CULTIVATION AND TECHNOLOGY Proper ridging important weapon against green discolouration compensate shallow ridge preparation with an inter-row rotary cultivator,’ according to Den Boer. Harrow puddles more than cultivator Two other focal points – again when preparing ridges – are puddling and soil crumbling/breaking clods. These factors can also considerably influence the percentage of green discolouration in potatoes. Den Boer explains how that works. Puddling is compacting the subsoil during soil cultivation when the soil is not dry enough. This happens more often in soils with a bad soil structure, and it is there that a grower may decide to adopt a short-cut soil cultivation process. The top layer of such soils is almost just as dry as that of soils with a proper structure, but the subsoil remains wet for longer. In clay and loam soils, the danger of puddling is greater than in peat and sandy soils. The degree of puddling can also depend on the type of farm machinery used for ridging. It depends on the surface structure of the machine parts which touch the subsoil and the way this is done. Den Boer shows the outcome of a ‘puddling’ calculation, in which the differences between a driven rotary harrow, a Multivator (special field cultivator for sowing and planting) and a field cultivator are shown. For example, an average driven rotary harrow touches 42 percent of the subsoil, a Multivator 19.5 percent and a field cultivator 21.4 percent. The calculation shows that one machine can, in theory, cause twice as much puddling as the other. Practice subsequently teaches that there is a difference in how the parts of the machine touch the soil. The teeth of a driven rotary harrow turn and pry up the wet subsoil, while the teeth of a cultivator beat the soil like claws and subsequently cause a more open structure. Puddling very bad A puddled subsoil is very bad, because it compacts the natural open structure of the soil and with it the transport of water to and from the location, and it also slows down or even blocks root growth. That is, of course, bad for plant growth, but it can also result in more green tubers. Whether or not that happens depends on the situation directly after the final ridge building, Rotary harrow - Multivator and lost of green potatoes, variety Lady Olympia, 2004, location PPO Westmaas lost in kg lost in kg Multivator Rotary harrow 40-50 mm 50-70 mm 0,17 0,25 0,43 0,93 lost in kg Multivator Rotary harrow 40-50 mm 50-70 mm 2 4 2 5 totaal > 40 mm 2.187 3.387 1.600 3.147 lost number of plants totaal > 40 mm 0,7 1,0 0,3 0,7 when the soil has not subsided yet. Imagine that it starts pouring with rain immediately after cultivation, like in many places in Holland last year. If you happened to have been driving through those heavy-soil areas, you could have seen immediately which subsoil was still intact and which soil was not. At places where, after a period of dry weather, the sides of the ridges had started to crack, you could be certain to look at compacted subsoil. The soil starts cracking when the structure is poor and/or puddled, as a result of which the rainwater remains between the ridges for too long. As a result the bottom of the ridge subsides more than the interior ridge containing the plants. This has already subsided unlike the soil beside the plateau which has become separate later due to the rotary cultivator. Small clods allowed, big ones are not In addition to differences in puddling, the various cultivation machines can also cause differences in crumbling. A field cultivator and a Multivator are better equipped at that point than a driven rotary harrow, is Den Boer’s experience. Tests and practical experience in heavy clay and loam soils show that a driven rotary harrow causes more clods than the other two. A small clod is not so bad but, for example, heavy clods in the ridge can cause tubers to grow too much against the sides of the ridge. Original ridge Subsided ridge Expanded ridge Cracks as a result of impermeable subsoil. A large clod causes green discolouration. 24 Potato World 2010 • number 1 Pagina 23
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