Monday, November 8, 2010

The Couve story: Day 15

Bare rooted seedlings start off slowly, and by day 15 the crop was still small. We irrigated every day for about 30 minutes, to keep the soil immediately around the seedlings wet. The block was divided in half, with a ‘wet’ side and a ‘dry’ side. The idea was to make sure the wet side always had enough water, and then to push the dry side as far as we could. However, over first two weeks both sides got just about the same – the equivalent of 1.7 mm per day on the wet side and 1.6 mm per day on the dry side.

For a few weeks we are going to follow the graphs below. On the left axis we see the soil 'wetness' (suction in Kpa) at depths of 20 and 40 cm as logged by the watermark sensors. On the scale below we consider a reading less than 10 kPa to be wet; 10-20 kPa to be 'OK'; 20-40 kPa to be getting dry; and greater than 60 to be dry.
The pink and red diamonds show us when wetting front detectors captured samples at 20 and 40 cm depth. In this case we plot the data as the nitrate concetration of the water collected (on the right hand axis).

Tiny plants plus daily irrigation meant the soil stayed very moist on the ‘wet side’, but the irrigation events were not sufficiently long to activate the 30 cm detector. So on day 13 we did a longer irrigation (1.3 L per emitter) just so we could get a water sample at 30 cm. The nitrate level was 440 mg/L - a surprisingly high value (the pink diamond).

Chicken manure had been applied to the previous crop, but just a few watering can loads of ‘compost tea’ to the young couve seedlings. The irrigation and nutrient strategy was obvious from here. No more fertiliser or manure, and irrigation needed to be short and frequent so as not to leach the nutrients below the shallow root zone.

Although the ‘dry side’ of the block received almost as much water as the 'wet side', the 40 cm watermark sensor showed the soil as slightly drier (the blue line - around 10 kPa). But the really big difference was the nitrate reading on day 13: 866 mg/L! This sandy soil had loads of nutrients (the pink diamond) despite applying nothing to this crop.

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