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The history and benefits of plant sap

History

Plant sap analysis or S.A.P. – special analytical procedure’ has a long history and dates back to the 1930s. However, it is only since the late 1970s that plant sap analysis has been used as a quick test for plant nutrient testing. By the early 1980s, standardisation of plant parts for sampling purposes took place and plant sap analysis caught on as an alternative to normal tissue analysis. In the UK, Omex Agrifluids has been using plant sap analysis since 1976 and it quickly expanded to the USA and Canada. In Australia, Agvita has been using this technology for over 20 years as a tool for early detection of nutrient imbalances and deficiencies. Currently, many respectable laboratories and fertilizer companies such as NovaCrop Control in the Netherlands and Hortus and Crop Health Laboratories in Ohia, USA offer plant sap analysis. NovaCrop Control offers the service in more than 15 countries worldwide.

In South Africa (SA), Omnia Fertilizer acquired sap technology from Omex in the late 1990s and registered it as OmniSap®. It was also dubbed the ''blood test for plants.'' At Omnia, the OmniSap® service is offered to all SA business units and also to Zambia, Zimbabwe, Mozambique and Australia by means of outsourcing to CSBP Laboratories in Perth.

Figure 1: Omnia's state of the art sap press facility at Chemtech Laboratories, Sasolburg
Figure 1 Figure 1

Advantages of plant sap analysis with specific reference to OmniSap®

Plant sap analysis gives an overview of a plant's current nutrients available for plant growth and plant nutrient reserves as compared to normal leaf-tissue analysis where only built-in nutrients or nutrient history can be viewed. Therefore, a sap analysis will be the final result of soil interactions and is a valuable tool for crop management through the season.

With OmniSap® the standard test range are: nitrate (NO3), ammonium (NH4), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), zinc (Zn), boron (B), sodium (Na),copper (Cu), manganese (Mn), iron (Fe), aluminium (Al) and silica (Si). Each of these nutrients are expressed as a percentage of the norm and plotted on normalised report graphs in order to easily view nutrient imbalances or soil interactions. Each nutrient is also plotted against the seasonal norm to give an indication of future nutrient requirements of the specific crop (see Figure 2).

Figure 2: The OmniSap® report showing results of nutrient levels and balances on the first page and nutrient uptake curves on the second page of the report. Each nutrient is plotted against uptake curves and a specific point in time, giving insight into the crop's performance and future demand as seen on the third and fourth pages of the report.
Figure 2

At Omnia, we focus on continuous monitoring of crop growth and vitality to reduce the producer's risk and to optimise yield. Figure 3 (page 20) shows a summary of this concept. The season is started with a soil analysis and fertilizer recommendation to get the basics right. OmniSap® is then introduced and used throughout the season at critical stages to monitor nutrient levels and to correct deficiencies. The cycle is then completed with a total tissue analysis to take stock of nutrient levels at the end of the season and to identify problem nutrients that will be addressed in the next season's planter mix. This constant monitoring ensures optimum production and that soils are not mined or over fertilized, thereby ensuring long-term sustainability for farmers.

Figure 3: The concept of continuous crop monitoring to ensure long-term sustainability
Figure 3

An added advantage of OmniSap® is that by applying a little bit of plant physiology and statistics, a whole new world opens up. This include nitrogen metabolism and management, legume nodulation, sugar production (Brix), plant health and vitality in the form of insect and fungal stimulation, root diseases, osmotic potential and irrigation scheduling, soil compaction and soil acidification,  ammonium toxicity, and yield classification to name only a few. Omnia has developed the last two by incorporating discriminant and principal component analysis into models which make early season detection of ammonium toxicity and yield constraints possible. With OmniSap® nutrient levels, ammonium toxicity models and yield prediction models are therefore all condensed into one report (Figures 3, 4 and 5),  thereby reducing the producer's risk. While optimising yield, the application of OmniSap® also leads to the responsible use of resources, thereby looking after the environment.

Figure 4: Yield classification model using discriminant and principal component analysis
Figure 4

Figure 5: Ammonium toxicity model (a) and output (b)
Figure 5

The yield classification models used in OmniSap® is a fairly new concept developed to aid producers to identify yield constraints caused by nutrient supply early in the season and to address them, thereby reducing risk and maximising profit. The current maize and wheat models use discriminant and principal component analysis to group nutrients, predict yield and classify the crop into a low or high yield potential group to obtain average long-term yield data. This enables the producer to make corrections to increase his crop yield from a low yield potential to a high yield potential if it is not already the case.

Another fairly recent introduction into OmniSap® is the use of ammonium toxicity potential models. The aim is to aid producers with nitrogen management, specifically the use of nitrate nitrogen (N) which is the preferred N source for most crops. The model is based on the ratio between ammonium-N and nitrate-N in the sap as well as on cation levels. The model gives an indication of low, medium or high ammonium toxicity situations and thereby warns the producer to implement corrective measures to ensure limited yield loss, as the ammonium toxicity models and yield class models work hand in hand. Ammonium toxicity occurs when the balance between ammonium-N and nitrate-N is elevated, normally due to the use of only urea-based products and cold or wet soil conditions. This depresses cation uptake, resulting in stunted yellow plants due to the shift from shoot to root biomass, and large-scale translocation of sugars towards the roots to counter the high ammonium levels.

Data management and research

With several hundred thousand OmniSap® samples in the database, the data is used to look at long‑term nutrient trends for specific crops and areas. Figure 6 shows, for example, how five years of sap data was used to map sulphur trends in dry-land maize in the Free State. The reports provide valuable information regarding product development for specific areas as well as for nutrient content of foliar products, thereby adding yet another dimension to OmniSap®. Combining these OmniSap® trends with long-term soil analysis trends also serves as a warning for producers regarding soil nutrient mining.

Figure 6: Mapping of OmniSap® sulphur trends for maize based on five-year data
Figure 6

Figure 7: OmniSap®'s new Bellingham & Stanley Bench Refractometer that will be used for Brix measurement
Figure 7

Current and ongoing research in OmniSap® also includes the development of new crop norms (focusing on pastures), expansion on yield and ammonium toxicity models, analysis of low level nutrients and secondary applications of plant sap analysis. Glasshouse detection trials are also constantly run to ensure that our OmniSap® technology stays relevant. The latest project close to completion is the inclusion of Brix in sap analysis. This service should be available from early 2015.

Brix is the percentage (%) of dissolved solids in the sap. A high Brix sap has a reduced water activity. With this low water activity, a high Brix sap has a reduced freezing point and subsequently a greater frost resistance. Each additional Brix unit protects a plant by a further 0.5 °C. At the other extreme, this reduced water activity produces a proportionally greater tendency to retain moisture and increase heat wilt resistance. Higher Brix levels prevent bacterial and fungal infestations and thus increase storage life. Most bacteria, for example, do not grow at water activities below 0.91 (around 9 mole % dissolved solids, corrected for molecular weights) and most moulds cease to grow at water activities below 0.8 (over 20 mole % dissolved solids). Water activity is thus a critical factor in determining shelf life as well as field success. In addition, a high Brix level provides proportionally greater nutritional content of the food and ensures a good old-fashioned, true nature-ripened flavour.

By Willem Jonker: Specialist Agronomist - OmniSap®