(a) What do you mean by soil consistency and soil plasticity ? Briefly discuss the various forms of soil consistency found in…

(a) Soil consistency and plasticity (20 marks)

Soil consistency is the resistance of soil to deformation or rupture, the manifestation of the forces of cohesion (attraction between soil particles) and adhesion (attraction of soil particles to other substances such as water films and tools) at different moisture contents. It decides how the soil behaves in tillage and engineering, and depends on texture, clay content and type, organic matter and moisture.

Soil plasticity is the ability of soil to be moulded into a shape by an applied force without rupture, and to retain that shape when the force is removed. It arises when the soil is moist enough to allow particles to slide over each other on water films, and is greatest in clayey soils, particularly those rich in 2:1 minerals such as montmorillonite, and is reduced by sand. It is measured by Atterberg limits: the plastic limit (lower moisture limit at which the soil is plastic), the liquid limit (upper limit, beyond which it flows), and the plasticity index, the difference between the two. A soil with a high index is difficult to till.

Forms of consistency by moisture condition:

• Wet soil: stickiness (adhesion to other objects) graded as non-sticky, slightly sticky, sticky and very sticky; and plasticity graded as non-plastic, slightly plastic, plastic and very plastic.
• Moist soil: the friability, graded as loose, very friable, friable, firm, very firm and extremely firm, and the friable state is the best for tillage.
• Dry soil: hardness, graded as loose, soft, slightly hard, hard, very hard and extremely hard.
• Cemented consistency, caused by cementing agents (calcium carbonate, silica, iron oxide), graded as weakly, strongly cemented and indurated.
• Two other terms: shrinkage limit (below which the soil does not shrink) and sticky limit. Importance: consistency determines the correct moisture at which to plough (tillage at the friable stage avoids clods and puddling), the workability of soil, resistance to erosion, and design of foundations and roads. Heavy soils show more of the wet forms (sticky, plastic), and sandy soils are loose or non-sticky in most conditions.

(b) Nitrification and factors affecting it (20 marks)

Nitrification is the biological oxidation of ammonium (NH4+) to nitrate (NO3-) by aerobic chemoautotrophic bacteria, in two steps:

• Step 1: NH4+ + 1.5 O2 → NO2- + 2H+ + H2O, by ammonia oxidisers such as Nitrosomonas, Nitrosospira and Nitrosococcus (and ammonia-oxidising archaea).
• Step 2: NO2- + 0.5 O2 → NO3-, by nitrite oxidisers such as Nitrobacter, Nitrospira and Nitrococcus. The process makes nitrogen mobile, since nitrate is not held by soil colloids, so it is easily taken up by crops but also leached or denitrified. The first step is rate-limiting, and nitrite rarely accumulates.

Factors:

• Supply of ammonium: the substrate is provided by mineralisation, urea hydrolysis or ammoniacal fertiliser; high concentrations of free ammonia inhibit Nitrobacter and cause nitrite accumulation.
• Aeration (oxygen): the process is strictly aerobic, so it is favoured in well-drained soils and stops in waterlogged or compacted soils.
• Moisture: optimum at about 50 to 60% of water-holding capacity; low in dry soil and in waterlogged soil.
• Temperature: optimum 25 to 35 degrees Celsius; slow below 10 and stops around 5 or above 45.
• Soil reaction: best at pH 6.5 to 8.5; inhibited below about 5.5 (acid soils), and lime promotes it.
• Organic matter and C:N ratio: a wide C:N ratio leads to immobilisation of ammonium, reducing nitrification; the organic matter supplies ammonium and energy for other organisms.
• Nutrients and salts: adequate calcium, phosphorus and micronutrients help; high salinity inhibits it.
• Nitrification inhibitors and toxic substances: nitrapyrin, dicyandiamide, neem cake and karanja, and certain pesticides slow the process, prolonging availability of ammonium; used to save fertiliser nitrogen.
• Soil texture and microbial population, and cropping, through root exudates. Management uses these factors to improve nitrogen use efficiency, for example by splitting fertiliser, using neem-coated urea and controlling drainage.

(c) Rise in available phosphorus after flooding (10 marks)

Flooding increases the availability of phosphorus in most soils, because of these changes:

• Reduction of ferric to ferrous: ferric phosphate, and phosphate held on ferric oxide and hydroxide coatings, is reduced to more soluble ferrous phosphate, releasing phosphate (reductive dissolution of the occluded, unavailable phosphorus).
• Change of pH towards neutrality: pH of acid soils rises and pH of alkaline and calcareous soils falls, towards about 7. In acid soils, the solubility of iron and aluminium phosphates increases; in alkaline soils, that of calcium phosphates.
• Release of anions: organic acids and carbonate formed by decomposition displace the phosphate held on clay and oxide surfaces (anion exchange).
• Greater mineralisation of organic phosphorus, and its solubilisation by microorganisms.
• Greater diffusion of phosphate through the water film, since the water-filled pores increase mobility to the roots.
• Reduced formation of insoluble aluminium-phosphate compounds in the presence of organic anions; and lower fixation. These effects explain why rice, grown flooded, often responds less to phosphorus than upland crops, though the effect is temporary and phosphorus can be fixed again when the soil dries.