Calcium & how to use it better
Good calcium levels in potato tubers can reduce multiple quality problems including Internal Rust Spot (IRS), internal browning and hollow heart. Calcium also plays a role in reducing susceptibility to bruising and post-harvest diseases. However despite this, farmers do not always get a good response to calcium fertilisers – Here we explore why, and what can be done to improve it.
So what exactly do potatoes do with calcium?
Lets start by looking at why potatoes (and other crops need good calcium levels). Calciums main function within cell walls is to give cell wall rigidity & strength. The most obvious symptom of calcium deficiency is the disintegration of cell walls and the collapse of affected tissues. It’s this tissue collapse that contributes to IRS, internal browning, and premature rotting and bruising post-harvest.
Potatoes don’t actually need very much calcium, the quality problems associated with calcium result from tiny local deficiencies, but these minor deficiencies (in terms of the amount of tuber affected) can make crops unsellable.
While tubers may have small areas of calcium deficiency, the rest of the plant rarely suffers any shortage at all and is often precipitating Ca out from leaves as in excess.
The Maths should make us ask questions
If a 35t/ha crop of potatoes had complete loss due to internal rust spot, the actual quantity of Ca-deficient tissue (2% of each tuber is actually affected) is only 700kg/ha. The difference between the affected and healthy part of the potato is typically only 4mg/kg.
Therefore the amount of calcium required to prevent an entire 35/ha tonne crop of potato from having internal browning is only 2.8g/ha. This should raise a few questions for growers.
- Why are small parts of the tuber deficient when the area right next to them isn’t?
- Why are these small areas of tissue deficient in Ca when there’s no whole plant deficiency?
- Why doesn’t applying large amounts of calcium reverse the deficiency?
In order to answer these questions, it’s important to understand how calcium behaves in a plant. There are two factors to be considered in plant Ca availability –– transport and absorption.
Unlike most other mineral nutrients, Ca isn’t phloem mobile and can only be transported through the xylem. Ca enters the plant with water and is transported upwards with transpiration, where it’s either absorbed and stored, or is precipitated from the leaves as excess.
Ca only moves upwards. This is why targeting and correct placement of applications is so important. Ca applied to leaves can’t correct problems in the roots.
Therefore, foliar sprays of Ca fertilisers will never put the nutrient into tuber – it’s physiological impossible for the plant to move Ca down.
Ca is absorbed into cells using polar-auxin transport –– as auxin moves out of the cell, Ca enters. Parts of a plant that are low in auxin can’t absorb the nutrient effectively, regardless of how much is available.
High auxin-producing areas include new shoots, new flowers, and new leaves. Low auxin-producing areas include fruits, roots and tubers.
This is why applying Ca to correct physiological disorders can be so ineffective. It doesn’t matter how much is applied, parts of the plant with low auxin levels such as tubers can’t absorb it properly.
So how can we improve tuber Ca levels?
Now we have identified the two main drivers in low tuber calcium problems, we can use this to improve our agronomy, and the products we use to help it. Here are some ways we can improve.
Target the tubers
Don’t apply it to foliage and expect it to get to tubers. For best results it must be placed near the stolon roots inside the tuber zone. Depending on available equipment this can placed through drip lines, or incorporated into hills at planting. Remember that it is the Stolon roots that supply the tubers, the main root system tends to bypass them and take calcium past them and up to foliage, so placing in the tuber zone is crucial.
Target optimum absorption stages
Time applications to when tubers can absorb it. Tubers produce very little auxin once they start growing, so to get conventional Ca sources into a tuber it really needs to be done during the cell division stage. Once tubers reach 5mm in size there’s very little new cell formation, and auxin levels decline. For Ca to be able to get in the tuber it needs to be available between hook eye and 5mm tuber size.
Manage haulm growth
Any weather conditions or agronomic practice that creates rapid vegetative growth will dramatically increase the risk of Internal rust spot and other problems associated with low tuber calcium levels. it is therefore advisable to manage nitrogen applications so as to prevent too much vegetative growth. A good way to do this is to use supplemental applications of Lono which supplies Amine Nitrogen stabilised with LimiN technology which is proven to give a better growth habit where the plant produces a more robust roots system and less top growth. For more information on this you can read Understanding Nitrogen – The key to Potato Yield
This approach has seen good results on varieties sensitive to internal rust spot, but works primarily by creating a ‘shape’ that supplies more Ca to the tuber (growing the stolon roots) and reducing the strain caused by excessive vegetative growth on calcium transport.
Use LoCal chemistry
For varieties and growing conditions that are really problematic then we need to get calcium into tubers during bulking. Levity have developed LoCal, a chemistry that allows low auxin parts of plants (like tubers) to absorb calcium.
Calcium fertilisers incorporating LoCal like Levity’s Albina supply calcium alongside a chemistry that enables tubers to absorb it better as they grow and mature.
Albina comes in a variety of formulations both liquid and granular that can be applied to the tuber zone to supply calcium that can be accessed better by tubers.
For specific advice on varieties/soil types/ growing conditions and how to get the best out these products you can contact firstname.lastname@example.org