This is the title of a book published in 1927 by two researchers associated with the University of Nebraska; John Weaver and William Bruner. If you have ever wondered what your garden plants are up to underground this is well worth a look. I provide a link below to this work which in turn is a collection of links to the individual chapters which excepting the Introduction chapter each provide the research results for a single crop. The Introduction chapter is worth a read to understand their methodology as well as to appreciate the huge amount of work that was expended. I first became aware to this work years ago via a book by Steve Solomon who reproduced a number of their diagrams in one of his garden topic books. I have since referenced the original work from time to time and never cease to be amazed by it. Some of their results are to me counter-intuitive. Taking beets as an example, when I pull these up to harvest they seem to hardly have any roots at all. The reality is the roots can attain a spread of 4 feet from the beet and a depth of up to 10 feet. This is a spread and depth greater that that of sweet corn!
There are many take away conclusions from their data but one is that there generally is less need to obsess about watering if the surface layer of soil is dry since direct seeded plants very rapidly attain considerable root depth. If the seed managed to germinate and emerge it will be rapidly developing roots downwards to where the moisture is.
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