The following article applies particularly to avocados, but parts have relevance to kiwifruit, citrus, pip fruit, summer fruit, vineyards, olives, nuts, berries, feijoas, row crops and pasture.
Soil biology has been neglected for the last 60 years. It is the microbes that make minerals available to plants, and prevent the minerals from being tied-up or leached. When healthy, the underground workforce far exceeds the mass of the above ground biomass. Look after it. In natural circumstances, the microbes are in balance. No one fungus, bacteria, nematode or any other parasite can gain dominance to cause a problem. Under most current growing practices, the balance is disturbed and diseases, pests and weeds are common, necessitating chemical intervention.
The Australia National Bank carried out a three-year survey of 800 farms. They wanted to know what factors drove profitability. The answer had nothing to do with finances. The answer was organic matter. That is, the higher the organic matter, the higher the profitability and sustainability. New Zealand properties are collectively losing organic matter. If you want to be profitable, that has to be turned around by each grower or farmer regardless of any external factors.
Available calcium levels in NZ soils are too low. If your calcium is below 60% of base saturation, it is too low. There are many products to raise it, but don’t rely on gypsum to increase calcium until that 60% mark is attained. Below 60%, because of the sulphate component of gypsum, gypsum will decrease soil calcium if 60% of BS is not reached. Aim to get calcium to 68% on a soil test from Perry Agricultural Laboratories, (PAL) USA. With magnesium aim for 10.1-12% on the base saturation, but do not exceed a total of 80% between calcium and magnesium with a PAL test.
Potassium needs to be 7-7.5% for trees, vines, and berry fruit including strawberries, but 3.5-5% for pasture. Avoid potassium chloride but use potassium sulphate, which is far friendlier for the soil microbes. Do not make your choice based on price. You will be financially better off with the sulphate which gives a more complete energy release, and better tasting food or forage.
Even when using a high analysis crop-mix, go for one with the sulphate form, such as Yara Mila Complex. Plants high in Cl ions will be lower in sulphate and other negative ions. That applies to any nutrient. If too much potassium is supplied, the plant will not have enough Ca, Mg, Cu, etc.
Finally, and still with the base saturation, get sodium in the range of 0.5-2%. Do not allow potassium and sodium combined, to exceed 10% of the BS between them, or manganese uptake will be affected.
Once the soil is balanced to the above guidelines, pH will automatically adjust to about 6.4, which is where it should be. If you deliberately try to alter pH, the chances are you will not have balanced the soil properly. pH must be the result of your fertiliser programme, not the cause of it.
When applying fertiliser including nitrogen and lime, always add a carbon source. Some fertiliser firms already do this. Carbon sources are manure, compost, humates, humic and fulvic acids, sugar and molasses. Just 5% of humates or 1kg/ha of sugar are sufficient. Carbon is food for the microbes. Microbes have a carbon:nitrogen ratio of 5:1. If they are fed nitrogen without carbon they will mine the soil carbon, hence the reduction of organic matter. 1kg of excess nitrogen will account for the loss of 100kg of soil carbon as CO2.
There has not been a crop on earth that can be called “bumper” that has not had luxury amounts of the Big Four. The Big Four are calcium, magnesium, phosphorus and boron. Sulphur may make up “the Big Five”. Get those Big Four elements up when measured by a PAL soil test.
Boron is a calcium synergist. It is responsible for opening the trap-door in chloroplasts that transfer 60% of the plants’ sugars into the roots. From there, 50% of that is transferred to the soil, to feed the microbes which in turn, make minerals available to the plant. It is this process that girdling prevents by stopping the transfer of sugar from the leaves into the roots. The microbes are starved for sugar, and in return the plant is starved for minerals. Dig a hole under a girdled vine and observe the root mass from the hole. Dig under a non-girdled vine and observe the difference in the roots.
Avocados arguably use more boron than any other crop and there can be good gains in using a natural form of boron (Organibor) to counteract the leachability of this trace mineral. It is unbelievable how much boron they can utilise and how much must be applied to achieve the luxury leaf levels needed for maximum production.
Boron is extremely important for avocados that have such a poor fruit to flower ratio. There is only a very small percentage of the mass of flowers that are converted to fruit and this is a central role of boron. This mineral increases the length of the pollen tube to boost the efficiency of pollination.
A problem with applying boron, eg. ulexite, to the soil is that it is very easily leached, particularly if organic matter is low. It is a negatively charged anion that can only be stored in the humus component of the soil. Many of us have lost a percentage of our humus over the past few decades and this has reduced the capacity to store boron. It has been found that we can compensate for this decline in organic matter if we apply boron that is complexed with humic acid.
To encourage the onset of flowering and fruiting, an acidic mixture may be applied by foliar spray. Phosphoric acid pH = 0.8; apple cider vinegar pH = 3.9, Citric acid: 100g/100L to halve the pH, (1g/L citric acid reduces pH 7.27 to 3.14.) Acid shock and reproductive minerals stimulates flowering. Calcium, potassium, chlorine and nitrate nitrogen are vegetative minerals. All the other nutrients are reproductive minerals.
Grow red clover or lucerne or other legumes and diverse species between the rows of trees. Legumes help to make phosphorus available. Sprays of fulvic acid are excellent to stimulate legume growth. Be aware that too much phosphorus will lock up zinc and iron.
If nitrogen is high, potassium needs to match it. Maintain a 1:1 N:K ratio in the leaf. We often find K too low in tree crops. The leaf readings of manganese and zinc can be a tell-tale sign for potassium shortage. When manganese is high and zinc is low, this indicates a potassium deficiency, regardless of the reported potassium level.
Sooner or later, you will reduce applied nitrogen. Too much N attracts insect pests, and generally results in extra pruning. If you are doing things right, you should be tapping into the 74,000 tonnes of nitrogen above every hectare. To do that, five soil conditions must be met. Appropriate calcium:magnesium ratio. Available phosphorus. Available iron. Molybdenum. Cobalt.
Phytophthora is a major avocado problem and there are viable biological management strategies that are less intrusive than injecting with phosphorus acid. The avocado tree evolved in America in fertile, well drained soils with plentiful rainfall and minimal disease pressure. The tree is very sensitive to dry conditions and it also hates wet feet. In this context, drainage and poor irrigation practices become the major yield limiters as the trees are far more likely to be ravaged by root diseases like Phytophthora.
There are other factors that can increase root rot pressure including the form of nitrogen present in the root zone. Nitrate nitrogen stimulates Phytophthora while ammonium nitrogen is antagonistic towards this pathogen. Unstabilised urea, the most common choice of nitrogen in most crops, can be counterproductive because it converts to nitrates so rapidly.
Conversely, there can be considerable gain in stabilising urea with humic acid to slow the conversion to nitrate nitrogen. The conventional treatment for Phytophthora involves injecting the trunk with phosphorus acid which is translocated down to the roots to kill the pathogen. This acid can compromise tree health and is not sustainable.
Research released at the International Silica Conference in South Africa suggests that drenching the roots with liquid silica can be at least as effective as phosphorous acid in the management of Phytophthora.
Some cellulose digesting fungi also double as predators that eat Phytophthora. If they are supplied the right form of nitrogen they can more vigorously hunt down pathogens like Phytophthora.
To summarise, look after the microbes. Do this by feeding them and by buffering fertiliser applications. Add carbon. Build organic matter. Get the Big Four elements up to luxury levels in the your crops will be larger and earlier. They will not go off in cool storage. Insect attack will reduce to a minimum. Phytophthora cinnamomi will be kept in balance. Above all, the fruit will taste and store better.
Lastly, for those that spray herbicides. This action will delay your success. Try to replace spraying with other methods, including not being so fussy. You do not need a park-like setting. Allow different species to develop in the understory. Spraying reduces organic matter. That in turn increases the effects of drought and pest attack. Halve your spray rates and add fulvic acid to the spray at 40ml/100l. Maintain correct N:K levels. Tap into free nitrogen.
Maintain an appropriate Ca:Mg percent that will allow correct oxygen:water proportions.
Phosphorus is an enigmatic element. Manufactured from rock phosphates and sulphuric acid, superphosphate ties up in the soil within about six weeks (sometimes in as little as six hours) into a form that is even more insoluble that the rock phosphate it was manufactured from. It has a triple negative charge that readily attaches to calcium, iron, manganese and aluminium. It is toxic to mycorrhiza fungi, as are DAP, MAP and herbicides. Be careful with these products. Those fungi are crucial for trees and vines, indeed most plants, to obtain phosphorus from the soil. They must be protected at all costs. Always buffer the phosphate fertiliser with humates or similar. The acid phosphate MAP and the alkaline DAP are buffered and stabilised by both boron and potassium humates.
Sechura RPR offers highly citrate soluble and slow release phosphate so there is a complete release pattern throughout the crop cycle. Boron ensures that the 35% calcium in RPR can really perform in the root zone. Look after the fungi.
Acknowledgements: NTS Certificate of Sustainable Agriculture, NTS Nutrition Matters