High Soil Strength Research Articles

Eastern Gamagrass Root Penetration in Adverse Subsoil Conditions

Eastern gamagrass [Tripsacum dactyloides (L.) L] (EG) is reported to exhibit acid tolerance and root penetration through claypans. To study its root growth in these conditions, a greenhouse column study was conducted under simulated soil stress conditions, using EG and sordan [Sorghum × drummondii (Steud.) Millsp. & Chase] as a comparison species. Treatments consisted of incomplete factorial combinations of plant species (EG and sordan), soil water potential (−10 and –300 kPa), lime (pH 3.5 and 4.8), and soil bulk density (Db) (1.3 and 1.7 g cm−3). The treatments were arranged as a randomized complete block design (RCBD). For some of the response variables that were analyzed at different depths, depth was considered as a subplot factor. Treatments were applied to Al‐toxic Tatum Bt horizon material (clayey, fine, mixed, semiactive, thermic Typic Hapludults) used in the middle 30‐cm section of 15 by 60‐cm polyvinyl Cl (PVC) columns. Soil strength was determined at harvest by cone penetrometer resistance. Results indicated that EG tolerated acid, Al‐toxic conditions, while sordan roots were sensitive to the low pH conditions. Eastern gamagrass roots penetrated high soil strength layers that inhibited sordan root growth. The characteristics of acid, Al tolerance, and penetration of high soil strength make EG valuable in establishing grassed buffers, vegetative conservation barriers, and pastures in extremely acid or dense soils.

Eastern Gamagrass Root Penetration in Adverse Subsoil Conditions

Eastern gamagrass [Tripsacum dactyloides (L.) L] (EG) is reported to exhibit acid tolerance and root penetration through claypans. To study its root growth in these conditions, a greenhouse column study was conducted under simulated soil stress conditions, using EG and sordan [Sorghum × drummondii (Steud.) Millsp. & Chase] as a comparison species. Treatments consisted of incomplete factorial combinations of plant species (EG and sordan), soil water potential (−10 and –300 kPa), lime (pH 3.5 and 4.8), and soil bulk density (Db) (1.3 and 1.7 g cm−3). The treatments were arranged as a randomized complete block design (RCBD). For some of the response variables that were analyzed at different depths, depth was considered as a subplot factor. Treatments were applied to Al-toxic Tatum Bt horizon material (clayey, fine, mixed, semiactive, thermic Typic Hapludults) used in the middle 30-cm section of 15 by 60-cm polyvinyl Cl (PVC) columns. Soil strength was determined at harvest by cone penetrometer resistance. Results indicated that EG tolerated acid, Al-toxic conditions, while sordan roots were sensitive to the low pH conditions. Eastern gamagrass roots penetrated high soil strength layers that inhibited sordan root growth. The characteristics of acid, Al tolerance, and penetration of high soil strength make EG valuable in establishing grassed buffers, vegetative conservation barriers, and pastures in extremely acid or dense soils.

Soil mechanical behaviour of sandy loams in a temperate climate: case studies on long‐term effects of fertilization and crop rotation

Abstract. Two case studies on Danish sandy loams investigated the long‐term effects of fertilization and crop rotation. Case Study 1 compared a diversely cropped organically farmed soil (DFG(1)) with a conventionally farmed soil predominantly growing annual crops (DFA), both receiving animal manure. In Case Study 2, a diversely cropped organically managed soil (DFG(2)) receiving animal manure was compared with an almost exclusively cereal cropped conventionally farmed soil receiving no animal manure (CCC). A multi‐level experimental strategy was followed to compare integrating field methods with specialized laboratory methods. Ease of tillage was described in the field and characterized using a drop shatter field test and by measuring aggregate tensile strength. Fitness as a seedbed was characterised visually in the field and from the drop shatter test. Impedance to seedling emergence and root penetration was evaluated by measuring bulk soil shear strength. For Case Study 1, soil tilth was better for the DFA soil than for the DFG(1) soil (i.e. lower soil strength and higher ease of fragmentation and friability index for DFA). However, a crumbly structure and a moderate bulk density suggested that the DFG(1) soil was also a favourable medium for plant growth. For Case Study 2, the CCC soil had a lower organic matter content, higher bulk density, higher soil strength, and lower ease of fragmentation in comparison with the DFG(2) soil.

Non-inversion tillage effects on soil mechanical properties of a humid sandy loam

Optimisation of soil tilth is of paramount importance in organic plant production in order to enhance crop growth. Non-inversion and reduced tillage systems are often claimed to be preferable for organic farming. The purpose of this study was to evaluate the early stage effects of converting a mouldboard-ploughed soil to a non-inversion tillage system. A multi-level experimental strategy including in situ, on-site and laboratory methods was followed in order to relate quantitative measures of soil physical properties directly to soil behaviour in the field. A non-inversion deep soil loosening (0–35 cm) tillage system (NINV) was compared to a conventional mouldboard ploughing and harrowing tillage system (CONV). The experimental site was located on an organically managed sandy loam soil. The tillage treatments were applied to plots in two fields (B 3 and B 4) at the experimental site. Limited numbers of measurements were performed in the B 3 field during the 1997–1999 growing seasons. A more comprehensive programme was carried out in the B 4 field in May and September 1998. A root-restricting plough pan was detected in the CONV treated soil. The NINV treatment effectively loosened the plough pan resulting in a visibly improved soil structure and a decrease in soil strength. The penetration resistance in the plough pan was reduced from about 1800 kPa in CONV to less than 1000 kPa in NINV when measured at field capacity. The loosening of the plough pan was still evident after 2 years without tillage operations in a perennial grass/clover crop. The topsoil of the CONV treatment had a more desirable tilth than that of the NINV treatment, which had higher soil strength at the 7–14 cm depth. In accordance with this, the CONV treated topsoil fragmented more readily than the NINV soil in the field. The laboratory measurements on soil from the September sampling showed that the NINV treatment had lower friability index (i.e., friability index of 0.16 and 0.22 for NINV and CONV, respectively) and higher tensile strength of air-dry aggregates. The differences in topsoil tilth were not eliminated by natural soil meliorating processes during the growing season. This paper discusses the early stage effects of converting to non-inversion tillage. A number of years of continued treatment may be required before beneficial effects of non-inversion tillage are manifested in improved topsoil tilth.

Spatial distribution of Eucalyptus roots in a deep sandy soil in the Congo: relationships with the ability of the stand to take up water and nutrients.

Spatial statistical analyses were performed to describe root distribution and changes in soil strength in a mature clonal plantation of Eucalyptus spp. in the Congo. The objective was to analyze spatial variability in root distribution. Relationships between root distribution, soil strength and the water and nutrient uptake by the stand were also investigated. We studied three, 2.35-m-wide, vertical soil profiles perpendicular to the planting row and at various distances from a representative tree. The soil profiles were divided into 25-cm2 grid cells and the number of roots in each of three diameter classes counted in each grid cell. Two profiles were 2-m deep and the third profile was 5-m deep. There was both vertical and horizontal anisotropy in the distribution of fine roots in the three profiles, with root density decreasing sharply with depth and increasing with distance from the stump. Roots were present in areas with high soil strength values (> 6,000 kPa). There was a close relationship between soil water content and soil strength in this sandy soil. Soil strength increased during the dry season mainly because of water uptake by fine roots. There were large areas with low root density, even in the topsoil. Below a depth of 3 m, fine roots were spatially concentrated and most of the soil volume was not explored by roots. This suggests the presence of drainage channels, resulting from the severe hydrophobicity of the upper soil.

Soil strength and soil pore characteristics for direct drilled and ploughed soils

Direct drilling has often been reported to increase density and strength and to affect pore continuity and tortuosity of the upper soil layers. In this study these aspects were studied for three texturally differing soils 4–6 years after initiation of continuous trials with direct drilling and mouldboard ploughing. The soils studied were a coarse sandy soil (Korntved, 5% clay), a sandy loam (Ballum, 8% clay) and a silty loam (Højer, 19% clay). The crop rotation at Korntved was spring barley and winter rye while at Ballum and Højer it was spring barley and winter wheat. Both crops were grown every year. All fields had been mouldboard ploughed for decades prior to the trial period. The ploughed treatment (PL) was imposed in the autumn and the seedbed preparation and drilling were performed with an S-tined seedbed harrow and a traditional drill. The direct drilled (DD) treatment received no tillage other than the drilling which was performed by a triple-disc drill. Straw and stubble were burned. In the 4th, 5th and 6th years of the trial period, minimally disturbed soil cores were taken from the 4 to 8, 14 to 18 and 24 to 28 cm depths, i.e. two horizons above the ploughing depth of 20 cm, and one horizon below this depth. Longer cores were sampled in the 18–27 cm depth in order to include this transition layer. Furthermore, in the 4th year of the trial period shear strength was measured in the field at 2-week intervals in the spring with a vane shear tester in the two upper layers mentioned. All samplings and measurements took place in the field grown with spring barley. In the laboratory air diffusivity and air permeability were measured at field-sampled water content and again when the soil cores were drained to a matric potential of −100 hPa. Cone penetration resistance was measured with a 2 mm diameter penetrometer. Separate core samples from the 14 to 18 cm depth of the Korntved and Højer soils were used for estimating soil cohesion and soil internal friction by a shear annulus method at field-sampled water content. For all soils, DD increased soil bulk density in the two upper soil layers. The shear vane tester also generally estimated higher shear strength for the DD compared to the PL treatment. The shear annulus measurements in the laboratory revealed no differences between tillage treatments for the Korntved soil, while a tendency of higher cohesion and internal soil friction was found for the DD treatment on the Højer soil. The cone penetration measurements indicated a stronger top-soil and fewer high-strength soil elements in the 24–28 cm horizon for the DD than for the PL treatment. Generally the DD treatment had lower volume of macropores (i.e. pores>30 μm ) in the 4–8 and 14–18 cm depths than the PL treatment. This was reflected in reduced air diffusivities and air permeabilities for these horizons. An exception was the 14–18 cm depth of the Ballum soil, where increased air diffusivity and air permeability was measured at field-sampled water content. Continuity indices calculated from air diffusivity and air permeability measurements showed that the DD soil from the 4 to 8 and 14 to 18 cm depths had less continuous and more tortuous macropores than the ploughed soil.

Soil Compaction and Growth of Woody Plants

Although soil compaction in the field may benefit or inhibit the growth of plants, the harmful effects are much more common. This paper emphasizes the deleterious effects of predominantly high levels of soil compaction on plant growth and yield. High levels of soil compaction are common in heavily used recreation areas, construction sites, urban areas, timber harvesting sites, fruit orchards, agroforestry systems and tree nurseries. Compaction can occur naturally by settling or slumping of soil or may be induced by tillage tools, heavy machinery, pedestrian traffic, trampling by animals and fire. Compaction typically alters soil structure and hydrology by increasing soil bulk density; breaking down soil aggregates; decreasing soil porosity, aeration and infiltration capacity; and by increasing soil strength, water runoff and soil erosion. Appreciable compaction of soil leads to physiological dysfunctions in plants. Often, but not always, reduced water absorption and leaf water deficits develop. Soil compaction also induces changes in the amounts and balances of growth hormones in plants, especially increases in abscisic acid and ethylene. Absorption of the major mineral nutrients is reduced by compaction of both surface soils and subsoils. The rate of photosynthesis of plants growing in very compacted soil is decreased by both stomata1 and non-stomata1 inhibition. Total photosynthesis is reduced as a result of smaller leaf areas. As soils become increasingly compacted respiration of roots shifts toward an anaerobic state. Severe soil compaction adversely influences regeneration of forest stands by inhibiting seed germination and growth of seedlings, and by inducing seedling mortality. Growth of woody plants beyond the seedling stage and yields of harvestable plant products also are greatly decreased by soil compaction because of the combined effects of high soil strength, decreased infiltration of water and poor soil aeration, all of which lead to a decreased supply of physiological growth requirements at meristematic sites. Many protocols have been developed, with variable success, to alleviate the adverse effects of soil compaction on the growth and development of woody plants. These include planting of compaction-tolerant species, controlling vehicular and animal traffic, amending soils by adding coarse materials and/or organic matter, replacing compacted soils with uncompacted soils, loosening soils with aerating equipment, installing drainage systems and judiciously applying fertilizers. Prevention of soil compaction before planting is usually much preferred over post-planting treatments because the latter are expensive and difficult to apply, may not be adequately effective and may injure plant roots.

Atributos físicos do solo e crescimento radicular de soja em latossolo roxo sob diferentes métodos de preparo do solo

A presença de camadas compactadas no solo, resultantes das operações de preparo, pode causar restrição ao crescimento radicular e, conseqüentemente, reduzir a produção. A determinação de atributos físicos do solo, entre eles a densidade e a resistência, tem sido utilizada para avaliar a compactação, visando a recomendações de manejo. Neste estudo, procurou-se determinar a relação entre esses atributos e o crescimento radicular, em um ensaio com cinco sistemas de preparo de solo e cultura de soja (Glycine max L.), localizado no município de Tarumã (SP), em Latossolo Roxo muito argiloso. As avaliações foram realizadas em janeiro de 1993, sete anos após o início do ensaio. Os sistemas de preparo combinaram preparos de verão e de inverno com grade pesada, arado escarificador e semeadura direta. O tratamento com semeadura direta, no verão e no inverno, apresentou valores elevados de resistência e densidade desde a superfície até 0,3 m, enquanto o tratamento com grade pesada apresentou valores elevados para esses parâmetros entre 0,1 e 0,2 m de profundidade, caracterizando camadas compactadas. Nos tratamentos com escarificação no verão, a resistência e a densidade do solo foram menores, com pequena diferença entre as profundidades analisadas. A semeadura direta apresentou maior densidade de raízes, seguida dos tratamentos com escarificação. Cerca de 50% do sistema radicular ficou concentrado na camada superficial do solo no tratamento com grade pesada, 40% no tratamento com semeadura direta e 30% com escarificador. As análises de regressão e correlação indicaram que, com o aumento da densidade do solo, a resistência e a umidade do solo foram maiores e, em decorrência dos valores obtidos no sistema de semeadura direta, a densidade radicular, nessas condições, também foi maior. A relação entre os parâmetros avaliados indicou que o sistema radicular foi reduzido em profundidade, quando a densidade e resistência foram elevadas na camada de 0,1-0,2 m ou quando foi alto o gradiente de densidade e resistência entre as camadas de 0,1-0,2 e 0-0,1 m. Os valores de resistência e densidade de um solo, tomados isoladamente, não puderam ser considerados como indicadores do crescimento de raízes.

Genotypic Variation for Root Penetration of a Soil Pan

ABSTRACT Cotton production has increased dramatically on the Coastal Plain of the southeastern USA since eradication of the boll weevil (Anthonomus grandis [Boh.]) in the late 1980s. Most of the cotton production in this area occurs on soils possessing subsoil pans. Soil pans limit root growth, requiring mechanical disruption to increase the root zone and facilitate cotton growth. Mechanical amelioration of a soil pan has several disadvantages including expenses for equipment and energy, equipment to break up the pan may not be available, and the effects of mechanical disruption are temporary and contribute to soil erosion. The need for soil pan disruption might be eliminated or reduced with cotton germplasm capable of rooting through high strength soil. Our objective was to identify germplasm with superior rooting ability and thereby initiate breeding efforts incorporating this trait into cultivars. Cultivars and Pee Dee germplasm lines were evaluated at Florence, SC for 2-yr without irrigation for root penetration of a naturally occurring soil pan. Roots were excavated at maturity and rated on a 1–5 scale based on magnitude of the root penetration of the pan. Combined analysis of variance over years revealed significant genotypic variation and a genotype yr interaction for root penetration of the pan. Despite the genotype yr interaction, we identified germplasm that could partially root through the pan. Additionally, we were able to eliminate those genotypes with poor rooting characteristics from further consideration as parents in the breeding program. Cultivars with ability to penetrate a soil pan would make a valuable contribution to sustainable cotton production systems for the USA and developing countries.

Soil Compaction and Growth of Woody Plants

Abstract Although soil compaction in the field may benefit or inhibit the growth of plants, the harmful effects are much more common. This paper emphasizes the deleterious effects of predominantly high levels of soil compaction on plant growth and yield. High levels of soil compaction are common in heavily used recreation areas, construction sites, urban areas, timber harvesting sites, fruit orchards, agroforestry systems and tree nurseries. Compaction can occur naturally by settling or slumping of soil or may be induced by tillage tools, heavy machinery, pedestrian traffic, trampling by animals and fire. Compaction typically alters soil structure and hydrology by increasing soil bulk density; breaking down soil aggregates; decreasing soil porosity, aeration and infiltration capacity; and by increasing soil strength, water runoff and soil erosion. Appreciable compaction of soil leads to physiological dysfunctions in plants. Often, but not always, reduced water absorption and leaf water deficits develop. Soil compaction also induces changes in the amounts and balances of growth hormones in plants, especially increases in abscisic acid and ethylene. Absorption of the major mineral nutrients is reduced by compaction of both surface soils and subsoils. The rate of photosynthesis of plants growing in very compacted soil is decreased by both stomata1 and non-stomata1 inhibition. Total photosynthesis is reduced as a result of smaller leaf areas. As soils become increasingly compacted respiration of roots shifts toward an anaerobic state. Severe soil compaction adversely influences regeneration of forest stands by inhibiting seed germination and growth of seedlings, and by inducing seedling mortality. Growth of woody plants beyond the seedling stage and yields of harvestable plant products also are greatly decreased by soil compaction because of the combined effects of high soil strength, decreased infiltration of water and poor soil aeration, all of which lead to a decreased supply of physiological growth requirements at meristematic sites. Many protocols have been developed, with variable success, to alleviate the adverse effects of soil compaction on the growth and development of woody plants. These include planting of compaction-tolerant species, controlling vehicular and animal traffic, amending soils by adding coarse materials and/or organic matter, replacing compacted soils with uncompacted soils, loosening soils with aerating equipment, installing drainage systems and judiciously applying fertilizers. Prevention of soil compaction before planting is usually much preferred over post-planting treatments because the latter are expensive and difficult to apply, may not be adequately effective and may injure plant roots.

Soil strength and water content influences on corn root distribution in a sandy soil

Initial field observations revealed a shallow corn (Zea mays L.) root system on a Zimmerman fine sand in a corn/soybean (Glycine max L.) rotation. Since root distribution influences crop water and nutrient absorption, it is essential to identify factors limiting root growth. The objective of this study was to determine the factor(s) limiting corn rooting depth on an irrigated fine sand soil. Bulk density, saturated hydraulic conductivity, and soil water retention were measured on undisturbed soil cores. Corn root distribution assessed at tasseling over a 3-yr period showed an average of 94% of total root length within the upper 0.60 m of soil with 85% in the upper 0.30 m of soil. Mechanical impedance was estimated with a cone penetrometer on two dates with differing water contents. Cone penetrometer measurements greater than 3 MPa indicated mechanical impedance in soil layers extending from 0.15 to 0.35 m deep. Penetration resistance decreased as soil water content increased. However, soil water contents greater than field capacity were required to decrease penetration resistance below the 3 MPa threshold. Such water saturated conditions only occurred for short periods immediately after precipitation or irrigation events, thus roots usually encountered restrictive soil strengths. The soil layer from 0.15 to 0.60 m had high bulk density, 1.57 Mg m-3. This compacted soil layer, with slower saturated hydraulic conductivities (121 to 138 mm hr-1), held more water than the soil above or below it and reduced water movement through the soil profile. Crop water use occurred to a depth of approximately 0.75 m. In conclusion, a compacted soil layer confined roots almost entirely to the top 0.60 m of soil because it had high soil strength and bulk density. The compacted layer, in turn, retained more water for crop use.

Effect of long-fallow on soil quality and cotton lint yield in an irrigated, self-mulching, grey Vertosol in the central-west of New South Wales

Reduced crop growth rates (‘long-fallow disorder’) can be a feature of long-fallow cotton (cotton, alternating with a bare fallow, is sown every other year). This is usually attributed to decreased development of arbuscular mycorrhiza (AM), although associated soil physical, chemical, and biological properties are very rarely reported. A study was conducted from 1993 to 1997 in a grey, self-mulching Vertosol in the central-west of New South Wales to characterise soil properties under long-fallow cotton with a view to identifying soil factors other than AM that could contribute to cotton growth rate reductions. Soil quality indicators monitored were compaction (bulk density and air-filled porosity), strength (cone resistance), plastic limit, exchangeable cations, nitrate-N, pH, organic C, development of AM, and incidence of cotton root diseases. In comparison with continuous cotton, long-fallow cotton had lower soil strength, and lower plastic limit. Exchangeable Ca and Mg were higher with continuous cotton only in 1994. Higher nitrate-N was also observed with long-fallow during the first fallow phase of the experiment. Long-fallow did not have any significant effect on soil organic carbon. However, a net decline in soil organic C and exchangeable Mg occurred with both treatments. During the cotton phase, subsoil nitrate-N and incidence of black root rot were lower with long-fallow cotton. Uptake of nitrogen by continuous cotton may have been reduced by greater severity of black root rot. Vegetative and reproductive growth, water extraction, and cotton lint yields in long-fallow cotton plots were higher than those in continuous cotton plots. AM development was similar with continuous cotton and long-fallow cotton. Compared with long-fallow cotton, the lower lint yield in continuous cotton was thought to be due to the interactive effects of declining nutrient availability, higher soil strength, and greater severity of black root rot causing decreases in nutrient and water uptake.

Root and shoot response of field-grown lettuce and broccoli to a compact subsoil

The direct effect of subsoil compaction on the root and shoot growth of lettuce (Lactuca sativa ‘Classic’) and broccoli (Brassica oleracea L. Italica ‘Bushido’) was investigated in field experiments. Possible secondary effects of the compact subsoil were minimised through careful supply of water and nutrients to the soil. Roots were sampled at 3 times to determine the timing and extent of root growth into the compact subsoil. The compact treatment had a distinct layer of high strength soil at 0·17-0·35 m, maximum soil strength of 3·1 MPa at 0·27 m. In the loosened treatment, deep ripping reduced the maximum subsoil strength to 1·9 MPa at 0·35 m. The roots of both crops penetrated the compact subsoil approximately half-way through the vegetative growth phase of the shoots. Root growth was restricted by the compact subsoil, with root length densities 60-75% lower than in the loosened subsoil. As a result, only 6-13% of the total root system was present in the compact subsoil. The reduced root length in the subsoil was compensated for by increased root growth in the topsoil, for the broccoli crop only. The compact subsoil had no effect on the shoot growth or yield of either vegetable crop when water and nutrientswere well supplied. This may have been due to the small proportion of roots growing in compact soil and/or the advanced vegetative growth stage of the shoots when the roots grew into the compact subsoil. Consequently, under dield conditions, a compact subsoil had no direct effect on vegetable crop shoot growth or yield.

Pre-emergent shoot growth of maize ( Zea mays, L.) as a function of soil strength

Poor crop establishment is a common problem in summer cropping regions. The physical environment predisposes seedbeds to high soil strength and temperature. It is frequently difficult, however, to determine the relative contributions of these stresses to emergence failure. Our capacity to determine the effect of soil strength is limited by: (a) a limited understanding of the effect of soil strength on pre-emergent shoot growth, and whether there is an interaction with soil temperature and (b) difficulty in measuring the mechanical stresses experienced by the pre-emergent seedling in a way that is applicable to a wide range of soils. We used a uniaxial compression cell to quantify the rate of shoot growth of maize as a function of soil strength (0 to 956 kPa), and to assess if temperature affected this relationship. Soil strength was calculated as spherical cavity expansion pressure, with field estimates taken from blunt penetrometer readings adjusted for soil friction. Coleoptile and first internode growth rates fell by more than 50% when strength increased from 0 to 118 kPa. By combining functions describing the strength dependence of shoot growth with functions developed previously to describe the temperature dependence of shoot growth, a model was developed to predict the response of shoot growth to strength at a range of temperatures. The model successfully predicted the outcomes of laboratory experiments where seedlings were grown at 118 kPa and exposed to diurnally varying temperatures in both the physiological temperature range (10–40°C) and at supra-maximum temperatures (> 40° C). This suggested that at low soil strengths, temperature did not interfere with the shoot growth response to strength. However, when seedbed strength increased to 174 kPa, and seedlings were exposed to supra-maximum temperatures, the model significantly overestimated first internode growth, suggesting that high temperature stress had reduced the expansion pressure of the first internode. Maize shoot growth in the laboratory was more than five times as sensitive to cavity expansion pressure, than in field measurements. One factor which may have contributed to this is the increase in second node width in response to high temperature and/or high strength. As a result, the theoretical analysis used may have underestimated the mechanical forces imposed on the elongating shoot.

Simulating Maize Emergence Using Soil and Climate Data

AbstractExperimental programs aimed at improving seedling establishment in the semiarid tropics require quantitative information concerning the interaction between the seedbed environment and preemergent seedling growth. The model here described links a soil heat and water model, a soil strength model, and a temperature‐dependent maize (Zea mays L.) shoot growth model to predict the response of preemergent maize seedlings to a physical environment typical of the semiarid tropics. In comparisons with field measurements taken in the semiarid tropics under high drying rate conditions, the model simulations yielded accurate representations of soil water, temperature, strength, and preemergent shoot growth of maize. The model was also used to compare the effects of radiation‐ vs. wind‐dominated evaporative demand on preemergent seedling growth in a hardsetting seedbed. Results showed that under radiation‐dominated conditions, the early onset of high soil temperature limited seedling growth, while under wind‐dominated conditions, the early development of high soil strength affected seedling growth first. Simulations using surface mulch as an ameliorative management practice showed that mulch greatly reduced soil evaporation, thereby eliminating high soil strength as a limitation to seedling establishment. Results indicate that the model is a potentially useful tool for investigating the likely effects of soil and climate on seedling emergence.

Drought Resistance Aspects of Turfgrasses in the Southeast: Root‐Shoot Responses

Knowledge of relative drought resistance is important for selecting turfgrasses that persist during drought stress. Seven of the most commonly used turfgrasses in the Piedmont region of the humid Southeastern USA were evaluated in a field study under edaphic stresses common to the region for drought resistance and spatial rooting responses as a component of drought avoidance. Grasses were ‘Tifway’ bermudagrass (Cynodon dactylon × C. transvaalensis); common bermudagrass [C. dactylon (L.) Pers.]; ‘Meyer’ zoysiagrass (Zoysia japonica Steud.); common centipedegrass [Eremochloa ophiuroides (Munro.) Hack.]; ‘Raleigh’ St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze]; and ‘Rebel II’ and ‘Kentucky‐31’ tall fescue (Festuca arundinacea Schreb.). Grasses varied in drought resistance, as determined by wilt and leaf firing during dry‐down periods, from very high (Tifway and common bermuda), high (Raleigh St. Augustine, common centipede), medium‐high (Rebel II tall fescue), medium (K‐31 tall fescue), and medium‐low (Meyer zoysiagrass). High soil strength and acid soil complex stresses were present on the site. Under these stresses, deep rooting (20‐ to 60‐cm zone) in late summer ranged from Tifway bermuda (best) > Rebel II tall fescue = common bermuda > K‐31 tall rescue > Raleigh St. Augustine > common centipedegrass > Meyer zoysiagrass. Meyer produced only 4% of the root length density that Tifway did within this zone due to low genetic tolerance to the edaphic stresses. These results illustrate that genetic tolerance to edaphic stresses can markedly affect turfgrass rooting and drought resistance rankings. Plant breeders must give attention to edaphic stress tolerances if drought avoidance via roots is to be improved.

Modeling Preemergent Maize Shoot Growth: I. Physiological Temperature Conditions

AbstractPrediction of seedling emergence is an important first step in predicting crop establishment. There are numerous models that, under nonlimiting seedbed physical conditions, can adequately predict time to emergence using heat sums based on average daily temperatures. Under more adverse seedbed physical conditions, however, such as occur in the semiarid tropics, use of such coarse indices will tend to obscure the strong influence that even temporary extremes in soil temperature or strength can have on the time and extent of emergence, yet it is in these situations that final crop establishment and ultimately yield will be particularly sensitive to emergence predictions. One way to quantify the effect that temporary extremes in seedbed conditions have on emergence is to model the emergence process (namely, preemergent shoot growth). In this paper, we model preemergent maize (Zea mays L.) shoot growth as a first step in characterizing seedling response to a high soil temperature and strength environment. The model, developed from shoot length data obtained under constant temperature conditions ranging from 20 to 40°C, uses an exponential function that predicts coleoptile and first internode growth rates as a function of temperature and sums the output to predict shoot length. Comparison of model predictions with independent measurements carried out in the 20 to 40°C range show that the model is capable of accurately simulating preemergent shoot growth under conditions where (i) temperatures are constant with time, (ii) temperatures of the whole seedbed vary diurnally, and (iii) temperatures within the seedbed vary with time and depth in the seedbed. However, the model was unable to predict shoot growth in situations where seedlings had been exposed to temperatures in excess of 40°C, indicating that further work is needed to assist modeling within the high temperature range.

Growth and morphology of eucalypt seedling-roots, in relation to soil strength arising from compaction

Information on the response of root growth and morphology to soil strength is useful for testing suitability of existing and new tillage methods and/or for selecting plants suitable for a specific site with or without tillage. Although there is extensive published information on the root growth-soil strength relationships for annual agricultural plants, such information is scarce for woody, perennial tree species. The purpose of this study is to examine growth and morphology of the root systems of 17-day-old eucalypt seedlings with respect to variation in soil strength. Soil strength in this study was varied by compaction of a well-aggregated clay soil to bulk densities of 0.7–1.0 Mg m-3 whilst maintaining adequate water availability and aeration for plant growth. Lengths and tip-diameters of primary and lateral roots were measured on the excavated root systems of seedlings. With increase in bulk density and also soil strength (expressed as penetrometer resistance), total length of primary and lateral roots decreased. There were 71 and 31% reduction in the lengths of primary and lateral roots respectively with an increase in penetrometer resistance from 0.4 to 4.2 MPa. This indicated primary roots to be more sensitive to high soil strength than the lateral roots. Average length of lateral roots and diameters of both primary and lateral root tips increased with an increase in soil strength as well. There was greater abundance of lateral roots (no. of lateral roots per unit length of primary root) and root hairs with increased soil strength. The observed root behaviour to variable soil strength is discussed in the context of compensatory growth of roots and overall growth of plants.

Coping with soil and climatic hazards during crop establishment in the semi-arid tropics

Climatic induced hazards (e.g. water deficit, high soil temperature and high soil strength) that adversely affect seedling emergence and establishment of annual crops on red earth soils (Kandsols) at Katherine in the Daly basin of the Northern Territory are reviewed and results of some recent simulation studies and experiments are presented. Simulation studies, using 100 years of historical weather data, have shown that maize and sorghum density at Katherine is rarely reduced by water deficit during crop establishment. However, the median number of days between 1 December and 20 January during which seedlings may be exposed to damagingly high soil temperature (>55�C between 2 and 7 days after sowing) was 5.5, out of an estimated 21 days suitable for sowing. While the exposure of a crop to inclement conditions during establishment may have immediate and dramatic effects on the mortality of pre-emergent and post-emergent seedlings, there may also be longer-term and less evident adverse effects on crop growth and development. The responses of developing seedlings to inclement conditions following sowing are described and management options (eg adjusting planting dates, changing crop species, changing seedbed configurations, using surface mulch) for the amelioration of the seedbed environment are discussed. Of the possible management options for ameliorating adverse seedbed conditions during crop establishment in the semi-arid tropics (SAT), it appears that the best practice is to maintain a soil surface cover (mulch) in close proximity to the emerging seedlings. The presence of surface mulch extends the window of opportunity for establishing crops by slowing soil drying, delaying the onset of high soil temperatures and high soil impedance, and by improving the availability of water to the young seedlings at this critical stage.

Early Pinus caribaea var. hondurensis root development. 2. Influence of soil strength

The influence of penetration resistance (PR), an easily measured indicator of soil strength, on the growth of Pinus caribaea var. hondurensis radicles and seedlings was investigated. Negative exponential relationships between PR and both radicle and primary root elongation were observed. All root elongation ceased at PR levels of 3.25 MPa. Tip diameters of radicles and primary roots were positively correlated with PR values up to 2.4 MPa, whilst numbers of primary roots, total root lengths and lengths of longest roots were all negatively correlated with PR. Hypocotyl elongation was also reduced by increasing PR, although the reductions occurred at higher PRs than those which inhibited root development. In contrast, primary shoot development was unaffected by PR levels which were sufficient to stop root elongation, but was reduced in soil with a PR of 4.8 MPa. There were significant family x soil type and family x PR interactions for radicle, hypocotyl, primary root and primary shoot development. 1f these interactions are correlated with performance in the field, then they may serve as useful indicators of family suitability to both soil type and high strength soils.