The living environment: Soils

-aeration of soil by ploughing and drainage -addition of soil nutrients -irrigation -soil compaction, increasing bulk density -pH control.

-rain splash -wind blow -surface runoff.

-vegetation -soil organic matter -high infiltration rate.

The Universal Soil Loss Equation (USLE) can be used to estimate erosion rates.

-ploughing vulnerable soils -vegetation removal -overgrazing -reducing soil organic matter -reducing soil biota -cultivating steep slopes -soil compaction by machinery or trampling.

-reduced productivity -sedimentation in rivers and reservoirs -downstream flooding -coastal sedimentation -increased atmospheric particulates -desertification -landslides.

-long-term crops -contour ploughing -tied ridging -terracing -windbreaks -multicropping -strip cropping -mulching -increasing soil organic matter.

Soil fertility is the ability of soil to sustain plant growth.

• water content • soluble materials • air content • dead organic matter • pH • soil biota • soil texture • soil structure • soil depth

Water is essential for all organisms that live in the soil, including the plants. A fertile soil allows good drainage so it does not become waterlogged but still retains enough water for the survival of the soil biota. Plant nutrients are absorbed in ionic form, dissolved in water.

Fertile soils contain macronutrients such as nitrogen, phosphorus, and potassium, present in ionic form mainly as nitrates, phosphates, and potassium ions. Fertile soils also contain micronutrients, including boron, cobalt, copper, iron, manganese, and magnesium. In a fertile soil, toxic ions such as aluminium and heavy metals are adsorbed onto the surface of mineral particles, usually clay, so they cannot dissolve in the water where they could harm soil organisms.

Most living organisms in soil and many processes that increase fertility are aerobic, so well aerated soils are likely to be more fertile.

Fertile soils usually have a high dead organic matter content, which releases plant nutrients as it decomposes. Dead organic matter also increases water retention and provides food for soil biota.

The pH of fertile soils is usually in the range pH 5.5 to pH 7.0 which is the range of tolerance for most plants and other soil biota. Acidic soils can increase the leaching of plant nutrients and damage root cell membranes. Under alkaline conditions phosphates become insoluble.

Living organisms are involved in many soil processes that affect soil fertility. - Detritivores such as beetle larvae, millipedes, woodlice, and slugs break up dead organic matter and release nutrients into the soil. Worms and detritivores also increase soil drainage and aeration by creating tunnels in the soil. - Decomposers, including bacteria and fungi, break down dead organic matter. They secrete digestive enzymes and rely on detritivores to physically break up the dead organic matter and increase its surface area. - Nitrogen-fixing bacteria converts gaseous nitrogen into ammonium ions. - Nitrifying bacteria oxidise ammonium ions to nitrite ions then to nitrate ions. - Mycorrhizal fungi form symbiotic relationships with plant roots and aid phosphate uptake by the plants.

Soil texture is the property controlled by the proportions of the different size categories of mineral particles present in the soil.

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0.002 - 0.02

0.02 - 2.0

The larger pore spaces of sandy soils allow rapid drainage which reduces the water content but increases aeration. Clay soils are poorly drained (due to capillary action and surface tension).

The tiny pore spaces between clay particles allow water underground to rise towards the surface. There is no capillary rise of water in sandy soils.

The pore spaces in well-drained sandy soils are filled with air. Clay soils are more likely to be waterlogged with a low aeration rate.

Nutrient ions absorb easily onto clay particles but not onto sand particles.

The high water content of clay soils produces a high thermal capacity so they warm up and cool down slowly. Sandy soils have lower thermal capacities so they warm up more rapidly after cold weather.