User:Alandmanson/Soil fertility management

Soil fertility management in maize-based cropping systems
In KwaZulu-Natal, maize is grown in a wide variety of situations (see Annotated Bibliography - Maize). This page will focus on soil fertility management for maize grown in rain-fed systems, where the principles of conservation agriculture are recognised.

The soils range from shallow soils with moderate weathering to deep, highly weathered soils. Soil texture ranges from sand to clay, although soils with a clay content of between 15% and 50% predominate; kaolin is the predominant clay type. The internal drainage is moderate to good, many of the deeper soils have red or yellow-brown subsoils with little evidence of mottling due to waterlogging. Plant-available phosphorus (P) and zinc (Zn) concentrations are low in most soils that have never been fertilized. Many soils with a high-rainfall (>800 mm per annum) climate exhibit a high P-sorption capacity and are highly acid (pHwater <5.0).

Soil acidity and liming
Soil acidity severely limits the productivity of many soils in KwaZulu-Natal, and satisfactory production of most crop types on these soils is impossible without the application of lime.

Soil reaction (pH)
The most widely used measure of soil acidity is soil pH. Methods of measuring soil pH frequently vary from one laboratory to another. Since the pH value of a particular soil may vary considerably with the method of measurement, it is important to specify the method used.

Soil pH is measured in a suspension (mixture) of soil and distilled water, or of soil and a salt solution. Salt solutions commonly used are 1 M KCl (used at Cedara) or 0.01 M CaCl2. Soil pH measured using a salt solution is generally lower than if distilled water is used. The difference between KCl pH and water pH is usually 0.5 to 1.0 pH units. When measuring soil pH, it is also important to use a standard soil:solution ratio. That used most commonly is 1:2.5. At Cedara, 25 mL of KCl solution (one molar) is added to 10 mL of soil.

Soil pH is a useful measure of soil acidity (or alkalinity) for a wide range of soils, but is less useful for the determination of the lime requirement of highly acidic soils. The following generalizations can, however, be made:
 * KCl pH < 4.3 (acidic) — Aluminium toxicity likely
 * KCl pH 4.4 - 4.9 — Manganese toxicity possible
 * KCl pH 5.0 - 6.5 — Few pH-related problems
 * KCl pH > 6.5 (neutral to alkaline) — Possible Zn, Cu, and B deficiencies

Extractable acidity
Extractable acidity is the acidity that is extracted by 1 M KCl. The acidity is determined by titration with a base (sodium hydroxide). This acidity is comprised predominantly of exchangeable aluminium (Al3+), but includes some exchangeable H+. It is this acidity that must be neutralised by liming if aluminium toxicity is to be eliminated.

The quantity of extractable acidity is usually only significant when the KCl pH is less than 4.5, but at any pH lower than that, a wide range of levels of extractable acidity is possible if different soils are considered.

Acid saturation
The degree to which plant roots are affected by aluminium toxicity is usually related to the level of Al3+ in the soil solution. This, in turn, depends on the relationship between the quantity of exchangeable Al3+ and the quantity of exchangeable bases (Ca2+, Mg2+, K+ and Na+), rather than on the level of exchangeable acidity alone.

Acid saturation (expressed as a percentage of the total cations) is, therefore, a good index of likely aluminium toxicity over a variety of soils. This philosophy is discussed in more detail by Farina and Channon (1991) and Farina, Manson and Johnson (1993).

Calculations
The level of soil acidity that is tolerable in any situation is determined by the permissible acid saturation (PAS) of the crop to be grown. If soil acid saturation exceeds the PAS, excess acidity must be neutralised by liming.

The lime requirement in t/ha is calculated according to the formula below and reported to the nearest 0.5 t/ha.

Lime req. = ["Acidity(Al+H)" - ("Total cations" x PAS/100)] x F

"Acidity(Al+H)" is the measured extractable acidity and "F" is a factor indicating the amount of lime required to neutralise 1 cmolc/L of exchangeable acidity. Where crop PAS is 20% or higher, then

F = 4

Where crop PAS is less than 20%, the F must be calculated as:

F = 5 - PAS/100 x 4

According to this formula F increases from 4 to 5 as PAS decreases from 20% to 0%. The reason for this is that proportionally more lime is required to attain very low levels of acid saturation in the soil. This results from stronger buffering capacity at the lower acid saturation levels. F-values for the different levels of PAS used in the program are given in Table 3.

In addition to F varying with PAS, both lime quality and depth of incorporation have an important influence on the magnitude of F. The F values listed in Table 3 assume a lime neutralising value (calcium carbonate equivalent, or CCE) of 75% and a depth of incorporation of 200 mm. If lime of lower quality is used, or lime is incorporated to a depth of greater than 200 mm, proportionately more lime would be required.

As indicated in Table 3, for topdressing of established pasture, F is fixed at 2.0, irrespective of the PAS of the pasture species. This is because the lime is not mixed with the soil, and so a smaller volume of soil is affected.

For the establishment of lucerne and clovers, a pH (KCl) of at least 5.0 is considered necessary to ensure adequate persistence of the legume. Carrots are also very sensitive to acidity and a pH (KCl) of 5.0 is recommended. The attainment of this pH generally requires a quantity of lime in excess of that required to reduce the acid saturation to below 1% ("Al+H" x 5.0). This lime requirement is likely to be lower in sandy soils and therefore dependent on sample density. It is calculated as follows [lime requirement (LR) in t/ha, and sample density in g/mL]:

LR = "Al+H" x 5 + 3  if  sample density < 1.10  and  pH < 4.6 LR = "Al+H" x 5 + 2  if  sample density < 1.10  and  pH < 4.8 LR = "Al+H" x 5 + 1  if  sample density < 1.10  and  pH < 5.0 LR = "Al+H" x 5 + 2  if  1.10 < sample density < 1.35  and  pH < 4.7 LR = "Al+H" x 5 + 1  if  1.10 < sample density < 1.35  and  pH < 5.0 LR = "Al+H" x 5 + 1  if  sample density > 1.35  and  pH < 5.0

Lime recommendations are rounded to the nearest 0.5 t/ha unless the calculated lime requirement is between 0 and 1 t/ha, when 1 t/ha is recommended.

Many farmers choose to apply more lime than recommended, at times when capital is available. This allows a longer period before reacidification results in a need for more lime, and is usually a sound policy.

Choice of lime
The neutralising value of the lime chosen is obviously important. In a study of 39 commercial lime samples, it was found that the calcium carbonate equivalent (CCE) ranged from 54% to 108% (Engelbrecht, 1983). The most effective source was, therefore, twice as effective as the lowest quality lime. Consideration of the neutralising value is obviously critical when determining the effective cost of a source of lime. Use of the following formula is suggested: Effective cost = Total cost x 100/CCE, where Total cost = Price at source + cost of transport + cost of spreading

The CCE gives the long-term value of the lime. Limes that contain large particles, and/or are crystalline are slower to react than others. These slow-reacting limes should be allowed more time to react with the soil, or alternatively, higher rates should be used (Engelbrecht, 1983; Farina, Channon & Sumner, 1981).

The calcium (Ca) and magnesium (Mg) content of the lime may also be important: both are essential in plant and animal nutrition. If a soil is low in Mg, dolomitic lime (which contains at least 4.5% Mg) would be preferred to calcitic lime. Calcium deficiencies are rare if soils have been limed to the correct acid saturation.

The type of lime recommended by the Cedara fertilizer advisory service depends on the soil Ca and Mg levels.

If soil Mg < "Target Mg", then dolomitic lime is recommended. If soil Mg > "Target Mg" and Ca x 0.6 > Mg, then either calcitic or dolomitic lime may be applied. If soil Mg > "Target Mg" and Ca x 0.6 < Mg, then calcitic lime is recommended.

Magnesium applications may be necessary even if no lime is required; this generally only occurs in sandy soils with a very low cation exchange capacity, or if exchangeable K levels are very high. Magnesium applications are probably advisable if soil Mg is less than "Target Mg" in the crop parameter file, or if the K:Mg ratio is greater than 4:1. Dolomitic lime, magnesite (MgCO3), magnesium oxide (MgO), or magnesium sulphate (MgSO4) may be used as a Mg source.

Consequences of Acidity
The most critical effect of soil acidity is that root growth is inhibited. Short, stubby roots are a common symptom of severe soil acidity, but the symptom is not useful for diagnosis under less severe conditions. Restricted root growth results in increased wilting, as plants cannot take up sufficient water, even from fairly moist soils. Uptake of N and P can also be limited by the small root system, and nutrient deficiency symptoms may occur.

Aluminium toxicity is also associated with inhibited uptake of Ca and Mg, and Mg deficiency symptoms (interveinal chlorosis, starting at the oldest leaves) often indicate excessive soil acidity. Red colouring in maize and sorghum leaves may also reflect excessive soil acidity.

Manganese toxicity occurs in broad-leafed crops grown on some acid soils (pHKCl <5.0) with high Mn soil tests (Soil Mn > 40 mg/L). In KwaZulu-Natal, manganese toxicity has been observed in soyabean, dry bean, cabbage and lettuce. Symptoms include stunting of the plants and leaf chlorosis. For these crops, if PAS indicates that aluminium toxicity is unlikely (Acid saturation less than PAS and lime not recommended), but soil Mn is greater than 40 mg/L, lime should, nevertheless, be applied.

Molybdenum (Mo) availability in the soil is decreased at low pH, and Mo deficiency symptoms may also indicate soil acidity.

Negative responses to lime application
Zinc (Zn) deficiency: Lime application can reduce the availability of Zn by increasing zinc sorption. Consequently, lime application can induce severe Zn deficiency in acid soils that are low in Zn. Advisers and farmers working with acid soils should pay particular attention to concentrations of plant-available Zn if lime is to be applied. Zinc-containing fertilizers should be used if plant-available Zn (soil-test Zn) is low (<1.5 mg/L) or marginal (1.5-3 mg/L). Additional applications of zinc fertilizer should be applied if soil-test Zn is very low (<1 mg/L); application of 100 kg zinc sulphate per hectare is a typical recommendation for most crops in KwaZulu-Natal.

Overliming: Lime application rates greater than those recommended may increase soil pH to greater than 6.5, especially in sandy soils with low buffer capacity. Uneven application of lime can result in overliming in patches of fields. Iron (Fe) deficiency and Zn deficiency may both occur at high pH.

Subsoil Acidity
Subsoil acidity can severely restrict subsoil root development in certain crops (such as maize and lucerne). If topsoils are limed, and remain moist, these crops may suffer no yield loss. However, if topsoil moisture is depleted in dry periods, poor subsoil rooting can result in severe moisture stress and yield loss. Amelioration of this subsoil acidity can markedly improve yields in seasons when moisture stress is severe.

Lime applied to topsoils has very little effect on subsoil acidity, as it is immobile and reacts with the soil where it was placed. Lime or limed soil must therefore be incorporated to the rooting depth required; alternatively one can use compounds that can be leached into the subsoil and neutralise acidity there. Gypsum and calcium fulvates (derived from coal, chicken litter, green manure or sewage sludge) added to topsoils have been shown to decrease subsoil acid saturation by decreasing exchangeable acidity and increasing exchangeable calcium in highly weathered soils (Shainberg, Sumner, Miller, Farina, Pavan & Fey, 1989; Van der Watt, Barnard, Cronje, Dekker, Croft & van der Walt, 1991; Liu & Hue, 1994).

Improved subsoil rooting of maize and lucerne growing on highly weathered, clayey soils has been shown to occur after gypsum additions to their topsoils. Maize yield responses are highly economic in the long term, but may be delayed for one or two seasons if there is insufficient precipitation for leaching to occur. On a dystrophic Avalon soil at Geluksburg, the effect of 10 t/ha of gypsum has persisted for more than 12 years, the average yield response being more than 1 t/ha/annum. At Greytown (dystrophic Hutton soil), less gypsum was needed, but the effect did not last as long. There was no response to gypsum on acid sandy Avalon soils at Dundee. There is no way at present of determining the quantity of gypsum required by different soils.

Gypsum should never be used to ameliorate topsoil acidity if there is an opportunity to incorporate lime. At best, gypsum has never been shown to be better than about a third as effective as lime, and the two materials usually cost about the same. In addition, the effect of gypsum in the topsoil is temporary (as it will be leached into the subsoil), whereas the effect of lime is more permanent.

Tolerance of acidity: Species and cultivar differences

Grasses such as Eragrostis curvula and kikuyu are the most acid tolerant species grown in KwaZulu-Natal (PAS of 60% and 40% respectively); lime is recommended at high acid saturation to improve mineral balance for animal nutrition rather than improve yields. Potatoes are relatively acid tolerant (PAS of 30%) Both maize and soyabeans have a PAS of 20%, but the effect of high acid saturation on yield is far greater in the case of maize, if the soyabeans have sufficient molybdenum. Crops such as lucerne, clover and carrots are relatively intolerant of acidity.

In certain species, there is considerable variation in tolerance between cultivars, and this can be used to the farmer's advantage if liming is not possible. Differences between local maize cultivars has been shown, as have differences between overseas bean cultivars.

Other considerations
Lime should be applied early enough to allow time for it to react with the soil before the crop is planted. Liming one to two months before planting is recommended. It should also be remembered that lime reacts more rapidly in a moist soil, with very little reaction in a dry soil.

Lime should be spread as evenly as possible and mixed thoroughly (discing followed by ploughing), so that the reaction with the soil is rapid and so that the entire volume of the tilled soil is ameliorated. This maximises the volume of soil that roots can utilise without damage due to acidity. Where subsoils are acid, lime should be incorporated as deep as possible.

Lime has a long-term effect and liming should be considered a long-term investment. However, re-acidification does occur, and soils should be tested regularly to monitor the acidity.

Nitrogen (N)
Nitrogen is an essential constituent of chlorophyll, therefore a deficiency in N gives rise to inadequate chlorophyll production with characteristic yellow-coloured plants i.e. chlorosis. Sufficient N thus leads to dark-green coloured plants. An excess of N can lead to excessive vegetative growth; it also often increases succulence in plants which has been associated with greater disease susceptibility. This is especially important if crops are produced for grain, fruit or flowers as an excess of N can delay the formation of these reproductive organs and can lead to reduced yield and/or quality. An N deficiency affects the whole plant, but the older leaves are affected more than the younger leaves. The N concentration of plant foliage is usually 2-3% (on a dry matter basis) but this depends on the age of the plant and which leaves were analysed; whole-plant N concentrations are generally lower as roots, storage organs and wood have lower N levels. For example, maize silage (total above-ground biomass) in KwaZulu-Natal, South Africa generally contains between 0.9 and 1.5% N (dry-matter basis), so a silage crop that comprises 20 t ha-1 of dry plant matter will include 180-300 kg of N per hectare per annum.

Most of the N in the earth and its atmosphere exists as the inert gas N2, which represents about 78% of the earth’s atmosphere. This is the primary source of soil nitrogen, and the nitrogen cycle in soil forms an integral part of the overall cycle of nitrogen in nature. Only a limited number of microorganisms have the ability to utilize elemental nitrogen; all other living organisms on the earth require combined nitrogen for carrying out their life activities. In the soil the combined N is largely bound to organic matter and mineral material. The total amount of N in many soils is appreciable, often exceeding 4000 kg/ha to the depth of ploughing. Of this, more than 90% usually occurs in organic compounds and only a few kilograms per hectare exists in available mineral forms (such as nitrate NO3- and ammonium NH4+ ions), some of this being held on clay mineral surfaces.

Nitrogen is commonly the most important fertilizer element applied to soil, its effects being manifested quickly on plant growth and ultimately in crop yields. The marked increases in yields of crops that have occurred during the past 50 years may largely be attributed to increased use of N fertilizers. Nitrogen is, however, often used ineffectively. Much of the fertilizer nitrogen applied to soils is not utilized by the crop. It is lost either in solution form, by leaching of nitrate, or in gaseous form as ammonia, nitrous oxide, nitric oxide or dinitrogen. The leached nitrate can contaminate rivers and ground waters, while the emitted ammonia can contaminate surface waters or combine with atmospheric sulphur dioxide to form aerosols which affect visibility, health and climate. These losses have large economic implications.

When land is cultivated, soil organic matter declines rapidly, releasing large quantities of plant-available nitrate and ammonium (much of which is lost by leaching or to the atmosphere). In successive years, rates of tillage-induced mineralization decline, and eventually new equilibrium levels of soil organic matter are established which are characteristic of the climate, cultural practices and soil type (this can take as little as 5 years for sandy soils, or over 40 years for clays). As equilibrium is approached, the nitrogen removed by harvested crops must come from external sources. Systems of agriculture which rely heavily on soil reserves to meet the long-term nitrogen requirements of plants cannot continue to sustain high crop yields. In the past, biological nitrogen fixation was the chief means of supplying nitrogen for cultivated crops; in recent years, nitrogen fertilizers have become available, which, when used to augment the nitrogen supplied by natural processes, can increase yields and improve the quality of crops. A major concern of present-day farmers is the effective use of nitrogen fertilizers.

An outline of the nitrogen cycle in soil is depicted in Figure 1. Under natural conditions, gains in total soil nitrogen occur through fixation of elemental nitrogen by microorganisms, and from the accession of ammonia and nitrate in rain water; losses occur through crop removal, leaching and volatilization. Within the soil, an internal cycle is operative, through which mineral nitrogen becomes immobilized through production of organic matter. The formation of organic-mineral complexes protects nitrogenous constituents against attack by microorganisms. The positively charged ammonium ion (NH4+) undergoes substitution reactions with metal cations on the exchange complex and can be fixed by clay minerals.

The nitrogen cycle
There are two “tanks” of nitrogen in the N-cycle:
 * 1) Atmospheric N2 – 78% of atmosphere is N2
 * 2) Soil organic N – mainly proteins, peptides and amino sugars

Dinitrogen gas (N2) can be regarded as relatively inert, contrasting with ‘reactive N’ (Nr) which includes N in organic matter, gasses such as nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O) and ammonia (NH3), water-soluble urea (CO(NH2)2), and the water-soluble salts, ammonium (NH4+) and nitrate (NO3-). .

Soil N is influenced by inputs, losses and transfers.

Inputs - increased total soil N

 * Biological N2 fixation
 * Application as N fertilizer (mostly after industrial N2 fixation)
 * Additions of organic matter (such as plant material, manures, composts and organic waste)
 * Rainfall (usually <5 kg N/ha/annum)

Losses - decreased total soil N

 * Leaching of NO3-
 * Denitrification (N2 and N2O returning to atmosphere)
 * Removal by plants and animals.

Transfers - unchanged total soil N

 * Immobilization and "humus formation" (incorporation of N from NH4+ and NO3- into plant and microbial biomass and necromass)
 * Conversion to "slow pool" SOM - absorption, adsorption, aggregation, occlusion, complexation, pyrolysis
 * Release of "slow pool" SOM - dispersion, desorption, physical disruption, mineral dissolution, depolymerization (chemical action of extracellular enzymes and ROS)
 * Decomposition of accessible SOM (microbial mineralization of DOC to NH4+)
 * Residue decomposition (microbial mineralization of plant residue directly to NH4+)
 * Nitrification (NH4+ to NO3-)
 * Root uptake (of NH4+, NO3- and small, soluble organic compounds such as amino acids)

Immobilization and mineralization
At any one time, the great bulk (95-99%) of the nitrogen in a soil is in organic compounds that protect it from loss but leave it largely unavailable to higher plants. Much of the organic N in soil is present as amine groups in proteins or as part of humic compounds. As organic compounds are attacked by soil microorganisms, the organic nitrogen is broken down into ammonium ions (NH4+) and some of the nitrogen appears in the nitrate (NO3-) form. This conversion of organically bound nitrogen into inorganic mineral forms (NH4+ and NO3-) is termed mineralization. A wide variety of soil organisms perform this process. Approximately 1.5 to 3.5% of the organic nitrogen of a soil mineralizes annually. The exact rate of mineralization depends largely on the temperature, aeration and moisture status of the soil.

The opposite of mineralization is immobilization, the conversion of inorganic nitrogen ions (NH4+ and NO3-) into organic forms. Microorganisms in the soil incorporate mineral nitrogen ions to synthesize cellular components, such as proteins. When residues with high carbon content are incorporated into the soil and are broken down, the growth of the microbial colony may require more nitrogen than is contained in the residues themselves. The microorganisms then use inorganic nitrogen in the soil, making it unavailable to plants. When these organisms die, some of the organic nitrogen in their cells remains bound in the organic compounds that make up the soil organic matter complex and some may be released as NH4+ and NO3- ions. Mineralization and immobilization occur simultaneously in the soil; whether the net effect is an increase or a decrease in the mineral nitrogen available depends largely on the ratio of carbon to nitrogen in the organic residues undergoing decomposition.

Ammonium fixation by clay minerals
Like other positively charged ions, ammonium ions (NH4+) are attracted to the negatively charged surfaces of clay and humus where they are held in exchangeable form, available for plant uptake, but partially protected from leaching. However, because of the particular size of the ammonium ion, it can become entrapped within cavities in the crystal structure of certain clays. Several clay minerals with a 2:1-type structure have the capacity to fix both ammonium and potassium ions in this manner. Vermiculite has the greatest fixing capacity. Ammonium and potassium ions fixed in the rigid part of a crystal structure are held in a nonexchangeable form, from which they are released only slowly. Ammonium fixation by clay minerals is generally greater in subsoil than topsoil due to the higher clay content of subsoils. While ammonium fixation may be considered an advantage because it provides a means of conserving nitrogen, the rate of release of the fixed ammonium is often too slow to be much practical value in fulfilling plant needs.

Ammonia volatilization
Ammonia gas (NH3) can be produced in the soil-plant system and much nitrogen may be lost to the atmosphere in this form. The source of the ammonia gas may be animal manures, fertilizers, decomposing plant residues or even the foliage of living plants. Soil colloids, both clay and humus, are capable of adsorbing ammonia gas, so ammonia losses are greatest where little of these colloids are present, or where the ammonia is not in close contact with the soil. Incorporation of manure and fertilizers into the top few centimetres of soil can reduce ammonia losses by 25 to 75% from those that occur when the materials are left on the soil surface.

Nitrification
If suitable conditions prevail, ammonium ions that begin to accumulate in the soil will be enzymatically oxidized by certain soil bacteria. These bacteria obtain their energy from ammonium oxidation rather than from organic matter oxidation. Bacterial oxidation of ammonium is termed nitrification. Ammonium is converted to nitrite by a specific group of autotrophic bacteria (including Nitrosomonas) and then to nitrate by others (such as Nitrobacter). Nitrification significantly increases soil acidity as H+ ions are released from the reaction. This is the reason why fertilization with ammonium-containing fertilizer causes the acidification of soils.

Nitrate leaching
In contrast to ammonium ions, which carry a positive charge, the negatively charged nitrate ions are not adsorbed by the negatively charged colloids that dominate most soils. Therefore, nitrate ions move downwards freely with drainage water, and are thus readily leached from the soil. The nitrogen lost by leaching can contaminate drinking water and contribute to the eutrophication of surface waters.

Gaseous losses by denitrification
Nitrogen may be lost to the atmosphere when nitrate ions are converted to gaseous forms of nitrogen by a series of widely occurring biochemical reduction reactions termed denitrification. The anaerobic bacteria which are responsible for these biochemical reductions are heterotrophs, which obtain their energy and carbon from the oxidation of organic compounds. The gasses released by denitrification are dinitrogen (N2), nitric oxide (NO) and nitrous oxide (N2O). Denitrification is an important process in waterlogged soils; once oxygen levels have been depleted, the denitrifying bacteria use nitrate as their oxidant (source of oxygen).

Biological nitrogen fixation
Next to photosynthesis, biological nitrogen fixation is probably the most important biochemical reaction for life on earth. Through this process certain organisms convert dinitrogen gas from the atmosphere (N2) to nitrogen-containing organic compounds that become available to all forms of life through the nitrogen cycle.

Symbiotic fixation with legumes: The symbiosis of legumes and bacteria of the genera Rhizobium and Bradyrhizobium provide the major biological source of fixed nitrogen in agricultural soils. These organisms induce the formation of root nodules that serve as the site of nitrogen fixation. In a mutually beneficial association, the host plant supplies the bacteria with carbohydrates for energy and the bacteria reciprocate by supplying the plant with fixed-nitrogen compounds (reactive N). Nearly 200 species of non-legumes are also known to develop nodules and to accommodate symbiotic nitrogen fixation. Non-nodule nitrogen-fixing systems also exist and these mostly involve cyanobacteria. Certain free-living microorganisms present in soils and water are also able to fix nitrogen. Because these organisms are not directly associated with higher plants, the transformation is referred to as nonsymbiotic or free-living.

Addition of nitrogen to soil in precipitation
The atmosphere contains various nitrogen compounds (e.g. ammonia gas, nitrates dissolved in water vapour, etc) which are added to the soil through rain, snow and dust. Although the rates of addition per hectare are typically small, the total quantity of nitrogen added annually is significant and may be tens of kilograms per hectare near highly polluted areas.

Phosphorus (P)
Phosphorus (P) in most soils is virtually insoluble. This is true for the whole pH range suitable for agricultural crops. Most of the inorganic P in soils is in the form of phosphate (PO43-), and most of the phosphate is bound within iron, aluminium and calcium phosphates or bound to clay particles such as iron and aluminium oxides and hydroxides, organic matter-aluminium complexes, and calcium carbonate. The iron and aluminium forms predominate at low pH (KCl pH <6), and the calcium forms predominate at higher pH (KCl pH >6).

Between 25% and 90% of the total P in the soil is bound in the organic matter. This organic P is not plant-available, but it can be an important storehouse of P as it can be released by the process of mineralization (when organic matter is decomposed by micro-organisms).

Plant utilization of P
Plants are adapted to the very low levels of phosphate found in soil solutions; they can extract sufficient P from very dilute solutions as long as those low concentrations are maintained. However, the P near roots is soon exhausted and diffusion of more P towards the root is very slow because the concentration of P is low.

For good P nutrition, therefore, the following factors are critical:
 * good root development (allowing more soil to be explored);
 * good water supply (allowing faster diffusion of P towards the root);
 * the ability of the soil to replenish (or buffer) the solution P; and
 * soil temperature (low soil temperatures decrease the rate of diffusion of P towards the root and slow down the metabolism of the plant).

Plant-available soil P
Plant growth depends on the "labile pool" of P available to supply the plant with sufficient P via the soil solution through the growing season. If the level of this "plant-available P" is too low for optimum plant growth, the farmer needs to make up the shortfall with fertilizer.

A variety of soil tests are used to determine the pool of soil P that is available to plants. Generally an extractant (usually several chemicals dissolved in water) is added to a soil sample. After mixing for a set length of time, the extractant liquid is filtered from the soil, and the amount of P extracted is analysed. A variety of laboratories are used by KwaZulu-Natal farmers, and the methods used by these laboratories vary considerably. Consequently soil P tests results for a particular sample will vary depending on the laboratory used. At Cedara the "AMBIC 2" extractant is used.

What is meant by "Target P test"?
This is the minimum P test required in the soil under consideration for optimum growth of the indicated crop. Target P test can depend on both the soil type, the soil texture, and on the crop to be grown.

Variations in Target P test values for different plant species are due to differences in their ability to utilise P in the soil.

Calculation of the P recommendation
This can be calculated using a formula of this type P Recommendation (kg/ha) = (Target P - soil P) x PRF where PRF is the "P requirement factor", which reflects the ability of the sampled soil to fix P, and thereby reduce the availability of P to the plant.

The soil sample density is used to determine the PRF because since clay soils (low sample density) require more P to increase the soil P test by 1 mg/L than do sandy soils (high sample density), sample density has been found to provide a reliable measure of the PRF. Table 2 shows how PRF is related to soil sample density.

Where P is incorporated into the soil, the PRF ranges from 10.78 (clays) to 3.00 (sands). For the topdressing of established pastures, however, the maximum PRF is 5.00 as there is little mixing of fertilizer with soil and therefore less fixation of P.

What if "Sample P" is greater than "Target P"?

A soil P test value that exceeds or is equal to the Target P test value indicates good soil P reserves for the crop in question. In these cases the P recommendation is limited to a "starter" application of 20 kg P/ha for the establishment of pastures and most field crops. For potatoes and vegetable crops, higher "starter" applications are recommended.

Optimising root growth
Liming of soils to the correct acid saturation improves root growth of many crops grown on acid soils. Improved root growth means that more soil is explored, and more P is accessible to the roots, so P-uptake is improved. The apparent "optimum” P is much higher if the soil is highly acid and it is not limed.

Root parasites such as nematodes and certain fungi can also severely limit the extent of the root system and induce P deficiency in soils with levels of P that are normally adequate for optimum growth. If the roots of seedlings are damaged during planting, or if the seedlings are root-bound, subsequent root growth is likely to be poor and, again, P deficiency is likely.

Placement of P in a band near the seed at planting can help to stimulate early root growth when the root system is still small.

Moisture status
A good moisture status is necessary for optimal use of soil P because diffusion of P towards the roots is much slower in dry soils. In drier years, the level of soil P required for maximum profit is not any lower than in seasons with adequate moisture, even though crop yields may be lower. This is different to the situation with N fertilizer, where higher rates of N fertilizer are only beneficial in wet years.

Under rain-fed conditions, the soil surface frequently dries out, so for dryland crops it is essential that all the required P is incorporated into the soil before or at planting.

Temperature
Higher temperature results in better P-use as long as the overall growth of the plant is not adversely affected by the heat. In cold soils, the rate of P-diffusion to the root is slower, root growth is slower, and root respiration declines, depriving roots of the energy needed to absorb P. Mineralization of organic P by micro-organisms is also slower.

The effect of temperature is important in pastures: In the case of kikuyu, a fairly high level of soil P is needed for good spring production, whereas in summer, less P needed for maximum fodder production. In fact, in summer, high rates of P uptake results in a low Ca/P ratio in the forage and this can be detrimental to animal production. Optimum yields of Italian ryegrass in winter and early spring require fairly high soil P levels, whereas maximum production in late spring is possible at moderate levels of P.

In the case of maize, both shortages of moisture and cool temperature may limit P-use in spring. Even if the soil P is high, it is advisable to use a dressing of "starter" P applied in a band near the seed. This ensures that there is sufficient P for the small plant while the root system is still poorly developed.

Should P be band-applied or broadcast?
In the case of maize, if the soil P is low (if the P recommendation is greater than 40 kg/ha), it is advisable to broadcast at least half of the recommended P, as this encourages the root system to explore all of the topsoil. If only 20-30 kg P/ha is needed, all of the P can be applied in the band.

Soyabeans are less likely to respond to banded P on most soils, so it is again advisable to broadcast at least half of the recommended P if the soil P is low. However, in the case of wheat, banding of P stimulates tillering; as this is important for maximum yields, some P should always be banded.

"Build-up" of P
If the soil P is initially low and the fertilizer recommendations are followed, the soil-test P should rise every year. The rate of "build-up" depends on the soil type.

However, the aim of each recommendation is to maximise the profitability of the crop in the year of analysis. The "build-up" of P is a positive side-effect; less P is needed for subsequent crops, as they can use some of the P that was not used by the previous crops.

Water-soluble sources

 * Single superphosphate (8.3% or 10.5% P),
 * double superphosphate (19.6% P),
 * monoammonium phosphate (MAP) (22% P, 11% N),
 * diammonium phosphate (DAP) (20% P, 18% N),
 * ammoniated superphosphate (12.2% P, 3.8% N).

Plant availability of water-soluble sources of P is limited only by soil fixation.

Rock phosphorus
Plant availability of rock phosphorus is limited by both dissolution & fixation. As a result, the use of rock P is limited:
 * The use of rock P should be limited to acid soils where the low pH and low Ca levels can assist dissolution. Low P levels also assist dissolution of rock P.
 * Powdered rock (< 0.15 mm) dissolves much faster than granular rock P, so the granular form should be avoided.
 * Only use rock P if it is to be broadcast and incorporated into the soil, because close association of rock P particles placed in a band inhibits dissolution.
 * "Starter" water-soluble P placed in the band is essential for annual crops, even if rock P is used.

Potassium in the soil
Soils generally contain large amounts of K relative to plant requirements, but much of it is usually unavailable to plants. Soil K can be grouped into three fractions according to its availability: non-exchangeable mineral K, exchangeable K, and solution K. Non-exchangeable K is found in the crystal lattice of relatively unweathered or in the interlayers of clay minerals such as illite and vermiculite. It is available to plants only after weathering to the exchangeable form. Exchangeable K is held by electrostatic forces to negatively charged clay and organic matter surfaces. Solution K is the small fraction which is present in the soil solution.

Reactions transforming exchangeable K to solution K, and vice-versa, are rapid. As solution K is depleted, it is rapidly replaced by exchangeable K, and when K is added as fertilizer, it is rapidly adsorbed on negatively charged surfaces, ie. transformed to exchangeable K. In most soils, short term plant K supplies are derived mainly from the exchangeable and solution forms. Non-exchangeable K is released slowly into the plant available forms, and in some less-weathered soils, this release can supply plant-available K for many years. However, if soils containing appreciable amounts of illites, vermiculites and chlorites become depleted in K, fertilizer K may be trapped as interlayer K, thus reducing its availability to plants.

Most KwaZulu-Natal cropping soils are highly weathered, with kaolin minerals dominating the clay fraction. Reserves of non-exchangeable K are therefore generally low, and K fixation is seldom a problem in the province.

Role of potassium in plants
The primary role of K in plants is metabolic – there are numerous physiological and biochemical functions for which K is necessary (Mengel and Kirkby, 1987). These include involvement in enzyme activation, water relations through control of stomata and turgor, ATP production (energy relations), translocation of carbohydrates, nitrogen assimilation & protein synthesis, and starch synthesis. Important effects of K deficiency include yield loss, reduced tolerance to drought and frost, and increased susceptibility to fungal attack and lodging.

Potassium is the nutrient cation taken up in the largest amounts in crop plants, and K generally makes up more than 1.5% of the dry matter of non-woody crops. Because of this, the K reserves in most KwaZulu-Natal soils can be depleted rapidly, and yield loss due to K deficiency is common. Because K is highly mobile in plants, under conditions of deficiency it is transported from older to younger tissues. Hence, deficiency symptoms, usually in the form of browning or ‘firing’ of leaf tips and edges, first appear in older leaves.

Relations of considerable practical importance exist between K and N, Ca, Mg, and Na. The N status of plants has been shown to have a considerable influence on the requirements for K. Where K supplies are adequate, its uptake increases with increasing N supply, and the required K concentration (critical level) for maximum yield has been found to increase with increasing N concentration. A strong antagonism in nutrient uptake exists between K and the cations Ca, Mg, and Na; excessive K supply may lead to decreased uptake of Ca, Mg, and Na. Although this is generally not sufficiently severe to influence plant growth, the resultant mineral balance in pasture species may be detrimental to the health of grazing animals. Low Ca uptake can also have a detrimental effect on fruit quality (as with ‘blossom-end rot’ in tomatoes, and ‘bitter pit’ in apples).

Potassium fertilization: practical considerations
In their virgin state, most of KwaZulu-Natal soils contain sufficient plant-available K to sustain crop production for a few years. However, long-term K reserves are generally low, especially in the highly weathered soils of the high-rainfall areas. Fertilizer K is therefore generally needed to sustain profitable crop production.

Leaching of K is restricted to sandy soils (<15% clay); in loam and clay soils, K is essentially immobile. Therefore, other than through crop removal, and losses through soil erosion, K losses from KwaZulu-Natal soils are generally small, and applied K is generally efficiently used.

Fixation of K may greatly affect fertilizer K requirements. It is negligible in highly weathered soils, but in soils where clay minerals such as illite and vermiculite dominate, K fixation may appreciably increase fertilizer K requirements. In KwaZulu-Natal, the amount of fertilizer K required to increase the soil test in the top 200 mm by 1 mg/L, is generally less than 2.5 kg/ha for weathered soils. However, the requirement may be as high as 8.8 kg/ha for structured high base status soils (Johnston et al. 1999).

Potassium fertilizers
Potassium may be applied in the inorganic form as a ‘straight’ fertilizer or in NPK compounds or blends. The most widely used and cheapest inorganic K fertilizer is KCl (muriate of potash) which contains 50% K. Other ‘straight’ K fertilizers include the more expensive potassium sulphate and potassium nitrate which are generally used for specialist crops such as tobacco, and in tunnels using soil-less culture.

All the K contained in organic fertilizers is plant-available. Manures, slurries, composts, and green manures, although often neglected, are potentially valuable on-farm sources of K.

Potassium recommendations based on soil testing
What does the sample K test indicate?

The K test value indicates the amount of exchangeable K in the soil sample that is submitted for analysis; the unit of concentration is milligrams (mg) of K per litre (L) of soil. Laboratory methodology involves the determination of the quantity of K extracted from the soil by the "Ambic 2" extractant through the displacement of K from the soil particles by ammonium ions.

What is meant by "Target K test"?

The concentration of exchangeable K in the soil is widely regarded as the most reliable index of plant-available soil K. The "target K test" is the minimum concentration of soil K required for target growth of the indicated crop. This level of K depends on the crop to be grown, and the norms used are based on the results of field trials conducted locally and internationally.

Variations in target K test values for different plant species are due to differences in ability to utilise exchangeable K in the soil and differences in the amount of K required by the plant. Table 1 shows the target K test values for several crops.

Table 1. Target soil K test values for selected crops.

Crop	Target soil K (mg/L) Kikuyu	140 Eragrostis curvula	180 Italian ryegrass	140 Lucerne	180 Maize (Bio 3, 4 & 6)	120 Soyabean	80 Cabbage	200

How is the K recommendation determined?

This is calculated using the formula K Recommendation (kg/ha) = (Target K - soil K) x 2.5. The factor 2.5 is used for situations where the K is incorporated into the soil and corresponds to the amount of K (in kg/ha) required to increase the soil test by 1 mg/L. In the case of pastures where maintenance dressings of K are to be applied, only 1.5 kg K/ha are required to raise the soil test by 1 mg/L, and the formula used is   K Recommendation (kg/ha) = (Target K - soil K) x 1.5.

What if "Sample K" is greater than "Target K"?

A soil K test value that exceeds or is equal to the target K test value indicates good soil K reserves for the crop in question. In such a situation, the soil can supply all the K required by that crop and no K fertilizer is required. However, if uneven distribution of K across the sampled land is suspected, an "insurance" dressing of K may be justified, especially in the case of a high-value crop such as potatoes.

Crop-specific information

More crop-specific information on the potassium fertilization of different crops in KwaZulu-Natal is available for maize (Farina et al., 1992, 1993), soyabean (Birch et al., 1990), potatoes (Katusic & Manson, 1997; Manson & Katusic, 1997), lucerne (Phillips & Miles, 1996), other pastures (Miles, 1991), vegetables (Allemann & Young, 2001) and crops grown using no-till (Thibaud, 2000).