Ecological sustainability of potato-based rotation in the Western Free State of South Africa

Abstract

The escalation of agricultural input expenditures has decreased profit margins associated with agricultural yields. Increased attention ought to be directed towards the adoption of sustainable agricultural practices and crop production systems. Previous studies showed large variation in Resource Use Efficiencies between South African farms, especially WUE and NUE. Decades of misuse, over fertilisation and over irrigation, have resulted in a cascade of challenges and environmental degradation. Potatoes are often prone to over irrigation and fertilization, especially on sandy soils. It remains crucial to find innovative methods to improve fertiliser and irrigation management practices to ensure that the correct amount of fertiliser and irrigation water is applied at the correct time as the potato plant requires it. This will ensure the effective utilisation of fertiliser and water by the plant and will reduce the loss of nutrients to the environment as well as contribute to ecological sustainability of potato-based rotations in the Western Free State of South Africa. The loss of water and nutrients from the soil in potato-based rotation systems on sandy soils has persistently posed difficulties to crop production. This especially applies to shallow-rooted crops such as potatoes when produced under irrigation on sandy soils from which the nutrients are easily leached. Efficient water and nutrient management are essential for improved yield, profit margin, and sustainability of potato production. The overall objective of this study was to conduct detailed field studies to monitor nutrient and water dynamics of potato fields, and carry-over effects of nutrients and water to the subsequent rotation crop. This research project aimed to determine the water and nutrient use efficiencies of potatoes produced on sandy soils, and how efficiently farmers in the Western Free State region manage their input resources. This was investigated by measuring the water and nutrient balance components, as well as following the same process for the follow-up crop. This study was conducted in the Western Free State and seven centre-pivot fields were monitored. Seven irrigated fields producing potatoes on sandy soils were selected and monitored during the 2021/2022 summer season. Detailed measurements were carried out, including the measurement of water and nutrient input rates, soil moisture and temperature, drainage, leaching, crop growth analysis, and yield determination. Flow meters and pressure transducers were used to measure irrigation. Rainfall was monitored over the growing season by installing automatic rain gauges. Drainage lysimeters were installed to measure drainage and leaching throughout the season. Nutrient status was determined by collecting water and soil samples. Capacitance probes were installed to monitor soil temperature and moisture over the season. Tuber yield was determined at the end of the crop season. Potato plants and tubers were sampled every three weeks to determine biomass accumulation and final tuber yields were determined at the end of the growing season. Growth analysis was determined three-weekly by analysing the wet and dry biomass of the potato leaves, stems and tubers. Incoming and intercepted solar radiation, leaf nitrogen content and canopy cover were measured, and thermal images taken, which were compared with satellite imagery and indices. The collected data was used to estimate nutrient balances and water use efficiency. Results showed that the inputs and losses varied substantially between fields. Total rainfall received during the potato season for the seven fields ranged from 292 – 429 mm, with an average of 345 mm. While gross irrigation ranged from 179 – 439 mm. Total evapotranspiration ranged from 225 – 466 mm, with an average of 406 mm. Total drainage ranged from 1 – 89 mm, with an average of 79 mm. Final tuber yield ranged from 63 - 110 t ha-1, with an average of 88 t ha-1, while potential tuber yield ranged from 81 – 109 t ha-1, with an average of 100 t ha-1. Potato water use efficiency (WUE) varied from 117 – 184 kg ha-1mm-1, while irrigation water use efficiency (IWUE) varied from 207 - 481 kg ha-1 mm-1. Partial Productivity Factor (PFP) for nitrogen, phosphorus and potassium ranged from 196 - 346 kg tuber kg-1 N, 485 – 712 kg tuber kg-1 P, and 223 – 348 kg tuber kg-1 K. Nutrient uptake efficiency for nitrogen, phosphorous, and potassium ranged from 44 – 79 % N, 30 – 47 % P, and 93 – 174 % K. This study revealed that the subsequent crop, such as oats in this instance, exerts a positive influence by effectively utilizing residual nutrients left in the soil after the potato season, preventing their leaching into the underlying soil layers. Additionally, it was observed that during the fallow period, characterized by the absence of crop cover and living roots in the soil, substantial amounts of nutrients leach from the soil profile to the underlying soil layers. Limited associations were observed between PAI (Photosynthetic Activity Index) and SPAD (Soil Plant Development Analysis), PAI and leaf nitrogen concentration, and SPAD and leaf nitrogen concentrations across various positions within each field, as well as between field averages and the cumulative measurements for all fields. Despite these poor correlations, this study highlights the potential applications of PAI, SPAD, and leaf nitrogen concentrations in enhancing nitrogen management in irrigated potato production. Differences between fields are due to different water and nutrient management strategies being followed. This included the use of irrigation scheduling programs, in-field monitoring equipment, and improved fertiliser management programs, and difference in environmental conditions.

Key words: water use efficiency, nutrient use efficiency, drainage, leaching, follow up crop, nitrogen