Operation of a Subsurface Drip Irrigation (SDI) system under National Organic Plan (NOP) Standards

Operation of a Subsurface Drip Irrigation (SDI) system under National Organic Plan (NOP) Standards

Summary

This project focuses on evaluating how a subsurface drip system (SDI) can be successfully operated under National Organic Program (NOP) standards. Subsurface drip – where the drip tape is permanently buried in the ground – is a standard growing method for many commercial vegetable growers. There are many advantages to having the drip tape buried in the ground: water is delivered to the root zone; evaporation is minimized; drip tape does not have to be installed and taken up every year; and the reduced cost for tape as it does not need to be replaced as often. However, subsurface drip tape has the same limitation that surface drip tape has, being susceptible to plugging. In addition there are a number of different problems that subsurface tape has that surface applied tape does not have: gophers chewing thru the tape; emitter plugging caused by dirt ingestion when the tapes deflate; challenges with germinating small cover crop seeds planted on the surface; and roots growing into the emitters.

The traditional way of addressing most of the limitations of subsurface tape is to inject sulfuric acid into the lines. This accomplishes a number of things including: clearing out “scale buildup;” burning out small roots; and killing slime/algae at the emitter zone. Since we are certified organic, Rio Grande Community Farms (RGCF) cannot use sulphuric acid, due to the organic standards. The main challenge of this project is to determine how to operate the SDI system under organic standards, without the use of sulphuric acid and other harsh chemicals.

The system was installed on 16 acres in October, 2009. We fired the system up, and in 2010 and 2011 we have used the system to grow cover crops and cash crops. RGCF has a contract with the City of Albuquerque to manage 50 acres of publicly-owned land in Albuquerque, New Mexico. We have been certified organic for a number of years and, prior to this project, have grown all crops on flood irrigation using water from the Rio Grande river, delivered through a series of canals. This was our first foray into drip irrigation.

Objectives/Performance Targets

Overall Project Objective:

To clearly document the procedures, processes and activities involved in operating a subsurface drip irrigation system under NOP organic certification.

Goal #1: The following information will be quantified and recorded:

– exact sequence of all tillage procedures, including specific SDI implements used
– schedule of all crop rotations, including cover crops, green manure and cash crops
– schedule of all traditional organic methods used: mulching, side dressing, foliar feeding, etc.
– schedule of all irrigation events
– exact brand, quantity, potency and proportion of all inputs injected into system
– exact injection procedures and equipment used, including dilution procedures
– detailed descriptions of all system maintenance procedures, including emitter diagnostics
– records of all water use by zone
– crop yields and outputs
– soil and water test results

Goal #2: Using information gathered in #1 above, produce and publish a Monograph entitled:

“The Operation of a Subsurface Drip Irrigation System for Vegetable Production for Small Farms of 10-100 acres under Organic Certification: Processes, Procedures and Schedules”

Goal #3: Actively disseminate the above Monograph (and other materials) thru a variety of venues: Western SARE, conferences, extension service, USDA, professional publications, etc.

Accomplishments/Milestones

Here is a summary of the “Top Ten” lessons we have learned so far.

1. SDI leads you into no-till technology.

Our drip tape is buried approximately 6” below the grade of the land. When it was injected we mounded up over the tape, that left a “row” that was about 6” above the grade, placing the tape at about 12” below the top of the row. We injected the tape using GPS technology. Our buddy Dean has a 90 hp John Deere and had successfully installed SDI tape in his fields two years prior. We spaced the tape 36” on center. While we knew that we could not subsequently disturb the ground as we had done in the past, we did not fully understand the implications of never disturbing the field more than 6” below grade. We automatically adopted “no-till” out of necessity. The learning curve for a whole new suite of implements and processes has been steep. We are glad we have made this transition, but as Dan our Farmer says: “It is not a learning curve, it is a learning ladder- straight up!”

Gone are the plough, shank and heavy discs. Hello to the no-till drill, air seeder, flail chopper, lister/peeler, rotovator and roller. These latter implements only disturb the soil slightly, allowing us to perform a number of tillage operations while affecting only the top layers of soil. This is the fundamental principle of no-till technology: disturb the soil as little as possible to allow the soil microorganisms to thrive. It has been a wonderful journey of discovery and “re-learning” many of the practices we have been doing for years.

It was also a surprise that being organic does not necessarily require you to be no-till. Many of our traditional organic tillage practices have destroyed the soil as readily as conventional farming methods with chemicals.

Lesson learned: If you install SDI, better be prepared to retool and rethink many of your conventional tillage practices.

2. GPS technology is essential.

While we injected the drip tape using GPS technology (Dean’s tractor is fully GPS automated), we thought we could maintain the field without the technology. We have three tractors at the farm – none of which have the GPS technology installed. The cost for GPS alone is about $30,000, and we just did not have the funds. Also we visited three farms where the rows were maintained by eye, and the farmers said that they were able to manage just fine.

While it is true that a skilled farmer can maintain the integrity of the rows based on visual observations, you become much more restricted in how you can manipulate the rows into various configurations. Yes, we can maintain the rows on 36” centers, but what about combining two rows into one bed covered in plastic? Tilling the tops of three rows to make one flat area with three tapes? Precision drilling of corn seed into a cover crop that has been rolled and which totally covers the outline of the row? All are much more difficult. Once the basic configuration of the rows starts to break down, recreating those rows becomes more of a challenge. If we had GPS technology, recreating the original rows using the lister/peeler would be a piece of cake – just insert the data disc into your GPS reader and press “Go.” Without the technology, we must measure the exact location of the drip lines every couple years and “rebed up” using visual flags as guides.

It is a lot more work in the long run. Part of the issue is one of scale. Normally because of the startup costs, you do not see SDI on small acreage. SDI with GPS technology works great on large fields laid out in perfect symmetry, often in a single growing zone. Our scale is much smaller – somewhere between manual hand harvest scale and small tractor scale.

3. Bed/Zone design is critical.

We put a lot of thought into our design – and it has paid off. We created seven zones, each operable by an independently controlled valve. Each zone is a little over two acres. Within each zone we have “beds.” Each bed is composed of 12 drip tapes, and each tape is on 36” centers. Each bed is therefore 36 feet wide. Each zone has either two or three beds. Knowing that we are still essentially a hand scale operation, we injected 12 tapes, then left out three tapes; then injected 12, left out three, etc. This way we have an approximately 10’ wide roadway between each bed. Beds are approximately 600 feet long. This roadway does not get watered (no tapes), and we are able to till the road at the beginning of the season to create a nice clean road zone.

We have been growing cover crops on all of the zones and slowly building up the number of rows and beds in cash crops. Because we can control the water flow to each zone, we are able to group crops by water needs at the zone level. As long as we keep similar water need crops in the same zone, we are o.k. Our first year we combined crops into one zone: some got over watered and some got under watered. Our goal is to ramp up to the point that we have a complete bed (12 rows) planted in one crop. Last year we combined two rows into one “minibed” and covered this with black plastic. Thus each “minibed” is approximately 6’ wide with two tapes underneath it. This is ideal for growing crops such as melons. This year we are experimenting with growing some rows in cover crops and some adjacent rows in cash crops.

The other area that we thought through is the tape spacing compared to the spacing on our tractor tires. It is critical that your tractors be able to drive down the furrows of the rows. All three of our tractors are on 6’ wide tire spacing, meaning that both front and rear tires fit exactly down the furrows. We had to make some modifications to some of our implements to assure that the tractor tires fit down the rows. Our widest implement – the flail chopper- has tires exactly 12’ apart.

4. You must understand your water source.

We have a well on the property, but the City only has water rights for approximately 12 acre feet of water. This is not enough to grow crops on 16 acres all season, so we knew from the beginning we needed to design the system to accommodate surface water – water from the ditch. We did this by beefing up our sand filtration system and installing a pre-filter with a fine mesh that screens out much of the debris before it enters the filters. We thought that the well water would be cleaner than the surface water. We were wrong. While the amount of suspended solids in the surface water is higher than the well water, the water itself from the ditch is cleaner than the well water. Our well water contains a high amount of sand and has iron, manganese and calcium in the water. Our ditch water has high particulate matter and a very fine silt, but very few impurities. The ditch water cleans up better than the well water. This is fortunate since we use the ditch water for the majority of the season. We use the well water at the beginning and end of the season when the ditch is turned off.

There is one exception to the above. The surface water from the ditch originates from the Rio Grande river. Water in the Rio Grande originates in Colorado from mountain run off but gathers water from a variety of sources on its way down to Albuquerque. We have learned to not use the ditch water after storms. We also learned an important lesson the hard way. In 2011, there were many fires in northern New Mexico in communities whose water runoff ends in the Rio Grande. For one month in August, 2011, the water in the Rio Grande was black as ink. We thought our filtration system could handle it … think again. I have a vivid picture of one of our interns slowly backing away from the filters as the pressure gauges started climbing – 50, 60, 80, 90, 110 psi…

Lesson learned: closely monitor the quality of your water.

5. Germinating small seeds is a challenge.

Cover crops are a critical piece of no-till farming. The goal is to never have bare ground – you are always growing a crop that will enrich the soil, either through adding nutrients (legumes) or adding organic matter (grains and grasses). Many seeds, such as clover, are very small and need to be planted no more than 1”-2” below the surface. On a mounded up bed this can put the drip tape anywhere from 6”-10” below the seed. The SDI system is ideal for transplants where the transplanted plant already has roots 6” or more below the surface. With small cover crop seeds it is necessary to bring the water up to the seed. Once germinated, roots quickly drill down to the water source and excessive watering is not necessary. But for germination we have often had to leave the system running 48 hours to completely soak the surface.

This is also where it would help to have GPS technology. If we were able to till the rows down to the grade level, or even slightly below, we could place the seed within a few inches of the drip tape. Because we must always maintain some level of visual distinction of the bed, however, we can never get the seed that close to the tape.

We have experimented with a couple of alternatives. When we installed the SDI system, we did not eliminate our alfalfa flood valve turnouts that are on a buried pipeline. We have used flood irrigation to germinate seeds, but surprisingly, this did not work as well as letting the drip system run for an extended period. Surface water from the ditch carries a large number of weed seeds, and these compete with the cover crop seeds. We are also considering trying to set up radial sprinklers that rotate in a 200’ circle. Water applied from above in this manner may be more effective in sprouting small seed.

6. “Gopher Dan” is a problem.

In our research prior to installing the system, every farmer we talked to said that gophers were a major problem. We have not found this to be the case. In 2011 we repaired only about a dozen leaks caused by gophers. More work was created by “Gopher Dan,” our Chief Farmer who would often nick the tape performing some minimal till operation. In general, however, leaks in the tape have not been as big a problem as we anticipated.

7. Ionizing water treatment really does work.

ZetaCore has developed a process for treating water through a device that changes the molecular structure of water and the minerals that are dissolved in water. All drip systems suffer from “scale,” which is a generalized term for elements that react with air and soil and form solid compounds. The three most common are calcium carbonate, manganese carbonate and iron oxide. Each of these minerals exist in water in solution and turn into a solid precipitate upon reacting with the air. This scale and rust can quickly plug up drip tape emitters. In above ground drip tape systems, the tape is usually thrown away every season or two before the emitters can become plugged. With SDI, tapes can be in the ground as long as 10 years. It is essential to keep the emitters clear.

The Zetacore system works by having the water flow over a “core” of fused metals that are electrified with a dc current. The electrification of the metals changes the molecular structure of the dissolved elements, thus enabling them to not precipitate out when they hit the air. Thus no scale.

We were initially skeptical of this technology, however, it has been used for many years at golf courses and commercial greenhouses. We installed a unit that can treat up to 600 gallons per minute. We have had zero problems with scale. Recently we performed an “autopsy” of a section of drip tape that has been in use for 2.5 years. There was no evidence of calcium buildup, manganese scale or rust- the three killers of drip tape. No evidence at all. The proof is in the pudding, as they say, and we are sold. This is one of our “secret weapons” in organic operation of SDI.

8. Fertilizer injection is not a magic bullet.

One of the advantages of a drip system is that you can use the system to fertilize. Fertigation is a well-developed technology, and there are many commercial products available. The challenge is using fertigation both organically AND where the drip tape emitters are expected to last several years. Fertilizers injected into drip tape create nutrient rich zones at the emitter, which in turn can create algae, slime and other living organisms. Without treatment, these living organisms can plug up the emitters. In conventional technology, sulphuric acid does the trick to kill the slime. In conventional SDI systems, it is common practice to continuously inject a low level of sulphuric acid. As with the creation of scale, the one- to two-year lifespan of the tape, for organic growers using drip tape above ground, is not long enough to plug up the emitters with slime. What to do for an organic SDI system? After 2.5 years, we still have not injected fertilizers into our system. For 2012 we will be doing some injection on one zone to trial this aspect of SDI.

Instead of injection, we are using the traditional organic methods of providing nutrients to the plants: pellet chicken manure, side dressing with guano, foliar feeding through spraying and soil enrichment through cover cropping. It is still unclear if these methods are adequate and cost effective for fertilization. We would like to try and find a liquid injectible nutrient that will not create slime.

One thing that has worked well for slime and which is an alternative to sulphuric acid is hydrogen peroxide. Not an acid, it is an oxidizer that reacts with any organic matter and produces only water and oxygen as by-products, both allowed under organic standards.

9. Flush, baby, flush!

Our irrigation specialist who helped us install the system recommends that we flush the drip tapes each time we use them. This has worked well. Every incident of irrigation includes at least five minutes of the system open at the ends, allowing the water in the tape to flush out any impurities that may have accumulated in the tape. We know that this is effective, because at the beginning of every flush cycle we see a stream of brown water coming out the flush lines. We will continue this practice indefinitely.

10. High tech is better.

At least some high tech is helpful. In particular, we installed a very expensive ($4,000) flow rate meter that is based on electromagnetic fields – no moving parts. The device measures the flow rate of the water in gallons per minute and the total gallons used. We installed it in the main line to measure total water flowing out of the filters. While we installed this device primarily to have an accurate count of how much water we are using, it quickly became apparent that it is a valuable diagnostic tool as well. The way our system works is that the pump is designed to create a constant pressure in the line (psi), regardless of the flow rate. For example let’s say that each zone flows at 100 gallons per minute (gpm) at a given pressure (16 psi). When one zone is open, we see 100 gpm flowing, two zones, 200 gpm, etc. If we see that the flow rate goes up – for example one zone is flowing at 120 gpm – there is only one conclusion: we have a leak. If the flow rate goes down, say, to 90 gpm – we have the tape plugged somewhere. The system is so finely tuned that we can tell exactly what type of leak it is: a 25 gpm leak is a gopher all the way through the tape; a 6 gpm leak is a pinhole in one tape, etc.

• 2011 Photos

Impacts and Contributions/Outcomes

Our conclusion at this point is that it is possible to operate an SDI system under organic standards, though the challenges and limitations are different than what we expected going into the project. For the past two years, we have established a “brain trust” to help us evaluate all aspects of this project. This group is made up of farmers, experts from the USDA and other agriculture professionals (expert in composting, water treatment and soil amendments). The chairman of the group, Lee Orear, is a retired hydrology engineer who worked at Sandia National Laboratories for many years, specializing in water flow analysis. This group addresses not only the actual functioning of the system but also crop choice, crop rotations, soil quality, tillage practices, water utilization, fertilization and a multitude of other factors that contribute to the overall success of the project.

In 2012, we plan to gather more data on water usage by crop type, yield and overall cost of running the system. One of the most important reasons to use SDI is to conserve water, and we intend to gather the data to verify how much water can be saved.

We are also refining how we interplant cover crops within the cash crop zones. This is an important feature of organic farming.

We will also in 2012 begin to inject fertilizers into the system and evaluate this.

We believe that as water becomes a more valuable and scarce commodity, drip systems will become more and more important. Already we are seeing many farmers in southern New Mexico who grow alfalfa on flood irrigation switch over to SDI. As water becomes more scarce, we need to have proven models. We are also demonstrating how surface water from the ditch can be used in drip systems.

Collaborators: