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  The higher the CEC, the greater the soil’s capacity to store nutrients – and the higher the level of fertility of the soil. All the best soils have a high clay and organic-matter content to maximise the CEC. In horticulture, we usually increase the CEC by adding organic matter such as well-rotted compost to the soil, but you can also use clays such as bentonite and vermiculite.

  The major cations present in soils – sodium, hydrogen, aluminium, potassium, calcium and magnesium – balance on the CEC. It just happens that the best balance of the major cations for plant growth and soil structure is:

  sodium – less than 5 per cent

  hydrogen and aluminium – less than 2 per cent

  potassium – 5–15 per cent

  calcium – 60–75 per cent

  magnesium – 15–25 per cent.

  If your soil shows any dispersion (see Stability and dispersion), chances are that it is deficient in exchangeable calcium and has excessive amounts of sodium or magnesium on the CEC; in other words, the soil is sodic, magnesic or both. This is very common in clay subsoils and results in them being poorly structured, sticky (or ‘cloying’, the origin word of ‘clay’), hard setting and badly behaved. Such soils will respond to gypsum.

  Adding compost, worm castings and fresh organic matter to your soil … greatly encourages the beneficial microbes that are already present in the soil to multiply. Nothing else is needed.

  Normal compost contains many beneficial microbes, so you don’t need to buy them as special soil additives.

  Worm ‘wee’ or compost tea has soluble nutrients in an immediately available form for plants, and it encourages many useful soil organisms.

  Other suspicious preparations (usually involving microbes or organic humus extracts) claim to ‘unlock’ nutrients that are tied up in the soil. You can’t replace the 30 kilograms of phosphorus and 300 kilograms of potassium per hectare taken out of the soil every year during crop cultivation by ‘unlocking’ more nutrients – you need to add more nutrients to the soil. Even if these preparations did work (which they don’t), it would be, in effect, nutrient ‘mining’ and not sustainable. Our philosophy is simple, and it works: what we take out of the soil, we must put back into the soil.

  Our advice? With the exception of Rhizobium and some strains of fungi (mycorrhizae), there is no evidence to suggest that there is any benefit to manipulating soil microbes by adding any particular strain of them to soils. Rather, incorporate what microbes need to live on, and they will soon colonise the soil in their millions. Adding compost, worm castings and fresh organic matter to your soil on a regular basis is not only good for the plants, but it also greatly encourages the beneficial microbes that are already present in the soil to multiply. Nothing else is needed.

  Facts about soil microbiology

  The particular form of organic matter added to the soil influences the types of microbes that are present. Put cheese or milk (protein) in a soil, and more likely than not you’ll get penicillin-like fungi growing in it; straw (cellulose) invites a different kind of fungi to colonise your soil. Place meat in soil, and you’ll get all kinds of smelly bacteria. Each type of organism has a complex range of dietary preferences, and organisms will proliferate when their needs are met.

  There is good evidence that well-composted organic matter contains a wide spectrum of microbes that seem to suppress certain root diseases of plants. We conducted a trial once using compost on root rot of beets. Our compost treatment controlled the disease at least as well as using twice the recommended rate of the strongest fungicide.

  There are two types of microbes for which the benefit to plant growth has been very well documented. Bacteria in the Rhizobium genus fix nitrogen in legume roots, while mycorrhizae live in plant roots and help the roots to extract nutrients such as phosphorus by greatly increasing the surface area of the roots. Both varieties of microbes are very choosy when it comes to their plant friends; a particular strain will only usually benefit one plant genus, or even just a single plant species. The bottom line with these beneficial microbes is that they can only be introduced to your soil in specific circumstances, so research the subject very carefully before spending any money on these organisms.

  In the world of soil biology, always remember: there is a lot of muck and magic. If it sounds too good to be true, it probably is. And, even more importantly, making and using your own organic composts and worm castings is the cheapest, best and simplest way to introduce beneficial microbes into your garden.

  Compost heaps don’t have to be pretty to be useful soil microbe farms – they just have to be well fed with kitchen scraps and green matter!

  ASSESSING SOIL PH

  The level of acidity or alkalinity (as measured by the pH) in a soil is a fundamentally important property that affects a soil’s physical, chemical and biological fertility. There are three main reasons for this:

  Some plants and their roots simply cannot survive outside a narrow pH range.

  The level of acidity influences chemical reactions in the soil and determines how available most nutrients are to plant roots.

  The vast majority of microorganisms that are vital for maintaining a living and productive soil environment require a certain level of acidity to be active.

  Soil pH is a good indicator of the general state of health of your soil, so measuring it is like a doctor taking your blood pressure – it will give a pretty good idea of whether there is a balanced, healthy environment below the surface. Various factors, including the long-term use of fertilisers such as ammonium sulphate, can gradually alter pH over time, so testing your soil’s pH every few years is a good ‘check-up’ for your soil.

  The pH scale ranges from 1 (highly acid) to 14 (highly alkaline). An extreme pH at either end of the scale (think battery acid at one end, and caustic soda at the other) equates to a very corrosive situation that will kill any plant exposed to it. The vast majority of soils fall within the range of 4 to 8, and most plants prefer a pH of between 5 and 7. Most natural Australian soils are acid, except in coastal areas of South Australia and Western Australia and pockets of New South Wales and Victoria, where basalt and limestone occur. Granites and most sedimentary rocks such as sandstone give rise to acid soils.

  If a plant’s veins are green but the cells between the veins are yellow (known as interveinal chlorosis), this is a sure sign of iron deficiency due to alkaline soil.

  Problems caused by incorrect soil pH

  Nutrient availability is strongly affected by pH level. This is because the solubility of some ionic nutrients is dependent on pH. The classic example is iron, which forms very insoluble iron oxide in soil with an alkaline pH level. This makes it unavailable to plants that are not specifically adapted to alkaline soil. A deficiency of iron results in severe yellowing of a plant’s new leaves (which is known as interveinal chlorosis). Interestingly, the same symptom shows up in plants with manganese deficiency thanks to alkaline soils.

  Plants that are intolerant of alkaline soils have traditionally been known as acid-loving plants, and they include:

  Rutaceae: pretty much every Citrus species

  Rosaceae: raspberries (Rubus species) and strawberries (Fragaria species)

  Proteaceae: macadamia nut (Macadamia integrifolia)

  Ericaceae: blueberries (Vaccinium species).

  However, rather than calling them acid-loving plants, it would be better to describe them as ‘iron-inefficient’ plants because of their inability to take up iron when soil pH is alkaline. On the other hand, some plants – formerly known as ‘lime lovers’, but now called ‘iron-efficient’ plants – have mechanisms that allow them to obtain iron even from alkaline soils. They do this by exuding acids that convert the insoluble iron oxide into a soluble form. Plants in this group include:

  Lamiaceae: rosemary (Rosmarinus officinalis), lavenders (Lavandula species) and thyme (Thymus vulgaris)

  Fabaceae: garden pea (Pisum sativum), beans (Phaseolus species) and lucerne (Medicago sativa)
.

  In very acid soils, with a pH below 5.5, two very important ions can rise to toxic levels: manganese and aluminium. Some plants are tolerant of high levels of these two ions, but most are not – excess manganese and aluminium depresses plants’ productivity or kills them. Plants susceptible to toxicity from these two elements include most species in the Fabaceae (legume) family, one of the most important of all food families. This is why you are commonly advised to put lime on the soil before planting peas and beans, although this will only be necessary if the pH is 6 or less.

  TESTING THE PH OF YOUR SOIL

  There are two products within the urban farmer’s reach for testing pH. One is a small colour-test kit, which is relatively inexpensive and can give accurate results if the instructions are followed. (1) A small amount of moist soil is placed onto a white tile. (2) Drops of indicator solution are added until the soil is just saturated. (3) Special white powder is sprinkled on top of the wet soil, and the powder changes colour based on the soil’s pH. (4) The powder colour is then measured against the colour chart provided.

  Another tool for testing pH is a small battery-operated pH meter. The only ones that are of any real value are those with glass bulb probes. These give very accurate readings and cost around $200 each. This may sound like a lot of money, but not in comparison to losing all your produce due to an acid or alkaline soil. We are continually confronted by commercial growers who are reluctant to spend $200 on a pH meter and yet will spend $2000 a year on lime and dolomite that they may not need.

  The width of the colour band is an indication of the availability of the nutrient at a given pH – the wider the better.

  TESTING FOR TOO MUCH LIME

  Overliming the soil is a common problem. Often people are told by ‘experts’ that the soil in their area is acid and needs lime, and many seed packets suggest the addition of lime at sowing. If this is done regularly for many years, lime levels will build up and the soil’s pH will rise. This is disastrous for acid-loving plants, as they will be forever affected by iron deficiency. They will have distorted leaves and stunted plant growth.

  To test for lime, place a small amount of soil on a dish or in a glass jar, and add a few drops of a diluted acid, such as one part hydrochloric acid (sold as swimming-pool or brickie’s acid) and five parts water. Fizzing or bubbling indicates the presence of calcium carbonate (lime), as the carbonate reacts with the acid to produce carbon dioxide gas. The more violent the fizzing, the greater the lime content and the harder it will be to acidify the soil. Consider growing alkaline-tolerant plants.

  Raising soil pH

  Often known as sweetening or liming the soil, raising a soil’s pH to reduce acidity is accomplished with liming agents. There are a number of different liming agents, which vary both in their mineral composition and their liming strength (see the Liming Agents to Raise pH table).

  The two most common liming agents are lime (calcium carbonate) and dolomite (calcium magnesium carbonate). Rarely is builder’s lime used, but its great advantage over lime is that it’s more soluble and hence reacts faster. It can also be applied to the surface and washed in, but be careful – it is very caustic and can burn your eyes. Note that it is much stronger than lime, so you don’t need as much.

  Which liming agent you should use depends on the amount of calcium relative to magnesium in your soil. Ideally there is about three to five times more calcium than magnesium. Note that liming agents should not be utilised unless a pH test shows that they are needed. Excess lime has its own set of problems, which are even more difficult to correct than an acid soil.

  The use of lime is appropriate for many unimproved acid soils when they are first brought into cultivation, as they often have enough magnesium but insufficient calcium. For long-term maintenance, however, always utilise a mixture of equal parts lime and dolomite. This provides two important nutrients, calcium and magnesium, in exactly the right balance.

  Some common fertilisers acidify soils, and the worst offenders are the ammonium forms of nitrogen: urea, ammonium phosphate and ammonium sulphate. Continued use of these will cause a gradual drop in pH, and this will need to be offset periodically (possibly every two to five years) with a liming agent.

  The amount of lime or dolomite needed to raise a soil’s pH depends on soil texture and the starting pH. Clays need much more than sands to obtain a given pH rise. The Using Lime or Dolomite to Raise pH table on the opposite page will help you work out the approximate amount of lime or dolomite required per square metre to raise the pH in the top 100 millimetres of your soil. And remember: the importance of testing your soil’s pH regularly cannot be stressed enough.

  Members of Fabaceae (such as peas and beans) are intolerant of acid soils. They will need lime if the pH is below about 6.5.

  When spreading lime over soil, note that one standard cupful equates to about 300 grams.

  LIMING AGENTS TO RAISE PH

  LIMING AGENT AMOUNT EQUIVALENT TO 1 KG OF LIME COMPONENTS

  Lime 1 kilogram Pure calcium carbonate

  Magnesite 840 grams Pure magnesium carbonate

  Dolomite 920 grams About two parts calcium to one part magnesium carbonate

  Builder’s lime 740 grams Pure calcium hydroxide

  USING LIME OR DOLOMITE TO RAISE PH

  SOIL TEXTURE FROM PH 4.5 TO 5.5

  (PER SQUARE METRE) FROM PH 5.5 to 6.5

  (PER SQUARE METRE)

  Sandy and loamy sand 85 grams 110 grams

  Sandy loam 130 grams 195 grams

  Loam 195 grams 240 grams

  Silty loam 280 grams 320 grams

  Clay loam 320 grams 410 grams

  Clay 360 grams 500 grams

  Lowering soil pH

  Acidifying soil is not as commonly necessary as liming. It is sometimes needed for acid-loving plants, where a high pH has been induced by overzealous liming or natural alkalinity (such as limestone-based soils).

  The safest thing to use on your soil is a handful of iron sulphate per square metre every month until the pH is down to around 6.5. Use a hose or watering-can to wash off any iron sulphate that gets on the foliage of plants straight away, as it will burn. Agricultural sulphur (or ‘flowers of sulphur’) can be used if the pH is very high, as it is much stronger than iron sulphate – but only use one dose of half a handful per square metre. Its action is much slower than that of iron sulphate, and it is very insoluble so it must be dug in to the soil. Water your soil well afterwards to help it dissipate more quickly. Measure the soil pH three months later to check how much it has dropped – if you have a heavy lime soil, you may find that you need to apply another dose of agricultural sulphur at this time.

  ASSESSING SOIL SALINITY

  As we mentioned earlier in this chapter, elements supplied as either organic or mineral fertilisers must break down into very simple forms known as salts before they can be absorbed by plant roots. If you are not careful when using fertilisers, it is quite easy to overfeed your plants; salts then build up in your soil until it becomes saline, and plants start to die. Salinity due to overfeeding of plants is a relatively common occurrence, thanks to the misguided belief that if some is good, then more is better.

  To measure the salinity in the soil, we can make use of an interesting scientific phenomenon: pure water does not conduct electricity, but salty water does, and the higher the salt content, the higher the conductivity. These days, there are inexpensive handheld electrical conductivity (EC) meters available that will measure the amount of soluble salt in water extracted from your soil. We highly recommend one if you have a commercial operation, especially with a hydroponic set-up.

  Low conductivities equate to low total salt levels, which means that there is little nutrition in the soil. High conductivities mean that there are excessive salts in the soil. This may be due to the presence of common salt (sodium chloride), or the overenthusiastic application of fertilisers when poor plant growth is blamed on low soil fertility. Soil purchased from commercial centres comm
only has too much compost, and is therefore quite saline.

  In urban farms with good drainage and good-quality water, salinity is rarely a problem. However, the issue can arise when:

  poor-quality water such as bore water is used – we should always get the water tested in a new growing situation

  capillary or ‘wick’ watering is used – this is because we are operating in a ‘closed system’ where it is difficult or impossible to flush excess salts from the soil

  overfertilisation has occurred – it is always crucial to ensure that you are using fertilisers at an appropriate strength, whether they are mineral or organic in nature.

  In summary, measuring the amount of salts in your soil is just as important as measuring the soil pH. A low-salinity reading is a sure indication that your plants are hungry and are almost certain to respond to an appropriate dose of fertiliser. On the other hand, if you have high salinity, then you need to take action in the form of heavy watering to wash the excess salts away.

  Commercial composts can be quite high in salts, so always ask for a nutrient analysis.

  Most vegetables, such as cabbages and parsnips, are salt sensitive and suffer if the salinity level is too high.

  URBAN SOILS

  As it turns out, cities are particularly conducive to plant life because of the incidental nutrients that are provided by simply being in an urban environment. The atmosphere is enriched with nitrogen, carbon and sulphur; soils are often enhanced with phosphorus and calcium; the pH levels are usually higher (the soils are less acid); and the important micronutrients zinc, copper and boron are available from a variety of sources. However, poor management choices and overzealous fertiliser use on urban farms can lead to soil chemistry imbalances. The Symptoms of Soil Chemistry Imbalances table details some of the most common imbalances and their cures.