Richard Bell

Bangladesh government has targeted to secure food security, particularly by increasing rice production, in response to the global climate change, increasing food demand, and political outlook. To achieve this, they are using all-strategies that include improved crop management practices, cultivated area expansion, and genetic improvements. To talk about on rice alone, as of July 18, 2022, the production of rice was over 38.4 million tons (Mt), but the government aims to increase it to 47 Mts by 2030 and then to 54 Mts by 2040. They also target to produce more than 60 Mt by 2050, while in 2021, rice was grown on 78% of Bangladesh's total arable land. In the context of Bangladesh's intensive agricultural production system, agriculture remains the predominant sector of employment, engaging a substantial 40.6% of the labor force in 2022 (BBS, 2022).

Over eight million hectares of land in south-west Western Australia required applications of micronutrients, particularly copper, zinc and molybdenum, in order to grow profitable crops and pastures when first cleared for agricultural production. Initial fertiliser applications, particularly of copper and zinc, had very long residual effectiveness – research showed that the once-off application of copper continued to correct crop deficiencies for over 30 years.

Soil acidity and aluminum (Al) toxicity are significant soil constraints for agricultural production worldwide. Soil acidification, or a decrease in soil pH, is a natural process increased by agriculture due to product removal and nitrate leaching. Its effect on plant production is measured by dividing the soil profile into layers based on nutrient availability. In most regions of Australia, a 0-10cm soil layer is used to define the topsoil, while the subsoil is defined as soil layers below 10cm. The soil acidification process results in an acidic layer typically forming in the 10-30cm soil layer. Hence, subsoil acidity or the occurrence of soil Al is a greater issue than topsoil acidity. Acidity is used to describe the overall problem, but soil Al is the soil factor resulting in reduced root growth and agricultural production. The main toxic species in soils is @equ_0001.eps@, which is abbreviated to Al3+. A soil pH measurement defines the degree of soil acidity, while soil Al3+ content measures the degree of Al toxicity. Both pH and Al3+ are commonly measured using 0.01M CaCl2 at a soil:solution ratio of 1:5 (pHCaCl2 and AlCaCl2). Topsoil with pHCaCl2 less than 5.0 and subsoil with pHCaCl2 less than 4.5 are classified as acidic. At the same time, soils with AlCaCl2 greater than 2.5-4.5mg Al/kg in 0-30cm soil layers restrict wheat (T. aestivum) production.

Lime application to the soil surface effectively treats topsoil acidity/Al3+ toxicity but is ineffective in treating subsoil Al3+ toxicity. The low effectiveness is due to lime's low solubility combined with soil, environmental, and lime quality restrictions which limit lime dissolution and alkalinity movement from the topsoil to the subsoil. Hence, subsoil Al3+ toxicity management requires combining lime with gypsum applications, using strategic tillage to increase lime dissolution in the topsoil and redistributing lime into the subsoil, and growing Al3+ tolerant crop species. Topsoil alkalinity movement into the subsoil occurs when sufficient lime is applied to maintain the topsoil pHCaCl2 between 5.5 and 7.5. Maintaining pHCaCl2 within this range creates a pool of excess lime alkalinity which is leached into the subsoil to mitigate subsoil Al3+ toxicity. The conservation farming system or the no-tillage seeding system results in the stratification of surface-applied lime in soil layers near the surface. Strategic tillage (surface and deep) is used to increase the effectiveness of surface-applied lime in ameliorating subsoil Al3+ toxicity. Strategic surface tillage increases lime dissolution in the topsoil to achieve the target pHCaCl2 range faster, resulting in greater rates of alkalinity movement. Strategic deep tillage practices redistribute a lime-rich topsoil layer into the subsoil, typically to a depth of 40cm. Gypsum is more soluble than lime, and the applied calcium and sulfate are leached rapidly to treat subsoil Al3+ toxicity. Its application with lime can increase alkalinity movement into the subsoil when measured using a change in AlCaCl2 at pHCaCl2 less than 4.5. Further research is required to understand the soil chemistry process involved and to identify soil types or properties where gypsum and other organic amendments applied with lime can improve alkalinity movement.

Plant and soil analyses are complementary tools for the diagnosis and prognosis (prediction) of crop nutrient status and environmental quality. Their use depends on well-established relationships between nutrient concentration (in plants or soils) and plant growth/yield or environmental quality. Based on a calibrated relationship, nutrient concentrations are categorized into deficient, adequate, and toxic ranges. From these relationships, the critical concentration range is defined, corresponding usually to concentrations required to obtain 90% or 95% of maximal growth/yield. In addition, use can be made of visual plant symptoms to recognize nutritional disorders. Fertilizer recommendations are often based on plant or soil analysis results. Critical deficiency ranges are dependent on plant age, plant part, concentrations of other nutrients, as well as on environmental factors. Plant analysis can be used to either diagnose existing disorders in plants or predict future nutrient limitations based on various calibration relationships. Soil tests are complementary to plant tests and are used primarily to predict future nutrient limitations.

Deep sands occupy globally over 900 million hectares. In addition, large areas of soils have sandy A horizons overlaying B horizons with significant amounts of clay. The sand content of these soils varies but more importantly variations in the minor clay and organic matter constituents have a major bearing on the edaphic properties of sands. The fundamental limitation of sandy soils is their low reactive surface area. The limited capacity to supply resources, water or nutrients, to the roots of crops is the core reason for the low productivity of crops and pastures on sandy soils. Sands commonly suffer from multiple limitations, including multiple nutrient deficiencies, acidity, dense packing, low water-holding capacity, low fertilizer use efficiency, and water repellence. In addition, leaching of nutrients and agricultural chemicals, acidification, subsoil compaction, and wind erosion are degradation risks of sands. Breakthrough technologies to increase the performance of sands involve permanent increases in the reactive surface area with added clay, recalcitrant organic matter, or both to address the inadequate supply of resources. There is still an incomplete understanding of why and when amendment options on sands will be effective. The opportunity exists to develop cost-effective, novel amendments with high reactive surface area for long-term amelioration of sandy soils.

In the mega delta of the Ganges, Brahmaputra and Meghna rivers, the landscape has low elevation and agricultural land use is highly vulnerable to changes in upstream flows, climate variability and sea-level rise. The lack of freshwater in the dry season is a major impediment to agriculture in south-western Bangladesh. This study was conducted to assess the quality and quantity of water from river, canal, pond and groundwater sources during the dry season in Khulna, Bangladesh. Overall, salinity (ECw) in a controlled canal and in ponds increased from 2 dS m−1 in February to a maximum of 4.2 dS m−1 in mid-April. The relative ECw of canal and pond water increased with the decrease in relative water volume in a manner consistent with the view that increased ECw was caused by evaporation and irrigation water extraction. Pond ECw increased with lower site elevation, but regardless of elevation, ECw increased with time. Groundwater can be considered to be a prospective supplementary water source (ECw: 2.3 to 5.7 dS m−1) if canal water is insufficient. River water in this period was never suitable for irrigated crops (maximum ECw: 24.6 dS m−1). We conclude that water availability in ponds and controlled canals is important for crop irrigation and that the ECw is regulated by both the volume in storage, cumulative evaporation and irrigation water extraction.

Most investigations of edaphic processes are confined to the topsoil even though plant roots explore a much greater volume of soil than the topsoil. However, plant growth is commonly restricted by adverse physical, chemical, or biological properties of subsoils or combinations of such constraints. Testing surface soils does not reveal these limitations, and hence their significance in crop and soil management is often ignored. There is growing body of research on the impact of subsoil constraints on crop production. To synthesize the insights from this body of knowledge for soil scientists, agronomists, and land managers so that due attention is given to the subsoil in soil, crop, and land management, we have commissioned the 15 following chapters dealing with subsoil constraints for crop production.

Approximately 5 and 29% of soils used for crop production globally have a sand or gravelly subsoil. The proportion of cropping soils with sand subsoil is greatest in Africa and Australia and Oceania. The countries with the greatest area of soil with gravel subsoil used for cropping are India and China. Sand and gravel subsoils have a limited capacity to supply soil and water to crops. Sand subsoils by definition have low clay content and, as a result, low water and nutrient storage capacity. The capacity of gravel subsoils to store water and nutrients decreases as gravel content increases. Although crop roots can access water and nutrients from these subsoils, the depth of these resources and physical constraints to root growth limit the efficiency of their use. Sand and gravel subsoils can constrain root growth although the mechanisms differ. Root growth is constrained in sand subsoils by constraints that can develop under crop production: compaction and aluminium toxicity. The impact of gravel subsoils on crop growth depends upon the penetrability of the gravel layer by crop roots. For impenetrable gravel layers, the properties of the topsoil will have the greatest influence on crop growth. For penetrable layers, root depth or length decreases as gravel content increases. There is potential to adapt agronomic management to maximise production on soils with sand or gravel subsoils. Split applications of nutrients can minimise leaching risk. There is evidence that the constraints that develop on sand subsoils due to crop production can be ameliorated profitably.

With the wide adoption of conservation agriculture (minimal soil disturbance, stubble retention, crop rotation), soil nutrient stratification is becoming more prevalent especially for poorly mobile phosphorus (P), potassium (K), copper (Cu), zinc (Zn), and manganese (Mn) that concentrate in the fertilized topsoil (0–10 cm). In water-limited environments, surface soil drying limits root access to the topsoil nutrients, but the nutrients in moist subsoil may play a substantial role in crop nutrition and growth. Although the subsoil is generally lower in available nutrients and organic matter than the topsoil, there is strong evidence that subsoil can contribute significant amounts of nitrogen (N), P, and K taken up by crops. Placing fertilizers deeper in soil profiles increases plant nutrient efficiency in low rainfall regions, because deep fertilizing can induce deeper root growth and leave fertilizer-supplied nutrients in moist subsoil for longer periods during the growing season. However, the contribution of subsoil nutrition to crop growth is limited by subsoil constraints that restrict deeper rooting, including physical constraints, e.g. gravel layers and soil compaction, and chemical constraints, e.g. acidity, alkalinity, salinity, sodicity, nutrient deficiency, and element toxicity. On the other hand, crops and genotypes efficient in nutrient uptake under drought are likely to have an extensive, deep root system and thus a large surface area of contact between roots and soil. The uptake of soil water from moist subsoil and its release into dry topsoil by roots – hydraulic redistribution – may maintain the growth of fine roots and thus prolong nutrient uptake from drying surface soils. A good understanding of subsoil nutrient acquisition by crop species and their response to subsoil constraints is required for designing crop rotations and nutrient management programmes that allow for effective use of subsoil water and nutrients, especially in rainfed agriculture.

The existence, prevalence, and severity of subsoil constraints for crop production globally are under recognized and under-reported. Subsoil constraints (acidity, acid sulphate horizons, alkalinity, compaction, deep sand layers, gravel layers, high-density horizons, pans, pathogens, salinity, sodicity, waterlogged horizons) may be natural features of soil profiles or induced by land use and management practices. The subsoil in this chapter is considered to be the layers of the root zone below the depth of sampling for soil analysis, which typically corresponds to soil below 10–25 cm depth, depending on the soil sampling conventions of the region. Tropical regions, in particular (in Africa, Asia, Northern Australia, and Latin America), contain large areas of deeply weathered profiles that commonly have hostile subsoils that constrain root growth. The main consequence of subsoil constraints is that water and nutrients contained in subsoils are not accessed or efficiently utilized, and hence crops fail to reach their yield potential. Even when best management practices are applied to the topsoil, yield of crops is depressed by subsoil constraints. Crops may acquire up to 75% of N, 85% of P, and 70% of K uptake from the subsoil if root growth is not constrained. Technologies to sense, identify, map digitally, and ameliorate subsoil constraints represent a promising frontier for soil management, with the potential to substantially lift crop productivity in many parts of the world.