Department of Soil Science

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Deficit irrigation and canopy management practices to improve water use efficiency and profitability of wine grapes
(Stellenbosch : Stellenbosch University, 2024-03) Lategan, Eugene Lourens; Hoffman, Josias Eduard; Myburgh, Philipus Albertus; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Grapevines irrigated at low plant available water (PAW) depletion levels required more than double the irrigation volumes compared to those irrigated at high depletion levels. The accelerated sugar accumulation of sprawling grapevine canopies resulted in earlier harvest dates, reducing pre-harvest irrigation requirements. Diﬀerent canopy manipulations did not aﬀect total leaf area (LA) per grapevine within an irrigation strategy, but negatively aﬀected LA as less water was applied. Non-suckered grapevines produced more shoots and more vigorous shoot growth, while non-suckered vertical shoot positioned (VSP) grapevines tended to produce lower cane mass. The LA distribution provides a good indication of canopy orientation, and non-destructive measurements of primary and secondary shoots can estimate winter pruned cane mass. This would allow viticulturists, producers, or irrigation consultants to estimate the maximum cane mass and use the VINET© model to predict grapevine water requirements in real-time throughout the season, as the LA is estimated using cane mass. Grapevines with sprawling canopies had lower mid-day leaf water (ΨL) and stem water (Ψs) potentials compared to those with VSP canopies. Grapevines experiencing severe water constraints ripened more rapidly than those without or with medium water constraints. Low frequency irrigation increased water constraints compared to high frequency irrigation. Diurnal ΨL cycles showed that grapevines with sprawling canopies had lower ΨL after 18:00 and throughout the night, suggesting that their water status could not recover as fast as VSP grapevines. High irrigation frequencies led to higher grapevine row evapotranspiration (ETGR) losses, with losses from sprawling grapevines, particularly those irrigated at ca. 30% plant available water (PAW) depletion, being higher in January and February than those with VSP canopies. Seasonal full surface evapotranspiration was more sensitive to irrigation frequency than to canopy manipulations. Grapevines irrigated at ca. 30% PAW depletion had higher mean full surface crop coeﬃcient (Kc) values compared to other strategies, with those irrigated at ca. 90% PAW depletion having the lowest Kc values. The mean peak Kc was generally obtained in February for grapevines irrigated at frequencies, while the lowest Kc was found during the same period for low frequency irrigation applications. The fraction of soil wetted during irrigation applications under grapevine row (Kc,GR) could be a more realistic coeﬃcient than Kc for producers and consultants to use in irrigation scheduling requirements. Irrigation frequency had a more signiﬁcant impact on yield than canopy manipulation. Higher rainfall in 2013/14 increased vegetative growth and yield, with low frequency irrigations resulting in higher production water use eﬃciency compared to medium and high frequency irrigations. The incidence of grey rot was higher during the wetter season, with grapevines with sprawling canopies experiencing higher yield losses due to sun burn and less frequent irrigation. The highest incidences and yield loss to grey rot were found in grapevines left un-suckered and irrigated at ca. 30 PAW depletion, while irrigation at around 90 PAW depletion resulted in the absence of grey rot. Grapes were harvested near the target total soluble solids level of 24ºB, with severe water constraints enhancing berry maturation. Non-suckered VSP grapevines produced poorer quality at lower levels (30% and 60% depletion levels), with the highest overall wine quality obtained when irrigated at ca. 90% PAW depletion. Less frequent irrigations reduced summer canopy management requirements, but grapevines with more shoots required higher labour inputs at harvest. Pruning labour input requirements were aﬀected by the number of shoots produced per grapevine and the mass per individual shoot. Sprawling canopy grapevines generally required lower labour costs, and pump costs were aﬀected by the frequency of irrigation applications. During low to normal rainfall seasons, grapevines with sprawling canopies irrigated at ca. 60% PAW depletion produced the highest gross margins incomes, followed by box pruned grapevines irrigated at ca. 90% PAW depletion. In high summer rainfall seasons, box pruned grapevines and non-suckered VSP canopies had the highest gross margins. Grapevines with sprawling canopies, particularly those irrigated at ca. 60% PAW depletion, produced the best balance between yield and quality, ensuring the best gross margin incomes. The gross margin water use eﬃciency (WUEGM) increased with an increase in PAW depletion level irrigation, with box pruned grapevines consistently having the highest WUEGM. The study found that grapevines with sprawling canopies experienced lower diurnal and cumulative evaporation losses compared to VSP grapevines, regardless of PAW depletion levels. The higher mean leaf area per grapevine resulted in denser canopies, and treatments irrigated at approximately 30% PAW depletion were always within stage 1 of evaporation. Grapevines irrigated at around 60% PAW depletion occasionally went into stage 2, particularly in sprawling canopies. The water content of soil under grapevines irrigated at around 90% PAW depletion spent most of the season in stage 2. The vegetation coeﬃcient (Kv) of sprawling canopies was lower than VSP grapevines, irrespective of PAW depletion. The VINET© model generally underestimated transpiration rates in wet soil regimes and overestimated them during dry soil regimes. Adjusting the model by addition of Kv and adapted transpiration water predictions can be done using two multilinear regressions after a few grapevine canopy measurements inputs have been considered.
The effects of selected soil physical properties on the soil water balance, soil water redistribution and pyrrolizidine alkaloids loads in two diferent textured soils in the nieuwoudtville rooibos tea production area
(Stellenbosch : Stellenbosch University, 2021-03) Olifant, Kagisho Ignitious; Hoffman, Josias Eduard; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Rooibos (Aspalathus linearis) is an endemic crop of South Africa produced only in the south western parts of the country. This includes the Northern Cape where the production was reported to have declined over past five years. The first aim of this study was to look at the soil water dynamics in relation to the Normalized Difference Vegetative Index (NDVI) on Rooibos plantation around Nieuwoudtville area in the Northern Cape. Pyrrolizidine alkaloids (PA’s) contamination on Rooibos has been reported from several markets around the globe and pose a health risk towards consumers. Hence the second aim of the study was to investigate the PA’s in Rooibos plantation. Field trials were conducted at Rogland, Meulsteenvlei, Oorlogskloof and Klein Blomfontein farms near Nieuwoudtville while a pot trial was conducted at Vaalharts Research Station near Jan Kempdorp in the Northern Cape Province. Soil water content (SWC) was monitored at an hourly basis throughout the growing season (October 2017 to February 2019) using ECH2O sensors in the field. At the end of 2018/19 growing season, soil water balance and NDVI of selected sites around Nieuwoudtville were determined. The total rainfall received at Rogland, Meulsteenvlei, Oorlogskloof and Klein Blomfontein was 373, 495, 413 and 391 mm, respectively. The cumulative evapotranspiration ( surrounded the Rooibos plants and known to contain PA’s. Further investigation was conducted in pots to evaluate PA’s uptake by Rooibos plant. Soil collected from Rogland was treated with three different weeds (Chrysocoma oblingifolia [0.225 ppm total PA’s], Othonna coronopifolia [0.377 ppm total PA’s] and Raphanus rhaphnistrum [46.008 ppm total PA’s]). Weeds were applied at a rate of 1% of soil volume and replicated 6 times. The pots were planted with Rooibos seeds obtained from Oorlogskloof on the 25th of July 2018 and the experiment continued up until the 1st of February 2019. The total weed population density between the study sites was not significantly different. However, the significant difference between the study sites was only found on Arctotheca calendula, Chrysocoma oblongifolia, Cleretum papulosum, Cynodon dactylon, Ehrharta longiflora, Juncus capensis, Senecio arenarius, and Ursinia weeds species. The dominant weeds species found to contain PA’s at the study sites were Arctotheca calendula, Chrysocoma oblongifolia, Othonna coronopifolia and Raphanus rhaphnistrum with an average total PA’s of 5, 75, 2 817 and 15 330 μg.kg-1, respectively. The mean total PA’s concentration in Rooibos plantation at the study sites was 7.2 ppm (Klein Blomfontein). 15.5 ppm (Meulsteenvlei), 16 ppm (Rogland) and 43.2 ppm (Oorlogskloof). The higher total mean PA’s concentration at Oorlogskloof can be due to the high density of Chrysocoma oblongifolia as compared to the other study sites. The Chrysocoma oblongifolia could have released PA’s in soil during decomposition which could have let to the uptake of PA’s by Rooibos plants. Furthermore, this study confirm that Rooibos itself cannot produce PA’s but rather, it absorbs PA’s from the soil. The lateral transfer of PA’s from weeds to Rooibos was also found to be inconclusive.
Evaluating the effect of low flow drip irrigation on selected growth parameters of citrus trees
(Stellenbosch : Stellenbosch University, 2023-03) Pretorius, Johannes Jacob; Hoffman, Josias Eduard; Raath, Pieter Johannes; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: The use of low flow drip irrigation is preferred nowadays in South Africa due to easier irrigation management, increased ability of precision irrigation and claims of increased water-use efficiency. The aim of the study was to investigate the validity of such claims since there is mounting pressure for all industries to find new methods to save water. Furthermore, it is imperative to investigate the effect that any new irrigation method has on both the soil environment and plant productivity. This study investigated the performance of three different drip delivery rate treatments (T1-1.6 L.h⁻¹, T2-0.7 L.h⁻¹ and T3-0.4 L.h⁻¹) with regards to soil water content (SWC) distribution, soil electrical conductivity and pH distribution, water-use efficiency, tree stress, fruit yield and quality. The soil water content distribution showed that the use of pulsed irrigation with a higher emission rate such as T1-1.6 L.h⁻¹ was most successful in preventing the movement of water out of the active rootzone, followed by T3-0.4 L.h⁻¹ and T2-0.7 L.h⁻¹. For T3-0.4 L.h⁻¹ it was concluded that the application rate was very similar to the overall uptake rate of the trees since there was also a reduced movement of water beyond the active root zone and very small changes in the water content of the main wetted core even as irrigation was applied. The higher application rate of T2-0.7 L.h⁻¹, compared to T3-0.4 L.h⁻¹ led to a greater degree of water movement beyond the active root zone. The calculated water-use efficiencies showed very good agreement with the measured SWC distributions since T1- 1.6 L.h⁻¹ had the highest water-use efficiency, followed by T3-0.4 L.h⁻¹ and T2-0.7 L.h⁻¹ respectively. The electrical conductivity showed good agreement with the soil water distributions since there was an increase in salts at the periphery of the wetted volumes; this became more prominent as the drip delivery rate decreased. All treatments showed severe acidification directly beneath the drippers which was ascribed to the form and quantity of nitrogen that was applied to the trees. The predawn leaf and midday stem water potential measurements taken during the main phenological phases of fruit growth during the 2022 production season concluded that no single treatment was constantly subject to more stress compared to the other. The results did however indicate that all treatments displayed symptoms similar to trees that only have a fraction of the root zone exposed to sufficient water content ranges. Considering the results already discussed, it could be concluded that the availability of soil water to the plants was not only influenced by the physical distribution thereof, but it was also less available due to chemical constraints. The fruit yield of the 2021 production season was mainly influenced by differences in the applied irrigation volumes with T1-1.6 L.h⁻¹ having the highest yield, followed by T2-0.7 L.h⁻¹ and T3-0.4 L.h⁻¹ respectively. Alternate bearing also seemed to have played a role in the yield differences observed for the 2021 and 2022 production seasons. The yield of the 2022 production season was mainly affected by tree physiology and T1-1.6 L.h⁻¹ again had the highest yield, followed by T3-0.4 L.h⁻¹ and T2-0.7 L.h⁻¹ respectively. Overall, the fruit quality was always at acceptable export levels for all treatments.
Evaluation of old store-and-release covers on discard dumps and backfilled pits to improve and predict their performance for rehabilitated mines at Mpumalanga Highveld, South Africa
(Stellenbosch : Stellenbosch University, 2021-12) van Schalkwyk, Roeline; Hoffman, Josias Eduard; Van Zyl, Johan Hendrik Combrink ; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Store-and-release covers (SRCs) are an important mitigation method to protect the environment at rehabilitated mines in the Mpumalanga Highveld, South Africa. The long-term performance of SRCs can be influenced by soil cover-, soil hydraulic-, and vegetation properties. Currently, a Technical Guideline for Soil Covers Development is not in place in South Africa. In addition, data sets of well- and poorly constructed covers, and the availability of data on appropriate input parameters for predicting long-term performance of such covers are limited. This need includes data for saturated hydraulic conductivity (Ksat), soil water retention curves (SWRCs), and photosynthetic active leaf- area index (LAI). Moreover, the measurement of Ksat and SWRCs is time-consuming, labour intensive and costly. Consequently, a multidisciplinary study to investigate the impact of soil cover-, soil hydraulic- and vegetation properties on long-term performance of SRCs was initiated. Most importantly, pedotransfer functions (PTFs) to predict Ksat and SWRCs were developed from particle- size distribution, soil organic matter (SOM) and bulk density. Leaf area index values for good and poor vegetation covers were determined for rehabilitated mines in Mpumalanga Highveld. Soil cover properties viz. cover configurations, soil texture, Atterberg limits, bulk density and soil nutrient availability were determined. Saturated hydraulic conductivity were measured using two types of double-ring infiltrometer, a single-ring infiltrometer, and a constant-head permeameter. Soil water retention curves were established using the pressure plate apparatus. The SRCs data-set was split into training and testing sets to validate the SWRC model. After the SRCs data-set was split into moderately- and very dense SRCs data sets, and an additional site was used to validate the moderately dense Ksat model. The data-set of very dense SRCs was also split into training and testing sets to validate the very dense Ksat model. Monthly LAI from September 2018 to August 2019 was destructively measured using a LI-3100C Area Meter. The dual-layered SRCs were constructed with sandier growth medium (top layer) underlaid by a loamy to clayey water retention layer (sub-layer). Monolithic SRCs were constructed of sandy loam or sandy clay loam soil covers. After the SRCs were split into moderately- and very dense soil cover conditions, the moderately dense SRCs performed significantly better and had acceptable bulk densities, good vegetation covers with good root distribution in the growth medium, steep slope in the desaturation function of the growth medium and high water-holding capacity (WHC) in the water retention layers. The Ksat and WHC of the moderate SRCs over 20 years had values similar to that of the soils, but the values of sandier soil cover layers were lower than critical threshold values due to low resistance to compaction. The statistical analysis of best-fit moderately- and very dense Ksat. models yielded an adjusted R2 of 0.749 and 0.999, respectively from sand-, silt- and clay content, SOM and bulk density. The statistical analysis of the best-fit SWRC model of 14 matric potentials had an adjusted R2 = 0.827 from three fractions of sand-, two fractions of silt-, clay content, SOM, and bulk density. The photosynthetic active LAI for good and poor vegetation cover of rehabilitated mines at Mpumalanga Highveld was ~1.2 and 0.8 m2.m-2, respectively. Poorly constructed soil covers result in high bulk density, low to very low Ksat and WHC values and poor vegetation properties and should be avoided at any cost. The critical threshold values for bulk density, Ksat and WHC of soils can be used to evaluate long-term soil cover performance. The developed PTFs can be used to predict soil covers’ hydraulic properties having soil physical properties similar to the old SRCs. These results can be considered as a possible amendment to the Technical Guidelines on Soil Cover Development in South Africa.
Evaluation of the effect of an orange oil based soil Ameliorant on selected soil physical properties
(Stellenbosch : Stellenbosch University, 2013-03) Viljoen, Daniel Willem; Hoffman, Josias Eduard; Uys, Dirk Cornelius; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: A new orange oil based soil ameliorant is available on the market. Apart from the orange oil, the other main constituents of the product are a nonionic surfactant and an anionic surfactant. Surfactants are used in the agricultural sector, amongst others, as a countermeasure for soil with poor infiltrability or with hydrophobic characteristics. Farmers who applied the orange oil based soil ameliorant to the soil observed a positive growth response by the crop. However, the main concern about surfactants is that it can cause the soil to disperse and thereby decrease the infiltration and saturated hydraulic conductivity thereof. The aim of this study was therefore to determine the effect which this product might have on the following selected soil physical properties: bulk density, aggregate stability, soil strength and unsaturated hydraulic conductivity. The product was applied on four farms on soils with different textures: Dublin Farm (22% clay), Wansbek (20% clay), Toitskraal (7% clay) and Two Rivers (3% clay). Field studies were repeated at Dublin Farm and Toitskraal to study the longevity effect of the product. Differences in bulk density were not attributed to the effect of the product, but to spatial variation. The aggregate stability at the 50 mm depth tended to decrease after application of the product at Dublin Farm trial 1, Toitskraal trial 1 and at Wansbek. At Dublin Farm trial 2 and Toitskraal trial 2 the application of the product tended to increase the aggregate stability. For Dublin Farm trial 2 and Wansbek the shear strength at the 50 mm depth tended to increase with increased application rates. The opposite was observed at Toitskraal and Two Rivers. The unsaturated hydraulic conductivity tended to be higher at the 0 mm depth for the treated soils at all of the trials except Toitskraal trial 2. From the aggregate stability results it is clear that the initial effect of the product was detrimental which can be attributed to the anionic surfactant. The long term effect can be attributed to the effect of the nonionic surfactant. The differences in shear strength can be attributed to aggregate stability (for Dublin Farm trial 2) and bulk density (for Two Rivers). There is however no explanation for the results found at Toitskraal and Wansbek. From the linear regression of bulk density against unsaturated hydraulic conductivity for Wansbek and Two Rivers it is clear that the application of the product definitely had an influence on the unsaturated hydraulic conductivity. For both farms, the correlation between bulk density and unsaturated hydraulic conductivity was better for the control than for the treated soils. To conclude with, the application of the product according to the recommended application rate, resulted in a slightly detrimental effect to the soil on the short term, but on the long term it tended to have a slightly positive effect on the soils.
Evaluation of the effects of an orange-oil based soil ameliorant on soil water management
(Stellenbosch : Stellenbosch University, 2013-03) Wright, Nordely; Hoffman, Josias Eduard; Uys, Dirk Cornelius; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Soil amelioration and conditioning is desirable and in many cases essential, due to increasing food demand and the deterioration and exhaustion of soils. A new soil ameliorant, consisting of orange oil as a base and a mixture of surfactants, is on the global agricultural market. Use of this soil ameliorant by farmers has made an impact on crop production and plant growth on many farms. The effects of this soil ameliorant on selected soil properties as well as plant traits were evaluated by a field trial, a pot trial and a Water Characteristic Curve experiment. A field trial was performed in the Firgrove area near Somerset West, Western Cape (South Africa). It entailed the evaluation of the water content and lateral movement of water in a sandy soil after the application of the soil ameliorant. The field was already planted with Capsicum annuum crop at the initiation of the trial. The trial was performed in a drip irrigated field by taking soil water measurements using a Diviner 2000 probe over a nine week period. The trial showed significant increases in water content on the plots treated with the soil ameliorant. These increases are indicative of an increase in the lateral movement of the soil water, as the measurements were taken between two drippers. On average, the ameliorant treated soil had 17% higher water content than that of the control. A Water Characteristic Curve (WCC) experiment was conducted, which entailed establishing the WCC for a sandy soil treated with the soil ameliorant. The Sandbox apparatus, from Eijkelkamp Agrisearch Equipment, was used to perform the experiment and provides suction values of 0.1 to 10.1 KPa. The WCC showed that the ameliorant application increased water retention over all suctions, especially for the 10 l/ha ameliorant application. This substantiated the Field trial where water retention was increase in a sandy soil. A pot trial was performed in a greenhouse to evaluate the effect of the soil ameliorant on selected soil properties and certain plant traits. This experiment consisted of an ameliorant treatment and a control with a combination of four different Plant Available Water Depletion (PAWD) regimes namely, 10% depletion, 50% depletion, 80% depletion and 50%C depletion, where “C” refers to covered. The trial layout, with five single pot replicates per treatment combination, was according to a randomized block design. The surface covering of one of the 50% PAWDs was a plastic sheet which to prevent evaporation from the soil surface. The ameliorant treatment resulted in significant improvements in overall plant growth, total biomass production, especially dry root biomass. Leaf Area Index and plant height were also improved. The Biomass Water Use Efficiency was improved with the ameliorant application, especially for the 50%C PAWD illustrating the beneficial use of a mulch. Bulk density was decreased with application of the ameliorant but this difference was not statistically significant. Aggregate stability for the moist soils (10% and 50%C PAWD) was significantly improved with the ameliorant application. The application of this soil ameliorant made significant improvements in various facets of plant growth and certain soil physical properties. Especially water holding capacity in sandy soils and the overall improvement in plant growth. There is still much opportunity for research in this field and many questions remain, especially those pertaining to the mechanisms involved in the workings of a soil ameliorant containing a mixture of ingredients.
Influence of soil texture, water management and fertilizer N on the biomass production and antimicrobial properties of Mentha longifolia L.
(Stellenbosch : Stellenbosch University, 2013-03) Koetlisi, Andreas; Hoffman, Josias Eduard; Pienaar, Abel J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
ENGLISH ABSTRACT: Soil texture, plant available water and fertilizer N would influence growth, biomass production and antimicrobial properties of locally used medicinal plants.This research was aimed at investigating how various soil textures (loamy sand, sandy loam and loam) with varying amounts of plant available water (PAW) and nitrogen fertilizer rates would influence the biomass production and antimicrobial properties of Mentha longifolia L. In this research, a two-way factorial experiment was used. It was produced by 3 X 3 factors, viz. three different soil textures (loam, sandy loam and loamy sand) and three levels of PAW in the first trial (0 %, 50 % and 90 % depletion of PAW) and three levels of N fertilizer rates in the second trial. The elemental fertilizers KNO3, K2SO4, KH2PO4, KCl Ca (NO3)2.2H2Oz, CaSO4.2H2O and Mg SO4.7H2O were used to prepare a nutrient solution for fertigation to meet 0Kg ha-1, 150Kg ha-1 and 250Kg ha-1 fertilizer N. This was replicated four times. The experiment was conducted in a tunnel. From the first trial the highest biomass production was obtained from 0% depletion of PAW treatments whereas 50% and 90% depletion of PAW matched each other at lower biomass productions. In terms of soil texture a higher biomass production was gained from loamy sand followed by loam and sandy loam. In the second trial similar influences of soil texture were evident and the significant biomass productions were highest, intermediate and low from 250Kg ha-1, 150Kg ha-1 and 0Kg ha-1 of fertilizer N, respectively. Accordingly, Mentha longifolia L revealed a minimal bacterial inhibition activity at 20g 100ml-1 against Staphylococcus aureus (gram positive bacteria) under Minimum Inhibitory Concentration assay–susceptibility test. It was therefore concluded that soil texture does influence biomass production. In a like manner, the PAW had a significant impact on the total biomass production. An increase in N fertilizer increased vegetative biomass production. Plant material obtained from Mentha longifolia L has antimicrobial properties. Medically the plant can be used to combat Staphylococcus aureus – a major and ubiquitous pathogen for humans. The significance of this study is thus that it will benefit and help the medical community and future research as the guide to sustainable production and utilization of Mentha longifolia L.

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