Monday, 27 January 2014

Starting to clear and triple dig an allotment (16)

Why dig the allotment?

Digging has long been a method of clearing the ground of weeds and it is good to find that even in the past gardeners have struggled with the same perennial weeds as I have to on the new allotment.  

 Mawe and Abercrombie (1778) suggest "In the course of digging, all root weeds that are perennial should be carefully picked out particularly couch grass (Elymus repens) and bear-bind, the Convolvulus sepius, (what is now called Calystegia sepium) for the least bit of either will grow, increase greatly in summer and prove most troublesome weeds."  "But slight rooting weeds on the surface being turned clean to the bottom, will rot, and never trouble you again."

They also recommend skim digging - taking off weed turfs and putting them at the bottom of trenches.
"In all trenching, whether one, two or more spades deep, always, previous to digging, pare the top of each trench two or three inches deep or more, with all weeds and other litter into the bottom of the open [trench] which not only makes clean digging , and increases the depth of loose soil, but all weeds, as also the seeds of them that may have scattered on the surface are regularly buried at such a depth that the weeds themselves will rot, and their seeds cannot [germinate]."

If the allotment soil was clean of perennial weeds, I would be skim digging.  However, the safest way to remove all perennial weed rhizomes is to sieve trench dig.  

Now I have just read a blog on the myths of gardening and one that says that aeration of soil should not be the primary reason for digging.  

It could be said that adding carbon depletes nitrogen; adding nitrogen depletes carbon; adding air depletes both.  
The biology  is quite clear.  To avoid nutrient depletion the ground  should not be dug unless  large amounts organic material can be added to the soil.  It can be in the form of weeds, manure, compost, shredded brushwood, logs, paper and cardboard.  

I add copious amounts of organic matter to the soil and, if well rotted, mix it by sieving throughout the soil profile.  This will replace the organic matter that increased oxygen allows to be removed through the heightened activity of soil heterotrophs when they take carbon to burn for energy and carbon and nitrogen to build their bodies.  Carbon is removed as the gas carbon dioxide and nitrogen in the form of nitrous oxides or- eventually nitrogen gas.  

Jethro Tull in the 1700s asserted  that soil should be "moved" quite often and that pounded earth was plant food and this may well be based on a misunderstanding of plant nutrition.  However, he based his comments on the observation that when soil was ploughed plants grew well and free of competition. Digging does release nutrients through incorporation of weeds and organic matter throughout the soil profile.  

It has been suggested that organic matter can be incorporated into the soil by mulching the surface of soil and allowing worms and other soil animals to mix it through the top soil.  This mimics the natural way that soil is formed.  Yet there is evidence that the formation of soils naturally takes considerable amounts of time.(Alexandrovskiy 2007) and, while it might be good for the soul, I would rather not wait.

I cannot believe that digging will cause a depletion of bacteria in the soil.  They are too small to be affected greatly.  Digging will cause larger animals and fungi to be disrupted and this must be considered when digging.

So, at the cost of possibly breaking useful fungi hyphae, I will continue triple sieve trench digging to remove weed rhizomes.  However, after this initial clearance of the allotment, I doubt whether I will dig as deep again.

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