Efficient fertiliser application in orchards

Note Number: AG0240
Published: April 1995
Updated: February 2010
Reviewed: August 2013

Fertiliser is only a moderate expense in the overall production costs incurred in an orchard. However, inefficient application techniques will reduce the proportion of applied fertiliser taken up by the trees. Applying larger doses to compensate for losses is not the solution as inefficient fertiliser use can also acidify soil (lower soil pH) and in the long term pollute drainage water.

The long term answer is to rethink your fertiliser application strategy to ensure that it is as efficient as possible - given your conditions. In most orchards fertiliser is either broadcast over the orchard floor, spread along the tree-line or injected in with the irrigation water - a technique which is commonly called fertigation.

Normally growers select fertiliser on the basis of results obtained from previous applications, cost, and solubility in water if for fertigation. However, another aspect often overlooked is the ability of different fertilisers and nutrients to move through the soil. This 'mobility' in soil can have a big impact on leaching losses that can result from poor fertiliser application techniques and irrigation management.

Mobility of fertilisers in soil

Growers are generally aware of the solubility of fertilisers in water. This is especially the case when fertigation is used and the fertiliser needs to be dissolved in water before application. However, the ability of a fertiliser to dissolve in water does not always give a good indication of the potential for the fertiliser or its components to move through the soil. Table 1 gives a general guide to the mobility of fertiliser components in soil.

Table 1. Mobility of fertilisers or components in soil

For example, urea is very mobile in soil and is easily leached down the soil profile, past the majority of fine roots if fertigation run times are excessive. Ammonium 'sticks' to clay particles and is much less mobile. As a result ammonium (in MAP, ammonium sulphate etc) is much less likely to be leached down past the tree roots. An interesting example is ammonium nitrate - here the nitrate component is mobile and easily lost by leaching, while the ammonium component binds strongly or 'sticks' to clay particles and is much less likely to be leached. This does not mean that mobile fertilisers should be avoided, in fact some offer distinct advantages in their effect on soil pH, e.g. calcium nitrate or potassium nitrate may slowly increase soil pH.

Table 2. Effect of efficient and inefficient use of different sources of nitrogen on soil pH

However, mobile fertilisers do need to be applied carefully to minimise leaching losses. Efficient application strategies are discussed below. Another point to remember is that although you may apply a fertiliser of lower mobility (i.e. ammonium), it will be naturally broken down in the soil to form nitrate, which is very mobile. As a result, care should be taken with all irrigation to minimise leaching of mobile nutrients. In cases where saline water is used for irrigation, leaching may be required to prevent the build up of salts in the soil profile and in this situation leaching management should be based around fertiliser applications. In lighter textured soils, even the relatively immobile fertilisers and components can be leached past the root zone when fertigating. In studies on peach trees planted in a clay-loam soil, small amounts of ammonium and phosphate were leached past 450 mm with an inefficient application technique using MAP as a fertiliser. Select the appropriate fertiliser for your situation, taking into account:

• previous leaf analysis results;
• soil pH results;
• proposed method of application (i.e. spread/fertigation);
• potential of fertiliser to alter soil pH (Table 2); and,
• cost per unit of nutrient and mobility of fertiliser in soil (Table 1).

A special note on Cadmium

Farmers should be aware of the potential dangers of using phosphatic fertilisers with high levels of cadmium. Research has shown that the use of phosphatic fertilisers which contain the heavy metal cadmium as a contaminant can increase cadmium levels in both soil and the produce. There are legal maximum levels of cadmium allowable for horticultural produce in Australia. Fertilisers containing cadmium in excess of 1 mg/kg are required to state the following warning:

"WARNING: this product contains cadmium. Continued use of this product in agricultural situations may lead to residue levels in plant and animal products in excess of the maximum level specified by the Australia New Zealand Food Standards Code and the accumulation of cadmium in soils."

It is in the grower's and consumer's interests to minimise the addition of cadmium to soils and agricultural produce. Growers should consult fertiliser suppliers or manufacturers for advice on the cadmium levels of fertilisers they are considering using. There are a number of low cadmium horticultural fertilisers marketed today. These have lower levels of cadmium than some superphosphate and other fertilisers.

Timing of applications

In general avoid large applications early and late in the growing season. Any mobile fertiliser that isn't taken up by roots can be lost by rainfall leaching the soil.

From spring to harvest

Early in the season trees have only developed a small proportion of their total leaf area and this combined with low evaporation limits fertiliser uptake. High levels of rainfall are not uncommon in early spring, which further increases the risk of leaching any fertiliser left in the soil profile. If possible apply smaller amounts of fertiliser more frequently in the periods of maximum tree demand. Smaller applications will give the tree an opportunity to take up all the applied fertiliser before the next irrigation or rainfall leaches the soil. Smaller applications will also have less dramatic short term effects on tree and fruit growth and on leaf analysis results as the tree gets a more even supply of nutrients. Avoid any large fertiliser applications for six weeks up until fruit harvest as applications in this period can adversely affect fruit colour, maturity and storage.

Post harvest

Early in the new season deciduous plants rely on nutrients stored from the previous autumn for flowering and to develop new leaves. Post-harvest fertiliser applications are often beneficial as they ensure adequate nutrient storage for the following spring. After harvest trees quickly reduce water uptake and after leaf fall tree water use drops to almost nil. As a result, fertiliser uptake late in the season is slow and it pays to apply fertiliser as soon after harvest as possible. If it is applied too late for tree uptake, winter rainfall will leach any fertiliser residues from the soil. In some late harvest situations, the post-harvest application will need to be applied particularly quickly as leaf fall occurs soon after harvest. There is some evidence that late post-harvest urea sprays will improve the nutrition of apples and possibly pears, in the following spring. A low Biuret (<0.4% Biuret) foliar urea spray containing 4% urea is often applied just before leaf drop. Post-harvest urea sprays will also aid in blackspot control in the following season by reducing spore numbers over winter.

Fertiliser application techniques

Fertigation - an option for micro and sprinkler irrigation users

Fertigation gives the operator a great deal of control over fertiliser timing and dose. Fertigation has the potential to be an efficient technique for fertiliser application. However, if poorly used it can be very inefficient and in a number of cases severe acidification of the irrigated soil has occurred.

Remember you are concentrating all of your fertiliser and its breakdown reactions into the irrigated soil. It is very easy to leach this soil and any mobile fertiliser/nutrients that it contains with over-irrigation.

Most problems occur when little attention is paid to irrigation scheduling and leaching occurs as a result of excessive run times. In particular, be careful with the actual fertiliser application and the following irrigation when fertiliser levels in the soil will be high. When fertigating, the irrigation run should be shorter than normal, with the fertiliser applied over 30 to 60 minutes towards the end of the run.

Leave enough time for flushing of fertiliser residues from the lines before shutting the pump off. If possible fertigate a day or two after rain or after a normal irrigation, so that the soil is moist beforehand. This will lengthen the time before the next irrigation is due, giving the tree maximum time for nutrient uptake before the next irrigation is needed.

A wide range of fertiliser injection methods are available. Make sure that the equipment you choose can inject all the fertiliser solution needed for any one application in about one hour or less. Also, because smaller and more frequent applications are more efficient, the convenience of some degree of automation will greatly help in adopting improved fertilisation strategies.

Tree-line applications - for micro and sprinkler irrigation users

Fertiliser type should be carefully selected as larger less frequent applications are more prone to leaching losses as a result of rainfall and irrigation. Again where practical split large applications and apply less fertiliser more frequently. Avoid long irrigation runs straight after application, to minimise leaching losses. If possible, apply fertiliser a day or two after rain or a normal irrigation, so that the soil is moist beforehand. Then apply a short irrigation to water the fertiliser into the topsoil. This strategy will maximise the time before the next irrigation is due, giving the tree maximum opportunity for nutrient uptake before re-irrigation.

Broadcast

There has been a trend away from broadcasting fertiliser over the whole orchard towards tree line application. Generally unless the sod/pasture requires fertilisation applying fertiliser to the tree line is more efficient as the majority of roots are found in the tree line. If for equipment or specific management reasons fertiliser is broadcast, there are a few options that could increase efficiency. Fertiliser type should be carefully selected as mobile fertilisers are prone to leaching losses as a result of rainfall and excessive irrigation. Where practical split large applications and apply less fertiliser more frequently. Redirect your slasher output so that it throws onto the tree line. This will increase the proportion of applied fertiliser that ends up in the tree and provide a free source of fertiliser enriched mulch for the tree line.

Working with your irrigation system

Micro-irrigation

Fertigating with micro-irrigation systems can be precise and has the potential to be very efficient, as you can apply fertiliser almost directly to the tree roots under the emitter. Remember that fertigation concentrates all the fertiliser and its breakdown reactions in a small soil volume and inefficient fertigation with micro-irrigation can acidify soil rapidly. Care has to be taken in fertiliser selection and the application technique. Once acidified it is difficult to raise the soil pH underneath irrigation outlets in an established orchard. Check how deep into the soil your irrigation water is moving with a simple steel probe or shovel. Use the general guide above to optimise efficiency but remember to check on your irrigation depth. It is a good idea to test soil pH directly under the microjet/drip outlets on regular basis to ensure that acidification is not occurring. When using tree line fertiliser applications with microirrigation, only the soil that is both irrigated and fertilised can be managed - with the remainder left to rainfall. However, you can alter fertiliser type, timing, application rate and follow up irrigation strategies, all of which can affect tree uptake.

Sprinkler irrigation

Sprinkler irrigation usually wets the whole orchard floor. Use a suitable spreader to put most of the fertiliser in the tree line where the tree roots are most concentrated. Sprinkler systems offer reasonable control of irrigation run time. Apply the correct type of fertiliser and irrigate for the minimum period so that the fertiliser is kept in the topsoil. Subsequent irrigations run times should again be checked to minimise leaching of fertiliser residues and any breakdown products. Fertigation is not often used with sprinklers because 1/3 of the nutrients end up in the traffic row where there are few tree roots. If you do fertigate, use the same principles mentioned above.

Flood irrigation

It is difficult to alter the efficiency of fertiliser use in flood irrigated blocks as it is difficult to alter the amount of irrigation applied, while still watering the whole block. In general avoid large applications early and late in the growing season. Any mobile fertiliser that is not taken up by roots can be lost by rainfall leaching the soil. Early applications in spring are particularly risky when higher levels of rainfall occur and trees have only developed a small proportion of their total leaf area. This combined with low evaporation limits fertiliser uptake in early spring. Make sure your irrigation run time is just sufficient to water the whole area and that fertiliser is not lost in surface runoff as a result of excessive irrigation.

Irrigation run-times

Ensure that your normal (non-fertilising) irrigation run times are not excessive. Optimal run time will depend on factors such as soil type, moisture content, and emitter output. Fertilisers that contain ammonium (includes urea) and organic matter are continually broken down over time to release nitrate. Nitrate is the main form of nitrogen taken up by trees. Over-irrigation will continually leach this nitrate, reducing tree uptake and increasing soil acidification in the irrigated soil. In some cases where salinity is a concern, a controlled leaching strategy is required to prevent the build up of salts in the soil profile.

Leaf analysis is helpful

Leaf analysis can be used as a guide to ensure tree nutrition is kept at an optimum. Efficient fertiliser use will probably allow most growers to slowly cut application rates as less fertiliser will be lost and wasted. Regular leaf analysis is the key to ensure that the nutrient levels are kept in the adequate range. Leaf analysis costs are falling and some growers are checking nutrient levels in each block yearly in order to optimise their nutrition program and to avoid applying expensive fertiliser and nutrient sprays that may not be needed.

Efficiency is the key

Efficient fertiliser use will reduce soil acidification and as a result the amount of lime and mechanical incorporation required at replanting. It will also minimise the chance of nutrient pollution of drainage water and importantly reduce the amount and cost of fertiliser applied in your orchard.

Further information

Refer to the Chemical use section of our website for further information or contact us to speak to your local chemical standards officer.

Acknowledgements

This Agnote was developed by DEPI Victoria, April 1995.

It was reviewed by Harold Adem, Farm Services Victoria, February 2010.

ISSN 1329-8062

Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
Melbourne, Victoria

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