Have you ever grown something that, just doesn’t look great, or perhaps starts off ok, then succumbs to some sort of condition before coming to fruition? I think this is a scenario all growers encounter somewhere along the line. To deduce what’s missing we might try and visually diagnose what’s wrong, do some research, carry out soil tests, apply more fertiliser etc… but by this stage its probably already too late and we’re left trying to figure out what can be done to improve things next season.
Wouldn’t it be good to know what our crops need to get through the season in excellent health and deliver high quality production? Well, I’m here to tell you, there is a diagnostic tool being adopted all around the world that is proving to be nothing short of a revelation. It is enabling growers to achieve great results, and do so with significantly less input expense.
The technique they are using, termed Differential Plant Sap Analysis, involves taking samples of both young and old leaves and having the sap analysed to determine the nutrient levels. Plants prioritise new growth and when it comes to the allocation of mobile nutrients like Potassium and Nitrogen, differing levels in old and young leaves show if they’re getting too little or too much. Conversely with immobile nutrients like Calcium and Boron, lower levels in younger leaves indicate that plants are struggling to get an adequate supply to new growth in time. The requirements plants have for certain nutrients at different stages of growth can vary quite dramatically and differential sap analysis enables us to pick up these trends straight away and tend to matters before our crops are compromised.
One of the first places we started doing differential plant sap analysis with was a commercial organic avocado operation. Over previous years the soil had been tested and, based on the results, custom blends of fertiliser had been applied annually and fish hydrolysate, seaweed and an NPK type liquid fertilisers were put out regularly through a fertigation system. Each tree had also received two large applications of compost and had been mulched with silage. Despite all this the trees weren’t doing great, most were dropping a lot of fruit, dying at the tips and losing just about all their leaves through winter. An initial round of sampling, carried out in spring, revealed what was really going on and we changed the program straight away. It’s amazing how quickly things turned around. The photo above shows the trees eight months later, heading into winter. What is more, with subsequent sampling we were able to see and meet their changing requirements through all stages of growth and production. This was achieved without any applications of nitrogen or potassium, simply because these nutrients were showing up in excess and compromising the uptake of other nutrients like calcium.
We have learnt an awful lot over the last few years, and continue to gather relevant information. It has become evident from sampling programs throughout the South West and similar environments around Australia that certain nutrient issues commonly occur, regardless of the crop or soil type.
This happens for number of reasons; always somewhat linked to a lack of sufficient biological activity and associated soil function. You need organisms such as mineral solubilising bacteria and mycorrhizal fungi to acquire and supply nutrients like calcium, phosphorous and silica to plants. Energy rich root exudates from living plants are required to fuel this kind of activity. Hot dry summers and sandy soils aren’t conducive to the year long green cover that keeps this biology ticking over. Further to that, water infiltration and storage associated with soil carbon and aggregation is severely compromised by practices like repeated tillage, bare fallow, fungicide application, monoculture annual cropping and overuse of soluble fertiliser, which only exasperate the problem.
In such situations, humus formation goes backwards and the capacity to hold soluble anions nutrients like boron in the soil profile is limited.
Finally, nutrients like manganese and iron need to be in what is referred to as a reduced state to be utilised by plants. It is the reducing biology that resides within the kind of saturated environment you find in soil aggregates that get these nutrients into a plant available state.
So you can see, regardless of the nutrient profile in your soil, plant availability is easily compromised in our dry, oxidising Mediterranean climate and sandy soils. While we aim is to implement appropriate biological management measures that improve our growing conditions over time, differential plant sap analysis shows us how we are doing at any stage of growth and enables us to provide timely support as needed to ensure seasonal success.
To find out more about our plant sap analysis service, go to: