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Fish farm waste-water treatment plantation

Dave Charley runs a fish farm in Mozambique and got the idea of using the high nutrient run-off to grow a fruit plantation. This is needed because the area doesn’t produce a great variety or volume of fresh produce, and there is a fresh-water lake below the system that needs to be protected from any enriched run-off.

Together we took Dave’s plans for expanding his fish (and cray fish) tanks, and developed a system to grow primarily banana pits along with coconut and date palms (which are hardy to slightly below this latitude).

Each banana pit will have four banana plants and four papayas around the edge.  The mulch pit will be filled with mulch from the nitrogen-fixing hedges we will establish by multiplying out some of the shrubs already on the land, as well from pollarded moringa alleys.  Along the N-fixing hedges will be regular emergent nitrogen fixing trees such as local alibizias and the almost ubiquitously used african Faidherbia albidia (a great acacia often used in agroforestry). If we find the plantation needs more shade, some of these could be interspersed in the banana pit area. They can also be pollarded for mulch. The mulch pit will be planted up with sweet potatoes, and possibly tarot.

Introducing the dragon-fruit banana pit

As a twist on the banana pit, we’re going to experiment with having the bananas and the papayas slightly outside of a cage structure planted up to support dragon fruit – a high value fruit which does well here when it gets a moist and shady spot. We’ll see if we can take the classic banana pit up a notch!

Plastic versus organic mulch

The ‘soil’ is pure sand.  Therefore, we need to increase its nutrient holding capacity.  We’ll plastic-line half the mulch pits (on the left hand half of Trial 1 area) to start with, and leave the other half (on the right hand side of Trial 1 area) relying on thick organic mulches (e.g. coconut husks as well as hedge clippings). When clay can be brought in (there is a supply nearby although the soils are mainly sand), some of the pits can be trialled with clay amendment/lining. It’s our hope that because the pits are getting generously watered with high nutrient fish-waste, and the plantings will be dense, we may be able to stimulate the soil biology enough to transform the sand into something more loamy without needing to line with plastic. If that’s the case, the plastic may in the long run be sub-optimal as it would interrupt the rooting and sub-surface water flows.

Other yields

As well as existing coconut palms there are also mature cashew and marula trees, the latter of which are regenerating in the lower area and can be encouraged as a marula / food-forest / zone 5.

So, it has begun, and the fish waste will soon be fruit.

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