Backyard Aquaponics
A place to discuss aquaponics

Yes perhaps it would be a more useful comparison, however, where would this comparison take place? Are we only counting water from supplemental irrigation or do we also count rain water? Is this growing taking place in a dry climate or in a humid climate? What is the soil? certain types of soil will loose more water beyond the roots than other types of soil.

Good point TCL

Hi Rupert

I guess the grain was not the most obvious choice of crop for this thread, but there is a lot more information freely out there compared to vegetables which would have been a trip to the library or an article from a paid journal.

(It is still useful if you think grain as tomato fruit, straw as the tomato stems and leaves that are discarded).

The 300L per kg of dry plant material is a ballpark figure for water use efficiency - ie plants require about 300L to gain 1kg of dry mass via photosynthesis (or somewhere between 200 and 1000 as extreme values. The actual figure does vary with the weather).

In the grain example, 2 lots of 300L was required for the stems and straw and another lot of 300L required for the actual grain (okay, I rounded excessively, it should have been 333L to multiply back out to 1000, not 300, but the concept is still the same).

If you harvested the entire crop like hay where you take everything above the ground as useful product then you have 3kg of dry plant material for approx 1000L of water.

If you discard the stuff you don't want then you only have 1kg of useful stuff ie 2/3 of the water went to producing "wasted" leaves and stems.

If we go back to our lettuce example, we still need to convert the fresh head of lettuce back into dry matter as only the dry matter is counted as useful content that was produced by photosynthesis ie the plant is mostly water which obviously contains no energy.

Lettuce is about 95% water and 5% dry material which is the figure of 0.05 ie if you had 1000 kg of lettuce then you only have 50kg of useful energy containing material that was produced from photosynthesis.

Did you have a chance to look at the ICT article - they explain why plants need to use this much water to produce such a small amount of dry material.

Regards

Is it.... as determined by who... and where???



Sorry Gumby.... but the figures say quite clearly.... 1000L/kg of dried weight grain...

It means that it took 1000L to produce 1kg of grain.... even if the 1000L was the TOTAL amount of water used by the entire 3kg of plant, of which 2kg of matter is ultimately discarded...

It still means that it took 1000L of water to produce the 1kg of grain.... not 300L

You can't discount the amount of water the remainder of the plant needed/used to grow... just because you only want the grain.... and divide the 1000L by 1/3rd...

Either that, or the figure represents the actual water required to produce ONLY the 1kg of grain.... not including the water required by the other 2/3rds of the plant...

In which case...the actual water required to grow the plant to the stage where it could be harvested.... to produce the 1kg of grain resulting....

Used a TOTAL amount of 3000L of water/plant.... particularly relevant in terms of "transpiration"... which is where the original arguement began...



That's not what the figure says... it either says that it took 1000L of water to produce 1kg of dried... and doesn't include the water required for the other 2/3rds vegetative growth...

Or it includes ALL the water required by the plant growth necessary to produce an end product of 1kg of grain....

It's not 300L... it's at least 1000L


Not yet...