Irrigation scheduling of potatoes using Chameleon sensors with different switch point sensitivities

Abstract

Water availability is one of the main factors that hold back agricultural production. This means new ways must be found to help improve irrigation management through means such as irrigation scheduling. The potato plant is sensitive to water deficiencies. Nitrogen is important for potato growth and is one of the main factors that influence tuber yield and quality. The Chameleon soil water sensor is an easy to use tool that can help farmers schedule their irrigation by indicating “how hard a plant must work” to extract water from the soil. Chameleon sensors are separated into two groups (very sensitive 18 kPa and less sensitive 24 kPa), according to how sensitive they are to changes in soil water status. This study investigated the effect of irrigation scheduling and nitrogen (N) timing on the total fresh tuber yield, tuber size distribution and tuber quality. Field experiments were carried out at the University of Pretoria’s Hatfield Experimental farm on a sandy clay loam soil. The experiment was laid out as a randomized complete block design (RCBD) with three replicates. The study consisted of two trials, one in a rainshelter and another in an open field, carried out simultaneously. The study had six treatment combinations made up of combinations of three irrigation and two nitrogen treatments. Two irrigation treatments were based on scheduling irrigation using the 18 kPa (Irr1) and 24 kPa (Irr2) Chameleon sensors and the third was based on neutron probe readings (Irr3). There were two nitrogen treatments with equal seasonal N amounts, where either all nitrogen was applied at planting (N1) or split into equal portions applied over an eight-week period (N2). Regular measurements of soil water status, canopy cover and nutrient levels were taken. Water status measurements indicated that scheduling irrigation according to Chameleon sensors (Irr1 and Irr2)) led to drier soils than Irr3. This was due to the Chameleon treatments used in this study not being sensitive enough to changes in soil water content. Total fresh tuber yields were significantly lower for Irr1 and Irr2 treatments than for Irr3. Irr1 and Irr2 treatments had significantly fewer large size tubers and significantly more small size tubers than Irr3 treatments. The possible water stress experienced by the potato plants grown under Irr1 and Irr2 treatment conditions led to fewer large tubers and this led to lower total fresh tuber yields. Tuber quality (specific gravity and chip colour) was not significantly affected by the irrigation treatments. Water use efficiency (WUE) was significantly higher for Irr1 and Irr2 than Irr3, which may suggest that Irr3 was over irrigated at times. Nitrogen use efficiency (NUE) was significantly lower for Irr1 and Irr2 than Irr3, possibly because water stress conditions led to the potato plants not being able to take up adequate nitrogen from the soil. There were no significant differences in total fresh tuber yields, tuber size distributions, tuber quality, NUE and WUE between Irr1 and Irr2. This indicates that the differences in the sensitivities of the two Chameleon types may have been too small to induce a profound difference in crop performance on the specific sandy clay loam soil. Total fresh tuber yields, tuber size distributions, tuber quality, NUE and WUE were not significantly affected by the timing of nitrogen applications. The effect of nitrogen timing was possibly reduced by the water stress.

Keywords: Chameleon sensors, irrigation scheduling, nitrogen timing, fresh tuber yield, tuber quality