End Of Fallow Research Articles

Tillage timing to improve soil water storage in Mediterranean long fallow

Creation of a dry soil mulch to maintain near-surface moisture in dry environments is an ancient practice that is still used in many regions of the world for timely establishment of crops after long fallow. Before availability of herbicides, the first tillage during fallow was timed to kill weeds, but now the optimum timing for tillage depends only on the interplay of evaporation and infiltration of rain. The goal is to maximize total root zone water storage and seed-zone water at seeding time. Our objective was to measure the effect of delaying soil mulch creation progressively later in the spring and summer on weed-free fallow for winter wheat (Triticum aestivum L.) in the Mediterranean inland Pacific Northwest drylands of the United States. Over a period of six years and a total of 12 site-years, four to seven tillage timings were compared to an untilled treatment. Soil water was measured at seeding time in the seed zone and total root zone. Seed-zone water and total stored water produced by different tillage timings was surprisingly constant, with few statistically significant or substantial differences. On average, only 2% of stored water was lost in the top 1.0 m of soil over 80 days during the hot, dry period towards the end of fallow. Responses to late spring rain could be seen, and tillage near the end of June (early summer) often produced maximum soil water content.

Wheat stubble height effects on soil water capture and retention during long fallow

A 4-yr study was conducted in a dry Mediterranean region of the Inland Pacific Northwest USA (PNW) to measure winter wheat (WW) (Triticum aestivum L.) stubble height and orientation effects on overwinter precipitation capture in the soil and subsequent water retention during ensuing dry summer months. The stubble was left standing and undisturbed during the 13-mo fallow periods. Stubble-height treatments were:(i) short (8 cm), (ii) medium (25 cm), and (iii) tall (75 cm). An additional treatment: (iv) mow tall stubble in mid-June, was included in the final two years. Soil water measurements were obtained at the beginning, middle, and end of fallow in 15-cm increments to a depth of 180 cm. Additionally, seed-zone water content was measured at the end of fallow in 2-cm increments to a depth of 26 cm. Near-surface soil temperature was measured from June-August. On average, tall- and medium-height stubble captured 34 and 32 mm (p = 0.002) more overwinter precipitation, respectively, than short stubble and these values were particularly pronounced (91 and 79 mm greater than short stubble) during a winter of heavy snow drifting. However, from mid-April until late August, the tall stubble lost an average of 91 mm of soil water compared to 70 and 59 mm in the medium and short stubble treatments (p < 0.001). Mowing tall stubble in mid-June before the hot, dry summer did not improved water retention. Continuous soil temperatures at 3, 7, 15, 25, and 40-cm depths measured electronically from June-August were coolest with the short, mowed, medium, and tall stubble, respectively, with frequent significant differences of > 1 °C among treatments. We speculate that soil in the tall treatment was warmest and lost the most over-summer water because all stubble was standing and offered less soil shading than the other treatments. At the end of fallow, medium and tall stubble averaged 14 and 8 mm greater soil water, respectively, than the short treatment (p = 0.027). Short stubble was a disadvantage for overwinter precipitation capture but was equal or better than the other treatments for retaining soil water from April to late August; presumably because this treatment had the most residue lying flat on the soil surface for shading. These findings offer opportunity to improve current models for soil water dynamics during fallow by incorporating stubble height. For combined soil water retention and farm management factors, medium-height WW stubble is the best fallow option for farmers in the PNW drylands.

Can legumes provide greater benefits than millet as a spring cover crop in southern Queensland farming systems?

Cover crops grown during fallows can increase organic matter inputs, improve soil surface cover to reduce erosion risk, and enhance rainfall infiltration. An experiment compared a chemical fallow control with six different cover crops terminated at either 60 or 90 days after sowing. The commercial choice of millet (Echinochloa esculenta) was compared with two summer legumes (lablab (Lablab purpureus) and soybean (Glycine max)), and three winter legumes (field pea (Pisum sativum), faba bean (Vicia faba) and common vetch (Vicia sativa)). Cover crop biomass growth, atmospheric nitrogen (N) fixation, surface residue cover, and soil water and mineral N dynamics during the growth period and subsequent fallow were measured. Soil water and N availability and yield of wheat crops following the experimental treatments were simulated over a 100-year climate record using APSIM. Both experiments and simulations found the legumes inferior to millet as spring-sown cover crops, because they were slower to accumulate biomass, required later termination and provided groundcover that was less persistent, resulting in lower soil water at the end of the fallow. After 90 days of growth, the summer legumes, lablab and soybean, produced the most biomass and fixed more N (up to 25 kg N/ha) but also extracted the most soil water and mineral N. Legume N fixation was low because of high soil mineral N status (&gt;100 kg N/ha) and occurred only when this had been depleted. At the end of the subsequent fallow in April, soil water was 30–60 mm less and soil mineral N 80–100 kg/ha less after both millet and 90-day terminated summer legume cover crops than the chemical fallow control. Simulations predicted soil-water deficits following legume cover crops to be &gt;50 mm in the majority of years, but soil mineral N was predicted to be lower (median 80 kg N/ha) after millet cover crops. In conclusion, monoculture legume cover crops did not provide advantages over the current commercial standard of millet, owing to less effective provision of groundcover, low N fixation and possibly delayed release of N from residues. Further work could explore how legumes might be more effectively used as cover crops to provide N inputs and soil protection in subtropical farming systems.

A study of the effect of the interaction between site-specific conditions, residue cover and weed control on water storage during fallow

In the semiarid central region of Argentina the probability that rainfall meets crop requirements during growing season is less than 10%, therefore fallowing has been the most important practice to assure water availability during the growing season. Various site-specific and management factors have been identified as crucial for defining fallow efficiency (FE) and final available water contents (AW). The objective of the present study was to improve our knowledge about the interactions between residue cover, weed control, soil profile depth and water storage capacity (WSC) on FE. In 10 sites covering the environments of calcareous plains and sandy plains of the semiarid central region of Argentina and with different WSC, experiments with 3 different levels of residue cover (H, M, L) and with and without weed control (C and W respectively) during fallow were set up. A completely randomized block design with four repetitions and splits plots to consider weed control was used. Soil texture and organic matter were determined in samples of the A horizon (0.20 m). Bulk density, field capacity, permanent wilting point and soil water contents (monthly frequency) were measured at depth intervals of 0.20 m to the depth of the calcite layer or to 2.00 m depth. Soil temperature was taken in weekly intervals at 0.05 m depth and weed plants, separated by species, were counted at the end of fallow in 4 repetitions of 0.25 m 2 in each treatment. An empirical model was developed to predict final AW under these experimental conditions. Model parameters were: Residue level, weed control, WSC, profile depth, and rainfall during fallow. Site-specific conditions (WSC and profile depth) affected water storage during fallow; soils with highest values for both parameters showed highest final AW. Weed density was the most important factor that controlled AW, with on average 35 mm less AW in W than in C treatments. Residue level had a positive effect on final AW in both C and W treatments, with a difference of 18.5 mm between H and L. An interaction between residue level and weed density was observed, indicating weed suppression in H treatments. This was also confirmed by correspondence analysis between residue level and weed species which revealed that different species were related to each level. High residue levels also decreased soil temperature, thus affecting germination of post-fallow crops. The empirical model had an overall average prediction error of 13.7% and the regression between measured and predicted values showed a determination coefficient of 0.77.

Dynamics of soil hydraulic properties during fallow as affected by tillage

There is limited information on the effects of tillage practices on soil hydraulic properties, especially changes with time. The objective of this study was to evaluate on a long-term field experiment the influence of conventional tillage (CT), reduced tillage (RT) and no-tillage (NT) on the dynamics of soil hydraulic properties over 3 consecutive 16–18 month fallow periods. Surface measurements of soil dry bulk density ( ρ b), soil hydraulic conductivity ( K( ψ)) at −14, −4, −1 and 0 cm pressure heads using a tension disc infiltrometer, and derived hydraulic parameters (pore size, number of pores per unit of area and water-transmission porosity) calculated using the Poiseuille's Law were taken on four different dates over the fallow period, namely, before and immediately after primary tillage, after post-tillage rains and at the end of fallow. Under consolidated structured soil conditions, NT plots presented the most compacted topsoil layer when compared with CT and RT. Soil hydraulic conductivity under NT was, for the entire range of pressure head applied, significantly lower ( P < 0.05) than that measured for CT and RT. However, NT showed the largest mean macropore size (0.99, 0.95 and 2.08 mm for CT, RT and NT, respectively; P < 0.05) but the significantly lowest number of water-conducting pores per unit area (74.1, 118.5 and 1.4 macropores per m 2 for CT, RT and NT, respectively; P < 0.05). Overall, water flow was mainly regulated by macropores even though they represented a small fraction of total soil porosity. No significant differences in hydraulic properties were found between CT and RT. In the short term, tillage operations significantly increased K ( P < 0.05) for the entire range of pressure head applied, which was likely a result of an increase in water-conducting mesopores despite a decrease in estimated mesopore diameter. Soil reconsolidation following post-tillage rains reduced K at a rate that increased with the intensity of the rainfall events.

Influence of fallowing practices on soil water and precipitation storage efficiency in semiarid Aragon (NE Spain)

The use of long fallowing (16–18 months) for soil water conservation has been questioned in semiarid drylands of Central Aragon. We quantify the soil water loss, soil water storage (SWS) and precipitation storage efficiency (PSE) of long fallow under three management systems (conventional tillage; reduced tillage; no-tillage). The precipitation storage efficiency of long fallow relative to short fallow (5–6 months) was also evaluated. Over 4 experimental years (1999–2002), the soil water balance was calculated from fallow seasonal precipitation and volumetric soil water content (0–70 cm depth). During long fallowing, primary tillage implemented in conventional tillage and reduced tillage plots induced significant soil water losses from the plough layer (0–40 cm depth) for the first 24 h after tillage. However, secondary tillage appeared to have a positive effect on soil water conservation at the end of fallow. The division of long fallow into three sub-periods showed that the early phase (July to November) was the most efficient in terms of soil water storage. Both for the individual fallow phases and the entire fallow period, the precipitation storage efficiency increased when most of the seasonal effective rainfalls (≥10 mm day −1) were received in the last 2 months of each period. Long fallow precipitation storage efficiency was low (11% on average). Neither soil water storage nor precipitation storage efficiency was significantly affected by the tillage system. The average additional soil water at sowing after long fallow compared with short fallow was 20 mm. Accordingly, the average gain in precipitation storage efficiency of long fallow relative to short fallow was only 5.3%. We conclude that long fallowing might not be suitable for enhancing soil water storage in Aragon.

Matching Plant-Available Nitrogen from Biosolids with Dryland Wheat Needs

Biosolids are stabilized solids from municipal wastewater treatment that meet federal standards for land application. Good estimates of biosolids N availability are needed to develop sustainable biosolids management programs. We conducted this study to (i) determine the availability and fate of biosolids N applied to a dryland soft-white winter wheat (Triticum aestivum L.)-fallow rotation, (ii) determine if N availability predictions for biosolids are applicable under dryland conditions, and (iii) make practical recommendations for biosolids management. We applied dewatered (21-30% solids) biosolids (4.3-5.5% total N; 257-853 lb N/acre) to on-farm test plots at three locations in the 9-12 in. rainfall zone of eastern Washington. Fertilized (anhydrous or aqua ammonia [AA]) and unfertilized treatments were established at each site for comparison. We measured yield and N uptake of grain and straw at harvest, and determined soil profile nitrate N (plus ammonium N, 0-12 in. depth only) before application, during fallow, and post-harvest. We determined apparent N recovery from the soil at the end of the fallow (ANR fallow ). Nitrogen release from biosolids as measured by ANR fallow was dependable and consistent over the three sites, despite differences in environment among the sites. ANR fallow averaged 29%, similar to predicted values of 26-31%. The lowest biosolids rates (257-330 lb/acre) supplied more available N tban AA. Yield, grain N, and flag leaf N all indicate that N was sufficient at the lowest biosolids rates used, and that higher levels of biosolids did not benefit the crop. Drawbacks to higher rates include risks of reduced crop yield (from moisture stress) and quality (from increased protein), and increased risk of nitrate movement below the root zone. Storage of nitrate in the soil profile does not appear to be a reliable strategy for supplying N for a second crop. Lower biosolids rates seem to reduce economic risks to the farmer and reduce leaching risk Evaluation of low biosolids rates (100-300 lb total N/acre) and second crop response will be valuable in refining biosolids application recommendations.

Soil Mulch Effects on Seedbed Temperature and Water During Fallow in Eastern Washington

AbstractDepth of dry soil mulch affected summer soil temperatures and seed‐bed water at the end of fallow in the dryland wheat region of eastern Washington. Increasing the depth of the tillage mulch from 6 to 11 cm reduced summertime seed zone drying sufficiently to benefit wheat emergence. Reduction of drying was greatest when the seed zone had good capillary continuity with the deeper soil layers. Drying depth and intensity was greater with a cloddy soil mulch than with a fine mulch.The seed zone water‐conserving effect of a fine soil mulch was related to the lowered temperatures and temperature gradients across the seed zone associated with the increased mulch depth. The deeper soil mulch conserved seedbed water through increased resistance to water flow from moist layers to the atmosphere, and through increased thermal insulation of the moist soil below the dry mulch. Surface‐applied straw (4,000 kg/ha) decreased seed‐zone temperatures under the shallow mulch only.The key factor in conserving seedbed water through extended periods of hot, dry weather is that the loss rates from the seed zone be balanced by upward unsaturated flow from deeper layers. As the soil dries, the relative importance of loss rates through evaporative loss across the dry layer and thermally induced flow downward increases because of decreasing return flow from the deeper layers along matric potential gradients. Under these dryland conditions, it appears that seedbed water could be best conserved through using a soil mulch of maximum resistance to vapor and liquid water flow, and maximum thermal insulation, overlying a seed zone having good capillary continuity with the deeper soil layers. The stratification produced by moderately deep rod‐weeding appears to provide such a mulch.