Fertigation and Gravity Fed Drip Irrigation

In the early 60s, in a kibbutz in the southern part of Israel, the members faced a daunting problem. The lands they inhabited needed to be irrigated to produce food, and the existing systems did not work for them. The sprinkler systems that moved on pipes and rolled across the field used too much water with puddles forming in low spots drowning crops. The sprinklers could not go up hills and required land to be flattened.

Members of the kibbutz invented drip irrigation and started a company called Netafim. The drip pipes produced by Netafim could be used on hilly ground and allowed a steady amount of water to be applied to the roots of the plants. The pipe insured that each plant got an adequate supply of water. Since the water was only applied to the plant roots of the crop, weeds were controlled because there was no excess water. The drip irrigation system also conserved water, using much less than the sprinkler systems they replaced.

While drip irrigation was being developed, Israeli scientists were using hydroponics to grow high value crops. As one of the scientists Meier Schwarz said, “We found that we could grow plants without soil, so what could we do with our technology in soil?”

Soil was analyzed, the water supply was analyzed and a nutrient was designed to add whatever the plants would need, not in the soil or water. This nutrient was added to the drip irrigation line and the new science of fertigation was born.

Today fertigation is performing what seemed like a miracle 40 years ago. Where cornfields worldwide average about three tons per hectare, fertigation in corn can produce 90 to 100 tons per hectare. It is a land revolution because fields under fertigation reduces the land required to produce food.

In the past, lands considered suitable for agriculture were areas where plants naturally grow, where rainfall is greater than the amount of water that evaporates from the ground. If there is not enough rainfall to grow crops the necessary water must be supplied by irrigation.

In the Negev desert of Israel where rainfall is only 150 mm a year, orange sands used go be the only landscape. Today huge orchards of oranges grow on these sands producing 50 to 80 tons of oranges per hectare. They are grown with drip irrigation lines supplying the necessary water and minerals for life.

Fertigation can be performed automatically from a pressurized water line, or for smaller more remote areas, set up in simple gravity fed systems using a water barrel and drip pipe.

There are now several other companies producing drip irrigation systems. It can be confusing to make a choice. Netafim is one of the most expensive, and they have advanced the technology so the drip holes are protected by an inline device to maintain the pressure and keep the holes clear of dirt or media. Pipes have held up under 30 years of use in several parts of Israel.

When using the system under fertigation, special nutrients are mixed to protect the pipes from clogging. Haifa chemical, another Israeli company, offers premixed nutrients optimized for four stages of growth. Some other companies are now producing fertigation formulas of their own. But all premixed formulas make assumptions about your soil and water.

These premixed formulas are often not suitable for your plants, water and soil conditions. The mixes are heavy with ammonia and phosphoric acid, adjusted so the pipes are cleaned of deposit. Many hydroponic nutrients will form a calcium deposit in the holes of the pipes.

For a person used to hydroponics, going back to soil can be quite a challenge. Soil can build up acidity or salts, reducing growth and sometimes killing plants. Soil also has nutrients in it, such as calcium, or toxic elements such as aluminum. There are also diseases that happen in soil that we just do not see in hydroponics.

One method to use in this complex soil world is to dig a hole and fill it with a standard hydroponic media such as red volcanic rock or perlite. It can be beneficial to mix the media with larger stones for roots to grab. An outdoor top heavy tree such as pomegranate can easily be blown over in the wind.

Gravity fed systems

A gravity fed system uses an elevated water tank connected to a water line with a valve (to shut water on and off) and an inline water filter. This connects to a main waterline that flows to the ground, and this main water line is connected to the drip lines that go to the crop.

The simplest gravity fed systems are five gallon buckets hooked directly to a 30 meter drip hose. Systems such as this are being used in Ethiopia and other countries of hunger for family gardens. The systems only cost about $15.00 and are distributed by humanitarian groups such as Helping Hands International. These systems usually supply just water, with no added minerals. If no fertilizers are used this will eventually deplete the soil of minerals.

When putting together your drip irrigation system, the water supply tank has to be sized so it will hold enough water for at least one days watering. This system would need to be refilled every day. If one week´s water supply is provided, the tank will only have to be filled once a week.

For a gravity fed system to work, the tank has to be placed higher than the crop. For each meter in height the water pressure increases about 1.3 psi. Netafim suggests a tank be placed at one meter to 1.5 meters above the crop surface.

Many of the commercial drip irrigation systems are designed to work with household water pressure which can be 40 to 50 psi. Most of the small micro sprinklers require about 20 psi to work and so do not work in most gravity fed systems. What works well is a simple drip line with holes placed every 12 inches. There are special drip hoses made for low pressure operation.

It is important to install both a valve and filter in the water line that leaves the tank. The filters used for drip irrigation trap larger particles that can clog the smaller pipes and holes of the drip lines. These filters, kind of looking like a small thermos, are usually a screw together type that can be cleaned by opening and flushing with water. They should be flushed and cleaned often to keep the filter open and free running.

Design example

For a small field of 100 m2 a tank of 330 liters (88 gallons) is placed on a very sturdy stand 1.1 meters (3.5ft) high. A pipe is attached to the bottom of a tank with a valve. Beyond the valve is the inline filter that can be unscrewed from the line and cleaned.

The pipe from the tank is connected to a 28 to 32 mm feeder line which goes to the end of the field. The drip lines are then attached to this feeder line by punching holes with a special tool. The drip lines go to the end of the field. All surfaces covered by the drip lines much be lower than the tank water level.

Several companies now provide fertigation fertilizers. Haifa Chemical supplying a fertilizer line called polyfeed made for fertigation in soil. All fertigation nutrients have to be 100% soluble.

The fertilizers can be added directly to the water in the tank or injected with a small injector added to the main water line. Several of these are on the market including some for very small gardens. In a commercial pressurized system the nutrients are usually added to the water with injectors, usually in two separate units to keep nutrients that would combine in concentration apart until diluted qwith water. This especially is likely to happen if you mix a calcium nitrate with any phosphate of sulfate.

A fertigation formula will differ from a hydroponic nutrient in several respects. Perhaps most important is that precipitates such as calcium and magnesium carbonate do not cause much of a problem in a fast moving hydroponic system, but they can form within the fertigation lines and clog micro holes for the dripping water.

Water quality

The local waters being used to grow plants may have properties that complicate fertigation. The water may have excess salt (NaCl), or hardness with calcium and magnesium ions in solution, or a high or low pH. All of these considerations needs to be addressed when mixing the formula for fertigation.

If the pH of the water is above or below 6.5 you can adjust the water. Use phosphoric acid if too alkaline or potassium hydroxide if too acidic to maintain the solution pH at 6.5. A fertigation formula can be mixed that will be more acid or more alkaline to help balance water pH.

By using an ammonia based nitrogen fertilizer the pH will be reduced and the natural calcium and magnesium in hard water will precipitate out as calcium and magnesium carbonates. So a calcium or magnesium rich high pH water should not be mixed with an ammonia fertilizer.

To avoid the formation of the carbonates a few guidelines are suggested. Many irrigation waters contain enough calcium and magnesium for plant growth (100 ppm for calcium and 50 ppm for magnesium).

Soil should be analyzed

The soil also has properties that influence fertigation. The first is if the soil is clay, loamy or sandy. Clay soil requires smaller hole sizes in the drip tubes to deliver less water per hour (0.61 instead of 0.92).

The pH of a soil is a measure of acidity or alkalinity, with soils ranging from 5.0 to 8.5. A pH of 7.0 is neutral and those below 7.0 are acidic, and above are alkaline. A pH of 6.0 to 7.0 is usually adequate. Below 5.5 and above 7.5, soil treatment or modification often is necessary.

For the low pH soils, dolomite lime can be added to the soil to increase pH, and provide both calcium and magnesium. Be sure to use the finely ground dolomite for the course grade can take years to dissolve. If the soil is too alkaline sulfur can be added but often does not work well.

Fertiliser chemicals to use

Fertigation mixtures use some of the same chemicals used in hydroponics, and most commercial farmers mix their own nutrients according to the water and soil tests.

Nutrients that should be tested for in soil are nitrogen, calcium, phosphorus, potassium and magnesium. When any of these nutrients already exists in the soil the amounts required in the nutrient solution can be reduced. For commercial systems or larger production a soil and water analysis should be made and a customized fertigation formula can be blended for the site conditions.