Fertigation for fresh vegetables Part 2 Planning and application BY Dr Terry Mabbett

Fertigation is the application of irrigation water and fertiliser within a single integrated system. Where used fertigation should be an integral part of an overall nutrient management plan for the individual crop or field. The mass (weight) of nutrients delivered to the crop by fertigation when added to other applications of fertilizer should not exceed 100% of the planned total nutrient application rate.

A range of numerical units can be used to describe the mass of nutrients applied to or required by a crop over a growing season with g/plant, g/unit area with g or kg/m² and g or kg/linear m of plant bed as obvious examples.  When area measurements such as square metres (m²) are used the area considered is usually the entire cropped section of the field and not just the area of the plant beds or the crop rows. That said, for permanent plant beds or very widely spaced beds the area of the bed can more reasonably be considered when calculating the quantity of nutrients required. And it is more practical and helpful to consider nutrients applied in irrigation water in the same units as those used for other methods of nutrient application. This makes for a much clearer picture when assessing how fertigation fits into the overall nutrient management plan.

For instance, a tomato producer may know that his/her crop, growing together with a leguminous cover crop, will require in total about 50 kg of nitrogen/unit area over the growing season. He estimates that the established legume cover crop will supply about 25 kg of this amount thus leaving a further 25 kg of nitrogen to be applied via fertigation. That 25 kg/unit area could be applied over a range of time/mass scales including 5 kg/week for 5 weeks or 7 kg/week (equivalent to 1.0 kg/day) over a longer period of 7 weeks.

In some instances, including field and greenhouse vegetable growing systems, nutrient application rates are more commonly based on the concentration of nutrients in the solution. For example, tomato seedlings in a greenhouse may be supplied via fertigation with a solution containing 75 ppm (parts per million) nitrogen, or pac choi (Chinese cabbage) grown in the field may be ‘fertigated’ with 150 ppm nitrogen. This type of terminology is more commonly employed in container production and when fertilizer is provided at every, or almost every, irrigation. However, the nutrient concentration will not furnish information about the mass applied per unit area or per plant, unless the total volume of water applied is also known.

Application scheduling for fertigation

The importance of having nutrients available in the soil when the plant needs them is clearly an advantage and well established. Equally well known and established is that nutrient uptake will generally parallel crop growth which essentially means that a fast growing crop has a higher nutrient uptake and utilisation. It therefore makes sense to have most of the nutrients needed by the crop in the soil by the time the crop begins its period of rapid growth and to apply the rest during the actual period of rapid growth.

If growth slows down or even stops, whether due to environmental conditions or simply because the crop is nearing harvest and plants are beginning to senesce, then nutrient application and provision can be cut down or even curtained as appropriate. Construction of a chart showing each week of crop production from planting through to harvest may help in fertilizer application planning. By indicating the approximate size of the crop at the beginning of each week the chart will allow scheduled fertilizer applications to take into account the timing of crop growth.

Injection timing for fertilizer

Fertilizer should be applied during the final stages of irrigation because this will ensure that most of the applied fertilizer is retained in the root zone. Sufficient time should be allowed after the end of fertigation for ‘pure’ water to flow through the system to flush out and remove any remaining particles that might clog the emitters.

Key information is required to determine when is the most appropriate time to begin injecting fertilizer into the irrigation system with the first being the length of time taken for the water to reach those emitters which are farthest from the point of injection. This should be calculated and noted during the first irrigation application. Secondly, is to determine how long it takes to inject the quantity of fertilizer required by the crop. This can be calculated by timing an actual injection or by calculations based on the volume of solution to be injected and flow rates of the irrigation system and pump. In this context the injection of a natural food colouring agent as a marker may be helpful in monitoring nutrient flow. An electrical conductivity (EC) meter may also be used to monitor solution at the emitters.

The operator may now calculate that moment in time before the end of irrigation to begin injecting fertilizer. This is achieved by adding:

• Time taken for water to move from injection point to furthest emitter
• Time required to inject fertilizer solution into the irrigation system
• Time taken for last portion of fertilizer solution to reach furthest emitter
• Time required to flush the system with ‘pure’ water

Therefore if it takes 30 min for water to travel from the injection point to the furthest emitter, 60 min to inject the solution, 30 min for the last portion of fertilizer to reach the furthest emitter and another 30 min to flush the system then fertigation should begin 2.5 hours before the end of the irrigation event.

Nutrients must be completely flushed out of the irrigation system after injection to keep drip lines clean and to prevent clogging. If clogging becomes a problem, it may help to open the ends of the laterals and flush water through the drip tape and out of the ends of the lines periodically during the season.

Delivery and distribution in the soil Nutrients delivered through drip irrigation via fertigation are distributed in a pattern that conforms to the wetting pattern of the soil. Soil wetting patterns are typically hemispherical or oval in shape. The widest portion of wetted soil will correspond with the position of the emitter with the deepest point of soil wetness directly below the emitter. The distance that water moves horizontally in the soil and the wetting depth are both dependent on soil texture, irrigation rate, and irrigation duration. Irrigation rate and duration should be based on crop water needs