Agricultural Glossary

The water a plant can actually use, calculated as: Available water content = water content at field capacity − water content at permanent wilting point It varies widely with soil texture. FAO gives the following ranges, in millimetres of water per metre of soil depth: • Sand: 25 – 100 mm/m • Loam: 100 – 175 mm/m • Clay: 175 – 250 mm/m This is why a sandy soil needs more frequent irrigation in smaller amounts.

The crop water need (ET crop) is the depth (or amount) of water needed to meet the water loss through evapotranspiration. It is calculated as: ET crop = ETo × Kc where Kc is the crop coefficient — the factor relating the reference grass to the actual crop, which varies with the crop type and its growth stage. This is why no single irrigation figure suits every plant: the number changes with the climate, the plant, and its stage of growth.

The state a soil reaches after drainage has stopped, when the large pores are filled with both air and water while the smaller pores are still full of water. It is the upper limit of the water a soil holds and a plant can use; anything applied beyond it drains below the root zone.

The velocity at which water can seep into the soil. It is commonly measured by the depth (in mm) of the water layer that the soil can absorb in an hour. It matters in practice: if water is applied faster than the soil can absorb it, the water runs off the surface instead of reaching the roots.

The soil water content at the stage where the plant dies. The soil still contains some water at this point, but it is too difficult for the roots to draw it out — which is why moisture being present in a soil does not mean the plant can use it.

The rate of evapotranspiration from a large area covered by green grass 8 to 15 cm tall, which grows actively, completely shades the ground, and is not short of water. It is expressed in mm per day, per month or per season, and is the basis on which any crop's water requirement is calculated.

The electrical conductivity of the soil saturation extract (ECe) is the accepted measure of soil salinity, expressed in decisiemens per metre (dS/m). Per FAO, a saline soil is one whose conductivity exceeds 4 dS/m at 25°C. The full classification is: • Non-saline: 0 – 2 dS/m • Slightly saline: 2 – 4 dS/m • Moderately saline: 4 – 8 dS/m • Strongly saline: 8 – 16 dS/m • Very strongly saline: above 16 dS/m

Soil in which salts interfere with normal plant growth, per the Food and Agriculture Organization (FAO) definition. It is divided into three types according to the amount and type of salts, the amount of sodium present and the soil's alkalinity: saline, saline-sodic, and sodic.

The amount of sand, silt and clay present in the soil determines the soil texture. Texture governs how much water a soil can hold, which is why plant-available water differs so widely between textures.

A soil rich in calcium carbonate (lime). FAO notes that calcareous soils sometimes contain more than 25 percent calcium. Its importance to anyone fertilising is that it explains much of what is commonly misread: • Its pH is high; FAO describes arid-region soils as neutral to slightly alkaline. • Micronutrients are therefore fixed: iron and zinc deficiency is frequent because of fixation under alkaline reaction, and acute iron deficiency occurs as lime chlorosis — FAO adds that the calcareousness of these soils plays an additional detrimental role. • Available manganese decreases as pH and calcareousness rise. • Conversely such soils are rather well supplied with potassium, sulphur, calcium, magnesium, boron and molybdenum, and copper deficiency in them is rare. The practical conclusion: adding calcium to a calcareous soil is rarely the answer, and yellowing on it is usually a problem of availability rather than absence. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 4.

A soil's capacity to hold cations — such as potassium, calcium, magnesium and ammonium — in exchangeable form, that is within the plant's reach and not washed away. It is a measure of the net negative charge of a soil, expressed in cmol/kg. It depends on the type and proportion of organic matter and clay minerals present. Clay soils have a higher CEC than sandy soils. What that means practically on a sandy soil: a low CEC means a poor ability to store nutrients, so they leach quickly with irrigation water. FAO lists low storage capacity for mineral nutrients in sandy soils among the problems of arid soils. See leaching and soil texture. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 4 and Glossary.

A micronutrient — iron, zinc or manganese — bound to an organic molecule (a ligand) that protects it from being fixed in the soil and keeps it in a form the plant can reach. Common ligands in commercial products include EDTA and EDDHA. Chelates also occur naturally: FAO notes that nutrients can be mobilised from soil mineral reserves by chelating substances excreted by roots or by microbes. Why they exist: iron is absorbed as Fe²⁺ and to a lesser extent as iron chelates, and for chelated iron to be used, the iron must be separated from the organic ligand at the root surface after Fe³⁺ is reduced to Fe²⁺. They are suitable both for soil application and for fertigation. Note: this page recommends no product, no particular ligand and no application rate. Choosing an appropriate chelate depends on soil pH and on the analysis result, and needs a specialist's sign-off. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapters 3 and 4 and Glossary.

An organic manure produced by aerobic, anaerobic or partially aerobic decomposition of a wide variety of crop, animal, human and industrial wastes. What is prepared with the aid of earthworms is called vermicompost. Its nutrient content is low compared with mineral fertilisers. FAO gives typical figures: rural compost about 0.5% N, 0.2% P₂O₅ and 0.5% K₂O; urban compost about 1.5%, 1.0% and 1.5% respectively. On average it also contains about 10 ppm zinc, 6 ppm boron and 12 ppm manganese. The essential point FAO makes: the nutrient status of a compost depends largely on the nutrient content of the wastes composted — so compost is not a specification, and its content cannot be assumed. Why it matters on dry sandy soil: FAO lists among the problems of arid soils a low to very low organic matter content, a low content of available nitrogen, and low storage capacity for nutrients in sandy soils. Organic matter raises cation exchange capacity. Note: poorly decomposed compost can damage roots — see fertiliser burn. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Glossary and Chapters 4 and 5.

The line on the ground beneath the outer edge of a tree's canopy — where rainwater drips from the tips of the branches. It is traditionally used as a rough guide to where the active absorbing roots lie. A note on the name: drip line in this sense is not the drip irrigation line meaning the pipe, and the two are easily confused in both languages. The canopy edge is what is meant here. Why it is worth knowing: it answers the question of where to put fertiliser far better than placing it at the trunk. Absorbing roots spread well away from the trunk, so fertiliser pressed against the trunk sits outside the active zone and may do harm. An important limit on the rule: under drip irrigation, roots do not necessarily follow the canopy edge but concentrate where the water wets the soil. If a tree is drip-irrigated, the wetted area is a better guide than the canopy edge. This is a rough practical rule, not a measurement. We give no distance, depth or quantity here — those depend on species, age, root spread and irrigation system, and are settled by a site assessment. See root zone. Source: FAO Irrigation Water Management Manual No. 5; FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 3.

Expressing a nutrient quantity as the element itself — phosphorus (P) or potassium (K) — rather than as its oxide. This is the form used in scientific literature. FAO notes that scientific literature mostly expresses nutrients in elemental form, while industry, trade and extension services continue to express phosphorus and potassium as oxides, so care is needed when converting research data into practical values. Conversion factors: P₂O₅ to P × 0.436, and K₂O to K × 0.83. FAO's own example: an application rate reported in research as 26 kg P per hectare translates into 60 kg P₂O₅ per hectare — the same quantity expressed two ways. See nutrient oxide and NPK. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 5 and the units-and-conversion-factors table.

Applying fertilisers together with irrigation water rather than as a separate operation. FAO records that it is more often advocated with drip irrigation systems than with conventional flood irrigation, and that in principle all required nutrients including micronutrients can be applied this way. Products used for drip irrigation should be highly water soluble, and include products containing major nutrients, micronutrient salts and chelates of EDTA and EDDHA. Why it matters in this region: FAO notes that if the irrigation water contains dissolved salts, drip irrigation is particularly suitable, as less water is applied to the soil than with surface methods — so the same network that suits saline conditions also suits fertigation. The trade-off: FAO is equally clear that drip and sprinkler irrigation are technically more complicated methods, and that sediments may clog emitters — a serious consideration where dust storms are frequent. See water-soluble fertiliser. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Glossary; FAO Irrigation Water Management Manual No. 5.

The statement of a product's nutrient content as the manufacturer declares it on the packaging: which nutrients it contains and in what proportion. It is the basis from which you calculate what a given weight of product actually supplies. For solid fertilisers the stated percentage is by weight. For liquid fertilisers it may be by weight or by volume — FAO gives the example that 20 percent N by weight in a solution of specific gravity 1.3 corresponds to 26 percent by volume, that is 260 g of nitrogen per litre. Confusing the two is a common error. The product label is the binding reference for amount, timing, method of application and safety precautions. Any figure that contradicts the label — whatever its source — should not be relied on. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 5.

Supplying nutrients by spraying them onto the leaves rather than applying them to the soil. It is possible because leaves can take up nutrient ions and even some molecules such as urea. FAO notes that nutrients enter the leaves either via the stomata, which serve for gas exchange, or mainly via small micropores of the cuticle. The decisive safety point: FAO states explicitly that foliar application is carried out through dilute solutions in order not to damage the leaf cells by osmotic effects. Increasing the concentration does not increase the benefit — it causes damage. When it is used: typically to correct a micronutrient shortage quickly, and where root uptake is impaired — as on alkaline calcareous soils where micronutrients are fixed. It is a fast corrective, not a substitute for fixing the underlying soil problem. Note: we give no concentration, product or spray timing here. That authority is the product label and it needs a specialist's sign-off. Spraying in extreme heat increases the risk of damage. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 3.

A laboratory examination of the irrigation water itself, not of the soil. It is an entirely separate analysis from a soil test and answers questions the other one cannot. Why it matters here in particular: in a region dependent on irrigation, the water is a renewing source of salts. Whatever enters the soil with each watering accumulates there. FAO lists an occasional excess of soluble salts, adsorbed sodium and boron among the problems of arid soils. Boron specifically is the strongest reason to ask for this analysis before any boron is applied: FAO records that boron toxicity arises where irrigation water is rich in boron, above 1–2 ppm. Adding boron in a region whose irrigation water may already carry it is a real risk, and only a water analysis can tell you. Irrigation-water salinity (ECw) is a completely different measure from soil salinity (ECe) and has its own thresholds. We have read no source giving those thresholds, so we state no figure — a gap we name rather than fill by guessing. See soil test and saline soil and electrical conductivity. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapters 3 and 4; FAO Irrigation Water Management Manual No. 5.

The downward movement of dissolved substances — nutrients or salts — with water travelling through the soil, until they pass below the root zone. It has two exactly opposite faces, and confusing them causes a great deal of muddle: • The harmful face: nutrient loss. Soluble nutrients — nitrogen above all — are washed beyond the reach of roots, and the loss is greater on sandy soils with low cation exchange capacity. FAO lists low storage capacity for mineral nutrients in sandy soils among the problems of arid soils. • The useful and deliberate face: leaching salts. Salts are washed below the root zone by water, and this is the primary means of reducing soil salinity. It only happens if the water has somewhere to go — so any salinity management begins with drainage, not with a product. The consequence is that the very water carrying salts away carries nutrients with it. See cation exchange capacity and salt-affected soil. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 4; FAO Soils Portal: Salt-affected soils.

An essential nutrient the plant needs in relatively large amounts. There are thirteen essential mineral nutrients, divided by convention into macro and micro according to the quantity required rather than to importance — FAO stresses that physiologically all of them are equally important regardless of amount. The macronutrients are nitrogen, phosphorus, potassium, sulphur, calcium and magnesium. Nitrogen and potassium together make up about 80 percent of the mineral nutrients in a plant. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 3.

An essential nutrient the plant needs in very small amounts, measured in the plant in micrograms per gram (parts per million) rather than as a percentage. The micronutrients are iron, manganese, zinc, copper, boron, molybdenum and chlorine. All of them together make up under 1 percent of a plant's mineral nutrients — the ratio of nitrogen to molybdenum is about 10,000 : 1. A small requirement does not mean a small consequence: iron and zinc shortage is among the commonest problems on alkaline calcareous soils, and the cause is usually that the element is present but unavailable rather than absent. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapters 3 and 4.

A layer of organic or inorganic material spread over the soil surface around a plant without being mixed into it. Its usual purposes: reducing water loss by evaporation from the surface, moderating soil temperature, suppressing weed growth, and protecting the surface from wind. Why it matters in Kuwait specifically: summer maxima reach 42–46°C in Dry Summer and 45–46°C in Wet Summer, with the highest recorded temperature 51.3°C at Kuwait Airport on 20 August 1998; and wind is not incidental — thunderstorms averaging 65 km/hour and severe dust storms in the Al-Sarayat season. Bare soil in those conditions loses water fast. A distinction often confused: mulching is not fertilising. Organic mulch decomposes slowly and adds organic matter over time, but it is not something to rely on for correcting a nutrient shortage. A practical caution: do not pack mulch against the stem or trunk. Poorly decomposed material can also draw nitrogen down temporarily as it breaks down — FAO notes that short-term fixation of nitrogen, phosphorus and sulphur into micro-organisms may create a transient deficiency, particularly at wide ratios of carbon to these elements. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 3; Kuwait Meteorological Department.

Three numbers printed on fertiliser packaging in a fixed order that never changes: nitrogen (N), then phosphorus expressed as phosphorus pentoxide (P₂O₅), then potassium expressed as potassium oxide (K₂O). For example 20-20-20 or 12-12-17. They are percentages by weight: 20% N means 20 kg of nitrogen in every 100 kg of product. They are not a quality score, and bigger numbers are not better. They describe three different jobs, and the right bag is the one that serves the job you need. Note carefully: the second and third figures are given as oxides, not as elements — so 20-20-20 does not supply equal amounts of the three elements. See nutrient oxide and elemental nutrient. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 5.

When one nutrient interferes with the uptake or use of another, producing a shortage of the second even though it is present in the soil in adequate amounts. The example FAO states explicitly: excess copper induces iron deficiency, which is why chlorosis is a common symptom of copper toxicity. A yellow leaf can therefore be caused by an excess of a different element rather than by an iron shortage. The general principle FAO sets out is balance: plants require nutrients in balanced amounts, growth is limited by whichever is shortest relative to need (the law of the minimum), and a plant reaches its full potential only when all nutrients are in the optimal range — that is, without deficiencies AND without excesses. The practical consequence: over-applying one nutrient is not a neutral act. It can create a shortage of another that was not short before. A note on the limits of this entry: antagonism is not a headword in the glossary of the source we read. What is given here is the specific interaction that source documents, plus its balance principle. We give no table of nutrient interactions, because we have read no institutional source documenting one. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapters 3 and 6.

The state of inadequate supply or low availability of an essential nutrient for optimal plant growth. In quantitative terms, the nutrient status is below the critical level. Deficiency symptoms are the visible signs of that shortage in a growing plant or its produce, usually visible to the naked eye. Common descriptors include chlorosis — loss of chlorophyll, resulting in loss of green colour and paleness — and bronzing, the development of a bronze or copper colour on the tissue. An important distinction: chlorosis is reversible, because the cells remain largely intact, so leaves can become green again once the missing nutrient is supplied. Necrosis is irreversible destruction, and a necrotic leaf cannot be recovered by adding the nutrient — though the plant may survive by forming new leaves. The essential caution: FAO warns that chlorotic and necrotic leaves might also result from the toxic effects of nutrients, from pollution, and from disease and insect attack, and that confirmation of the cause is important before corrective measures are taken. A visual symptom alone is not a confirmed diagnosis. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 3 and Glossary.

Expressing phosphorus and potassium as their oxides — P₂O₅ and K₂O — which is the form printed on fertiliser packaging and used in trade and extension. FAO points out a paradox in this convention: neither nitrogen nor phosphorus exists in soils, plants or fertilizers in elemental form in the first place — so both forms are a convention rather than a description. The practical effect: a bag marked 20-20-20 does not supply equal amounts of the three elements. From every 1000 g of it: 200 g of nitrogen, 200 g of P₂O₅ equal to 87.2 g of actual phosphorus, and 200 g of K₂O equal to 166 g of actual potassium. See elemental nutrient. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 5 and the units-and-conversion-factors table.

An adverse reaction in plants caused by certain constituents in the soil or water that are taken up and accumulated to high concentration, resulting in plant damage, reduced yields or even death. The degree of damage depends on the element, its uptake, its concentration in the plant tissue and the sensitivity of the crop. Why this must be read alongside deficiency: excess and shortage can produce similar-looking symptoms, so damage caused by excess gets diagnosed as hunger and treated with more fertiliser. Examples FAO gives: excess phosphorus produces a watery edge on leaf tissue that becomes necrotic and in very severe cases can kill the plant; high ammonium can be toxic, particularly in alkaline conditions; excess copper induces iron deficiency and therefore chlorosis; and boron toxicity arises where irrigation water carries more than 1–2 ppm. In arid-region soils FAO lists an occasional excess of soluble salts, adsorbed sodium and boron among the problems — so excess in this environment is a real possibility, not a theoretical one. See fertiliser burn in the symptom reference. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapters 3 and 4 and Glossary.

The volume of soil that a plant's roots actually occupy, and the only place from which the plant can take water and nutrients. Its importance is that it defines where fertiliser has an effect and where it does not. Fertiliser placed outside the root zone never reaches the plant; fertiliser placed hard against the stem can burn the roots. FAO lists among the conditions for optimal nutrition: sufficient available nutrients in the root zone of the soil, rapid transport of nutrients in the soil solution towards the root surface, satisfactory root growth to access available nutrients, and unimpeded uptake with sufficient oxygen present. A practical observation: under a drip system the root zone is not a full circle around the plant but concentrates where the water actually wets the soil. Emitter position therefore effectively decides where roots grow — and so where fertiliser should go. Root-zone depth is also the depth at which soil moisture should be checked, rather than the surface. See drip line and fertigation. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 3.

A common trade and extension term for calcium, magnesium and sulphur — the nutrients needed in smaller amounts than nitrogen, phosphorus and potassium but far larger amounts than the micronutrients. Together with phosphorus they make up about 19 percent of a plant's mineral nutrients. An important caution: FAO classifies these three among the MACROnutrients, as the least abundant of them, and does not use secondary as a formal category. The word means less in quantity, not less necessary. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 3 and Table 6.

A fertiliser that is not readily soluble but releases its nutrients slowly over a period of time. FAO records that some nitrogen fertilisers and some micronutrient forms are slow release, and that they are similar to controlled-release fertilisers. Its opposite is the readily soluble fertiliser that becomes available at once — see water-soluble fertiliser. Choosing between them is not a question of quality but of fit: slow release lowers the risk of burn and reduces loss by leaching, but gives a slower response and is not suited to correcting an urgent shortage. Note: release period and application rate are set by the product's own label, and vary with formulation, soil temperature and moisture. We state neither a period nor a rate here. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Glossary.

A measure of how acid or alkaline a soil is, defined as the negative logarithm of the hydrogen ion concentration. The property is also called soil reaction. Its practical importance is that it governs how AVAILABLE nutrients are to the plant rather than whether they are present. A nutrient can be abundant in the soil and still be beyond the plant's reach, because the pH holds it in an unavailable form. On alkaline soils — which is the condition of arid-region soils — FAO states the effect plainly: owing to the high soil pH, micronutrient availability poses several problems, and in particular acute iron deficiency occurs frequently in the form of lime chlorosis. FAO also notes that optimising soil pH is a precondition for the success of nutrient management. Your own soil's pH is known only from a laboratory analysis. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapters 3, 4 and 7.

A laboratory examination of a sample from your own soil, showing its actual condition rather than its assumed one. Why it comes before buying: feeding without a test may add a nutrient already present while leaving the real shortage untreated. On salt-affected soils, over-application adds salts to a soil that is already loaded. What is usually requested for fertiliser purposes: pH, the electrical conductivity of the saturation extract (ECe) to gauge salinity, available nutrients, organic matter, and soil texture. What cannot be known without it: your soil's salinity and pH cannot be judged by eye, by tasting the water, or from how the plants look. Choosing a plant or a fertiliser on the assumption that the soil is ordinary is the costliest error gardeners make in this region. FAO notes that optimising soil pH is a precondition for the success of nutrient management — and the figure is known only by measuring. See irrigation-water analysis. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapters 4 and 7; FAO Soils Portal: Salt-affected soils.

A fertiliser that dissolves in water so its nutrients become available to the plant quickly. It is the form required for feeding through irrigation water: FAO notes that products used for drip irrigation should be highly water soluble. The declared percentage on liquid fertilisers may be given by weight or by volume, and confusing the two changes the calculation — see fertiliser analysis. The trade-off: rapid availability means a faster response, but also a higher chance of loss by leaching on sandy soil, and a higher risk of root burn if over-applied or applied to dry soil. See slow-release fertiliser, fertigation and leaching. Source: FAO Fertilizer and Plant Nutrition Bulletin 16 (2006), Chapter 5 and Glossary.