Soils: A living legacy
Soil is a synergistic amalgamation of different parts. Giving it form is a framework of mineral matter ranging in size from large boulders down to fine clay particals. Pores between the mineral parts, are the space where air and water resides within the soil. Added to this mix is all manner of plant and animal life, and organic matter in varying stages of decay. When the mineral nutrients are also factored in, the result is soil with the capacity to support the long-term production of healthy and health-giving crops.
Soils are naturally divided into layers or horizons that are best seen by digging a pit. The upper layer is the topsoil, which is generally darker in colour than the lower layers due to the accumulation of organic matter. The topsoil is the substance most often associated with the word ‘soil’, and is where most of the soil’s organisms and much of its fertility reside. Just below the topsoil is the lighter-coloured and less fertile subsoil. The depths of these two horizons vary, and soils should always be cultivated in such a way that does not mix them together.
If the pit is dug down deeper, it will come to a third horizon consisting of the parent material from which the topsoil and subsoil originated. Digging a pit can also reveal the presence or absence of a hard pan – a distinct layer of compacted soil that prevents root penetration and the downward movement of water. Though they sometimes occur naturally, hard pans are often caused by rotavating when the soil is too wet or repeatedly at the same depth.
The soil is a thriving menagerie of life forms from both the plant and animal kingdoms. They include the more obvious plant roots and moles, as well as snails, slugs and insects. There are also the microscopic organisms such as nematodes, fungi and bacteria, unseen but no less significant in the roles they play.
The organisms residing in a soil affect its physical and chemical properties while influencing the health and productivity of the crops growing in it. The do-gooders decompose organic matter or extract gaseous nitrogen from the air, while the pathogens infect plants and cause disease. Earthworms make their presence known, too, by burrowing their way through the soil and improving its drainage, structure and fertility. Like any other community, the soil is a mix of the good and the bad, all contributing to the make-up of an ever-changing ecosystem.
Pores make up between 40 and 60 per cent of the volume of a soil. They are filled with either water or air, with the proportion of water being determined by the amount added by rainfall and irrigation; the volume taken up by plants through their roots; and the quantities lost by evaporation from the surface. The pores vary in size from small to large, and good soil management encourages a balance between air-filled pores and a crop’s need for water.
Solid as a stone
The solid part of most soils is dominated by mineral matter that comes in a range of sizes. Some of the larger material such as stones and gravel can interfere with cultivation and seed sowing while wearing down rotavators tines and tyres. Though small amounts can be tolerated, very stony soils should be avoided, especially where annual crops are grown.
The smallest-sized mineral matter in soils are particles of sand, silt and clay. Sand particles are the largest, and can be anywhere from 0.06mm to 2mm in diameter, while clays, the smallest, are less than 0.002mm in diameter. Silts, of course, fall in between.
Soils usually contain a mix of all three particles, and the relative quantities of each determine a soil’s ‘texture’, chemical properties and physical attributes. Though often measured in the laboratory, texture can be determined manually by the ‘feel’ method that uses a sample of wet soil rubbed between the fingers and thumb:
Sandy soils feel gritty and fall apart when rolled into a ball.
Clay soils are sticky and can be rolled into a sausage shape.
Silt soils are neither sticky or gritty, but have a buttery, silky or soapy feel much like talcum powder.
These are dominated by large pores, and so are free-draining and need frequent watering during a drought. They also tend to be low in fertility since their nutrients leach out with excess rain or irrigation water. On the plus side, sandy soils warm up quickly in the spring; are easy to cultivate; and can be worked soon after wet weather.
These have pores that are small and slow to drain. Leaching losses of nutrients are less, and the fertility and organic matter levels are higher, than those of sandy soils. When these soils dry out, they tend to turn hard and are difficult to dig. They are slow to heat up in the spring, especially if wet.
These soils are less water retentive than clay, but are slower draining than sand. Their nutrients are also less prone to leaching than they are in a sandy soil.
Loams are a mix of sand, silt and clay in proportions that give each type of particle equal control over the chemical and physical properties of the soil. The best soils are loams, and properly managed, they are as easy to work as sands while having the moisture retention and nutrient levels of silts and clays. Sand and silt soils are often called ‘light’ because they are loose and easy to work. Alternatively, sticky and difficult-to-work clay soils are deemed to be ‘heavy’, while loams, naturally enough, are considered as ‘medium’.
The particles of sand, silt and clay in a soil do not act individually, but are stuck together into ‘aggregates’. This aggregation is called ‘structure’, and occurs throughout the soil profile. Water movement, aeration and porosity are influenced by structure, which in turn affects the speed at which soils heat up in spring; the extent of root growth; and the biological activity of the resident life.
A goal of soil management is to encourage a stable granular or crumb structure in the topsoil. This sort of structure produces the fine tilth needed by germinating seeds and growing plants, and is promoted by digging in bulky organic matter; keeping tillage operations to an absolute minimum; and including fibrous-rooted green manures in the cropping programme.
Structure is weak when soils are wet, so cultivations must be carried out only when conditions are dry enough to prevent damage to the aggregates. Improving the drainage of water-logged soils also helps. Since wet aggregates are easy to crush, vegetable gardeners can use a bed system to limit foot traffic to paths bordering the growing areas.
Light-textured silt and fine sand soils low in organic matter have poor structural stability. When they are pounded by heavy rains or irrigation, their aggregates break down, and form hard crusts or caps that restrict seedling emergence. Soils prone to capping should not be watered after sowing, but if it cannot be avoided, then the water should be applied gently as fine drops.
Soils contain organic matter in varying degrees. Most top soils, for example, have between 1% and 6%, while subsoils usually have less than 2%. The organic matter in the soil is made up of dead and decaying plants and animals that are gradually broken down into humus, a chemically-complex, dark-coloured and rather stable substance synthesised by microbes.
Organic matter has an influence far beyond the modest proportions found in soils, and its importance cannot be overstressed. For example, it provides mineral nutrients to plants and is a food source for soil organisms. It also improves soil structure and may help control plant diseases. In addition, organic matter protects nutrients from being leached out by rainwater, indirectly influencing soil fertility.
Clay soils are not only easier to work but are also better drained and aerated when plenty of organic matter has been added, while the water holding capacity of sandy soils is increased by such treatments. In addition, organic matter applied as a mulch prevents water loss and reduces soil erosion.
Organic matter naturally breaks down, and its loss from a soil is a constant phenomenon accelerated by high temperatures, improved aeration and liming. Consequently, soils need to be constantly topped up by mixing in old crops, adding manure and compost and including green manures in the rotation.
The acidity or alkalinity of a soil is measured in pH units that range on a scale from 0 to 14. Neutral soils have a pH of 7.0; alkaline soils are over 7.0; and acid soils fall below 7.0. The majority of soils are in the pH range of 4.5 to 7.5, though some chalky or calcareous soils can reach 8.0 to 9.0.
The pH affects the availability of nutrients and the biological activity of the soil. For example, bacteria are less active and molybdenum is less available to plants in acidic soils, while manganese and iron deficiencies occur at high pH values. A low pH inhibits potato scab, and a pH of 7.0 and above keeps brassica clubroot in check. Some crops such as blueberries thrive in acid soils while others prefer more alkaline conditions.
Except for chalky or calcareous soils, the natural tendency is for soils to become more acid. There are simple-to-use proprietary tests that determine whether a pH adjustment is needed. If this proves to be the case, then lime, made primarily of calcium carbonate, can be applied to neutralise the acidity.
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