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Expertise

Using pivots for fertigation - Products, limitations, concentrations and management

By Kevin Fourie (Agronomist: Northern Cape)

Fertigation is often referred to as the practice of applying fertilizer in a liquid or water soluble form to a crop through the irrigation system. When properly executed, fertigation has been consistently proven, compared with traditional techniques, to increase fertilizer efficiency and crop growth by closely controlling the rate and timing of water and nutrient delivery, (Kafkafi and Tarchitzky, 2011).

The Omnia fertigation optimisation system or concept, (FOS™), delivers all of the above and also monitors the growth of the plant per stage. This system can be applied in highly technical drip-irrigation systems and in long-cycle micro-irrigation systems, as well as in a normal pivot irrigation system. The value of this concept is that it accomplishes nutrient use efficiency (NUE) as well as increased productivity for the farmer (yield/mm of water, or yield/kg of nutrients used, or improved quality aspects). This is accomplished through the use of the program in the management of the crop throughout the season. The crop can therefore be easily manipulated throughout the growing season without any additional products.

Products and limitations

Best management practices for improving fertilizer use efficiency includes applying nutrients according to plant needs, placing the fertilizer correctly to maximise uptake, using correct quantities to optimise growth and using the most appropriate nutrient source. The choice of product type used is restricted and mainly determined by the product pH, solubility and compatibility with other fertilizers, as seen in Table 1 (at the end of this article). Chemical reactions between fertilizers can take place, and some fertilizer- irrigation-water combinations can cause detrimental reactions such as corrosion of the metallic components of the system such as uncoated steel or galvanised pipes, valves, filters and injection units. In some cases precipitation of insoluble particles can also cause blockages of the emitter. Thus, knowledge of the fertilizer characteristics as well as the irrigation water quality is of the utmost importance.

As can be seen from Table 1, a wide range of products can be used for fertigation, from single elements to fully balanced nitrogen, phosphorus, potassium and sulfur combinations.

Table 1: Revised fertilizer compatibility chart (Roddy, 2008)

 UreaAmmonium
sulfate
Calcium
nitrate
Potassium
nitrate
Potassium
chloride
Potassium
sulfate
MAPFe, Zn, Cu,
Mn sulfate
Fe, Zn, Cu,
Mn chelate
Magnesium
sulfate
Magnesium
nitrate
Phosphoric
acid
Boron
Urea                        
Ammonium
sulfate
                     
Calcium
nitrate
                   
Potassium
nitrate
                 
Potassium
chloride
               
Potassium
sulfate
R R              
MAP            
Fe, Zn, Cu,
Mn sulfate
R          
Fe, Zn, Cu,
Mn chelate
R        
Magnesium
sulfate
R      
Magnesium
nitrate
   
Phosphoric
acid
R  
Boron

✔=Compatible, ✖ = Incompatible, R = Reduced compatibility

Balanced nutrition throughout the growing season

Thanks to fertigation, it is possible to create fertilization programmes based on the optimal nutrient solution concentrations for plants and in the proper ratio’s between the ions (Figure 1). Described in Figure 1 is the effect of the nutrient rate on the potential wheat yield if the right concentration and balanced nutrition is applied. The application of fertilizers and water through an irrigation system requires a consideration of plant needs, soil properties and the technological requirements of the fertigation system.

The higher the plant requirements and the technologically advanced system, the higher the quality demands will be. Improving nutrient use efficiency (NUE) and improving water use efficiency (WUE) have been listed among the most critical and daunting research issues currently (Thompson, 2012). NUE is a critically important concept for evaluating crop production systems and can be greatly impacted by fertilizer management as well as soil- and plant water relationships. Therefore, management practices such as fertigation which improves NUE without reducing productivity, or the potential for future productivity, are likely to be the most valuable.

Figure 1: Effects of nutrient supply on wheat yield, with potential deficiency and toxicity effects when the right amount of nutrients are not applied (IFA World Fertilizer Use Manual, 1992)

Fertilizer quality and use

Choosing the best fertilizer product with the best quality is critical in fertigation. The chemical form of each element, i.e. ammonium vs nitrate, or chloride vs sulphate, will affect the efficacy of the product. With drip fertigation, fertilizer has to go through a 120 micron filter system to prevent emitter blockage, but with pivot fertigation coarser products can be used. Thus, product formulations for the two different fertigation systems can differ with the various raw materials being used. The right product will more likely lead to higher yields and/or better quality with the same amount (kg/ha-1) of fertilizer used. Different raw materials and combinations can be used to supply a certain concentration of a specific nutrient, but the same yield results cannot always be expected as seasons differ. As seen in Figure 2, a remarkably higher yield was obtained when a different product, i.e. CUAN (28), with more agronomic benefits, was used and the same quantities (kg/ha-1) of nitrogen was applied additionally on maize in a trail conducted in the Northern Cape under irrigation.

Figure 2: Effect of an additional 46 kg/ha N applied additionally in two different forms of nitrogen on maize yield in the Northern-Cape under irrigation

Concentrations

Overhead irrigation systems such as pivots are easily adapted for applying chemicals and nutrients, but the high volumes of water relative to the added fertilizer makes for a relatively diluted solution.

Knowledge of each different dosing pump’s capacity and flow meter will determine the success of pivot fertigation with all the different pivot designs. The correct litre per ha-1 fertilizer per mm water should be obtained on each and every pivot. Thus, referring to NUE, the optimum concentration (parts per million) of a specific element is crucial in the success of fertigation. Again the solubility of products at different pH levels, irrigation water salinity (ECi) levels and water temperature all influence the concentration of the fertilizer in the water.

One of the greatest factors influencing fertilizer concentration is the risk of corrosion of the irrigation system, but there are a few factors that can contribute towards the accelerated corrosion of pivots. These factors include:

  • Physical pipe friction caused by fluid flow
  • Sand / silt contents of irrigation water
  • Thickness of galvanizing
  • Oxygen load of water
  • The salinity of the water
  • Bacterial colonies
  • Water temperature
  • pH of the water
  • The hardness of the water (high Ca and / or Mg levels)

Looking at all these factors, it is evident that fertigation is more than just applying fertilizer in the irrigation water. Knowledge of the different sources of water are of the utmost importance. Crop damage is something that can happen, but with the correct data it can be avoided. Specific fertilizer threshold values for each crop are needed to achieve optimisation. All these norms can be used the irrigation water salinity threshold norms, to provide a good platform from which to fertigate.

Management

To determine the possible variables according to parameters, the Omnia nutrient management system also takes the following into account:

The nutrient application systems

It is of the utmost importance to keep the incompatibility of specific elements or raw materials in mind when products are combined to ensure that implementation of the system will be easily manageable on the farm. Depending on the different conditions found on every farm, (i.e. locality of the pump house, tanks, offload area, distance between pump house and application areas, irrigation system, pressure in the system) multiple inlets can be used. The cost of implementation of a multi-channel fertigation system will also depend on how sophisticated the farmer wants to be. The most important factor is that the agronomist and farmer must be aware of the different water application rates between the different pivots on a farm.

Liquid or water soluble products

The pH, EC and bicarbonates in the water will not only influence the solubility and efficiency of different products, but also the rate at which the products can be applied. Figure 3 illustrates the effect on water pH of two different water sources treated with the same product at different concentrations. Clearly water source A can only be treated with 0.41 % of the product before the water pH drops below the acceptable norm of pH 5.5, compared to water source B where the water can be treated with nearly double the percentage of the product. This is only due to the two different water sources’ inherent qualities. The pH of the products and the solution in the application system will have an influence on the monitoring and managing of the applications throughout the season. Risk management on the farm, e.g. prevention of product contamination, or the prevention of pivot corrosion, needs active involvement from fertilizer companies.

Figure 3: The effect on water pH of two different water sources (A and B) treated with the same product at different concentrations.

Nutrient quantities vs. growth stage

The challenge will be the application and managing of different volumes of nutrients through the growing season, for different crops, soils and irrigation systems. It is also important to have the right nutrient balance during a “feeding” irrigation cycle. The success of FOS™ does not only lie in its ability to apply the correct volumes, but also in its ability to place the optimal nutrient concentration in parts per million in the water solution as well as in the root zone.

Measurements during the growing season:

The growth of the plant will determine adjustments to the proposed recommendations. The presence of the Omnia agronomist on the farm during the season can be seen as a value that Omnia can add to the farmer’s business, together with soil and leaf analyses, OmniSap® and OmniBio™ analyses, EC and pH measurements, 2:1 dilutions, determining of the application time, NDVI (normalised difference vegetation index), drones, censors, etc.

Conclusions

A multi-channel fertigation system forces the person who is making recommendations to have a clear understanding of the nutrient application systems in place. The person needs to know which nutrient management system is going to be used in the root zone throughout the growing season, and check quality of the fertilizer products. The water quantity and quality will not only have an influence on the reaction time and efficacy of the fertilizer solution, but also the integrity of the application systems, be it drippers, pivots, pipelines, flow meters, dosing pumps etc. Each pivot is designed to deliver a certain amount of millimetres in 24 hours. Farmers usually fertigate at 100% pivot speed, but not all the pivots deliver the correct minimum amount of water required for fertigation. When making a recommendation, the knowledge of how much water (mm) per hectare is applied, is of the utmost importance to insure successful fertigation and to manage risks effectively on the farm.

References
  • Kafkafi, U. & Tarchitzky, J., 2011. Fertigation: A tool for efficient fertilizer and water management. International Fertilizer Industry Association and International Potash Institute, Paris, France.
  • Roddy, E. 2006. Fertigation Fertilizer Sources. www.omafra.gov.on.ca/english/crops/hort/news/vegnews/2006/vg0406a2.htm
  • Thompson, Helen., 2012. Food science deserves a place at the table – US agricultural research chief aims to raise the profile of farming and nutrition science. Nature, July 12.
  • IFA., 1992. World Fertilizer Use Manual. W. Wichman (editor) Cotton. 457-471.