health and structure of the soil. Irrigation is the most important consideration, particularly in drier winters. A thick healthy stand at the end of winter is important if the sod is to survive the intense shading and reduce soil compaction of orchard traffic through the tree fruit growing season.

Under irrigated orchard conditions heavier soil types tend to set hard after several years of sod culture without soil disturbance. If high productivity of the orchard is to be maintained top dressing with soil amendments may be required. In some cases the soil may need to be broken up and a new sod established. Although light textured soils are not prone to setting, soil compaction can still pose problems along wheel tracks in the longer term.

Machinery should not be operated on wet orchard soils unless this is unavoidable, for example fungicide spraying.

Invasion by weeds can reduce the life of sod species.

Tree line
Complete sod cover is rare. Eliminating sod and weed growth along the treeline is important as this is where the vast majority of a tree crops root system is located. Additionally, sod growth along the tree line is difficult to slash, harbours snails and can hide and/ or interfere with under tree sprinklers. A 2–3 metre wide bare strip left along the tree line reduces the diversion of water and nutrients by the sod species from the tree crop.

Band application of pre-emergent herbicides assisted by spot spraying of knockdown herbicides is the general technique of maintaining this bare strip. A mulch layer of straw is also effective. Preservation of the mulch layer can be assisted by slashing the sod so that material is thrown onto the mulched strip. Do not bank excess mulch material up against tree trunks as this promotes trunk rots.

Cover crops

Benefits
Cover cropping has proved itself over time to be a cheap and effective way of maintaining soil structure and fertility. In areas where cover crops have been grown, the soil is generally more open and friable due to the addition of organic matter which improves water and air penetration.

On sandy soils in particular, the addition of organic material improves the ability of the soil to hold water, especially near the surface. On heavy clay or compact soils, the root activity of the cover crop and the organic matter produced can break up the clay, improving infiltration of irrigation water. Because the soil structure improves, tree roots are able to explore more of the soil for water and nutrients. This results in improved tree health, vigour and yields.

Other benefits from cover crops are:

• soils normally easier to work
• soil not as hard when dry, or as boggy when wet
• harbour many beneficial insects
• increase earthworm activity and population
• reduce orchard dust
• legume crops fix nitrogen

• winter weed control.

Limitations
Shading
Cover crops are frequently difficult to establish in stone fruit orchards due to excessive shading. Many cover crops need to be established in early autumn before the onset of cooler weather when growth slows. At this time trees are still carrying most of their leaves. Cover crops require sunlight for good germination and establishment.

Watering
Additional water is required during autumn and winter to meet the needs of the cover crop. Orchard irrigation systems are designed to water trees, not a cover crop. When the cover crop is established it blocks the throw of low set sprinklers. If not corrected, over-watering near the sprinklers occurs, leaching out nutrients and residual herbicides and central parts of the cover crop become droughted.

Frequent light irrigations are required to establish some types of cover crops, particularly small seeded types. Obviously this cannot be achieved with flood or furrow irrigation, thus the types of cover crop grown are limited to the large seeded types.

Types of cover crop
Legumes
Legume cover crops include medics, peas, beans, lupins and clovers. A well grown leguminous cover crop will produce nitrogen in proportion to the amount of top growth. In a healthy stand there appears to be no significant difference in the amount of nitrogen produced by the various types of legume in relation to dry matter produced.

A legume cover crop does not supply nitrogen to the soil while it is growing, although it does provide much of its own requirement. This nitrogen is available for uptake by the trees only after the legume crop is broken down in the soil. If a legume crop is harvested, eg baled, there is little input of nitrogen into the soil and the improvement in soil structure is greatly reduced.

Medics
Medics are the preferred cover crop in the Riverland area. Apart from producing nitrogen, medics have a high dry matter content which means the plant takes a long time to break down in the soil. This slow breakdown leads to better aeration and water penetration in spring and early summer when it is most needed.

Medics are planted between late February and mid April, as they prefer warm weather. Early planting also gives the medic crop more time to mature before it must be knocked down to reduce the chance of frost damage. If left to mature medics set seed, reducing the sowing rate required in the following year. Be cautious when sowing early, as medics that are planted during periods of extended hot weather may suffer reduced germination and burning off of seedlings.

Larger seeded legumes
Larger seeded legumes such as beans, peas and lupins are more suited to flood irrigated orchards than medics as they do not require frequent light irrigations to germinate and establish. These legumes produce less dry matter than medics, hence are less beneficial to soil structure.

Peas and beans require cool weather to germinate, hence should not be planted before April. Where planting of a cover crop in early autumn is not possible peas and beans are favoured over medics.

Cereals
Oats, barley and rye corn are the cereals most frequently used as cover crops.

Although cereals do not fix nitrogen some growers use them as an alternative to legumes because they have a vigorous root system which improves soil structure. In Addition they are less prone to insect damage, more easily established and can be planted later in the growing season suiting the production system of many growers. Cereals produce more dry matter than peas, beans or lupins.

In young plantings cereal crops, particularly rye, offer wind protection. Rye corn has a deep root system and grows well on sandy soils where wind erosion is a problem. Barley is more tolerant to salt than other cereals and is therefore preferred on heavier soils.

Mixtures
In some cases planting medics may not be possible because it is too late in the season or the irrigation system is unsuitable for their establishment. A cereal cover crop will improve soil structure, but unlike medics will not add to soil nitrogen levels. Large seeded leguminous crops such as peas or beans will fix nitrogen, but are not as effective as cereals in improving soil structure.

It is possible to use a mixture of legumes and cereals to achieve both. However cereals tend to compete with the legume crop, being more upright in growth. As a result legume growth is reduced and so is its nitrogen input.

Volunteer
Weeds can serve as a cover crop under some circumstances. Whereas planted cover crops can be difficult to establish, weeds if not controlled tend to grow profusely in most orchards. Volunteer cover crops of weeds work reasonably well except where problem weeds such as burrs and prickles or couch grass exist. It is difficult to prevent these weeds seeding and becoming more difficult to control the next growing season. Weed growth is never likely to be as useful as a properly planted and well established legume or cereal cover crop.

Management

Sowing
Methods of sowing have a large bearing on the success of a cover crop. A well worked weed free seed bed is important. Unprepared soils result in poor germination. This is especially the case where soil compaction is a problem. Restrict depth of cultivation to that required to plant the cover crop as deeper cultivation unnecessarily disturbs the tree root system. Best germination of these crops is achieved by shallow sowing, thus seedbed preparation need not extend beyond 5 cm depth.

Depth of seeding is very important. Seed must not be sown too deep, especially on heavy clay soils. Sowing depth varies between varieties, but as a general rule, seeds should be sown at a depth about two and a half times the length of the seed. Each seed has a limited amount of stored energy and it is quickly spent pushing through hard soil or from a depth in the soil. Deep sowing can result in reduced or no emergence.

Beans, peas or cereals are best sown into moist soil. If necessary irrigate a few days before sowing. If seed is planted into dry soil and then irrigated there is more chance of water accumulating around the seed and causing it to rot.

Medics and other small seeds should be sown into a dry shallow seedbed to maximise contact between the seed and the soil. Sow with a small seeds drill or mix with an inert material and broadcast with a fertiliser spreader. Incorporation of broadcast seed into the soil with light harrows can improve establishment. A light irrigation should be applied soon after sowing.

Rates
Seeding rate for the various crops is important. The aim is to establish a thick stand with vigorous, early growth to smother weeds. Where medic cover crops are left to set seed for the following year, the stand will gradually lose vigour due to run down seed reserves. To renovate the stand re-seeding at a lower rate is recommended every 3 - 4 years.

Table 1: Guide to cover crop seeding rates.

Inoculation of legume seed
Legumes are able to convert atmospheric nitrogen into a form available to plants. This is carried out by a bacterium called Rhizobium. These bacteria form nodules on the roots of the legumes. This is a symbiotic relationship which means that the bacteria obtain their food from the legume and in return supplies the legume with nitrogen.

For nitrogen to be fixed each type of legume needs a specific Rhizobium strain. Although many Rhizobium bacteria exist naturally in the soil, it is possible that the right kind of Rhizobium is not present. This is particularly so where the legume type planted has not been grown previously in that block. Table 1 shows which strain of inoculant is required for each legume.

If the required Rhizobium strain is absent from the soil it can be introduced artificially by inoculating the legume seed before planting.

Most of the legume species used for cover crops do not need artificial inoculation because there is sufficient natural inoculum in the soil. Seek advice on the possible benefit of inoculation when purchasing seed.

Fertilisation
Type and amount of fertiliser required vary according to the type of cover crop and soil type. It will also be affected by the nutritional program already in place for the trees.

Legumes
Legumes obtain their nitrogen from the air so generally there is no need to apply nitrogen. However in light sandy soils a small amount of nitrogen (10 kg/ha actual nitrogen) should be applied to start the crop off. Once the legume seedlings are established do not apply any nitrogen as they do not need it. Adding excess nitrogen can reduce the amount fixed from the air, defeating the purpose of using a legume.

Legumes have a high requirement for phosphorus to establish a healthy root system. Superphosphate at a rate of 200-250 kg/ha should be applied at or before planting a cover crop. If phosphorus has been applied recently, the application rate can be reduced by up to half. As phosphorus is not easily leached timing is not critical.

Cereals
Cereal cover crops require nitrogen and phosphorus to develop a healthy stand with a strong root system. Superphosphate and ammonia 2:1 at a rate of 300 kg/ha is a convenient way of applying these nutrients.

Mixtures
The nitrogen produced by the legumes in a mixture is not available for use by the cereal component of the cover crop, so 10-15 kg/ha of actual nitrogen is still required. Superphosphate will be needed at a rate of 250-300 kg/ha.

Irrigation
Cover crops vary slightly in their requirement for water. Lupins have the highest water requirement followed by peas, beans and medics. Cereals have the least requirement for water. Adequate water is essential for producing a large bulk of crop.

Check that the top soil is moist, remembering that cover crops are shallow rooted in comparison to tree crops. Regular irrigation is important during establishment, particularly if the cover crop is planted early in the season while temperatures are still high. For most cover crops, 25 mm of water per month from irrigation and rainfall during winter is sufficient.

When irrigation is required, check that the cover crop is not interfering with water distribution from low level sprinklers. If so slash with a nylon line trimmer or spot spray for a more permanent solution.

Pests
The main pests of cover crops, especially medics, are red-legged earthmite, lucerne flea, blue green aphid and spotted alfalfa aphid. Refer to Table 2 for more information.

Pests seldom warrant control in cereal cover crops.

Table 2: Guide to cover crop pest identification and control

Knockdown/incorporation
Growth of the cover crop should be curtailed before active growth of the trees begins, as there is evidence to suggest cover crops compete for water and nutrients to the detriment of the trees. When and how this is done will depend largely on the type of cover crop and whether it was planted primarily for nitrogen production or organic matter. Some of the options are;

• slash the cover crop and leave the cuttings on the soil surface.
• spray off with knockdown herbicide.
• use a blade plough to cut the roots and leave the cover crop on the surface.
• incorporate the cover crop using a disc or rotary hoe, a widely used technique on flood irrigated blocks.

If the cover crop is incorporated into the soil the breaking down process is faster which means the nitrogen becomes available to the trees sooner. This rapid breakdown means less long-term benefit in improving soil structure. Greatest benefit to soil structure is achieved by leaving the cover crop on the soil surface.

In frost risk areas the cover crop should be incorporated into the soil prior to budswell.

Legumes
Legumes produce nitrogen in nodules in the roots. As the plant grows some of the nitrogen produced moves up the plant to the stem and leaves. Once the legume starts to set seed this nitrogen moves into the seeds away from the main bulk of the plant. When mature 75 % of the nitrogen is in the seed with the remaining 25 % in the stem, leaves and nodules.

If nitrogen production is the primary aim of planting the cover crop, it should be slashed before it sets seed to maximise the amount of nitrogen in the plant. However using this method the cover crop will have to be reseeded each year.

A compromise is to slash after the first seeds have matured enabling high nitrogen input to the soil and some seed for next season.

Cereals
Cereals do not produce nitrogen, but still have some nitrogen extracted from the soil within the plant tissue. As with legumes, nitrogen moves into the seed at maturity. In order to reduce the amount of nitrogen removed from the bulk of the plant but maximise the organic material produced, slash or incorporate cereal cover crops just before plants begin to produce seed.

Bare Earth

In the past, flood irrigation necessitated keeping the soil bare of all vegetation. Repeated cultivation, a feature of this system, breaks down soil structure and organic matter, both of which are fundamental to maximising orchard productivity. Accordingly, keeping a stone fruit block cultivated all year round is not recommended in any orchard situation.

Cultivation
Cultivation has several negative effects on stone fruit orchards. Regular cultivation virtually excludes root growth of the tree crop from the top 10-15 cms of soil. In most soil types this is the most fertile and favourable layer for root growth and can make up a third of the potential rootzone. This is particularly the case where shallow carbonate layers are present. Allowing rootgrowth in this zone is particularly beneficial in soils where lime-induced iron chlorosis is a problem.

Although cultivation is frequently used to increase water infiltration for the next irrigation, in the longer term it is detrimental. Cultivation itself physically shatters the natural structure of the soil. The loose surface soil created by cultivation soon breaks down to a solid impermeable layer when irrigated.

Soil microbes are continually breaking down organic matter. By exposing fresh soil surfaces, cultivation accelerates this breakdown leading to a rapid decline in organic matter levels. Organic matter binds soil particles together into larger aggregates thus giving the soil a more permeable nature.

Cultivation is only a short term answer to poor water infiltration. Long term strategies such as cover cropping and application of soil amendments such as gypsum and organic matter must also be employed.

Cultivation around apricots grown on certain plum rootstocks results in suckering from damage points on larger roots, particularly if roots are drawn to the surface. Apart from being a physical nuisance, suckers divert water and nutrients from the tree and are difficult to control once present.

Herbicides
Total year round weed control with herbicides is not a sustainable system. In the absence of a cover crop or cultivation, soil in the inter-row is compacted by machinery movement restricting growth of tree roots and water infiltration. Allowing no plant growth also rapidly depletes soil organic matter levels and so compounds the above problems.