The final comparison

The two plots were planted 25 days apart, so the source of the irrigation water is not the only difference. For example:

1) the NaCl plot crop grew during cooler weather (ETo 371 vs 467 mm)
2) the NaCl plot required much less irrigation water and also received less rainfall

Here are some of the main observations:

1) Same water quality in terms of Electrical Conductivity (EC) and similar maximum EC (which is not very high).

2) Both plots had lots of nitrate

3) The washing machine water plots had lots more P and higher pH

4) The final yields were similar

Nitrate: kg in the soil

Up to now I have recorded the nitrate concentration in the water in the detectors (in mg/L or ppm). With a few assumptions, I can estimate the amount of nitrate in the soil in kg/ha, which is how we normally think of nutirent levels. Assuming the water content in the soil when the detector collects is 35%, and the collected sample is representative of the soil water as a whole, the mass of nitrate can be calculated.

The values in the graph are calculated over the top 50 cm of soil (I averaged the shallow and deep detectors), and are surprisingly high. It is probably a consequence of the relatively high organic matter levels. The decline in nitrate through the season is hopefully mostly plant, but there would be leaching losses as well.

pH

This is the pH of the water collected from the shallow detectors.

I was surprised to see how low the pH was - less the 5 in the NaCl plots! The plot irrigated with washing powder started a bit higher (5.7) and increased to 6.5. The washing machine water had a pH of 9.5 .

Phosphorus

This is the phosphorus level in the soil water solution collected in the shallower wetting front detectors (i.e. not a Bray or Collwell soil extraction).

See what a huge impact washing maching water has on soil P!!

Salt: The second sweetcorn plot

In both plots the irrigation water had an electrical conductivity of 1.27 dS/m. This equates to about 800 ppm salt. The second sweetcorn plot was irrigated with water made up to 800 ppm with common table salt (NaCl), whereas the first plot had 800 ppm salt from the washing machine water (top graph).

Once again the salt levels never rose very high. This was largely because such a small amount of irrigation was required (just 55 mm), and there was a substantial amount of rain.

Nitrate: The second sweetcorn plot

The wetting front detectors were installed deeper in the second plot - at 30 and 50 cm instead of 20 and 40 cm as in the washing machine plot.

The second corn crop (with NaCl added to the water) also had a lot of nitrate in the soil before any fertiliser was added, with over 700 ppm nitrate recorded by week 4 (above).

This time more nitrate was recorded in the shallow detector (which in this case was at 30 cm depth). In the washing powder plot, most of the nitrate was in the deeper 40 cm detector (top graph).

Such high levels of nitrate meant that no more additions of nutirents were reqired. Once again it was important to manage the irrigation so as not to leach the nitrate out of the root zone.

Water: The second sweetcorn plot

The soil water potential for the second plot stayed in the wet zone for most of the season (above). The early season rain and the cooler autumn weather towards the end of the season meant that there was no water stress.

This is in contrast to the crop grown with washing machine water which experienced soil water tensions up to 60 kPa just prior to the weekly irrigation (top graph).

Irrigation: The second sweetcorn plot

This time we will look at the whole season in one go rather than the week-by-week unfolding story. I will use the usual headings of IRRIGATION; WATER; NITRATE; SALT.

The new sweetcorn plot was irrigated with water made as salty as the washing machine water. But this time I used common table salt (NaCl) to increase the salinity of the water. Being planted later, the second crop experienced quite different growing conditions. The two plots can now be compared.

The graph above shows the rainfall and irrigation ratio (irrigation/ETo) for the second sweetcorn crop (NaCl). This plot was planted 25 days after the plot grown with washing machine waste water described in the previous posts (top graph).

The second plot received a lot of early rain and took a week longer to mature as it grew through late summer/autumn. Total ETo for the season was 371 mm, rainfall 246 mm and irrigation just 55 mm.

A Long Break...

It has been a long break between posts. My excuse is that I was waiting for laboratory results to get a bit more information on what was changing in the soil during the corn crop (particularly changes in Phosphorus and pH). Then I had to travel to South America, USA and Europe to run some workshops and I am now in Africa. So I've been out of the garden for over three months.

Nevertheless there is much to report.

1) If you want to get a full summary of the first sweetcorn crop you can download
"Science and the irrigator: a learning manifesto" from the link below
http://npsi.gov.au/products/npsi310

2) 25 days after sowing the sweetcorn crop, I planted a second plot of corn and irrigated with water of exactly the same salinity as the washing machine waste water. But this time I made the water 'salty' using common table salt instead of washing powder. I will post the whole of season results next.

3) Then I'll show how the soil pH and Phosphorus levels change when using salty water and washing machine water.