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Over the years, humans have compiled an impressive compendium of horticultural techniques through trial and error. The earliest written records of horticultural practices are from the first millennium b.c. in China, Mesopotamia (now called Iraq), and Egypt, followed by Greece and Rome. Some of the practices mentioned in these ancient writings include the use of iron tools; manure applications; crop rotation; double cropping; large-scale irrigation projects; pollination, pruning, and grafting of fruit trees; pest and disease control; as well as the identification, classification, and use of plants. Many of these methods are still applied today. These texts also describe methods that may not have been in common use even in ancient times. Similarly, modern texts on cultural methods include some practices that are not widely used but that have been documented nonetheless.
Ancient Egyptians created formal gardens with pools, a spice and perfume industry, and collections of medicinal plants. Mesopotamia had irrigated terraces, gardens, and parks. Significant contributions to taxonomy and plant physiology were made by the Greeks. Romans fostered the development of ornamental horticulture with topiary gardens, and they also used rudimentary
greenhouses made of mica to force vegetable production.
The majority of the popular edible plants we grow today were cultured by these ancient civilizations as well as those found in Central and South America. Many cultivars (cultivated variety) were generated from wild plants by 2000 b.c. Since our ancestors had a remarkable knowledge of wild, edible food plants—likely unsurpassed by contemporary humans—it is probable that they succeeded in the cultivation of the majority of plants that can be used for this purpose. Comparatively few new food plants have been domesticated in recent times, although many new varieties or cultivars of the ancient plants have been bred since then.
The origins of horticulture are vague because the first acts of plant cultivation by humans predate historical records. Archaeological data indicate that the cultivation of plants on a large and detectable scale coincided with global climate changes approximately 10,000 years ago. The warmer, wetter weather that followed the end of the last ice age caused changes in sea level, increased edible plant diversity, and caused human migrations into new areas. This large-scale cultivation event is referred to as the neolithic revolution.
Cultivation of plants on a small scale may have been practiced for many thousands of years prior to this. The protection and
encouragement of the growth of wild food plants through weeding, pruning, irrigation, and pest control, along with the simple
propagation of seeds or cuttings, most likely constituted some of the first human horticulture. The use of fire to remove dead vegetation and promote the new growth of desirable plants is another example of how ancient humans engaged in plant cultivation.
Archaeological evidence suggests that cereal crops were domesticated first. Domesticated crops have genetic and morphological
differences from their wild ancestors that make them better suited for human use. These differences were the result of natural mutations for characteristics such as a larger grain size in wheat, which was selected for over time because the early horticulturists
planted only the larger seeds that contained the genetic sequences for these traits. Most of the domesticated, edible plants we cultivate today are descended from wild plants found in the Near East, China, Southeast Asia, and the Americas.
The UNEP has estimated that about 20% of the world’s cultivated land and nearly 50%
of all irrigated land is affected by salinity (Flowers and Yeo 1995).
This is not a recent problem; many historians maintain that the ancient Sumerian
civilisation declined partly as a result of irrigation that caused salinisation—the toxic
build-up of salts and other impurities. Saline soils have a high concentration of ions, both Sodium cations (Na+) and
Chlorine anions (Cl-), and are generally unfavourable to the growth of most plants.
Alkaline soils above about 8.5 pH have only Na+ cations and normally cannot be used at all for crop production.
Some crops such as barley and cotton are quite tolerant of saline soils and can grow
in soils with more than 5000 ppm TDS (Total Dissolved Salts/Solids). Other more
sensitive crops such as beans and citrus trees suffer from salt stress in soils with only
960 ppm TDS. See list below.
Salinity of soil is normally measured in terms of its Electrical Conductivity (EC,
measured in deciSiemens/metre—dS/m) or Total Dissolved Salts (TDS—in ppm or
mg/litre). EC is a measure of the conduction of electricity through water, or a water
extract of soil. The EC value represents the amount of soluble salts in an extract,
providing an indication of soil salinity. Saline soils are defined as those with an EC of
greater than 1.5 dS/m for a 1:5 soil water extract and greater than 4 dS/m for a
saturation extract. It can be interpreted in terms of the salinity tolerance of plants.
Conversion Rates: 1 dS/m = 640 mg/litre = approx. 640 ppm TDS.
These units are used to indicate the extent of the problem of osmosis, the ability of
plants to take up water through their roots. The unit “ESP” (exchangeable sodium
percentage) indicates the percentage of absorbed sodium ions to other cations that could
be exchanged. Soils are categorised as being sodic with an ESP of 6–14% and strongly
sodic with an ESP of greater than 15%.
Some soils are naturally saline; other soils become more saline, under one or more
of the following conditions:
• when the irrigation water is saline, or when seawater inundates low lying areas;
• when the rate of evaporation is high (high temperature and/or wind speed);
• if the water table is high, as a result of poor drainage.
Some Notes on Saline Soils
• Most crops are more sensitive to soil salinity in hot, dry conditions than in cool,
humid conditions.
• Choosing to grow salt tolerant species and varieties may be the simplest solution;
• Plants growing in poor, infertile soils may appear to be more salt tolerant than
plants growing in fertile soils. In these cases it is the soil fertility and not the soil
salinity which is the more important factor limiting plant growth.
• As more and more water is lost from the soil, by drainage, evaporation and
transpiration, the soil moisture becomes more and more concentrated with salts. As
a result, plants experience increased salt stress as well as water stress when the soil
dries out.
• Crops are generally more sensitive when they are seedlings than when they are
mature plants. This is partly explained by the fact that soils are normally more
saline in the upper horizon, where the young seedling roots grow, than lower down
in the soil.
• The level of soil salinity is constantly changing, due to changes in rainfall and/or
irrigation, temperature and wind. Farmers who understand how and when these
changes occur can sometimes produce crops on saline soils where others would fail.
What Can be Done about Saline Soils?
••
Irrigate, applying extra water to leach (flush out) the salts.
• Install an underdrainage system to remove the saline drainage water away from the
roots.
• Add hydrated calcium sulphate (Gypsum); this replaces the sodium in the soil,
reduces the alkalinity and balances the salts in the soil.
• Add powdered sulphur; this makes sodium and chlorine more soluble, and allows
other elements such as calcium and magnesium to replace them.
Salt Tolerance of Plants
Crops differ in the degree to which they are affected by soil salinity; some species such
as barley can produce a reasonable yield in highly saline soils up to 18 dS/m, while
others such as beans and carrots grow very poorly in soils with only 5 dS/m.
In general, plants with low drought tolerance also have low saline tolerance.
Tolerant
Barley Hordeum vulgare Jojoba Simmondsia chinensis
Bermuda Grass Cynodon dactylon Leucaena Leucaena leucocephala
Cotton Gossypium hirsutum Saltbush Atriplex spp.
Date Palm Phoenix dactylifera Silt Grass Paspalum vaginatum
Durum Wheat Triticum turgidum Sugar beet etc Beta vulgaris
Guayule Parthenium argentatum Triticale Triticosecale
Moderately Tolerant
Chickpea Cicer arietinum Pineapple Ananas comusus
Fig Ficus carica Sorghum Sorghum bicolor
Mung Bean Vigna radiata Soybean* Glycine max
Oats Avena sativa Taro Colocasia spp.
Olive Olea europeaea Tepary Bean Phaseolus acutifolius
Papaya Carica papaya Wheat* Triticum aestivum
Pigeon Pea Cajanus cajan
Moderately Susceptible
Cabbage B.oleracea var. capitata Linseed Linum usitatissimum
Casssava Manihot esculenta Lucerne Medicago sativa
Castor Ricinus communis Maize Zea mays
Foxtail Millet Setaria italica Pepper Capsicum annuum
Grape Vitus spp. Pumpkin Cucurbita pepo
Groundnut Arachis hypogaea Sunflower Helianthus annuus
Broad Bean Vicia faba Sweet Potato Ipomoea batatas
Irish Potato Solanum tuberosum Tomato L.esculentum
Lentil Lens culinaris Watermelon Citrullus lanatus
Susceptible
Almond Prunus dulcis Mango Mangifera indica
Apricot Prunus armeniaca Okra Abelmoschus esculentus
Apple Malus sylvestris Onion Allium cepa
Avocado Persea americana Peach Prunus persica
Carrot Daucus carota Pear Pyrus communis
Cassava Manihot esculenta Peas Pisum sativum
Cherry Prunus spp. Plum Prunus domestica
Currant Ribes spp. Rice* Oryza sativa
Haricot Bean Phaseolus vulgaris Sesame Sesamum indicum
Lima Bean Phaseolus lunatus Strawberry Fragaria spp.
* Some varieties of rice, soybean and wheat show some tolerance to saline soils.
Some Observations on Growing Food in Saline Soils
• Varieties: a few examples of significant differences of salt tolerance between
different varieties of certain crops have been observed. This is not common, but it
has been observed in: barley, Bermuda grass, berseem clover, birdsfoot trefoil,
brome grass, creeping bentgrass, rice, soybean, taro and wheat.
• Fruit Trees: selection of the appropriate rootstock is important if fruit trees are to be planted in saline soils. Papaya may survive where other fruit trees perish in saline soil. Date palms can also tolerate high salt levels if the other growing conditions are favourable.
Food growers and farmers can help to some extent to reduce water stress of plants:
1. Conservation of Water. There are a vast number of techniques to conserve or “hold” water, some of which prevent the available rainwater from running off the field by the construction of ridges or bunds. These are raised rows of earth and stones constructed along the contours on sloping ground; or on level ground where they are built around small groups of plants, or even individual plants or trees. Some other water conservation techniques are discussed later on, such as mulching, green manure, use of shadow, crop rotation, catch crops, cover crops etc.
2. Crop Management. Farmers who make wise decisions about which crops and varieties to grow, when to plant them and how to take care of them can produce crops where their neighbours lose everything. Sensible farming practices such as mixed cropping, staggered planting times, mulching and crop rotations, together with soil and water conservation techniques, can all be used to produce healthy crops even in very arid conditions.
3. Fallow. If level land with a good soil depth of loam or clay is bare fallowed or clean 3. Fallow. If level land with a good soil depth of loam or clay is bare fallowed or clean fallowed for one or more rainy seasons, this makes stored moisture available for the
next crop. Bare (or “clean”) fallowing is the practice of leaving a field unplanted, with no crop growing—the field should be kept free from weeds and preferably covered with a mulch. Fallowing is not possible in many areas where land is in short supply, but it
can sometimes ensure that at least some yield is produced on some parts of the farm where continuous cropping could result in crop failure all over the farm.
4. Weed Control. In arid regions, and elsewhere also, a “clean” (weed-free) field can produce a bumper harvest where a field infested with weeds produces nothing.
5. Fertiliser. Application of fertiliser is not always successful in arid regions since plants can only use fertiliser if it is in moist soil and if it is near to the plant roots. Low rainfall often results in the fertiliser not reaching the root zone, because it remains too near to the soil surface to be used by the plant.
6. Windbreaks. Trees planted around field borders (or hedges/slatted fences) reduce the speed of the wind, reducing the water loss from both plants and soil. Appropriate tree species must be chosen; ideally they should also produce something useful such as
fruit or timber as well as providing shelter and shade. The trees should also have a modest water requirement and not be planted too close to crops, as they would remove too much water and nutrients from the soil.
Plants have a fixed capacity to utilise water from the soil. This water is transpired through very small holes, or pores, called stomata, which are found mainly on the underside of leaves. Stomata allow the plant to take in carbon dioxide (CO2) for the production of carbohydrate, and give out oxygen. However, as the temperature rises above about 32°C, and when there are strong winds, the root system cannot replace the lost water fast enough for the uptake of CO2 to continue, and the stomata close. As a result, water movement within the plant ceases and the plant wilts and stops making sugars.
If this process is repeated for a few hours every day, the plant begins to draw on the
moisture within its own plant cells after a few days, and the flowers and young fruits
begin to fall to the ground.
Some crops are more successful at surviving drought than others. For example, fast growing crops such as the millets, grass pea and sorghum often avoid drought because their life cycle is very short drought r — esistance via drought avoidance.
The correct choice of variety, or cultivar, is also important. Most crops have varieties that have been specifically selected for their drought resistance, such as Kalahari maize. Of course, these drought resistant varieties normally yield less than
varieties that need more water. The growth habit of plants is also relevant to the survival of plants in arid conditions.
Cereals, for example, can totally fail to produce grain if there are high temperatures and moisture stress at flowering, even for quite short periods. Other crops such as legumes may only lose a small part of their overall production in similar stressful conditions because they flower over a much longer period.
Plants are said to suffer from water stress, or moisture stress, when their growth is
slower than normal due to insufficient water in the soil. An indication of the
interrelationship between the soil, plants, and water is shown in picture below:
1. Soil Water. Soils lose water by drainage, by evaporation from the soil surface and by transpiration by plants. Plants take up moisture through their roots from the store of available water in the soil—this water is the difference between the volume of water held at field (moisture) capacity and that held at the permanent wilting point. Field capacity is the percentage
of moisture held in the soil 2–3 days after being saturated and after free drainage has ceased. The permanent wilting point is the moisture content of the soil at which plants fail to recover (to regain full turgidity) when water is added again to the soil.
The amount of available water in the soil depends mainly on the soil texture and profile:
2. Soil Texture. Sandy soils have a lower wilting point and field capacity than clay soils. In other words sandy soils hold less water in reserve for plant growth than clay soils, and so sandy soils need more frequent rainfall or irrigation to support a crop.
3. Soil Profile. Many soils have an impervious layer (a soil pan or hardpan) a short distance below the surface through which neither water nor roots can easily pass. Thus water is held near to the soil surface, and in addition roots cannot penetrate the soil deeply. Plants growing in these soils rapidly use up the available water, then suffer from water stress and fail to reach their full potential.
Plants need nitrogen (N) to make proteins and nucleic acids (DNA and RNA). However,
Nitrogen can only be used by plants when it is taken up by their roots, and since
Nitrogen is a stable and insoluble gas it has to be changed into soluble Nitrogen
compounds before it can be used by plants.
The nitrogen that is found naturally in the soil is mainly in the form of humus and
organic matter. Although almost 80% of the air is made up of nitrogen, plants cannot
use it (the nitrogen is said to be “unavailable” to them) until it has been broken down by
certain specialised soil bacteria, described below, or by lightning, Rhizobia etc.
Nitrogen is combined with other atoms to form molecules or ions, when it is said to be “fixed” ie converted from a gas to a solid:
Organic Nitrogen (N2) -> Ammonium (NH4+) ->Nitrites (NO2 )-> Nitrates (NO3–) (& other soluble compounds). The process is represented on the picture:
Three kinds of soil bacteria are involved in the Nitrogen Cycle:
1. Nitrifying bacteria, such as Nitrosomonas (which convert ammonium ions to nitrite
ions) and Nitrobacter (which oxidise nitrite ions to nitrate ions). They feed on
humus and animal excreta to produce soluble compounds that are available to
plants.
2. Nitrogen-fixing bacteria, which are found in nodules on the roots of leguminous
plants. They take in Nitrogen gas from the air in the soil and pass it on to the plant.
3. De-nitrifying bacteria, which break down humus and reduce nitrate ions to Nitrogen
gas, which returns to the atmosphere. Because this reduction requires anaerobic
conditions, these bacteria are most active in waterlogged soils.
Bacteria are most active, and multiply most rapidly, when the soil is warm, moist and
aerated; their activity is reduced as the soil dries out during the dry season, or if the soil
becomes cooler or less well aerated.
The function of the plant roots is to absorb water and nutrients from the soil, to anchor
the aerial (above ground) plant parts, and sometimes also to store food.
The young root that bursts out from the seed is called the radicle. Depending on the
species, this can either persist and become a deep growing primary root or tap root, or it
can be replaced by a more fibrous root system of secondary roots.
Adventitious roots are neither primary nor secondary roots, nor do they arise from
them, but are roots which develop in an abnormal position from stems or leaves.
Root hairs on the younger roots absorb water by osmosis and nutrients by active
selective absorption. This second process requires energy provided by root respiration,
which requires oxygen. If the soil is waterlogged, oxygen is unavailable and the roots
cannot respire, and so nutrients cannot be absorbed.
For optimum plant growth the leaves of crop plants should cover the ground area as
soon as possible after planting. By doing this, the plants utilise the sun’s energy more
efficiently, and they shade out weeds more rapidly; soil moisture loss is also reduced.
The relationship between the leaf area and soil surface area is known as the Leaf
Area Index (LAI), and is calculated by dividing the leaf area by the soil surface area.
If the LAI at any period in the growing season is less than one, then some of the sun’s
energy is wasted because some falls onto either bare soil or weeds.
The optimum LAI is different for each crop species. For Irish potatoes (Solanum
tuberosum) it is about 3, for sugar beet 4–5, and for grasses and most cereals 7–8.
If there is not enough leaf—ie if the LAI is too low—then yields will be reduced. If
there is too much leaf the lower leaves become too shaded, which also reduces yields
because losses due to plant respiration begin to cancel out gains from photosynthesis.
Correct plant spacing ensures that the LAI is optimum for plant growth. In temperate
climates, where sunlight can be an important limiting factor, the optimum LAI should
ideally be reached before the season of maximum light intensity ie during the longest
summer days.
From the above, it can be seen that the correct timing of planting (the planting date)
can be just as important as the spacing of plants to produce healthy crops.
If plants are growing too close together (ie they are “too closely spaced”) they compete
with each other for light, water, nutrients and air, and produce small plants of low
quality which are more susceptible to attack by pests and diseases.
If plants are growing too far apart, the yield per unit area is reduced and also the
plants may become too large and/or woody for consumption or sale. Weeds are also
allowed to develop more aggressively in the open spaces between crop plants.
The plant population, or plant density or spacing, is more critical for some crops
than for others. Wheat, for example, can be planted at very different spacings without a
big effect on the yield per hectare, because wheat plants can compensate for different
plant populations. Compensation in plants is the ability to grow large or to remain small
in response to the amount of space available to them. Other plants such as maize
compensate very poorly and so must be planted at much more precise spacings.
The picture below shows how maize should be planted more closely together—ie at a higher seed
rate – in fertile or highly fertilised soils than in infertile soils or soils with a low or zero
fertiliser input.
Figure picture below shows how the cobs of maize become smaller and smaller as the plant
population increases, even though the total yield per hectare continues to increase, up to
a certain plant population.
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