are cover crops & land drains mutually exclusive

shakerator

Member
Location
LINCS
If your using cover crops to moderate soil moisture instead of tillage.......

What happens to land drainage. We have all seen how strong osr roots can interfere and block the tiles functions. If we are looking at increasing subsoil root biomass in this way for most of the year surely it will have a negative impact on the flow?

It's such an investment to drain well, and I don't believe anything other than a perennial (cover/cash) crop would rip the equivalent water that flows away (wasted?) under this clay, but it is correctly argued good drainage is a pre requisite for no till.

Discuss!
 

martian

DD Moderator
BASE UK Member
Location
N Herts
I'd rather hoped that drainage would improve when you get a substantial underground biomass developing. As the roots die you'll get drainage channels forming surely?
 

Clive

Staff Member
Arable Farmer
Location
Lichfield
If we can achieve this perfect structure over time though low is disturbance, cover crops and increased OM are man made drains less important than they are in our current artificial structure situation ?

On some soils I think that might be the case
 
SilliamWhale said:
I think whilst tillage and lack of OM begets a need for more drainage I think you sometimes may have to start off with a element of decent drainage or intensive cover cropping.

We're back to the old symptoms/cause scenario again really.

I think this is correct but, to incorporate Clive's point, just how correct it is depends on the soil. For example, take a soil which has a subsoil with a high Mg content and a high clay content. This subsoil layer will have a very low hydraulic conductivity - water will not pass quickly through it - and it is unlikely that there will be cracking in the subsoil in periods of drought due to the Mg. Add on top of this the historic passage of heavy agricultural vehicles which have caused deep compaction and water will pass even more slowly through this layer. Put a mole through it in combination with some tile drains and suddenenly water has an easy passage downwards and away into the ditches.

From a cover crop + no till perspective the thickness of the layer is important. Say, for example, that the layer is 30 cm deep starting at a depth of 40cm. Further suppose that underneath this 30cm Mg clay layer there is free draining chalk with high porosity. In this scenario cover crops could plausibly eliminate the need for subsoil drainage; the reason being that the cover crop roots could penetrate the clay layer to give access to the chalk below. On the other hand, as as second example, take a soil whose subsoil has a 2m thick high Mg clay layer. In the case cover crop roots will not penetrate all the way through. In this scenario it may be that even after a long period of careful no till the need for subsurface drainage will remain to provide some way of allowing water to drain away.

One alternative thought on the second scenario is this: the cover crop roots penetrate to a considerable depth. Whilst they do not penatre through the impermeable layer they certainly increase the depth to which rainwater can easily flow. Therefore, although they do not provide a "way out" for rainwater, they do provide a "way down" which might be sufficient to lower the water table thus preventing waterlogging of cash crop plant roots.

There are advantages of not mole draining. After mole draining the leaching potential of residual pesticides and fertilisers is increased. In the absence of mole draining there is normally an associated rise in surface run-off which offsets the advantages of increased subsurface transport. With deep penetrating roots but no mole draining it might be possible to have the best of both worlds - low subsurface leaching and low surface run-off. Problem is knowing whether deep rooting cover crops will do enough to prevent waterlogging.

The above is all probably too academic. I think in soils that have needed mole draining in the past it would be madness to think that as soon as one turns to DD the need to mole ceases immediately. Waterlogging in heavy soils is the big problem when switching to no till IMO and there should be presumption in favour of mole draining until expereience in that particular system suggests otherwise.
 

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