Week 5: Irrigation

The first irrigation in week 2 was 7 mm, and this water went all the way down to the 40 cm depth detector. In weeks 2 ands 3 I gave 10 mm and 14 mm. These amounts reached the 20 cm detector, but not the 40 cm depth. This is understandable. The growing crop dries the soil, so each subsequent watering does not goes as deep. This week I estimated that around 20 mm of drip irrigation should reach 40 cm. But there was no response from the deep detector. I kept watering – 25 mm, 30 mm, 40 mm… By now water was starting to run off the beds into the furrows. Finally, after 46 mm had been added, the 40 cm wetting front detector collected a sample. The reason? Salt in the washing machine water has made the soil less permeable. Since the water cannot infiltrate as fast it spreads sideways, and therefore does not go as deep. The end result was that my irrigation ratio was just over 1. This means I applied just over the maximum amount of water I expect a mature crop could use in a week.

Week 5: Water

I was asked last week to turn this graph the other way round. Zero on the y-axis is at the top now (very wet) with 100 kPa at the bottom (dry). We were starting to get a bit dry (line falling) over the last week. This was intentional - I did not want to wash nitrate out of the soil, and I did not need to flush salt out. Last week I decided it was time for a big irrigation to fully rewet the soil to at least 40 cm. This gives me a water sample from both the 20 and 40 cm depth detectors.

Week 5: Nitrate

Last week I put on the equivalent of 10 kg of nitrogen fertiliser per hectare. The nitrate in the 20 cm detector shot up from 21 ppm to 264 ppm, but this week dropped down to 68 ppm. I do not know how much of this nitrate went up into the plant (probably most of it) or down into the lower soil layers. It was a big irrigation this week and we collected a water sample in the 40 cm depth detector. Plenty of nitrate there - over 300 ppm.

Week 5: Salt

I have irrigated 5 times now with wastewater at 800 ppm salinity but I am collecting water from the Wetting Front Detectors at 20 cm and 40 cm depths at substantially lower salinity (442 and 672 ppm respectively). Theory teaches us that plants will remove water and leave the salt behind, so the soil salinity levels should be starting at 800 ppm and going up. It has not rained, and I do not think I’m washing the salt deeper into the soil. Here is my guess at what is happening. If domestic water contains dissolved calcium and magnesium (‘hard’ water), these ions react with the detergent and inactivate it. Washing powders contain water softeners that bind with the calcium and magnesium to stop this happening. I think the water softener is reacting with ions in my soil making complex precipitates. Although I plot salt in parts per million on the graph, I actually measure the electrical conductivity and convert this to ppm using a fudge factor for common solutes. Maybe this is my problem?

Week 5: The crop

To the naked eye, everything looks OK with the crop. It’s about 70 cm tall now - grown another 20 cm through the week.

Commentary 1

I’m about to post week 5, and I know what you are thinking. Is it really necessary to make so many measurements just to grow a bed of corn? Well (real) corn is the most grown crop on earth (112 kg of grain for each person in the world). The decisions I’m making in my garden are much the same as you need to make in a hundred or thousand hectare field.

For other reasons behind this blog you may have to revisit the very first post “What is this all about?” And there is another important reason too. When we let a story unfold in real time like this one does, we have to learn on the job. Some of this week’s data does not make sense to me. I thought salt would be building up in the soil, but it’s not. I can’t discard the data because it does not fit my existing knowledge, so we’ll just have to live with it for now.

Week 4: Irrigation

No 'irrigation ratio' graph this week (I'll show weeks 4 and 5 next time). Instead I've been asked how I keep track of the exact volumes of water and the concentrations of salt and nutrients applied to the corn. Sorry it's very low tech - I mix it up in a drum and then gravity feed through the drip lines. The choice of washing powder brand is because i) it's mid range salinity but acceptable for plants ii) alkaline iii) contain phosphate - and I'm measuring salt, pH and phosphorus in the soil (but not showing all the detail here). The week's dose of potassium nitrate is shown in the test tube to the left of the washing powder.

Week 4: Water

Getting a little dry perhaps. My main aim at the moment is not to wash the nitrate out of the soil. The salt levels are OK, so no need for leaching. Next week I'll give a big irrigaiton and collect salt and nitrate readings from the 20 and 40 cm detectors.

Week 4: Nitrate

No compost, manure or fertiliser was applied prior to Week 4. This week I put on the equivalent of 10 kg/ha of Nitrogen fertiliser (as potassium nitrate). That's not a lot of fertiliser, but it shows up very strongly in the Wetting Front Detector sample.

Week 4: Salt

Salt is up a little this week, but still lower than the salt in the irrigating wastewater (which is about 800 ppm). Only the detector at 20 cm depth collected a water sample.

Week 4: The corn

The corn received 14 mm of irrigation last week (and 3 mm of rain)and is about 50 cm tall now. It also got a dose of nitrogen fertiliser, which shows up nicely in the data.

Week 3: Irrigation

This is the first of the "Irrigation" graphs, which will be updated each week. You can see the rainfall in blue. The green bars tell you about my irrigation. "Irrigation ratio" tells you how much water I applied as a proportion of the potential maximum water use (this maximum is calculated from a weather station). You can see I'm applying around 0.2 of the maximum. That's expected - the crop is still a baby. Plotting the irrigation ratio helps to keeps me on track because it takes into account whether the weather is hot and dry or cool and moist.

Week 3: Water

Since the rain stopped I have applied a total of 17 mm of irrigation and the soil at 30 cm depth is still moist

Week 3: Nitrate

The nitrate level has fallen from 220 ppm to 21 ppm. The crop will have taken some up, but I think most of the nitrate was washed downwards during the heavy rain in week 1. That's why I'm only applying a little bit of water and making sure the 20 cm detector is activated but NOT the detector buried at 40 cm. I'm hoping there is still a lot of nitrate between 20 and 40 cm depths.

Week 3: Salt

I have irrigated twice with waste water containing about 800 ppm salt. But the salt level in the 20 cm detector is still quite low. Most of the water in the soil is from the rain.

Week 3: The corn

It's been a really hot week and the corn is growing fast. Only two irrigations from the washing machine so far: 7 mm on the 6 Jan and 10 mm on 10 Jan

Week 2 Water

The water status of the soil is measured by Watermark sensors in units of suction i.e. how hard it is for the plant to suck water out of the soil. It’s easy for the plant to get water out of the wet soil demarcated by the green zone, harder in the orange zone and by the time we hit the red zone the plant will be suffering a bit. I am just measuring one depth (averaging two sensors) because this is about the middle of the root zone (our soil has a lot of clay in it below 40 cm and the roots of annual crops find it difficult to grow much deeper than 60 cm). The soil became very wet after the Christmas rain, and then dried out a little last week.

Week 2 Nitrate

We will track the concentration of nitrate in the soil, which is the most difficult nutrient to manage well. Again the graph is divided into three regions. I want to keep the nitrate levels between about 20 and 100 ppm (the green zone). After week 1 the levels were higher than the target in the 20 cm FullStop detector (orange zone) and much higher in the 40 cm detector (red zone). I did not apply any fertiliser. The source of the nitrate is decomposition of organic matter and this is something I have little control over. Already the heavy rain in week 1 has pushed nitrate deep into the soil below the growing roots. At the 20 cm depth there was a huge drop between weeks 1 and week 2. This nitrate is being leached downwards, not being used by the young crop.

Week 2 Salt

I am measuring salt in ppm (parts per million). The green zone is good, the orange is getting too salty and the red zone will most likely reduce the yield of my corn crop. The rain over Christmas gave me samples in my FullStops detector at both 20 cm and 40 cm depths (week 1). I’m in the green zone at 20 cm depth, but the 40 cm depth is a little salty. This is mostly good salt (fertliser) not the bad stuff in the washing powder. More on this topic next. After the first irrigation with washing machine water yesterday I collected water from the 20 cm FullStop detector only (week 2). Still low salt.

Week 2 The corn bed

The corn was sown on 20 December and 18 days later it is up and growing. I sowed seeds into moist soil and there was a lot of rain over Christmas (more than 50 mm), so no irrigation was needed for the first two weeks. Yesterday (day 17) was the first irrigation with washing machine water – 7 mm of water at 700 ppm (parts per million) of salt.

Installing the electrodes

The FullStop normally has a red or yellow float which pops up when water has been detected. In this case we have removed the float mechanism and installed an electrode. Electrodes were installed in wetting front detectors positioned at 200 and 400 mm depths. The electrodes will record when water passes these depths and record the salinity of the water.

Installing the gear

It all starts with digging holes and installing equipment. I am using an auger to install the FullStop wetting front detector. In the foreground there is a tensiometer (orange tube) and a Watermark sensor, both used for measuring soil water suction.

What this is all about

This blog accompanies my book “Out of the Scientist’s Garden”. The book is about how the world uses water in the business of feeding itself. It’s a serious book on a difficult topic, so I wrote in story form, through the lens of my fruit and vegetable garden. I kept the book free of tables and graphs because this type of information is for the specialist, and I was writing the book for everybody interested in water.

This blog will give a weekly update on growing sweet corn in my garden using waste water from the washing machine. I picked this subject because I am often asked about waste water and how to use it productively. More importantly, many irrigators around the world only have access to slightly salty water, and they use this to grow their crops. As we shall see, salt is a huge problem for irrigation.

Each week I will show a picture of the crop, report on how much water was used, and track the levels of nutrients and salt in the soil. This is also a pilot experiment to test some new scientific equipment we are developing. The equipment automatically measures how deep the washing machine water moves in the soil and the salt content of the water. The results are sent by mobile phone to the web. The weekly graphs will reveal what happens through the season.

The Scientist's Garden