Over the last few years there has been an increasing interest in the use of animal manures in macadamia plantations. This interest has been driven by a desire to move away from the use of ‘chemical’ or inorganic fertliser, and/or the increasingly high cost of these inorganic forms of nutrition. There are both ‘pros and cons’ for using manures and these must be taken into consideration if you are contemplating their use.
Most macadamias in Queensland are grown on soils derived from sedimentary rocks. These soils are generally very old, inherently low in nutrition and fragile. While the red volcanic krasnozems of northern NSW, Maleny and parts of the Mary valley are younger, higher in nutrition and deeper, they are none the less still relatively fragile and can be readily destroyed by poor farming practices.
With all these soils, organic matter is the ‘glue’ that keeps the soil particles together and provides structure. It is this glue which is destroyed by poor farming practices. Organic matter is also the glue that keeps nutrients, vital to the health and productivity of the trees, stuck to the soil particles, preventing those nutrients being washed away. A good analogy is that these soils are like a piece of particle board, quite strong and resilient when the glue is in good condition, but damage the glue and the particle board will rapidly lose strength and fall apart. Once this process has started it is very difficult to reverse, a fact readily confirmed by anyone who has tried fixing a piece of furniture made out of old, water damaged particle board.
When most people think of organic matter they think of layers or leaf matter and husk on the soil surface, however the bit that that does the ‘sticking’ is actually microscopic organic acids which you cannot see. These acids are stuck to the soil particles and each other, and are the end product of a long series of biological processes that break down the visible organic matter into the microscopic particles that you cannot see. It is this continual process of organic matter (leaf, husk, wood, grass, manures, composts etc) breakdown that keeps the soil healthy – glue renewal and maintenance. While you can skip this process to some extent by adding processed organic acids, such as fulvic and humic, these are an end stage process and may only be partially contributing to the maintenance of an overall healthy soil rich in biodiversity.
The use of manures is NO substitute for the loss of organic matter rich topsoil.
In our wet warm costal environment organic matter is readily broken down and lost. In addition, the often sloping nature of many orchards makes them prone to the loss of organically rich top soil through erosion. Preserving topsoil is vital to maintain orchard productivity into the future. Unfortunately, our management practices require us to remove all organic matter from under the tree for over half the year in order to facilitate harvest. This means the time available for organic matter breakdown and entry into the soil under the tree is limited and the bare soil required for harvest substantially increases the risk of erosion. Erosion control is therefore one of the most important steps in putting in place a holistic approach to orchard management
Manures are a good source of nutrition but a poor source of carbon and it is carbon that is required to provide the bulk fuel to for the ‘glue’ manufacturing process. If you were to apply sufficient manures to try to build up soil organic matter levels, the trees would receive excess nutrients which in turn would present additional problems. Manures are therefore usually described as having a low ratio of carbon to nitrogen ratio, generally less than 10:1 and in the case of chook manure it ca be as low as 3:1. So, for chook manure this means you get three carbons for every one nitrogen and consequently you are getting a very nitrogen rich product relative to the amount of carbon.. The use of manures therefore, should not be confused with building up soil organic matter levels directly and they are no ‘magic bullet’ for halting soil degradation and tree decline. For manures to work they must be seen as only part of a holistic approach to managing the orchard. Importantly, to maximize their effect requires additional planning and hard work.
Pros of using animal manures
Firstly, there is a philosophical side to using manures as they are seen a more ‘natural’ method of providing nutrition. If you are to go down this path, it is my experience that people who are committed to using manures tend to make them work better than people who may be paying ‘lip service’ to this approach. Also, manures do not work in isolation to improve soil health and again the most successful orchards have included manures as part of a holistic approach to maintaining orchard health and productivity over the long term. In short, you need to be committed to this approach if you want it to work as getting the nutritional balance right is harder using manures than when using a chemical fertliser approach
Manures will provide nutrition only when they start to break down. This will only happen under wet or moist conditions which favour biological life. Therefore, those growing macadamias in wetter, more evenly moist environments, or who have sprinkler irrigation will get nutrient release faster and thus find it easy to manage meeting the tree’s nutritional requirements from manures. Conversely, because manures are slower to breakdown you are more likely to have more nutrition left after heavy rain compared with ‘inorganic’ forms of nutrition, where the nutrition is readily available but also readily lost. Manures are therefore a good source of providing a relatively constant background release of nutrients but the release of these nutrients is often largely dependant on factors out of your control.
Manures release nutrients relatively slowly and therefore provide small amounts of nutrient on a constant basis. There seems to be an increasing body of anecdotal evidence that ‘little and often’ is a good approach to feeding macadamia. This fits in with the conditions under which they have evolved with many of the soils having a low nutrient holding capacity or being deficient in one or more of the essential elements. Which means, under natural conditions, the trees only had access to relatively small amounts of readily available nutrition. The macadamia tree has therefore evolved good mechanisms to access scarce resources eg phosphorous, and also mechanisms for storing some nutrients. Nitrogen, for example is stored in the old leaves and is withdrawn under periods of high demand eg flowering, when the soil cannot meet the trees requirements. You can sometimes see evidence of this in a heavy flowering year when flowering is followed by the loss of old leaves a few weeks later. Macadamias belong to the family Proteacea and members of this family have developed uniquely modified roots which enable them to extract phosphorous from soils containing very low amounts of P. However, the drawback is that this highly efficient mechanism, while good in low phosphorous environments, makes them very susceptible to phosphorous induced deficiencies of other elements under high P availability conditions. These mechanisms are an adaptation to low fertility conditions and help buffer the tree from the environment and it is important that you take factors such as these into consideration when you are developing a balanced nutrition program.
Manures can stimulate biological activity in the soil and this can lead to the release of some nutrients that are bound to the soil particles. They do this by not only providing a diverse source of nutrition, including elemental, N, P, K etc for the soil organisms, but also food such as carbohydrates bound up in plant cellulose. Manure based nutrients are relatively accessible to the soil flora and fauna, as the gut flora and fauna of the animal has already partially broken down the material. The enhanced biological activity that appears to be stimulated by manures can assist in the breakdown of organic material from side delivered grass clippings, prunings and husk that are naturally being produced in the orchard and this in turn will improve overall soil health.
Manures may also assist in the breakdown of other sources of carbon, such as composts that have been brought into the orchard. However, there often is insufficient time between the application of the mulch and the need to clean up under the trees for the start of harvest for the mulch to be fully broken down and incorporated into the soil. Manures may help this process considerably as only when the mulch has broken down and been incorporated, and is no longer visible is it truly being useful. While mulches on the soil surface may prevent some erosion, reduce water loss and keep the roots cool it is the breakdown process and particularly the breakdown of the small and microscopic particles that really stimulates the soil biology and leads to the final process of making the organic ‘gluing’ compounds that bind the soil system together. In conclusion, manures may therefore act as the catalyst to start the breakdown of a carbon source (fuel) and thus drive or enhance the speed of the glue manufacturing process that in turn will start to maintain and repair our damaged and broken soils.
Cons of using animal manures
There are a number of problems associated with manures with the main consideration in macadamias being their phosphorous content. As discussed earlier, macadamias have evolved in a relatively phosphorous deficient environment and have developed mechanisms to seek out and take up phosphorous. Consequently, this makes them very susceptible to phosphorous induced deficiencies such as iron and zinc, when soil levels of phosphorous get too high. With the exception of red the krasnozem soils, which readily tie up phosphorous, once soil phosphorous levels are high it takes a very long time for them to decline. Therefore, you need to be aware that phosphorous levels may build up over time with the use of manures and there is a need to be constantly monitoring soil P levels. The only method of doing this is to use a soil test, as there is no correlation between soil and leaf P levels. There is also a difference among soils in terms of their ability to make P available to the plant. In very sandy soils P may be more readily available and become toxic at much lower levels than in, say a much heavier red krasnozem. A test has recently become available called the phosphorous buffer index (PBI) which is a measure of the ability of the soil to hold onto and release phosphorous. I strongly suggest that if you are contemplating using manures then you should get a soil test that includes a PBI tests. This will tell you not only the P status of your soil now but may help you predict how your soils and therefore your trees may react when you apply large amounts of phosphorous.
Another drawback is that the nutrients available in manures are not in the right ratio required by macadamias, with excess P, possibly too much K and insufficient N and possibly very deficient in micronutrients such as boron. To compound this the ratio of nutrients will differ not only between types of manures but also between sources of on one type of manure and possibly between loads. For example, feedlot manure may have very high levels of salt (Na and Cl) which can lead to chloride burn. They may also have very high levels of iron while pig manure may also be salty and have high levels of copper, and chicken litter may have high levels of phosphorous.
NOTE : When applying manure you need to have a good idea of what is in it and be aware of not only its potential beneficial nutrients but also the potential for toxicities, eg P and Cl, and deficiencies, eg boron.
As discussed in the pros section, it takes a while for manures to release the nutrients they contain and this will only happen under moist or wet conditions. Due to this slow release, at certain times of the year the availability of nutrients may be insufficient to meet tree demands. Manures can only really go out from the end of harvest through to November and this period coincides with the greatest demand for nutrients from the tree as it sets and grows the crop. It is also historically the driest time of year, so just when the trees need nutrients the most the dry conditions may mean that the tree is unable to get at them, despite the fact there are sufficient nutrients available in the manure on the soil surface . In order to maximize production it may therefore be necessary to supplement production during the high demand and dry periods of the year with more readily available inorganic sources of nutrition.
Manures may initially be quite ’strong’ and have large amounts of readily available ammonium nitrogen which can lead to root damage and short term toxicity issues. This can be overcome by leaving the manures to compost for a few weeks before applying, but be aware that unless you manage this composting properly, it can lead to nutrient loss which means not only will the trees will miss out, but the lost nutrients could also result in offsite pollution.
There is increasing concern regarding pollution of streams, rivers, and the ocean with farm nutrients and it is likely that same type of restrictions will come into place in SE Queensland as those recently put in place north of Mackay to protect the Barrier Reef.
Storage and application of manures is therefore going to become a major issue in the future and we will need to develop techniques to minimize the off site loss of nutrients.
Manures also represent a health hazard in that they can contain high levels of harmful bacteria such as Salmonella, which can be transferred to the nut, then though the handling system and onto the kernel. There is an increasing emphasis on food safety, and manure use is therefore likely to come under closer scrutiny in this regard. A food contamination scare in say, Japan or the USA, traced back to the use of manures, could cause untold damage to the industry, from which recovery could be a slow, painful process.
Despite these issues there is a lot to be gained by the use of manures in macadamia orchards, especially if we are to halt the long term decline in productivity which can be attributed to the decline in soil health. However, manures are a double edged sword and it will require a dedicated focus to develop the necessary techniques to successfully incorporate manures into macadamia production.
The nutrient content of both manures will vary considerably according to it’s source. In the case of chicken litter, it will also vary considerably according to it’s batch classification. Determine if you have a single or double shed batch.The chart at the head of the page shows the analysis of a "double shed" batch.
Both feedlot manure and chicken litter are quite salty, and exhibit high conductivity. This may be quite OK, but if you have a saline soil or are using salty irrigation water then it could just as easily cause trouble. We recently had an example of this, where manure was applied to a very sandy soil in September but it did not rain until December. In the meantime the grower had used trickle tape irrigation from a dam which had become progressively saltier with the drought. When it did finally rain the trees received a pulse of chloride from the manure on top of the already salty conditions and this resulted in leaf burn.
Chicken litter has a very high ammonium content and if this was to be applied directly to the trees in large amounts it could also result in ammonium toxicity and leaf burn. The analysis also shows the chicken litter to have a relatively high phosphorus content, around 1.4%, relative to the levels of nitrogen and potassium and if large amounts were applied to a soil which had a moderate or low PBI then this could result in problems with iron or zinc uptake.
In summary, you should always get nutrient analysis conducted before you apply the manures so you know exactly what you are putting on because there may be both benefits and potential risks. These risks can be managed if you have sufficient information allowing you to make informed decisions.
Prepared by : Chris Searle. PhD
For Suncoast Gold Macadamias.