Introduction to integrated methods in the vegetable garden
Crop soil ; Essential laboratory analyses
Chapter : Crop soil
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⇒ Essential laboratory tests
Introduction to integrated methods in the vegetable garden
Chapter : Crop soil
Previous or next articles ; click on a title to go to the page
⇒ Essential laboratory tests
Before getting out the spade and the rake, I strongly advise any beginner gardener to evaluate the physical, chemical and biological characteristics of the soil of his garden with the help of a laboratory analysis in order to know its fertility potential and the cultivation systems it can support. This analysis also allows to know the pollutants and the level of contamination of heavy metals possibly present in the soil.
It is easy to find on internet sites or books specialised in gardening, descriptions of rudimentary experiments (pudding test, bottle test, leaching test, test of the presence of organic matter with hydrogen peroxide...) likely to indicate if the garden soil is rich in clay, or the fractions in clay and other elements. These experiments are not very precise and do not provide any information on the fertilising value of the soil. Some important biochemical characteristics can only be known from analyses carried out by a laboratory approved by the Ministry of Agriculture. These analyses, carried out according to standardised protocols, aim to establish a set of basic analytical data without which it is very difficult to take appropriate measures to improve soil fertility and correct any deficiencies.
A soil analysis carried out by a laboratory specifying soil particle size, organic matter content, pH value, iron and manganese content is already an interesting set of data. However, more precise measurements are needed to find out how much nutrients a soil is able to retain from in the absorbing complexes. Some deficiencies are impossible to detect without a more detailed analysis of the different elements present in the soil and their interactions. These deficiencies are often the cause of problems in the vegetable garden such as the prevalence of certain diseases or the production of stunted crops.
There are two forms of deficiency:
True deficiencies: One or more mineral elements are not sufficiently present in the soil.
Induced deficiencies: The minerals in the soil are sufficient, but their assimilation by the plants is blocked:
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· Either by the presence of another element in excess, such as the blocking of magnesium by an excess of potassium, the blocking of zinc by an excess of phosphoric acid, the blocking of potassium by an excess of calcium, or the blocking of iron in soils that are too calcareous.
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due to climatic conditions such as boron deficiency following a drought.
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Either an element is retrograded and stored in a non-assimilable form, which is often the case for phosphorus.
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Either because the pH is too acidic leading to a reduction in the absorption of magnesium and molybdenum, or too basic leading to a deficiency of trace elements, for example.
Every plot of land has a history of fertiliser inputs and crop exports (a), which means that a balance sheet of the fertiliser residues in the soil must be drawn up before starting new crops. Similarly, when a plot of land has not been cultivated for a long time, the assessment of its fertility can only be seriously apprehended on the basis of laboratory analyses. This is also the case if a cultivated plot of land has never been subject to such a study. These analyses must be carried out at least every 5 years, a period that can be reduced following major changes decided by the gardener to correct the physical and biological characteristics of the cultivated soil.
The surplus of inputs (b) that remains in the soil must be taken into account if it is quantitatively significant before the import of new fertilisers. This surplus forces a reduction in fertilisation for each excess element.
This more precise approach to fertilisation commonly used in integrated farming is often referred to as 'fertilisation reasoning', or ‘reasoning about inputs’.
These tips might seem exaggerated for a home gardener who wants to grow a small vegetable garden. Yet I know gardeners who lament the fact that some of their vegetables are stunted or frequently fall victim to disease. Gardening can be an interesting hobby, especially to combat the sedentary lifestyle that has become a real scourge nowadays. But how many efforts become useless leading to discouragement if the expected results are not there! A global analysis of the soil characteristics is only necessary once. The cost will be quickly recouped by strengthening the plants against diseases and increasing yields. Above all, you will avoid embarking on dubious or even dangerous treatments due to insufficient information on the physical and biological characteristics of the soil in your vegetable garden.
Source: table recomposed from conference data base 5-3-2013 - Biodiversité, Agriculture, Sol & Environnement
a) Exports are defined as losses of mineral salts as a result of harvesting.
b) In agriculture, inputs refer to everything that is introduced into the agricultural production process: fertilisers, plant protection products, composts and other amendments, seeds....
The plants are stunted with pale green to yellow leaves and eventually drop. Generalized chlorosis often indicates a nitrogen deficiency. Fruiting is often early as if the plant decided to reserve all its resources for the production of a few seeds to preserve its species. Leafy vegetables are quickly susceptible to nitrogen deficiency. This deficiency is easily corrected by applying potassium nitrate (when the potassium content in the soil is not excessive) to the leaves or to the roots during watering, or by applying pearl urea or ammonium nitrate before watering (slightly longer reaction time).
In vegetable crops, phosphorus deficiency is most easily noticed in legumes and fruiting vegetables such as tomatoes, cucumbers, aubergines, certain root vegetables such as potatoes and beetroot. This deficiency is characterised by stunted plants and delayed flowering. Older leaves are dark green before turning purple. The tips of the leaves may also turn dark green and the stems turn reddish. The fruits are small and few in number. To correct this deficiency, a small amount of available phosphorus should be added to the growing soil before planting or in a compost before spreading, such as 'rock phosphate' from solabiol.
The plant is flaccid indicating a lack of turgidity (lack of water inside the cells). Stems and leaves wilt easily and suffer from marginal necrosis, which can be confused with a magnesium or copper deficiency. The leaves then become reddish or brownish in colour. The leaves are often affected by chlorosis, which gradually spreads from the edge to the centre. Tomatoes, legumes and beetroot are very sensitive to potash deficiency, which occurs more easily in acidic soils. This deficiency is quickly corrected by adding potassium nitrate in the same way as for nitrogen deficiency, except if the latter is too abundant.
This deficiency is manifested by slowed growth, yellowing of the leaves without reaching the veins, or brown spots in certain plants with leaf fall. This deficiency, which is frequent in limestone soils, can be corrected every year by adding iron sulphate. However, as its action appears late (about 6 months), it is recommended to correct this deficiency the first year with iron chelate sprayed on the foliage. There are liquid specialities for a quick treatment of iron deficiency in the soil such as « Spinach H36 » from Aprochim.
Other possible deficiencies and confusions.
Disseminated yellowing in the form of pearls on the older parts indicates a magnesium deficiency. Chlorosis of the young organs indicates a calcium deficiency which may be confused with iron deficiency. Sulphur deficiency is characterised by chlorosis of the leaves; the veins are usually lighter than the interveinal tissue. Chlorosis of the veins indicates a magnesium deficiency which may be confused with a manganese or iron deficiency. Copper deficiency produces discoloration of the leaf tips. Root necrosis indicates a boron deficiency which may be confused with an attack of micro worms.
For the Provence-Alpes-Cote d’Azur region, one of the nearest laboratories approved by the Ministry of Agriculture is located at this address: Laboratoire Développement Méditerranéen (UDM) - 8 chemin des 2 Mas - Pist 4 - 30100 ALES - Tel: 04 66 61 02 97 - E-mail: laboratoire.ldm @wanadoo.fr - Website: click here. For the other regions of France, the list of laboratories approved by the Ministry of Agriculture in 2021 can be found here here.
UDM offers a series of analyses presented in a fact sheet accessible on its website for different situations (arboriculture, viticulture, arable farming, gardening ...). A fact sheet explains how to prepare a soil sample to be sent by post. This laboratory is able to carry out the specified analyses for a cost of approximately €90 including VAT (2016 price list), including the granulometric analysis which is carried out only once, unless you wish to change the texture of the soil by appropriate amendments. Data on microbial biomass, microbial activity and particle size fractionation of organic matter can also be obtained, but the price of the analysis will be higher.
The analysis sheet is accompanied by a page of comments on the physical, biological and chemical characteristics of the soil with advice on fertilisation. In order to obtain accurate measurements, I strongly advise you to check off the total soil nitrogen measurement among the choices offered, without which it is difficult to obtain accurate information on other parameters. For example, you will have an annual estimate of humus loss, which is important information for determining the volume of organic matter to be added each year, or the maximum supply of mineral nitrogen to be added expressed in U/ha (U = unit of measurement in kg of a pure element per hectare). Please note that not all laboratories offer such a complete analysis. Some important data are not specified, such as the ratio between certain elements, which may be the cause of certain induced deficiencies.
Subsequently, every 5 years, you can ask for a new, less expensive analysis to obtain some of the most useful data, such as humus and essential element reserves (potassium, phosphorus, magnesium, boron, etc.), the Carbon/Nitrogen (C/N) ratio, the Cation Exchange Capacity (CEC), and the specific ratio between certain elements. It should be noted that between two laboratory analyses, interesting data on the evolution of the characteristics of your soil (pH, nitrate content) are within the gardener's reach, provided that he or she acquires the laboratory utensils and their consumables (testers, test strips) which, depending on the equipment chosen, can be used for several years and is not necessarily expensive. This equipment is described in the following pages of this website.
Laboratory analyses are carried out on a fine soil sample and some parameters cannot be taken into account. As far as soil texture is concerned, particles smaller than 2 μm are considered as clay, although they may contain fine limestone elements. The clay content is therefore often overestimated. However, other laboratories include fine limestone particles separately in their reports. This is also the case for particles of 0.050 mm to 2 mm classified as "sand", even though they are not made up solely of silica, which inevitably has consequences for the physical and biological properties of the soil. The fixing power of cations (potassium, calcium, magnesium) and certain anions (phosphate, iron hydroxide, etc.) is different according to the type of clay. The CEC measured on the same sample with different control cations can vary significantly (1).
It is commonly accepted that the C/N ratio is also an indicator of the biological activity of the soil. A high C/N ratio (above = 12) would indicate a disturbed reduced biological activity. A C/N below 10 would indicate active biological activity. However, the introduction of long-term humus with a high C/N and including some humic acids is characterised by a reduced nitrogen consumption per year and an increase in CEC.
Laboratory analyses are sometimes denigrated because of their limitations related to analytical techniques. The techniques used by laboratories can sometimes hide biases that alter the conclusions.
A laboratory analysis can only give what exists at a given time. Its main objective is to provide a set of analytical data that the farmer can compare with other elements known to him that are inaccessible through laboratory analysis (such as fertiliser application periods, the influence of climatic conditions, the history of the plot, etc.), in order to refine his diagnosis. Some detractors of laboratory analysis prefer to focus on field observations: how plants evolve, their appearance, diseases diagnosed at a glance, etc. However, there is no good science without measurements. For example, for a deficiency that shows up as abnormalities in certain plants, how can we know whether it is induced or real without laboratory analysis! Moreover, it has long been known that the absence of symptoms does not necessarily mean that there is an optimal balance of mineral elements (2).
1) Ravina et Gurovich, 1977 ; Amacher et al., 1990
2) Symptômes et diagnostic des carences alimentaires ; Trocmé – phytoma N° 159 juin 1964
Laboratory analyses, next page: Soil texture and structure
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