Brix, Magnesium, Manganese, And Plant Health?

Written by James Warne of Soil First Farming

Ever measured the brix levels of your crops? Been told that if you achieve the magic figure of ~15% brix then you won’t
have an issue with pest and disease?

I have spent the last couple of years testing brix (during the spring) of most crops I visit and have never managed to get anywhere near to 15%. For a while I blamed my refractometer so got that tested against another one, no problem there. I then decided I must be measuring the sap from the wrong part of the plant. My understanding is that the sap should be taken from the newest ‘sink’, i.e. the newest emerging leaf. So I tried this as well with little success. However with another approaching growing season I intend to try again.

So what does brix measure? The general consensus is it measures the sucrose content of the plant sap, but this is only part of the story. It turns out that brix is actually a measure of all dissolved solids within the sap. So when measuring brix you are also measuring other solids contained within the plant sap although the largest proportion of these should be sucrose. These solids could be other sugars such as glucose and fructose although these are usually converted to sucrose for phloem transport; it could be amino acids, proteins and minerals also be found within the sap. Brix levels should be measured between 12pm and 4pm on bright days to get a meaningful result. The bris should then drop overnight as the sucrose has been delivered to the sink. If the brix does not fall by a considerable amount this may indicate a shortage of boron.

Why should you be worried at all about brix, after all its principally just a measure of the plant sugar? Well if your desire is to lower inputs and to get off the high input farming treadmill then creating an healthy plant community is essential to achieving a lower-input crop without reducing the output. Essentially it’s a simple measure of our abilities as crop managers at light capture and chlorophyll production. 

So what can affect brix levels? Brix can be negatively affected by most pesticide applications and some fertilizer applications such as ammonium nitrate as both place undue stress upon the crop. So how do we positively affect the concentration of sucose within the sap? As previously mentioned sucrose is the plants transport sugar and is made from the condensation of glucose and fructose, the primary products of photosynthesis. Plants produces glucose as the product of photosynthesis. Plants can also manufacture fructose, which is chemically very similar to glucose. It then combines these two monosaccharides into the disaccharide sucrose for transport from the source around the plant to the sink. The process of combining glucose and fructose into sucrose is known as condensation and is controlled by the enzyme sucrase-p-synthase. Potassium is well known to enable the sucrose to enter the phloem transport system. Once the sucrose has reached its sink the plant hydrolyses the sucrose into glucose and fructose to provide energy via the enzyme sucrase.

Photosynthesis is carried out in the chloroplasts by chlorophyll molecules. Magnesium and nitrogen are central to the chlorophyll molecule. We can assume that the crop is able to access sufficient nitrogen as most crops receive more than they can utilize and very rarely show any signs of deficiency, more likely than not most crops actually reveal an excess of nitrate. It is also worth noting that excess nitrate within the plant can actually depress brix as the crop uses more energy to assimilate nitrate than it does using ammonium. Can we ensure the crop receives all the magnesium it requires, especially in high pH soils or soils with excess potassium or low magnesium?

Of equal importance is manganese. Manganese is central to over 35 enzyme functions; it’s critical to chloroplast production, photosynthesis and the photosystem II process; nitrogen metabolism and nitrogen assimilation. Manganese is also believed to be essential to sucrose synthase. Manganese is also shown to be central to the plant ability to synthesis hydrogen peroxide which helps the plant defend against pathogens. Manganese is important for the process of lignification which give the plants strength to stand and resist pathogens within the roots.

Manganese deficiency is a common sight in cereals in the UK. The plant may be suffering from a shortage of manganese before we see physical symptoms. By the time we see the classic yellowing of the crop we are already two weeks too late, consequently yield will have been compromised. It takes around two weeks for the shortage of manganese to reveal itself with leaf symptoms so from that point on we are fire-fighting the deficiency. Over 95% of all the soil analysis we undertake shows very low levels of available soil manganese. While the obvious solution is to re-mineralise the soil using manganese sulphate or similar, in reality this proves expensive and doesn’t help with some of the underlying causes of deficiency such as soil pH.

Manganese deficiency is typical associated with high pH soils, loose well aerated (cultivated soils) and lighter textured soils. Soils high in iron can also reduce manganese availability. Manganese has low phloem mobility in the plant therefore regular foliar applications are necessary in situations where deficiency has been previously seen or maybe expected. Where the deficiency has already expressed itself in the form of visible symptoms a minimum of 750g of manganese as foliar manganese sulphate is required per application if you are using a straight un-chelated product. This can be reduced to 150g per application if using a quality chelated product. As with all foliar applications you always need to ask yourself the vital question; how does the positively charged metal pass through the negatively charged leaf surface? If your product or supplier cannot answer this question it probably means the product hasn’t addressed this fundamental question.


Mineral nutrition of higher plants. Marschner. 2012

Principles of plant nutrition. Mengel & Kirkby. 2001

Applied soil trace elements. Davies. 1980.