Major Soil orders of Nepal according to USDA taxonomy

a. Entisols:

• Youngest and least developed soils.
• These are the soils found on hill sides and adjacent to river courses and on the steeper, less stable slope throughout the mountain regions.
• These are the soils formed through deposition of colluviums and alluvium.
• This category of soil may also be found at constructions site. The parent materials are not exposed to soil forming factors. Soil productivity ranges from very high for certain entisols formed in recent alluvium to very low for those forming in shifting sand or on steep rocky slopes. (Brady & Well, 13th edition, page no.88).
• Lack of clear soil horizons is the distinguishing feature of these soils.

Major groups of entisols found in Nepal

I. Ustifluvents

• They tend to show little or more pedogenestic development and often exhibit strong evidence of recent sedimentation.
• Textures of these soils are generally coarse sandy with considerable inclusions of gravel within most of mountainous regions.
• Depending upon the type of materials carried by the rivers they can be calcareous or non-calcareous.

II. Ustorthents

• They describe those surfaces where older soils have been seriously eroded by surface erosion and origin diagnostic horizons are absent.
• Generally, orthents are found on steep slopes (over 30 degrees) or where landslide runouts have been deposited.
• They are shallow near the bedrock, coarse textured and poorly vegetated.
• These soils are used for grazing, fodder and firewood collection. Under the areas of gentle slope can be quickly reclaimed for agriculture, while steeper slopes are rapidly reinvaded by pioneer vegetation.

III. Fluvaquents

• These entisols are also found adjacent to rivers, are poorly to imperfectly drained, vary in texture and occasionally flooded. If suited to cultivation rice can be grown.

b. Inceptisols:

• Inceptisols show more significant profile development than Entisols, but are defined to exclude soils with diagnostic horizons or properties that characterize certain other soils orders (Brady & Well, 13thedition, page no 92). Inceptisols covers 9% of world land area.

Major groups of Inceptisols found in Nepal

I. Aquepts:

• Aquepts are relatively stable soils that are strongly affected by a high-water table- at least during the monsoon season. Subsoil is under anaerobic conditions for long periods and this inhibits the plant growth but is conductive to rice production.
• Aquepts are common in lower terai, in stable, low relief areas and are commonly associated with infilled back water channels.

II. Ochrepts:

• These are the commonest soils in the terai as well as in the middle hills, mostly below 1500 meters (higher on south facing slopes) and have developed on the acidic or neutral bedrock including lacustrine deposits. They have well developed B horizon and base saturation below 60%.
• Ustochrepts with high base saturation are most prevalent in the Far Western and Mid-Western Development regions of Nepal, where the climate is considered to be sub-humid (Annex 2, classification of Nepal’s soil, icimod.org).
• Subgroups under Ochrepts are described below:

i. Dystrochrepts and Udochrepts

• Dystrochrepts and Udochrepts have a lower base saturation percentage and are more acidic.
• They are more commonly found in Central and Eastern parts of Nepal, where more humid conditions create stronger leaching conditions.

ii. Cryochrepts:

• Cryochrepts are common at elevations above 3,500 meters, on moderately sloping lands throughout the country. They are of no importance for agriculture production.

iii. Eutochrepts:

• Eutrochrepts soils are similar to ustochrepts but developed on calcium rich parent materials.

III. Umbrepts:

• Umbrepts are the dark colored Inceptisols that usually occur above 2,000 m (1,500m on northern aspects).
• They have low base saturation and high organic matter levels in the surface soil.
• If organic matter oxidized off, their natural acidity becomes limiting to the growth of many agricultural crops
• Subgroups under Umbrepts are described briefly below.

i. Haplumbrets:

• Haplumbrets are the soils of high and middle hill regions (3,500m) and developed in cool temperatures on the acidic bedrocks under mixed forest.
• They have low base saturation.
• Soils under forest and on steep slopes are shallow and stony but the cultivated ones are fertile due to a high organic matter content, which inactivates the toxic effect of aluminum by its chelating action.
• Soil fertility-is maintained by grazing animals, and leaving fallow for 2-3 year periods.
• Barley, millet and potato are the main crops grown in this soil.

ii. Cryumbrepts:

• Cryumbrepts are also the soils of high Himalayan and high hill regions generally found above 3,000m but, depending on the local climate altitude vary.
• Soils of this group have dark A horizons, high organic matter with wide C/N ratio low base saturation and contain no free carbonate.
• They are silty in texture. These soils are under snow for at least three months of the year.
• Vegetation ranges from monsoon grasses to Rhododendron and Betula in this type of soil.
• Areas under these soils are extensively used for seasonal grazing.

c. Spodosols:

• Spondosols consists of spodic horizon (Bh,Bs) and indicate a stable but strongly leaching environment.
• Spondosols have strong reddish or black subsoil in which iron and organic have been deposited after initial leaching from the surface soil layers.
• They occur on stable landscapes at elevations above 3,000 meters where conifers dominate the forest.
• Spodosols have higher proportion of organic acids which accelerates weathering. This result in leaching of base cations. So, Spodosols are not fertile soils.

d. Mollisols:

• Soils with high organic matter content, usually under thick grass or forest, dark colour and high base saturation are classified under Mollisols.
• They develop on basic parent materials at higher elevations.
• They are formed on calcium rich parent materials and throughout rapid base recycling and/or low leaching, have maintained their high base saturation.
• Mollisols in humid regions generally have higher organic matter content and darker, thicker mollic epipedons than their lower-moisture-regime counterparts.

Some groups under Mollisols are as follows:

I. Haplustolls:

• They develop on alluvial materials and are distinguished by a soft and dark colored mollic Ah horizon with high base saturation and a well-developed Bm horizon under an ustic moisture regime.
• These soils are generally fertile and have high productivity for few years but later yield of crop decreases as organic matter decreases.

II. Cryoborolls:

• These differ from Haplustolls mainly in their development on base rich parent materials under thick grassland of the high mountain in high Himalayan regions.
• They are found in cooler climate and an udic moisture regime.

e. Alfisols

• Has trans-located clay in the subsoil and a high base saturation percentage.
• Alfisols are characterized by a subsurface diagnostic horizon in which silicate clay has accumulated by illuviation

Some groups under Alfisols are as follows:

I. Rhodustalf:

• They are the strong red soils common on ancient terraces, are among the oldest soils found in Western Nepal.
• They are also known to resource managers because of their tendency to crust on the surface after tillage.
• These soils have problems with phosphorous fixation and are occasionally subject to severe gullying.
• Strong local relief, low infiltration rates, slow permeability of subsoil due to clay accumulation and occurrence of high intensity rainfall are dominant characteristics that results in the gullying of these soils.

II. Haplustalfs and Hapludalfs

• The soils that do not meet the color criteria for Rhodic, soils are classified as Haplustalfs and Hapludalfs.

f. Ultisols

• Its properties are identical to those of the Rhodudalf, except that it has a low pH and a low base saturation.
• These soils are important to distinguish because soil acidification rapidly occurs through the use of chemical fertilizers.
• This soil is more weathered and acidic more than Alfisols but less than Spondosols.

g. Aridisols

• Water deficiency is the major characteristic of these soils. The soil moisture level is sufficiently high to support plant growth for no longer than 90 consecutive days.
• The natural vegetation consists mainly of scattered desert shrubs and short bunchgrasses.
• These are the soils that are dry for more than nine months of the year.
• They exhibit very little in the way of weathering and usually have free calcium carbonate and other salts at or near the surface. There is high accumulation of sodium salts.