Fertigation – technique, delivery and stock solutions

Many of our members have visited Barry's impressive garden. It is placed on a standard quarter acre block of Perth sand in Nedlands, yet it is a flourishing and immensely productive patch of fruit-bearing greenery. How does he do it?

There are two main techniques that he uses to keep his trees in tip-top condition: one is a comprehensive, almost industrial scale composting system. The other is a carefully-worked-out fertigation process. Here is the skinny on the fertigation, and the composting system plan is promised for future delivery.

The technique

In the broadest sense, fertigation is a technique where various substances are mixed in with irrigation water and then distributed to crop growing areas through the reticulation system.  It's easier to implement where all additives can be dissolved, but slurries are also used, eg in organic horticulture, provided particulate size is limited and emission jets are of sufficient diameter.  However, such applications require constant agitation to avoid unequal delivery or formation of sediments that could block the irrigation lines.  The present discussion will focus solely on use with soluble chemical fertilizers.  Fertigation is mainly used in high value crops such as fruit and vegetables where the costs of installation, use and maintenance make economic sense.

It has several advantages over the more conventional approaches of periodic granular or liquid application of chemical fertilizers, or reliance on organic matter to supply nutrients. These include:

Disadvantages are:

Fertigation is essentially somewhere between the organic and hand/machine spreading of granular chemical fertilizer approaches, and cherry picks some key features of each. With the former there's little control over timing of soluble nutrient release for uptake by plants at critical growing and fruiting periods throughout the seasons, whereas in fertigation it can be as precise as needed; both these approaches minimise leaching of nutrients below the root zone. The chemical approach gives better control of nutrient uptake than organic but is not as good as fertigation because the dissolution process can complicate uptake. Solid chemical fertilizer application can result in substantial leaching losses and is more harsh on plants and soil, whereas with fertigation this is minimised.

Nutrient delivery

To deliver fertilizer solution into the irrigation lines, a number of approaches have been used.

If a large tank is not available or feasible, then a system has to be selected that will deliver fertilizer solutions to the main irrigation line. Because this line operates under pressure from the pump (bore, pond or river water source), an injection system drawing from a nutrient reservoir is needed that can counter or use mainline pressure such that input occurs. A one-way valve should always be installed in the mainline, upstream from the injection system to prevent nutrient backflow when the system is not running as this could result in aquifer contamination.

Two other common approaches use the flow of water in the mainline to provide the means of injection. With both it is preferable to have a side line loop installed near the bore head or water supply source; fertilizer is then delivered into the system through this sideline. Within this loop, there should be a tap in the mainline and another in the sideline upstream from the injector, so that relative flow between the two parallel lines can be adjusted. This helps control how much fertilizer is injected. A one-way valve should be installed in the side line down-stream from the injector to prevent backflow or recycling of mainline water back towards the injector.

Stock solutions and calculations

These have been formulated for an injector system where sufficiently concentrated solutions are required to ensure adequate fertilizer will be delivered after dilution throughout the irrigation system. They're suitable for the Perth coastal sand plain where bore water and/or soil may be alkaline due to the presence of limestone. Other areas may not have to consider alkalinity. Fertigation solutions should be complete, with all macro-, meso- and micro-nutrients present. In concentrated solutions, the solubilities of individual ingredients and the formation of various complexes between the different ions may cause precipitation, and preventing this is a considerable challenge. My approach has been to avoid putting them all together with use of three stock solutions that are each rotated, one week at a time. You could avoid the do-it-yourself complications by simply purchasing a complete liquid fertilizer (ie with trace elements) of sufficient strength for fertigation. But you'll pay heavily for the convenience compared to buying the base ingredients separately in bulk quantities.

The three stock solutions are:

The quantities below are to make up 3L of stock for each.

For the MACRO/MESO stock solution, starting ingredients are:

Giving totals of:

(Mirco trace element mix – 1st 4 elements supplied as chelates)
300g containing

Giving totals of

P = 90ml H3PO4 (85%, density 1.68) 31.6% P
Total P = 90 X 1.68 X 85% X 31.6% = 40g

The MACRO/MESO stock contains the nutrients N, P, K, Mg and S in the approximate ratios of 12:1:12:1:2. It contains both NH4 – N and NO3 - N as most plants prefer a mix of the two rather than one or the other. Demand for K is high in fruit trees, so considerable KNO3 is incorporated, with the added benefit of NO3 – N to complement the supply of NH4+ – N from urea and (NH4)2SO4. Our coastal sands are particularly deficient in Mg so this meso-nutrient has to be specifically addressed. A similar situation exists in the sands of Florida where base fertilizers (those not including trace elements) are specifically formulated with Mg, so mixes will quote ratios of the four elements, eg 15:2:12:1 for NPKMg. The MICRO formulation really requires most salts to be in the form of chelates to avoid precipitation in concentrated stock solutions, and also to avoid precipitation in the soil, particularly if pH is alkaline. Sourcing the chelates individually can be expensive and the prepared mix is a suitable compromise. The problem with pre-made mixes is loss of control of element ratios, but this one is reasonable. The PHOSPHOROUS stock is the easiest way to include P without precipitation from other elements in concentrated solutions, and it contributes slight acidity.

N is lower here than in many commercial soluble fertilizer products (eg Yates's Thrive) but when distributed through the irrigation it's delivered more frequently than conventional periodic hand or solution application. The other meso-nutrient Ca doesn't really have to be provided as it's everywhere in our coastal sands, to the extent that its alkalizing properties usually need addressing. In addition, dolomite (Mg and Ca) can be sprinkled around once or twice a year when new batches of compost are applied throughout the garden. This also addresses the low Mg levels when compost relies substantially on local grass clippings.

The buffer reserve in calcareous alkaline sands that many of us experience near the coast is so large that it's not possible to permanently reduce pH, short of removing it all and replacing with better loams - an impossibly costly and disruptive exercise. The best we can hope for is more temporary adjustment throughout the growing seasons by adding powdered S which thiobacteria slowly convert into acidic sulphate. Fertigation is not a preferred technique for adjusting soil pH, primarily because of the corrosive nature of acids resulting in more rapid breakdown of pump seals, diaphragms etc. Scheme water in my area has a pH of 7.6, within the range of 7.3-7.8 quoted by the Water Corporation. My bore water at the sprinkler head is in the same range, ie slightly alkaline, so the HNO3 and H3PO4 acids included in the stocks ensure that at least the reticulated water is not more alkaline when delivering all the nutrients. HNO3 is by far the stronger acid of the two, but there's a limit to how much should sensibly be included. With the present addition the stock pH is 1.7 – as low as I'm prepared to risk it.

Each of the three stocks is used a week at a time, rotating as MICRO, MACRO/MESO, PHOSPHOROUS and MACRO/MESO – ie a 4-weekly cycle.

How much do these solutions contribute to my yard of approx 500m2 per month? In the hot months of the year, I run my bore delivering 80L/min for 50min sessions, 4 times/wk = 4kL water/session. This is equivalent to roughly 8mm precipitation. In peak summer days, pan evaporation in Perth can be 12mm or more. About 2L of concentrate is used per week, so with my 4-week rotation system this means approx 463g N, 27g P, 489g K and 40g Mg is delivered/mth. When moving out of the maximum growing period into the cooler months, I decrease delivered nutrient amounts by reducing run times, dropping back to 3 sessions/wk and changing the relative water flow through the main- and side-lines. Of course with winter watering restrictions the system is turned off, but for the evergreens from cooler climates that continue to grow slowly and still need some fertilizer, I rely on my heavy composting and retained nutrient build-up.

How do these fertigation rates compare with what has been reported as desirable for fruit tree yield elsewhere?

N is the single most important nutrient variable for yield (provided other nutrients are not limiting). The average N fertilizer rate for most tropical and sub-tropical fruit trees is about 100kg/Ha/year. Some nutrient-hungry crops like passionfruit might need 2-3 times this amount. The above fertigation rate of N when run for 8 months of the year represents about 76kg N/Ha/yr. So although less than the quoted 100kg, good composting can make up much of the difference. Remember also that fertigation will need less fertilizer than that applied in granular form for equivalent fruit yield. If you decide to fertigate, you should go through similar calculations to ensure that what you're delivering is meaningful, taking account of your soil fertility, delivery system and watering schedules.

Fertigation is a 'slow and steady' approach, so if you want results next week you'll be disappointed.  There are many ways you could speed up the action but if taken too far they inevitably have some disadvantages.  For example, with tank delivery systems it's relatively easy to add larger quantities of nutrients without precipitation, given the large water volumes.  But if the sprinkler system also wets foliage then the possibility of leaf burn with higher nutrient concentrations needs to be kept in mind.   Plus the problem of jet blockages increases as concentrations increase.  Precipitation and solubility limits figure more strongly with concentrated nutrient reservoir delivery systems.  You could also increase the session run times or their frequency.  With the former the possibility of nutrients being delivered below root zones should be recognised, especially with dripper systems and where soil water retention may be low.   Increasing frequency has to comply with prevailing government water restrictions.  Probably the surest way to speed and enhance growth if desired is to let fertigation carry the base load of fertilizing year round with supplementation by organic matter that has good nutrient retention characteristics, and only then adding granular forms sparingly if you're sure they're needed or you're impatient for results.  Fruit trees operate on yearly time scales, not weekly, so what's best for the long haul is the wiser strategy!

Spacer. Photo of Fertigation Pump.

Hydraulic-driven fertigation pump set-up. The nutrient reservoir is on the left with a delivery line to the pump which is in the lower right hand corner. Water from the bore runs horizontally from left to right in the mainline across the top and the smaller sideline looping off it is below. The taps allow control of relative flow between the two lines, and the brass check valve preventing nutrient backflow can be seen upstream from the pump.

Barry Madsen