SOIL PREPARATION

Soil re-compaction is clearly a function of time and, amongst others, also of soil type. In a soil preparation trial initially reported on by Saayman (1982), occurrence of re-compaction was investigated in terms of penetrometer soil resistance, bulk density and root distribution, 15 years after treatments were applied (Van Huyssteen, 1989). The soil type was a Clovelly/Hutton sandy clay loam soil at Stellenbosch. In this succeeding investigation, only three treatments were considered: i) Shallow delve plough; one direction to a depth of ca. 300 mm. ii) Ripper; one direction to a depth of ca. 700 mm and a spacing of 800 mm. iii) Delve plough; one direction to a depth of ca. 700 mm. The beneficial effect of the original deep tillage was still observable after 15 years in terms of bulk density and penetrometer soil resistance, e.g. in the deep-delved soil and under the ripper furrows these values were still lower than between ripper lines and in the shallow-ploughed treatments. This finding showed that this particular soil was stable against natural re- compaction, provided that it had been effectively loosened at the beginning. In contrast, the bulk density on the tractor tracks was as high as 1 710 kg/m 3 , which demonstrated the sensitivity of the same soil to wheel compaction after loosening. A study to determine the long-term effects of deep soil preparation, and the best method to measure the result, was conducted at three sites, namely Nietvoorbij (Stellenbosch), Broodkraal (Piketberg) and Kanonkop (Klapmuts) in the Western Cape of South Africa. These three sites varied with regard to (i) soil depth (ii) period since soil preparation and (iii) implements used (Hoffman et al ., 2016). The Cartref soil at Nietvoorbij was re-compacted after 26 years since soil preparation. This was evident from bulk density and penetrometer measurements that were compared to those taken one year after soil preparation. This soil therefore needed deep preparation again before a new crop was to be planted. In the case study at Broodkraal, bulk density measurements showed the positive effect of soil preparation. However, soil re-compaction took place within one year after deep tillage, albeit to bulk density values < 1 650 kg/m 3 that still allow grapevine root penetration. At Kanonkop, five years after the soil was deep-tilled, bulk density and soil shear strength measurements also clearly indicated the positive effect of two soil preparation methods compared to an undisturbed control. Both soil preparation methods gave similar results and effectively removed soil compaction down to 1.0 m.

146 | RE-COMPACTION