Soil analysis is the best means to establish the fertility status of a soil and whether there are deficiencies or an oversupply of
certain elements in the soil. From the analysis it is possible to determine the optimum fertilizer
application level to ensure a high yield of good-quality potatoes. In this way farmers can avoid the
excessive or insufficient application of fertilisers. Soil analysis is therefore an aid to both the
farmer and the agricultural expert.
The pH indicates the acidity of a soil. The lime requirement determines the amount of lime that
needs to be applied, and the magnesium content dictates whether it should be dolomitic or calcitic
lime. The percentage of clay is important in the application of most herbicides. It also regulates
the timing of irrigation on nitrogen containing fertilisers.
The macro elements potassium, calcium, magnesium and sodium are important for the growth, yield and quality of potatoes.
The equivalent values of these elements are used to determine the relative proportions needed. The nitrogen content (N) of
the soil is not normally determined. The available amount of N and the rate of its release are difficult to determine.
Determining N is also a very time-consuming process. Plant tissue (leaf) analysis can be done during the growing season to
monitor the nutrient status of plants and deficiencies of certain elements can be supplemented by means of top dressings
or foliar sprays.
Soil samples must be taken very accurately and representatively. If part of the field is visibly different with regard to
properties such as colour and texture, it must be sampled separately. The vigour of the previous crop can also provide an
indication of whether there are any problem areas in the field.
Various soil-samplers or augers are available, viz. the Beater, Endeman and Viehmeyer, but an ordinary spade can also be
used successfully. A topsoil sample is taken to a depth of 20 cm, and subsoil samples from 30 cm to 60 cm. Take 20 to 30
samples of topsoil (about 500 g of soil) and five subsoil samples for every 2 to 5 ha. If the fields are very extensive and
homogeneous, the soil can be sampled in units of 50 ha. Mix the topsoil samples thoroughly and then extract a collective
sample of about 1 kg. Place the soil samples in clean, new plastic bags. Write TOPSOIL, the number of the field, the name
of the farm and the name of the owner on a label and attach it securely to the outside of the bag. The label must not be
placed inside the bag. Do the same with the subsoil.
For recommendations to be made on fertilisation, certain information is needed, such as: will the potatoes be cultivated
under irrigation or under dryland conditions, the planting date, cultivar and target yield. This information must be submitted
together with the samples.
Soil samples can be submitted to various Institutes of the Agricultural Research Council (ARC), as well as to fertiliser
companies, for analysis and recommendations.
The potato plant has a poorly developed root system. Fertiliser is therefore mainly applied in the planting furrows at the time
of planting. It should preferably be placed at the same level as, or under, the seed tubers. It is recommended that potato
planters be equipped with bins for fertiliser application.
If the phosphorus requirement is very high, some of the phosphorus can be broadcasted and incorporated into the topsoil.
The rest is then applied in the planting furrows along with the nitrogen and potassium. Top dressings are applied on either
side of the plant, after which the rows are ridged and irrigated. It can also be applied through the irrigation water. Fertiliser
granules or liquids must be washed from the plant foilage as soon as possible after application to avoid burning of the
foliage. Some types of fertiliser, such as potassium nitrate and ammonium nitrate, are more likely to result in fertiliser burn.
Where large quantities of nitrogen have to be applied, it is preferable to divide the total amount into a number of separate
top dressings. The best times for applying nitrogen fertilisers on soils of different clay contents are discussed under nitrogen
fertilisation.
If necessary, trace elements can be applied as foliar sprays. Agricultural gypsum for supplementing the calcium content is
usually broadcasted and incorporated in before planting, but may also be applied in the planting furrows. Applications of
lime should be ploughed in during the previous season (before the potatoes are planted).
(Compiled in collaboration with Potatoes South Africa and National Potato Working Groups)
Nitrogen (N)
Nitrogen is one of the most important elements needed for growth because a shortage can result in yield losses. However,
an excess of this element can also be detrimental since it can result in excessive foliage growth at the expense of tuber
growth. Excess nitrogen also adversely affects the keeping quality of tubers, lowers the specific gravity, and may lead to
hollow heart, particularly for the cultivar BP1.
Generally, the nitrogen content of a soil is not determined by means of soil analysis. The soil nitrogen content is affected by
microbiological activities in the soil, which makes it difficult to interpret the results. Guidelines for nitrogen fertilisation for
various cultivation conditions are given in Tables 5.1 to 5.3.
These guidelines, however, must be adapted according to the rainfall, the soil’s history, the soil type, the planting density
and the planting date. Nitrogen is easily leached from the soil and irrigation must therefore be done judiciously. It is
therefore advisable to apply part of the total quantity of nitrogen as a top dressing, especially on sandy soils (see Table 5.3).
Table 5.1: Nitrogen fertilizer recommendations (kg/ha) for different yield potentials under rain fed production on soils
of different clay contents
Table 5.2 Nitrogen fertilizer recommendations (kg/ha) for different yield potentials under irrigation on soils of different
clay contents
Table 5.3 Percentage of total nitrogen fertilizer requirement to be applied before and after tuber initiation on soils of
different clay contents
Phosphorus (P)
Phosphorus is an important component of all plant cells. Sufficient quantities of phosphorus stimulate early root growth and
increase the plant’s water-use efficiency. A shortage of this element may result in the tubers having poor keeping quality.
Fertilisation recommendations for phosphorus are based on soil analyses. The phosphorus content of a soil can be
determined according to various methods. Guidelines for phosphorus fertilisation are given in Table 5.4. Phosphorus does
not easily leach from the soil and it can therefore be applied in an once-off application at or before planting.
Table 5.4
Phosphorus fertilizer recommendations (kg/ha) on the basis of different soil analysis methods
Potassium (K)
Potassium promotes the deposit of starch and is important for certain reactions in the photosynthesis process. An excess of
potassium in the soil can result in poor tuber quality. Deficiencies can also have adverse effects on quality (dry crisp colour
and relative density). Potassium requirement should not be determined exclusively on its concentration (mg/kg or ppm)
according to the soil analysis. The ratio of potassium to the cation exchange capacity (CEC) of the soil is also important.
Producers must therefore insist on having the CEC of the soil determined when the soil analysis is conducted.
If the CEC has not been determined but the acid saturation of the soil has been determined (titratable acid or H+ expressed
as percentage milli-equivalents (me %), the CEC of the soil can be calculated, as shown in Step 1. The total of the basic
cations (Na, K, Ca, Mg) and the titratable acid then represents the CEC.
Example:
Suppose the soil analysis is as follows:
The following steps are now followed to calculate the quantity of potassium as a percentage of the CEC:
If the concentrations of potassium, magnesium, calcium and sodium are indicated on the soil analysis in ppm or
mg/kg, it must be converted to me (milli-equivalents) per 100 g of soil (also expressed as cmol(+) kg-1). This is done
as follows:
The hydrogen concentration is then added to this total to give the CEC:
5.31 + 0.18 = 5.49 me
The CEC of the soil is therefore 5.49 me/100 g of soil.
The quantity of potassium is then estimated as a percentage of the CEC, e.g.:
me K/100 g soil x 100
0,21 x 100
__________________ = ____________
= 3.8 me %
CEC
5.49
If the CEC is not given and it can also not be estimated using the above method, the potassium ratio can be estimated as a
percentage of the total cations, as obtained in Step 1:
0.21 x 100
_________
= 4.0 me %
5.31
The potassium requirement of the soil can now be determined from Table 5.5 or Table 5.6.
Table 5.5: Potassium fertilizer (kg/ha) recommendations on the basis of analysis for soils with a CEC greater than 6
me %
Table 5.6 Fertilizer recommendations for potassium on the basis of soil analysis for soils with a CEC of less than 6 me
%.
Note: For the Sandveld production area the values in Table 5.6 may be adjusted 10% upwards. Consult a qualified advisor
in the area for more advice.
Calcium and magnesium fertilisation
Calcium and magnesium requirements can be estimated in the same way as for potassium. The following norms indicate the
desirable percentage saturation of these two cations:
Magnesium
15 to 20% of the CEC
Calcium
60 to 70% of the CEC
For soils with a CEC of greater than 6 me %, the calcium (as gypsum) recommendations given in Table 5.7 are used, and
for soils with a CEC of less than 6 me %, those in Table 5.8. Recommendations for magnesium fertilisation are shown in
Table 5.9.
The guidelines in Table 5.9 are valid if magnesium oxide is used. If other magnesium sources are used, e.g. magnesium
sulphate, magnesium nitrate or magnesite, their solubility must be taken into account in the calculations.
Trace elements
It is recommended that the hot water extraction method be used for soil analysis to determine the boron content.
Since problems can be experienced with soil analysis, it is recommended that the international norms for tissue analysis,
which are available in various text books, be used to determine whether or not the trace elements in the soil need to be
supplemented. When leaf samples are taken for analysis, it is important to take into consideration the stage of growth, the
part of the plant and the position on the plant, otherwise incorrect conclusions may be drawn. Trace element
recommendations must always be made by a qualified advisor.
Table 5.7: Fertilizer recommendations for calcium (as gypsum) based on the analysis for soils with a CEC greater
than 6 me %
Table 5.8: Fertilizer recommendations for calcium (as gypsum) on the basis of analysis for soils with a CEC of less
than 6 me %
Table 5.9: Fertilizer recommendations for magnesium on the basis of soil analysis
J. M. Steyn and K. P. Prinsloo
