This Winter, because of too much rain and time on our hands, @Simon C and I came up with a theory that we think explains all sorts of things. When we talked about it we found that he had one half of the jigsaw and I the other. Since then I have discussed it with other members of this forum and we are now conviced we might have a theory and also more importantly some solutions. I must also stress that this doesn't just apply to direct drilling. This is it:- Any residue of decaying graminaceous material ( sprayed off grass, volunteers or chopped straw from wheat, barley and oats etc. ) are, in anaerobic conditions ( normally wet ), producing considerable amounts of Phytotoxins. These mainly consist of acetic acid but some of the others are butyric and proprionic acids as well. These Phytotoxins are damaging seedling roots and making the plants more succeptible to pathogenic attack ( Fusarium and Phoma ) and, depending on the amount of material and lack of oxygen, can even totally wipe out a crop. Even if the crop survives we think it is affecting plant health and growth and the acidic conditioins are favouring weeds such as blackgrass allowing them to become competition for the crop. We think these phytotoxins are fairly immobile in the soil; acetic acid concentrations are more than halved 15mm away from the decomposing material. We also think this effect lasts for up to 8 weeks ( depending on temp.) starting from when the material comes in contact with the soil. Decomposing material from anything leguminous, rape and linseed etc. does not have this effect. We think this explains several things, one of which is declining yields, especially in rape. We also think that it explains why autocasting can be so hit and miss. If the chopped straw lies in a dry inert layer then your rape will probably be OK, but if the straw turns into a wet soggy matt then production of these phytotoxins will destroy your crop. We also think that often slugs, leatherjackets, wireworms etc are getting the blame for some of these crop failures. It isn't just small seeds and direct drilling, if you had a lot of grass weeds and you spread beans on the surface and ploughed them in you wouldn't have much of a crop either. We also think that Strip till is benefitting from having a super row cleaner that sweeps aside the straw to help the situation. However, in severe conditions even this isn't enough. So going back to our schoolboy chemistry we think the solution to neutralising these acids is lime, and so far we seem to have had excellent results. Of course you could just wait until all the straw had decomposed, then you wouldn't have the problem anyway. We also think that the reason that this hasn't become so apparent before is due to the effect only happening in wet conditions and therfore sometimes in drier conditions you can get away with it. We also think that for cover crops to become successful in the UK we need to think carefully about what to grow and how to destroy them for best effect. For example it would be OK to have barley in your cover if you are going to graze it hard with sheep, but you certainly don't want it in if you are going to dessicate it just before drilling. It will be interesting to see if the press want to publish our theory or whether they ignore it for fear of upsetting too many advertiserers of machinery and chemical supplies.