Deciphering a Soil Test

From the creators of the saga “deciphering a water analysis” comes … Anyway! Let’s get into trouble. Since we did the article understanding a little more the data that a water analysis showed us, we are going to do the same with a soil analysis . In it there are a lot of interesting data to correct our irrigation, our fertilizer plan and even what we grow. Attack!


The soil is the basic system of all crops. Okay, there are also hydroponic crops, but most of them are supported on substrates like coconut fiber, perlite, vermiculite, etc. Very few farmers work with pure hydroponic systems (with bags and 0% substrate).

In Gardenprue we have done a lot of content on the ground, to the point that we have a category dedicated to it. You can see it here .


The first practice when analyzing a soil is to classify it within a category. We all know that it can be made up of a fraction of clay, silt (mixture) or sand. These particles are differentiated according to their size, and in a soil analysis those whose size is less than 2 mm are studied.

A picture is worth more than 1,000 words. That is why it is better to see this structural triangle of the USD , where we can obtain the characterization of the different types of soil.


Within all the grouping of data that we obtain when the results are sent to us, we can make a clear division of said information as follows:

  • Fertility analysis.
  • Extract of the saturated paste.
  • Exchange complex.

The fertility analysis , as the word says, tells us how our soil is fertile. The organic matter that we have contributed through manure (animal) or compost (food scraps, leaves, etc.) has a lot to say here .

Therefore, a very important value is the% of organic matter in our soil. The ideal is to find a percentage higher than 1.5%, but it is normal that in certain “bare” soils we have values ​​below 1%.

In such cases, we consider that it is a low fertile soil and it must be corrected.

In this fertility analysis, we also find in a complete soil analysis the phosphorus (P) value measured in ppm. There is a bibliography that reflects the interpretation of said data as follows:

  • P less than 5 ppm: low phosphorus content.
  • P between 5 and 10 ppm: normal phosphorus content.
  • P greater than 10 ppm: high content.

It is normal, in intensive agriculture , to find values ​​above 50 ppm. However, it is a value that should not be neglected, because sometimes there are real nonsense (above 600 ppm).

With this amount of phosphorus and being in a limestone soil, the normal thing is that there are precipitates of calcium phosphate . This causes us to find a hardened soil, with crusts and a cemented appearance.


Another important value when determining the fertility of a soil . It is also measured in ppm. These are the normal values ​​that we can find:

  • Less than 125 ppm: very low content
  • Between 125 and 220 ppm: low content.
  • Between 220 and 250 ppm: normal content.
  • More than 250 ppm: high content.

More things. We continue …



Everyone knows how pH works and what values ​​it wants to be between. In an irrigation application it is not considered as important as, for example, in a foliar application .

The latter can cause phytotoxicity in the crop both due to high and low pH.

This occurs because there are certain active substances , such as insecticides and fungicides, that have a pH range where they are considered stable and do not cause problems.

A known characteristic of the soil is its  buffering capacity. This buffers the pH spikes that are applied through irrigation.

However, these values ​​must be taken into account when classifying a soil:

  • pH between 4.5 and 5.5: strongly acidic soil.
  • pH between 5.5 and 6.5: acidic soil.
  • pH between 6.5 and 6.8: slightly acidic soil.
  • pH between 6.8 and 7.5: neutral soil.
  • pH between 7.2 and 7.5: slightly alkaline soil.
  • pH between 7.5 and 8.5: alkaline soil.


In Gardenprue we have already talked about the issue of conductivity. You can see it in this article.  Although we talked in greater detail about the conductivity of water , today we are going to talk about soil.

Plants live and develop within a tolerance range of conductivity. Some are more sensitive than others. Therefore, it is a factor to take into account when choosing a crop. Let’s see in what values ​​we move:

Conductivity in saturated extract, measured in dS / m

  • Less than 2 dS / m: there is no risk of saline soil.
  • Between 2 and 4 dS / m: there is a low risk of salinity.
  • Between 4 and 8 dS / m: there is a moderate risk of salinity.
  • Between 8 and 16 dS / m: there is a high risk of salinity.
  • Greater than 16 dS / m: there is a very high risk of salinity.


We must not blindly trust the laboratory that has carried out the analysis. There is a simple way to verify that the data is real.

As in a nutrient solution, the sum of cations and the sum of ions, measured in meq / L, must coincide . That is, they must give the same value.

However, the laboratory allows a 10% error. If there is a difference between the sum of cations and ions greater than 10%, the analysis is considered invalid .


A novelty that all soil analysis laboratories should offer is to provide a table showing the mean values of each of the parameters.

In this way, even without knowledge, we can make an adequate interpretation of what happens in our soil. The ideal, later, is to consult an agronomist, but it serves as a reference initially.

Converting the data from meq / L to mg / L or ppm (parts per million) is relatively simple and we only need the molecular weight of each of the elements.

Nitrato (NO3-): 1 meq/L = 63 ppm = 1 mmol/L

Fosfato (H2PO4): 1 meq/L = 97 ppm = 1 mmol/L

Sulfato (SO4-): 1 meq/L = 48 ppm = 0,5 mmoles/L

Potasio (K+): 1 meq/L = 39 ppm = 1 mmol/L

Calcio (Ca2+): 1 meq/L = 20 ppm = 0,5 mmoles/L

Magnesio (Mg2+): 1 meq/L = 12,15 ppm = 0,5 mmoles/L

Knowing these values, we will be able to know if we have any values ​​above normal (we will therefore reduce the contribution) and those that are below the average (we will contribute an additional amount).


full soil test is around € 80-90. The water one a little less. A priori, it may seem like a very high amount, but we are going to do the following account to definitely open our eyes.

Let’s imagine that we want to grow a tomato in a greenhouse, with a net calcium requirement of 10 meq / L and 4 meq / L of magnesium.

If we have adequate calcium and magnesium values ​​in the  soil analysis , it is advisable not to reduce these levels and to maintain an adequate reserve or pantry. From here on, it can be increased as we periodically contribute organic matter.

If we have guides with a high conductivity load (> 2.5 mS / cm), it is very likely that these salts are being supplied to us by chlorides, sulfates, calcium, magnesium or sodium. Not so much nitrates, phosphates or potassium.

Let’s imagine that in the water analysis we have the following values:

  • Calcio (Ca2+): 13 meq/L
  • Magnesio (Mg2+): 5 meq/L
  • Sodium (Na +): 10.49 meq / L

The first thing we have to see is if there is a good relationship between calcium and magnesium . It is assumed that if there is twice as much calcium as there is magnesium, all these nutrients provided by water can be absorbed. And from then on (relation 2, 3, 4, etc.).

Otherwise, if we have more magnesium content than calcium or the Ca / Mg ratio does not reach 2, we will have to calculate the fertilizer so that, by adding calcium nitrate, this ratio increases and there is no blockage of the soil.

In this case, as the calcium and magnesium needs of the  greenhouse tomato  that we have discussed previously were 10 meq / L for calcium and 4 meq / L for magnesium, the irrigation water amply supplies these needs.

Therefore, we are already talking about savings in the contribution of fertilizers rich in calcium and magnesium. 

How much? Let’s see it.

Let’s put a water consumption of 4,000 m3 per campaign and 1 meq / L of calcium nitrate = 108 mg / meq.

To provide 10 meq / L of calcium and 4 meq / L of magnesium continuously, for this amount of water:

  • 4,320 kg of calcium nitrate
  • 1970 kg of magnesium sulfate.

Only by putting the economic calculation of calcium nitrate, at € 0.35 / kg as an assumption, we would be talking about an expense of € 1,512 per campaign . Now we should also add magnesium.

Not being all so drastic, it must also be said that for every meq / L that calcium is supplied, nitrogen is also made, so we would save on the contribution of ammonium nitrate or derivatives.


Of all the values ​​that the soil analysis offers us, and that we can compare with the table of reference values , there are very important numbers that we have to see.


Study the percentage of organic matter in the soil as a very important factor to know the soil pantry.

Not only because of the nutrients that it will provide in the future after mineralization, but also because it contributes to improving the properties of the soil (drainage, temperature, population of microorganisms, etc.) and the storage capacity of nutrients that we provide.

Definitely. A soil rich in organic matter (1.5-2%) makes the fertilizers that we provide in the cover increase their absorption by the roots and reduce their leaching or insolubilization.


Basically it is the phosphorus content that the soil has, and many or most of the time they are in very high content.

It is normal to find values ​​10 times above what is recommended in soils where the soils are worked continuously (intensive operations).

This is a drawback since this phosphorus in high quantities in the soil, added to the continuous supply of calcium from the irrigation water (or what we contribute with calcium nitrate), causes insoluble precipitates to form for plants, such as tricalcium phosphate. .

With this, we reduce the effectiveness of the nutrient supply and contribute to having a much harder soil (gypsum and phosphate precipitates) where the roots have problems for their development.


The saturated extract is telling us what the plants can take from that soil when we apply irrigation water and solubilize the minerals.

If the content of nitrates, potassium, calcium and magnesium is in the average of the reference values, we have a good reserve of soil to carry the crop forward.

All we need is to apply fertilizers to keep these values ​​constant, but not for their replacement.

On the contrary, having a very large “pantry” of these nutrients, excessively increases the conductivity of the soil, reducing the productivity of our crops.

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