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Reference values ​​of soil analysis

Knowing how to interpret a  soil analysis or a  water analysis is important to know what we are dealing with. As if it were a blood test, to take corrective measures (such as cholesterol), we first have to know that we have a problem.

In agriculture, less soil testing is done than it should be, and what we don’t know is that a lot of fertilizers are wasted and a greater amount of money is wasted than the analysis itself costs.

In addition, it is not worth thinking that what we contribute to the soil, even if it is in greater quantity than the plants need, will be able to recover it for the following years, since many nutrients, especially nitrogen, percolate and are lost to deep layers .

Overall, we have a double negative effect . The plant does not take advantage of these nutrients and we increase the contamination of the soil and water, which is already very affected.

We started this article by adding these two interesting topics that are very much related to what we are going to tell you now.

How to interpret a soil test

How to decipher a water analysis


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.

Until next time!

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