P. C. Nel

Allelopathic influence of Tagetes and Bidens-infested soils on seedling growth of certain crop species

Two naturally-occurring polyacetylene derivatives are found in Tagetes minuta L. and Bidens bipinnata L. which, when released into the soil, may affect crop growth. They are alpha- terthienyl in roots of T. minuta and phenylheptatriyne in leaves of B. bipinnata. Soil samples were collected from a lot where pure stands of the two weeds were found to be growing. Similar soil was also sampled for control purposes, where the two weeds did not occur. The effect of aqueous extracts, taken from the weed-infested soils, on seed germination of seven horticultural crop species was determined in the laboratory. Nine crop plant species were also grown in pots containing the sampled soil. The species were: carrot, cucumber, lettuce, maize, onion, radish, squash, sunflower and tomato. Seed germination was delayed by aqueous extracts from the weed- infested soils. Where soil was infested with Tagetes during the previous season, dry mass of top growth and plant height of all the test plants were greatly reduced. Soil fro...

Persistence of atrazine as affected by selected soil properties

The relationships between selected soil properties and the bioactivity of atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine) were investigated in field trials. Bioactivity was determined periodically for up to twelve months on a variety of soil types using oats (Avena sativa L.) as the test plant. The initial bioactivity of atrazine which was assessed 35 days after application was best correlated with the organic matter content (%C) and P reversion characteristics of the soils. Clay content and CEC were also important but at lower levels of significance. Six months after herbicide application, % C, soil pH and P reversion were important predictors of atrazine persistence, whilst both CEC and clay content were poor criteria. Persistence was negatively correlated with % C, but positively with both soil pH and P reversion. Since the dissipation of atrazine in most of the soils was virtually complete after twelve months, the prediction of persistence by the soil properties was poor at that stage...

Soil water use and rooting depth of Italian ryegrass (Lolium multiflorum Lam.) in a small plot experiment

Italian ryegrass is planted mainly under irrigation in the summer rainfall region of South Africa. Irrigation practices are often based on the assumption that Italian ryegrass has an extremely shallow root system. A small plot experiment was conducted to ascertain whether this assumption was true. It was shown that roots were, in fact, very shallow when plots were irrigated at weekly intervals, as is the usual practice. When stressed, however, the root system was much deeper and extracted water from depths of more than 1.0 m. Values for plant extractable water, determined in situ, ranged from 78 to 89 mm m−1 for the two experimental seasons. The water extraction pattern during the season did not differ much between the slightly and severely stressed treatments. Thus, compared with the control treatment, the pastures did not have to be stressed much before water was utilized from the deeper layers.

Importance of selected soil properties on the bioactivity of acetochlor and metazachlor

Bioassays with grain sorghum (Sorghum bicolor L. Moench) cultivar NK222 were conducted in a glasshouse. Thirty-four natural soils were used in each of two experiments with alachlor [2-chloro-2′,6′-diethyl-N- (methoxymethyl) acetanilide] and metolachlor [2-chloro-6′-ethyl-N-(2-methoxy-1-methylethyl)acet-otoluidide] respectively. Linear regression analyses were performed to determine the importance of certain soil properties in the prediction of herbicide bioactivity. The order of importance for alachlor bioactivity was as follows: % C > clay % ≥ CEC P reversion ≥ soil pH; and for metolachlor: % C > clay % 3 CEC > soil pH > P reversion. Inclusion of either clay content or P reversion with % C in multiple regression equations significantly increased the predictability of alachlor and metolachlor bioactivity. Organic matter content affected alachlor and metolachlor phytotoxicity, despite the relatively low organic matter levels generally found in South African soils.

Preliminary studies on the inheritance of atrazine tolerance in maize (Zea mays L.)

Thirty-three maize (Zea mays L.) inbreds were screened for tolerance to atrazine (2-chloro-4-ethylamino-6- isopropylamino-1,3,5-triazine) in a glasshouse using a sand culture technique. Tolerance ranged from 95,7–51,0% of the control. The continuous pattern of tolerance exhibited by the inbreds indicated polygenic inheritance. Six inbreds categorized as either highly tolerant or less tolerant were hybridized in all possible combinations. The six inbreds and Fl crosses were evaluated for tolerance to atrazine (6 p.p.m. v/v) under controlled conditions. Leaf mass data strongly suggested additive gene effects. Broad-sense heritability (0,66) indicated the potential for increasing hybrid tolerance to atrazine by breeding. The nature of gene action could not be established when veinal chlorosis was used as a measure of atrazine toxicity. S. Afr. J. Plant Soil 1986, 3: 27–30

Variable tolerance of some crop species to atrazine in various soils

The carry-over of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-s-triazine), which occasionally causes damage to susceptible crops grown in rotation with atrazine-treated maize (Zea mays L.), calls for a refinement of the specified recropping intervals. Bioassays were conducted in a glasshouse to evaluate, with nine soils, the tolerance to atrazine shown by dry beans (Phaseolus vulgaris L. cv. Teebus), grain sorghum (Sorghum bicolor (L.) Moench cv. NK222), oats (Avena sativa L. cv. SWK001), soybeans (Glycine max (L.) Merr. cv. Forrest) and sunflower (Helianthus annuus L. cv. S0222). Significant differences between the tolerance of test species in experiments where dry beans/sunflower and oats/soybean combinations were evaluated are inconsistent with the single recrop interval that is stipulated for all four species. The amount of atrazine that was required to elicit a significant response from a particular test crop varied from soil to soil. The variable availability of atrazine in different soil...

The influence of soil type, soil water content and temperature on atrazine persistence

Montmorillonite clay type soils have been implicated in cases where the excessive persistence of atrazine caused damage to sensitive follow-up crops. A study was conducted to evaluate the influence of temperature and soil water content on the persistence of atrazine in a clay soil (total clay 55%, 80% montmorillonite, 0.7% organic C, CEC = 48 me 100 g−1, pH = 7.3) and a loamy sand soil (total clay 7%, 0.34% organic C, CEC = 0.8 me 100g−1 pH = 5.4). The latter soil represented soils in which atrazine carry-over is not expected to occur. A factorial experiment (2 × 2 × 3 × 3) comprising the two soil types; atrazine rates of 1 and 2 mg kg−1; soil water contents of about 0 (air-dry), the amount at field capacity and 2× field capacity; and temperature regimes (day/night) of 30/16°C, 30/8°C and 16/8°C, was conducted in growth cabinets. Samples of soil incubated in the dark were analysed for atrazine on day 0 (herbicide applied), day 30 and day 60 by means of high performance liquid chromatography (HPLC). By day...

Allelopathic effect of sweetpotato (Ipomoea batatas) cultivars on certain weed and vegetable species

A glasshouse study was conducted to determine the allelopathic potential of sweetpotato (Ipomoea batatas (L.) Lam.) cultivars Bosbok, Brandal and Koedoe. Nine indicator plants, three weeds: yellow nutsedge (Cyperus esculentus L.), purple nutsedge (Cyperus rotundus L.) and common pigweed (Amaranthus hybridus L.), and seven crop species: carrot (Daucus carota L. cv. Kaapse Mark), tomato (Lycopersicon esculentum Mill. cv. Heinz 1370), cucumber (Cucumis sativus L. cv. Special Rust Resistant), radish (Raphanus sativus L. cv. White Icicle), onion (Allium cepa L. cv. Pyramid), lettuce (Lactuca sativa L. cv. Great Lakes).and oats (Avena sativa L. cv. SWK 001) were grown in soil from field plots previously cropped with these sweetpotato cultivars. The three cultivars were grown on separate plots. Soil from adjacent uncropped and weed-free fallow plots served as a control. Yellow nutsedge plants grown from tubers in soil from sweetpotato cv. Brondal field plots accumulated significantly less dry matter than plants ...

Quantitative bioassays for monitoring the dissipation of atrazine in soil

A simple bioassay technique was used to study the residual activity of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] applied at 0.25 kg ai ha−1 at different depths in a sandy clay loam (22% clay; 0.31% organic C) soil. Oats (Avena sativa L. cv. SWK001) was planted on treated and untreated plots and soil samples for glasshouse bioassay were taken from different soil layers (0–100, 100–200, 200–300 and 300–400 mm) −1, 0, 1, 30, 60, 90 and 120 days after atrazine application. The phytotoxic atrazine residues remaining in each soil layer were estimated by means of logarithmic dose-response curves. Bioassays for dose-response curves were conducted in parallel with the assays of soil samples collected in the field. It was estimated that 55% of the amount applied in the field was present in the 200–300-mm soil layer on day 30 after application. At days 60, 90 and 120 the percentages remaining in the 200–300-mm soil layer were 2.8%, 1.7% and 2.7%, respectively. Estimates on day 120 indicated th...

Inheritance of atrazine tolerance in South African maize.

Forty-five maize (Zea mays L.) inbreds were screened for tolerance to atrazine (2-chloro-4-ethylamino-6- isopropylamino-1, 3, 5-triazine) in a glasshouse using a sand culture technique. Ten inbreds displaying a spectrum of tolerance were hybridized in all possible combinations including reciprocals to yield 90 F1 crosses. The ten inbreds and 90 F1 crosses were evaluated for tolerance to atrazine (8 ppm v/v) under controlled conditions. A second diallel composed of eight inbreds and 28 F1 crosses without reciprocals were similarly evaluated. Leaf mass data strongly indicated additive gene effects and a degree of cytoplasmic inheritance was also evident. Significant amounts of heterosis also existed. Broad-sense heritability values (0,70 and 0,42) confirmed the potential for increasing hybrid tolerance to atrazine by breeding.