Agrochemophysica - Volume 13, Issue 4, January 1981

Lime requirements of 90 top- and subsoil samples of the Hutton, Avalon and Clovelly forms were determined using the following buffer methods: the Woodruff (1948) method; the 1961 SMP method (Shoemaker, McLean & Pratt, 1961); two modifications of the 1961 SMP method viz., the Highveld Region SMP method (HVR SMP method�Haumann & Volschenk, 1979) and the 1966 SMP method (McLean, Dumford & Coronel, 1966); the Adams & Evans (1962) method and the 1978 SMP method (McLean, Eckert, Reddy & Trierweiler, 1978) which is a newly developed double buffer method. The lime requirements obtained by these methods were compared with the limerequirement indicated by CaC03 incubation. All the methods were highly significantly correlated with the incubation method. The 1966 SMP method gave the highest correlation (r= 0 ,9219 ) closely followed by the HVR SMP method (r =0,9189) and the Woodruff method gave the lowest correlation (r=0 ,7690). Regression equations and graphs revealed that the 1978 SMP method gave the most accurate direct approximation of incubation lime requirement of all the methods. The conclusion, however, was that the 1966 and HVR SMP methods being twice as rapid as the 1978 SMP method, would be preferable for routine laboratory analysis after adjustment with the appropriate regression equation.

The confined penetrometer resistance of samples treated with polyvinylalcohol solutions was determined. The results were interpreted in terms o f the amount o f polyvinylalcohol adsorbed per gram clay content o f the soil samples. Incorporation of polyvinylalcohol in samples resulted in a considerable decrease in crust strength using the modulus of rupture test.

A medium-long term irrigation project with Malus communis cv. Granny Smith was conductedat the Welgevallen Experimental Farm of the University of Stellenbosch. The treatments, which were replicated four times in a randomised block design, comprised the application of irrigation when prechosen total available soil water (TAW ) levels were attained. Four TAW levels, viz. 85%, 65%, 45% and 25%, were implemented. The soils o f the experimental area consisted mainly of the Dundee and Westleigh forms and varied texturally from loamy sands to sandy loams in the 0-300 nun soil layer. Plant available soil water ranged from 42 mm to 45 mm per 300 nun o f soil, and bulk densities from 1 530 to 1 590 kg m-3. For the growing season lasting from October to May the mean rainfall was 353 nun and pan evaporation 1 241 mm. Mean daily maximum and minimum air temperatures were 24,4 and 11,7 �C, respectively. The effects o f the various soil water regimes on soil temperature, measured at three depths, were investigated during the course o f three growing seasons. The results showed that, at a shallow depth, soil temperature (Ts) was primarily a function of ambient air temperature (Ta). The former not only followed the normal diurnal course o f Ta but also the seasonal trends, with the former course superimposed upon the seasonal trend. The introduction of the concept of accumulated soil temperatures to detect treatment effects proved very successful: an 85% TAW level gave the lowest heat sums and, consequently, induced the lowest soil thermal regime. On the other hand, under the influence o f a 25% TAW level, the highest soil thermal regime was established because o f the highest heat sums. Soil thermal regimes for the 65 % and 45 % TAW levels were in between, with the 65 % TAW level the lower o f the two. Soil thermal diffusivity, K, was increased with an increase in TAW level from 25% to 85%, when comparing these TAW levels at the same soil depth. The higher the K value was at a specific depth, the better was the heat conduction of that soil, resulting in an effective spatial distribution of heat and, consequently, an enhanced cooling off, with time, of the soil. This explains why, at a particular depth, a soil at a 85 % T AW level was cooler than one, say at a 25 % TAW level.