Soil Science — Part 1 MCQs with Explanations

Amphiboles are a group of rock-forming minerals. Hornblende is the most common and most important amphibole in soil formation. It's important because it weathers relatively easily, releasing essential elements like iron and magnesium that contribute to soil fertility. Tremolite, Actinolite, and Diopside are also minerals, but they belong to different mineral groups (like pyroxenes for Diopside) or are less common/less reactive in soil environments compared to Hornblende. Therefore, Hornblende's abundance and weathering characteristics make it the most significant amphibole in soil science.

Silicate clay is the electro-microscopic clay mineral, which a diameter of fewer than 2 microns. The sources of negative charges on silicate clays are due to exposed crystal edges, Isomorphus substitution and anion exchange.

Nitrogen (N) is the most important nutrient for oil production in oilseed crops because it plays a fundamental role in plant metabolism, growth, and seed development. Why is nitrogen essential for oil production? Protein and enzyme synthesis: Nitrogen is a key component of amino acids, which form proteins. These proteins regulate metabolic pathways involved in oil biosynthesis. Chlorophyll formation: Nitrogen is essential for chlorophyll production, which supports photosynthesis —the process that provides energy for oil accumulation. Enhanced seed development: Oilseeds such as soybeans, canola, and sunflower require high nitrogen availability to maximize oil content in seeds . How does nitrogen impact oilseed crops? In soybeans and canola, nitrogen increases seed oil content by improving lipid metabolism . In sunflower crops, nitrogen improves both seed yield and oil percentage by supporting the synthesis of fatty acids. Adequate nitrogen supply ensures better seed filling , which directly influences oil quantity and quality . Comparison with other nutrients: Sulfur (S) is important for certain enzyme functions but is not the primary driver of oil synthesis. Magnesium (Mg) supports photosynthesis but has an indirect role in oil production. Zinc (Zn) helps enzyme activation but does not directly influence oil yield. Conclusion: Nitrogen has the most significant impact on oilseed crops because it directly influences the metabolic processes required for oil formation. A proper nitrogen supply enhances oil yield, seed quality, and overall crop productivity.

Flooded soils are a condition in which an area of soil is oversaturated with water, often due to natural occurrence or with the intended purpose of agricultural reasons like the cultivation of rice. The most conspicuous chemical change that takes place in the flooded soil is the reduction of iron (Fe). Competition generally limits the use of weaker electron acceptors, iron reducers may exist in the soil but will be dominated by the presence of denitrifiers, who will grow faster with access to their stronger electron acceptor.

Olivine is a silicate mineral that is rich in magnesium and iron and is commonly found in igneous rocks. Its chemical composition mainly contains magnesium silicate minerals , making it an important natural source of magnesium in soils and rocks. When olivine weathers, it releases magnesium (Mg) into the soil, which is an essential nutrient required for plant growth and chlorophyll formation. Although olivine may contain small amounts of other elements, it is primarily recognized as a good source of magnesium rather than boron, calcium, or molybdenum . Therefore, Option B: Mg is the correct answer.

To calculate the percent base saturation, divide the sum of the K, Mg, Ca, and Na (the bases) in meq/100g soil by the CEC. Multiply the result by 100%. Based on the formula A soil with 20 me/100g CEC and % me/100g exchangeable H will have a base saturation of 25%.

Sodium adsorption ratio is a measure of the amount of sodium (Na) Relative to calcium (Ca) and Magnesium (Mg) in the water extract from saturated soil paste. It is the ratio of the sodium concentration divided by the square root of one-half of the calcium and magnesium concentration. It can calculated by using formula: $${\text{SAR}} = \frac{{{\text{N}}{{\text{a}}^ + }}}{{{{\left[ {\left( {\frac{{{\text{C}}{{\text{a}}^{2 + }} + {\text{M}}{{\text{g}}^{2 + }}}}{2}} \right)} \right]}^{0.5}}}}$$

Nitrogen, phosphorus, and potassium are macronutrient uptake by crop yields and crop residues by attaining high-input potential yields compared to that low-input yields, and the number of fertilizers applied. On an average 1 kg of N, P 2 O 5 and K 2 O are added to increase grain yield by 10 kg.

Potassium feldspar refers to several minerals in the feldspar group containing potassium. It is observed the rate of K uptake by the plant is usually greater than the rate of K release from the weathering of feldspar it is because of less rupture of Si-O-Al bonds.

Edaphologists are the researchers who study soil by examining a variety of factors or soil properties that govern plant (flora) and animal (fauna) life in the soil. They study various properties of soil in plant production. Edaphic factors are those soil conditions that affect the organisms living in a particular area.

P 2 O 5 is Triple super phosphate and it is concentrated superphosphate. It contains 46% of P 2 O 5 . This P 2 O 5 in all fertilizers comes from Phosphate rock, a finite source formed over millions of years in the earth’s crust. About 90% of the world’s mined phosphate rock is used in agriculture and food production, mostly as fertilizer.

Apparent Nutrient Recovery is one of the more complex forms of NUE expressions and is defined as the difference in nutrient uptake in above-ground parts of the plants between the fertilized and unfertilized crop relative to the quantity of nutrient applied. This can be calculated if you have values of Nutrients removed from the fertilized plot, nutrients removed from the unfertilized plot, and nutrients added.

Molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Iron molybdate catalyst is a combination of the two oxides that produce the desired active and selective catalyst. The molybdate reagent is a solution containing both the molybdate and vandate ions. It is commonly used in the determination of Phosphate ion content.

Soil crusting is a thin layer of dense and tough material. It is more compacted and packed than the underlying material. Rain impact on exposed soil is the main cause of soil crusting. Rain separates the soil into very small aggregates and individual particles that cement into hard layers at the soil surface when drying occurs rapidly. The problem of soil crusting is prevalent in Silty-clay loam soils.

Quartz is among the group of primary minerals that is highly resistant to weathering. Its high hardness and lack of cleavage make it quite resistant to mechanical weathering. Quartz is itself an agent of mechanical wreathing in the form of blowing desert sand. However, Weathering is the breaking down or dissolving of rocks and minerals on Earth’s surface.

The zinc equivalent calculation is as follows : ZnEq = 100((Ag price in (g) × Ag Grade) + (Pb Price*2204.6 × Pb Grade(%)/100) + (Zn Price*2204.6 × (Zn Grade(%)/100))/Zn Price*2204.6). The numbers may not add due to rounding. Zn - equivalent is related to Heavy metal toxicity.

Basidiomycetes white-rot fungi including Clavaria, Cephalosporium and Humicola all are responsible for efficient lignin degradation in wood decay processes. It produces extracellular enzymes that are lignin-modifying enzymes that activate the decomposition process of lignin.

A soil, which has a pH ranging from 8.5 to 10 and ESP (Exchangeable Sodium Percentage) is greater than 15 and EC (Electrical conductivity) less than 4m.mhos/cm, the soil is called Alkaline soil . ESP is the relative amount of the sodium ion present on the soil surface, expressed as a percentage of the total cation exchange capacity (CEC). Electrical conductivity (EC) is the measure of the number of salts in the soil.

The arbuscules are the most characteristic structures, formed intracellularly and probably having an absorptive function. The vesicles are terminal swellings of hyphae formed inter and intracellularly having a storage function. Vesicles and Arbuscles are found in Endomycorrhizae and Arbuscular mycorrhiza (AM). AM is a type of mycorrhiza in which the symbiont fungus penetrates the cortical cells of the roots of a vascular plant forming arbuscles. Endomycorhizae is the mutually symbiotic relationship between fungi and plant roots.

The solum (plural, sola) in the soil profile consists of the surface and subsoil layers that have undergone the same soil forming conditions. The base of the solum is the relatively un-weathered parent material. The soil profile is a vertical section of the soil that contains all the secrets of the soil. A soil profile having two sola, one above the other is known as Bisequem profile .

The neutralizing value is a measure of the amount of acid a given weight of liming material will neutralize. Calcium carbonate is used as the reference and assigned a neutralizing value of 100 percent. Calcium hydroxide has a neutralizing value (calcium carbonate equivalent) of 136%.

Humic acids are the organic substances that coagulate when the strong-base extract is acidified. It is a chemical produced by decaying plants. It is used as medicine for stimulating the immune system and treating the flu, avian flu and other viral infections. A fulvic acid is a group of chemicals formed when plants and animals break down. It is found in the humus part of soil and peat. Nitrogen content in Humic & fulvic acids is 1 - 4%. Humic acid contains more carbon and less hydrogen than does the animal and plant residues from which it has formed through extensive biological decomposition.

Potash is produced worldwide in amounts exceeding 90 million tonnes per year, with Canada being the largest producer of potash, mostly for fertilizer use followed by Russia and Belarus. The most significant reserve of Canada’s potash is located in the province of Saskatchewan and is mined by the Mosaic Company.

Potassium deficiency symptoms in corn appear first on the lower leaves because potassium is easily moved within the plant from older to younger leaves. Leaf symptoms are yellowing to necrosis of the outer leaf margins. The critical value for K deficiency in maize is 1.0%. Potassium deficiency symptoms were observed when the tissue content was 2 - 3% of K and exchangeable K was less than 5 meq K/100g.

The microrelief of soils produced by expansion and contraction with changes in moisture is known as Gilgai microrelief. A gilgai is a small, ephemeral lake from a depression in the soil surface in expanding clay soils. Gilgai is found worldwide wherever cracking clay soils and pronounced wet and dry seasons are present. Gilgai is also called melonholes, crabholes, hogwallows or puff and shelf formations.

Phyllosilicate, are a compound with a structure in which silicate tetrahedrons each consisting of a central silicon atom surrounded by four oxygen at the corners of a tetrahedron are arranged in form of sheets or chains. A chain like phyllosilicate is Palygorskite-sepiolite. Both of them have similar fibrous or lath-like morphologies, but palygorskite exhibits more structural diversity.

Iron is a micronutrient and is required by plants in small amounts. Most of the annual plants require fe on the order of 1 to 1.5 lb Fe per acre. The deficiency of Iron starts when it is less than 55 ppm. Iron deficiency starts with a yellowing of the leaves in between the dark green veins, giving the leaves a spidery look. After some time, the leaves become whitish and start to die back, eventually resulting in stunting and drying back of the entire plant.

Soils high in swelling clays that crack widely upon drying resulting in shrinking, shearing, and soil mass movement are characteristics of Vertisols . The shrinking and selling of vertisols can damage buildings and roads, leading to extensive subsidence. A vertisol is a soil type in which there is a high content of expansive clay minerals present and typically forms highly basic rocks.

A hygrometer is an instrument used in meteorological science to measure the relative humidity or amount of water vapour in the air. The barometer is a scientific instrument used to measure atmospheric pressure. The thermometer is a device that, measures temperature or a temperature gradient. Rainguage is a meteorological instrument to measure the precipitating rain in a given amount of time per unit area.

Particle density is the density of the solid particles that collectively make up a soil sample. The value is commonly expressed in grams per cubic centimeter. The common range of all the soils is 2.55 to 2.70 grams per cubic meter. The particle density of clay soil is 2.83 gm/cm 3 .

Endepedon is a diagnostic horizon characterized by the accumulation of aluminum and organic matter with or without iron. And Argillic is the mineral soil horizon that is characterized by the illuvial accumulation of layer-lattice silicate clays. For consideration of Argillic endopedon, 1.2 times of clay content need to have than the overlying horizon.

A colloid is a glue-like organic or inorganic matter with too little particle size but a large surface area per unit mass or volume. The size of the clay particles is the smallest. It is less than 0.002 mm in diameter. Moreover, the size of sand is 2.0 to 0.05 mm and Silt is 0.05 to 0.002 mm.

Ammonium nitrate is a chemical compound with the chemical formula NH 4 NO 3 . It is a white crystalline solid consisting of ions of ammonium and nitrate. Both of them i.e. ammonium cation and nitrate anion are in 50% - 50%. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates. It is predominantly used in agriculture as a high nitrogen fertilizer.

Phosphate rock combined with finely-divided, elemental Sulphur and Thiobacillus is combined to form a fertilizer “Biosuper” . Biosuper is effective on soils of both low and medium phosphorus status, and also on soils differing in P retention capacity. Thiobacillus present in Biosuper oxidizes Sulphur, producing sulphates useful to plants.

A benchmark soil is one of a large extent within one or more major land resource areas that holds a key position in the soil classification system. A benchmark soil is a reference point that is adequately characterized in terms of its properties and environmental conditions. 64 benchmark soils have been identified in the different major land resource areas of Kerala for which soil monoliths were collected. The basic objective of this soil is to focus attention on extensively occurring soils and their agronomic concepts for wider acceptability of interpretations and extrapolations of research data.

Acis sulphate soil is the soil or sediments containing iron sulphides. Iron sulphides are micro-crystalline minerals such as pyrite that have formed naturally in soils where long-term water-logged conditions occur. When it is disturbed by drainage, lowering of water tables, or excavation, oxidation of the sulphides creates sulphuric acid which can trigger a range of flow-on effects. Thus, the useful practice to manage acid sulphate soil is keeping the soil waterlogged.

Plant analysis is a crop management tool that has been stimulated by the increased use of scouting programs and crop consultants. The information provided through plant analysis helps farmers with decisions on the effectiveness of fertilizer, the need for additional nutrients, and planning fertilizer programs for future use. It is required plant analysis is useful if done at an early stage.

Humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter by the act of micro-organisms i.e. bacteria and fungi. It is rich in nutrients and retains moisture in the soil. Organic matter is humified by a combination of saprotrophic fungi, like Alternaria and Aspergillus. Whereas Trichoderma strains are used for biological control of plant disease.

Oxisols are a soil order in soil taxonomy, for their occurrence in the tropical rainforest. The main processes of soil formation of oxisols are weathering, humification and pedoturbation due to animals. Oxisols are specified by a thick subsurface layer that contains kaolin-group clay minerals ie. Kaolinite and metal oxides in a finely textured matrix with very little or no easily weathered silicates.

Arkose is a coarse sandstone primarily made up of quartz and feldspar grains together with small amounts of mica, all moderately well sorted, slightly worn, and loosely cemented with calcite or less commonly, iron oxides. Closely resemble granite in appearance and is frequently utilized as a building material.