- A GUIDE TO THE MEANINGS OF SOIL ANALYSIS
To interpret analytical data fully in terms of crop requirements many factors must be taken into account, including the type of soil, its climatic environment, and its recent, history. (This is why the information asked for on the Soil Sampling Information Sheet is so important). It is the agricultural chemist’s job to do this when he makes his fertilizer recommendations, but farmers and agronomists often like to have a broad idea of what the figures mean so that they can compare the nutrient status after a number of seasons or between different lands. The following notes may help them to appreciate the significance of the analyses.
PH VALUES (CALCIUM‐CHLORIDE METHOD)
Soil acidity is described quantitively by the “pH value”. The accurate measurement of soil pH is not nearly as simple as most people think. The value obtained varies considerably with the amount of water present and, unfortunately, for technical reasons it is not practical to carry out the determinations with the relatively small amounts of water that are present under field conditions. This method employs a dilute solution of calcium chloride (M/100 CaCl), instead of distilled water as in the conventional methods. The calcium chloride method gives much more accurate laboratory results and, more important, it gives a truer measure of what the soil acidity will be under field conditions during the growing season. This is because it is not appreciably affected by the amount of water present, or by fluctuations in the salt content of the soil which are brought about by the use of fertilizers and manures.
Values obtained by the calcium‐chloride method cannot, however, be directly compared with those obtained by the water method, which are the ones that most people are accustomed to think in terms of, and which are most commonly referred to when optimum or desirable pH ranges are described in agricultural publications. On an average, the calcium‐chloride pH is about 0,7 units lower than the water pH (1:5 suspensions, as previously used), but in individual soils the difference may be considerably more or less than this figure, and it is in such cases that the calcium‐chloride value is a much more reliable indication of soil acidity under field conditions.
It is, therefore, necessary to draw up an entirely new scale of desirable and critical pH ranges by which to interpret the significance of calcium‐chloride pH values. Local experience shows that the following may be safely used as a broad guide:
Table 1: pH ranges of the calcium chloride method
|Strongly alkaline. Usually unsatisfactory and requiring further investigation.
|6.5 – 7.5:
|Alkaline. Usually on account of the presence of free lime. Satisfactory for most crops, though higher than desirable.
|6,0 – 6,5:
|Neutral. Highly satisfactory for lucerne, clovers, wheat and barley. Satisfactory for most other crops, but higher than desirable for tobacco.
|5.5 – 6.0:
|Slightly acid. Highly satisfactory for almost all crops, including tobacco. Lime is not required except in special circumstances.
|5.0 – 5.5:
|Medium acid. Satisfactory for most crops, but to maintain the pH in this range under regular cultivation liming will be necessary at a suitable stage in the rotation, especially in the better rainfall areas.
|4.5 – 5.0:
|Strongly acid. In the lower part of this range there is a progressive risk of fertility being adversely affected, and lime is therefore required.
|Very strongly acid. Severe infertility is likely and liming is essential before planting.
The total nitrogen content of a soil reflects its organic matter content. It varies greatly, being higher in higher‐rainfall areas than low‐rainfall areas, higher in vlei soils than upland soils, higher on heavy‐ textured soils than sandy soils, and higher in virgin soils than cultivated soils unless very large amounts of compost or manure are used regularly. It is, of course, much lower in the subsoil than the top‐soil, and abnormally low figures for the particular soil type may indicate severe erosion.
The following are characteristic ranges of total nitrogen in upland soils of medium and low rainfall areas:
Table 2: Total nitrogen of medium and low rainfall areas
|Total Nitrogen %
|0.02 – 0.05
|0.04 – 0.07
|Sandy Clay Loams
|0.06 – 0.10
|0.10 – 0,15
The term “mineral nitrogen” refers to nitrogen present in the form of ammonium salts (usually referred to for convenience as “ammonia”) and nitrates, and it represents the very small fraction of the total nitrogen that will become available to plants during the growing season. In a moist soil, this fraction is constantly replenished by decomposition of a small part of the organic matter, brought about by micro‐organisms in the soil. The rate which this occurs (known as the “rate of mineralization”) depends on the texture of the soil, its aeration, and its fertility. It is higher on sandy soils than clay soils and higher on well‐managed rotated soils than on soils that have been continually cropped for a long time with inadequate return of organic matter.
Table 3: The following are the average rates of mineralization, per season for cultivated upland soils:
|3 ‐ 5% of the total nitrogen
|Sandy loams and sandy clay loams
|2 ‐ 5% of the total nitrogen
|1 ‐ 2% of the total nitrogen
Two separate mineral nitrogen figures are quoted in soil analysis reports:
THE INITIAL MINERAL NITROGEN
This is the amount of nitrate‐nitrogen plus ammonia‐nitrogen, expressed in parts per million, actually present in the sample as received. Clearly this figure will vary enormously, depending on when the sample was taken and how much leaching has occurred e.g.: it is highest just after the commencement of the rains, decreases later, and rises again at the end of the rains. It also depends on recent treatment, being highest in soils that have recently been fertilized (with fertilizers containing nitrogen) and in soils in which residues of high nitrogen content are actively decomposing e.g.: some weeks after a leguminous green‐manure has been ploughed into a moist soil at the right stage of maturity.
THE MINERAL NITROGEN AFTER INCUBATION
This figure is the total amount of nitrate‐nitrogen plus ammonia‐nitrogen present in the sample after it has been maintained in an incubator for two weeks at the optimum moisture and temperature for microbial decomposition of organic matter. Experiments have shown that the amount of nitrogen mineralized under these optimum conditions is closely related to the amount that becomes mineralized in the field during the growing season. Since the figure obtained in the laboratory includes what was present originally (i.e. the initial mineral nitrogen) as well as what has become mineralized, it provides a good indication of the total nitrogen likely to be available to the crop, provided it is planted early before an appreciable amount of the initial nitrogen has been leached out
Table 3: indicates the significance of the mineral nitrogen figure and the conditions of which they are typical
|Mineral Nitrogen after incubation p.p.m
|Available Nitrogen Status
|Virgin or reverted lands and grass leys, un‐ ploughed or ploughed late.
|20 – 30
|As above, but ploughed early and lands that have been cultivated for some time with normal management.
|30 – 40
|Intensively managed cultivated lands and early ploughed, well fertilized, intensively managed grass leys.
|Well managed lands following a good leguminous green crop turned in at the right time and stage of maturity and lands that have received heavy dressings of compost or manure regularly, as in market gardening.
AVAILABLE PHOSPHORUS (RESIN EXTRACT)
The available phosphorus figures stated in soil analysis reports are determined by a new procedure. The soil is shaken for 16 hours with an “anion exchange resin” which progressively extracts the phosphate as it comes into solution, thus tending to simulate what occurs in the field. Experiments have shown that the new method provides a much better indication of the availability of phosphorus in soils than any other method, especially on fertilized soils, i.e. it is more sensitive to the residual effect of phosphatic fertilizers.
Table 4: Indicates the significance of available phosphorus figures and the yield increases that may be expected with adequate dressings of fertilizer phosphate:
|Available Phosphorous (resin extract) p.p.m. Phosphorus pentoxide
|Available Phosphorous Status
|Yield increases that may be expected with adequate dressings of phosphatic fertilizer, if other nutrients are also adequate
|Less than 7
|Very large: (up to double the yield)
|7 – 15
|Large: (increase of one to two‐thirds)
|15 – 30
|Small: (increases of less than one‐third)
|30 – 50
|No appreciable response likely with general crops, but maintenance dressings desirable.
|No response likely
The exchangeable potassium is the fraction of the total soil potassium that is held on the surface of clay particles in a form in which it can become available to plants.
The total amount of potassium (and similar elements, chiefly calcium and magnesium) that the soil can hold in this way depends on the amount and type of clay present. If the amount of exchangeable potassium actually present represents only a small proportion of what the soil is capable of holding, it will be more tightly held than if the proportion is high, and therefore it will be less available to plants. This means that the range of exchangeable potassium that corresponds to, say, a deficient potash status will vary according to the type of soil and will be higher in a clayey soil than in a sandy soil.
While the exchangeable potassium status of the topsoil is a useful guide to the potash supply power of the soil, there are other factors that may profoundly affect it. Some soils contain reserves of potassium in non‐exchangeable forms which may nevertheless be available to plants. The sub‐soil may also make a big contribution in some cases; while most of the available potassium is confined to the topsoil in heavy textured soils, it explains why no response to potash is frequently ‘obtained on such soils, even though analysis of the usual topsoil sample may indicate deficiency.
Table 5: For the above reasons the ranges given in the following table for the three main textural groups of soils should be regarded as a broad guide and not a firm index of availability of potash.
|Exchangeable Potassium (mm equivalents per 100 g)
|Typical Reddish Brown Clays
|Deficient (response to potash likely)
|Marginal (some response likely, if other conditions are suitable for high yields)
|0.05 – 0.1
|0.1 – 0.2
|0.15 – 0.3
|Adequate (response unlikely but maintenance dressings usually desirable)
|0.1 – 0.25
|0.2 – 0.3
|0.3 – 0.5
|Rich (no potash required)
OTHER DATA GIVEN IN SOIL ANALYSIS REPORTS
The texture of a soil refers to the proportion of the various particle‐size fractions of which it is composed. There are three main fractions, which are internationally defined as follows:
- Clay : particle sizes less than 0,002 mm
- Silt : particle sizes 0,002 – 0,02 mm
- Sand : particle sizes 0,02 – 2,0 mm
Anything coarser than 2 mm is classed as gravel and is not included in the soil analyzed.
Table 6: The textural classes commonly found in soils contain the following proportions of clay, silt and sand
|More than 50 % clay
|30 – 50 % clay : less than 50 % sand or silt
|20 – 30 % clay : less than 50 % sand or silt
|30 – 50 % clay : more than 50 % sand
|Sandy Clay Loams
|20 – 30 % clay : more than 50 % sand
|More than 20 % silt + clay : more than 50 % sand
|15 – 20 % silt + clay : more than 85 % sand.
|Less than 15 % silt + clay : more than 85 % sand
Other textural classes are: loams, silty loams, silty clay loams and silty clays, but they are not common.
These are carbonates of calcium chiefly (commonly known as “lime”) magnesium and, very occasionally in strongly alkaline soils, sodium. They normally occur in soils which contain as much exchangeable calcium and magnesium as they are capable of holding on their clay particles (i.e. they are “saturated” with exchangeable bases), and the excess is present in a “free” form as solid carbonates.
This refers to the electrical conductivity of a suspension of 1 part of soil in 5 parts of distilled water. The ability of such a suspension to conduct an electrical current is dependent on the quantity of salts that dissolve out of the soil, and so the measurement provides an index of the soluble salt content of the soil or its salinity.
Normally, the conductivity is very low (less than 100 micro‐ohms) but it may rise to 150 on heavily fertilized soils. The measurement is only made when other features of the analysis, or abnormal symptoms reported by the farmer, suggest that the soluble salt content may be agriculturally significant. The interpretation of conductivity figures depends on the texture of the soil and the type of crop to be grown, and it is not possible to give a simple guide.
This figure represents the approximate sum of exchangeable calcium, magnesium and potassium. It is used by the chemist in conjunction with the pH value to estimate the probable lime requirement of the soil. But a discussion of the relationship of the total base content to pH and lime requirement is beyond the scope of these notes.
2. THE SOIL SAMPLING INFORMATION SHEET
An example from the Iowa State University.
3. THE SOIL SAMPLE CARTON
SOIL ANALYSIS REPORT
All results are expressed in terms of the air-dried sample passed through a 2 mm sieve Reference Number……………………