How To Interpret SAP Analysis

In the third part of our series on sap analysis, Mike Abram learns how to interpret the results

You’ve taken your leaf sample, packaged it up and sent it to the Netherlands, and now the results have landed in your inbox. There’s a whole ray of bars and numbers for just about every nutrient you could possibly wish to measure. But where do you start when trying to interpret the results, what does it all mean, and how can you use the information to make better decisions? NovaCropControl’s analysis is split into four sections – at the top are the sugar, EC (total dissolved salts) and pH levels of the sap, followed by the cation nutrients, the anions and at the bottom the trace elements.

For each nutrient the report shows what the current level is for young leaves in a light green bar, and older leaves in dark green. There are target values for each nutrient, which are calculated from analysis of at least 500 samples from different fields and growers. The bars are split into three sub-sections representing levels that are too low, good, and too high in the sap. Broadly there are four stages to interpreting the results, suggests Eric Hegger, a consultant with NovaCropControl.

“Always start with which elements are high, which are low – are there any in excess or deficient. The next thing is the difference in uptake between young and old leaves, looking at nutrient mobility.” As explained in more detail in Direct Driller, July 2022, it’s important to understand which elements are mobile in the plant, and which are immobile to recognise which leaves are more critical to be looking at. “Then look at the interactions within the cations and / or anions, and if you still don’t have an answer to why any of the elements are too high or too low in uptake, look at what environmental factors might have influenced mineral uptake – for example, climate, pH, soil life, etc.

“In the end that should give most of the answers to know what corrections might need to be made,” he says. So what might it mean if an element is in excess or deficient in an analysis you receive? Let’s use examples of a barley crop from Australia and Ireland (see charts) to help explain the importance of each different variable analysed. Total sugars: Often this will be linked to level of nitrate in the plant, with high sugars meaning low nitrate and vice versa. “If the plant takes up a lot of nitrate it has to convert it to protein to use for growth. But if the plant takes more nitrate up than it can use for growth it starts to build up, and you measure that in the sap as higher nitrate levels.”

High nitrate in the plant means a lot of vegetative growth, which in turn means a lot of water in the plant which dilutes the level of sugars,” explains Mr Hegger.

“High levels of sugar in the plant could be an indication of good crop health, as we know that means lower nitrate levels and the moment nitrate gets too high the plant is more attractive to pests and diseases.” Another possible reason for low sugars is there is not enough photosynthesis for sugar production and active growth. Typically, growers should be looking for total sugar to be relatively high, especially in younger leaves, although too high levels can be caused when there’s not enough water movement – for example, very hot weather causing plant stomata to close drying the plant out and causing the sugars to concentrate. pH: In plant sap analysis, pH is not usually that informative, Mr Hegger suggests. “It is nothing to do with pH of the soil, usually doesn’t vary much from sample to sample, so in most cases I don’t look at this much.” 

EC: In contrast, EC, which is a measure of the total dissolved salts in the plant, is very important to look at as it gives a measure of whether there is enough nutrition uptake, or the transport of nutrition into the new leaves. “For example, if the new leaves are very low and the old leaves very high that could indicate the weather has been extreme and the plant has stopped evaporating and couldn’t transport enough nutrition to the new leaves,” Mr Hegger explains. “But in most cases the EC shows if there is enough nutrition uptake.” Cations (Potassium, Calcium, Magnesium): In the previous article, the antagonistic behaviour of the cationic nutrients was discussed – if one is too high, uptake of the others can be blocked.

In the example (chart 1, left) that’s clearly the case with potassium in excess, while calcium, magnesium and sodium to a lesser extent deficient, Mr Hegger says.

“It’s possible there is enough calcium and magnesium available in the soil but the plant cannot take it up because of the high potassium, so the grower has to do something about his potassium input. Mostly it comes from manure or compost, and maybe the input is too high.

“Eventually that could influence crop health and quality.”

Potassium deficiencies usually start in old leaves as it is a mobile element, and the young leaves will take potassium from the old leaves. For calcium it is important to look at the result for the young leaves because it is an immobile element and deficiencies will show there first. There are no target levels for the potassium to calcium ratio as even if the ratio is correct both could be too low or too high. It’s less important for grain crops, but in fruit crops, such as tomatoes, it’s critical for fruit quality. As with the other cations, if magnesium is too high it can often block uptake of potassium and calcium. It is quite common the cations are not optimally balanced in sap analysis and treating the excess rather than the deficiency is likely to have better results. However, in the field it can be relatively difficult to manipulate these elements very quickly, Mr Hegger admits.

“Before you plant the crop, when you use base fertiliser or apply compost or manures, you set the base for the entire season. During the season it’s not possible to do big corrections on the soil. You can do a foliar spray to try to make a correction if something is really too low, but don’t expect you’re going to make big changes in the crop.”

In the first year of analysis, often it is trying to learn from the results and understand how longer-term management, particularly of soil nutrients, can help, he says. “It can take more than a year to make a correction in the soil, before you see a reaction in the plant.”

Environmental conditions can also play a part. For example, high temperatures and light intensity can block calcium uptake. “It’s why it is  important to take samples every two to three weeks to see how levels change in the crop. If it is weather that is blocking the calcium and three weeks later the weather is normal, the levels might be back to the target levels.”

Sodium doesn’t have to be in the target values for grain crops, although it is important for some crops. “If it goes above the target values, then it can block the uptake of the other cations.” There are four measurements related to nitrogen: ammonium, nitrate, nitrogen in nitrate and total nitrogen. Ammonium levels can be affected by the time the sample is in transport, says Mr Hegger. “If it is high that’s something to consider, otherwise it is an indicator of stress in the plant.” In barley and wheat, nitrate is almost directly converted to proteins, so if it is too high it is an indication that too much has been supplied, Mr Hegger suggests. “Generally, it is important to get your nitrate levels as low as possible, as long as your total nitrogen levels are high enough.

“In barley and wheat total nitrogen is more important as this gives an indication of crop growth. If it too high, the plant will be more sensitive to pests and diseases.

“As long as total nitrogen is not getting below the target values, you know it is good enough, and fortunately nitrogen is easier to correct.”

N in nitrate is a measure of the health of the plant. A high proportion of N in nitrate compared with total nitrogen is an indicator of lower plant health in some crops, such as tomatoes. “Unfortunately we can’t use that trick in cereals because of the low nitrate levels in the crop, so it doesn’t mean so much.” Chloride, like sodium, is not particularly important as long as it is not too high. If it is higher than the target value it can block uptake of other anions. Sulphur is an important element for many processes in the plant. Like calcium it is relatively immobile so look at the young leaves for signs of deficiency.

Ideally, phosphorus levels shouldn’t be too high, so aim for these to be within the target values. “If it is too high, inputs can be reduced as it will block iron, manganese, or zinc uptake.” Aim for silica to be within the target values, as it helps with uptake of calcium and the health of the plant. “You can try to correct with a foliar spray, but it is not easy depending on crop.” Iron deficiency will show first in younger leaves, but it doesn’t matter too much if the old leaves are too high, while manganese can easily get too low. “For the plant it is like a sweet – it’s the easiest for it to take-up, but also one of the easiest to leach out of the soil.”

Trace elements, which also includes zinc, boron, copper and molybdenum are easier to treat with foliar sprays. No target levels are given for aluminium as ideally, for most crops, you don’t want any showing up, Mr Hegger says.