KNOWLEDGE
CENTRE

Barley [photo] 

Expertise

Using OmniSap® to mitigate risk under drought conditions

The availability of soil moisture and nutrient uptake go hand in hand in crop production. Nutritional imbalance under drought conditions depress plant growth and productivity by affecting nutrient uptake, transport, and distribution. Although the role of certain nutrients for plant resistance to drought is prominent as such, it is still important to monitor all plant nutrients constantly during the season to optimise production. Figure 1 clearly shows the detrimental effect of moisture deficit on nutrient uptake, highlighting the importance of monitoring and ensuring sufficient levels in the plant. Figure 2 shows the distance at which key nutrients are absorbed by plant roots, and highlights the importance of keeping these nutrients at optimum levels within the root zone.

Influence of soil moisture on nutrient uptake

Total nutrient content in 9 month-old Dalbergia sissoo seedlings as affected by varying levels of water stress. Mean of three replications

**Significant at <0,01

W1 - W4 were irrigated with 20, 14, 10 and 8 mm water respectively, and W5 received no further irrigation other than at establishment. Soil water content ranged from 82 mm to 4.2 mm.
Figure 1: Consequence of increased water deficit on nutrient uptake.


Figure 2: Distance from which the root may absorb key nutrients

The OmniSap® offer:

Nutrient Analysis: With a plant sap analysis, nutrient levels are measured in real time to quantify what is currently present in the plant's 'blood stream'. This will expose any deficiencies or imbalances in the plant at a certain point in time and create the opportunity to correct them before any yield and/or quality losses occur. Plant sap analysis is the missing link between soil and leaf analyses as it shows the actual nutrients available for plant growth in real time This will therefore expose any soil imbalances or production practices limiting nutrient uptake. The OmniSap® analysis includes the complete spectrum of plant essential nutrients: ammonium nitrogen (N), nitrate N, total N, phosphorus, potassium (K), calcium (Ca), magnesium (Mg), sodium, sulphur, iron, manganese (Mn), copper, zinc, aluminium (Al), boron and silicon. Fast turnaround times (48 hours at the laboratory) will ensure timeous results for quick correction. 

Ammonium Toxicity: Besides the regular nutrient analysis offered, OmniSap® also focuses on the use of the correct N source. Therefore an Ammonium Toxicity Index warning is included which warns the client of such situations. Ammonium toxicity is extremely detrimental to crop growth and normally occurs when urea or ammonium sources are used without the presence of any nitrate. Ammonium toxicity results in yellow stunted plants, with severely inhibited root growth, resulting in depressed cation uptake (especially Ca, Mg and K) and total metabolic disruption leading to low sugar levels and depressed plant growth and yield. It is also detrimental to the soil as it decreases soil pH levels significantly due to physiological acidification of the oxidation of unused nitrogen from reduced nitrogen sources to nitric acid. This allows elements such as Al and Mn to enter the soil solution and cause irreversible damage to plant roots. This index also advocates the use of sufficient nitrates. Under drought conditions, plants cannot be subjected to even more stress due to ammonium toxicity. Acidification of the soil and the limitation of root growth to make use of moisture and nutrients simply cannot be afforded under limited moisture conditions. Nitrate also has the added benefit that it helps control stomatal closure, thereby increasing the plant's water use efficiency.

Plant Sugar (Sucrose): OmniSap® offers the measurement of plant sugar or sucrose levels as part of the offer. The importance of sucrose in plants is summarised as follows: Sucrose is the primary product of photosynthesis (with fructose and glucose being the intermediate and short-life predecessors of sucrose). Sucrose provides mobile energy for plants during the day, whereas stored sucrose (starch) provides the substrate for plant respiration during the night to balance the plant's carbon balance. Sucrose provides the carbon skeletons for all plant compounds necessary for growth (biomass) and maintenance. Sucrose is transported to all plant parts and serves as the primary transport of sugars within plants. Sucrose, combined with nutrients from the roots, is converted to amino acids, proteins and enzymes via the TCA (Krebbs Cycle) which governs all plant processes. Magnesium (Mg) is the main driver of sugar production and distribution in plants. During drought conditions, free radicals are formed in plants and these break down chlorophyll, which is the main factory of the plant. Building Mg and sugar levels will protect chlorophyll, which will buffer plants against abiotic stress, since high sugar levels act as an osmolite, increasing drought and frost tolerance. Sugars are, of course, the energy base for the plant to combat stress situations such as drought. Monitoring the sugar levels in the plant under stress conditions will reveal much of its ability to withstand such conditions. In many cases timely remedial action may be taken, such as appropriate foliar nutrition, if the plant sap analysis shows deteriorating sugar levels.

Risk modelling: Yet another service under the OmniSap® banner is the yield classification models that use algorithms derived from principal factor and discriminant analysis to identify the plant's ability to reach a certain yield with available plant nutritional resources at any point in time of its growth cycle.

Besides yield potential prediction from principal factor or component analysis (PCA), it also offers very important information that can be used, in for instance drought conditions. During drought or moisture stress conditions, certain nutrients are taken up in greater quantities and some are metabolised more effectively than others. It is therefore possible to identify drought stress within the plant long before any physical symptoms emerge. An example of this is sodium and nitrate levels which increase substantially under drought conditions, as indicated in Figure 3.


Figure 3: OmniSap® analysis report showing elevated sodium levels due to drought stress.

It is not the focus of this article, but it needs to be mentioned that the PCA can also reveal many other problems besides pure nutritional stress such as root inhibition e.g. compaction, soil acidity, salinity and more.

Conclusion

The OmniSap® offer is much more than just a simple analysis of nutrients. It offers more information that can be used for the management of crop production. OmniSap® can be seen as an early warning system to identify plant stress and is an invaluable tool under drought conditions.

By Willem Jonker Specialist Agriculturalist: Strategic Agronomic Services