An ethnobotanical study of the effect of sand and gravel addition to soils on soil temperature and plant growth

Abstract

Kumara (Ipomoea batatas) being of tropical origin, was introduced to temperate Aotearoa/New Zealand by the Polynesian ancestors of the Maori. The pre-European kumara varieties were successfully cultivated in Aotearoa, at the southern-most limits of kumara's climatic tolerance, for hundreds of years. A cultivation method widely used by Maori, both spatially and temporally, was the addition of sand and gravel to garden soils. Evidence for this practice comes largely from Maori oral history, ethnological literature, soil surveys and archaeology. Sand and/or gravel were used both as surface layers and mixed into topsoils. Such modified soils have been termed 'made soils'. The practice has several important effects on soil properties and plant growth. Factors such as drainage, friability and soil aeration are discussed. Charcoal is often found in large quantities in made soils. Trial one investigated the effect of sand, gravel and charcoal addition on soil temperature. The trial was a single replication of a 24 factorial. The four factors were; sand, gravel, charcoal - with or without, as a surface layer or mixed into soil. Plots were 1 m2 and temperature was measured at 5 cm depth in plot centres using a data logger and temperature sensors. The charcoal surface layer depressed soil temperature rise through the day. This treatment was 3°C below the control treatment at 3 pm. All other treatments increased soil temperature through the day. The maximum temperature increases above that of the control ranged from 1°C for the mixed charcoal treatment to 4°C for the sand/surface and sand/gravel/surface treatments. The latter would add approximately 55.8 day-degrees to the kumara growing season in Kerikeri, 34.1 in Lower Hutt and 24.8 in Christchurch. These gains in day-degrees would significantly increase kumara growth and tuber production. Trial two investigated the effect of two sand:soil ratios, 1:2 and 2:1 on plant growth under two temperature regimes, over a 4 month period. Kamokamo growth far exceeded kumara growth. Kamokamo dry matter accumulation was limited by soil fertility dilution at the high sand rate. The trend for kumara was a depression of growth at high sand rates in the warm glasshouse where growth was significantly higher than the cooler glasshouse. Temperature was the main factor limiting kumara growth. The relative importance of the advantages and disadvantages of sand and gravel addition to soil, with regard to soil fertility maintenance and kumara growth are discussed.