Devon,
United Kingdom 
we have free draining light soil and yet we cannot seem to grow carrots or parsnips the parsnips have no length to them but a good sized top , we put in well rotted horse manure last year , can you advise a solution
2 Feb, 2010
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Are you sure there isn't a 'pan' or hard area a few inches down in your 'well draining soil'. It should be ideal for carrots and parsnips. Their natural habit is to root deeply, apart from stump rooted varieties.
The other possibility is that you are sowing too late. Parsnips should be started very early, in March or April if possible for really large roots.
Which variety of carrots are you sowing? The deep rooted winter ones like Colmar should grow well anywhere and we are successful in heavy clay soil.
Neither parsnips nor carrots should be sown into freshly manured ground as this may lead to forking, so probably the addition of horse manure was a mistake.
Carrots and parsnips normally come last in any rotation and are followed by manuring and things like potatoes or greens.
2 Feb, 2010