Key data in a soil chemical analysis: what do they mean? How do they impact soil fertility?

Through a well-performed soil analysis, it is possible to assess the degree of nutrient deficiency and determine the quantities to be applied in fertilization and liming. An important factor for high sustainable productivity in agriculture, chemical soil analysis is the basic instrument for transferring information about liming and fertilization from research to the farmer (DA SILVA et al., 2009).

There are five interlaboratory quality control programs in operation in Brazil: Rolas, for RS and SC; Cela, for PR; IAC, for laboratories that use the resin method in SP and eight other states; Profert, for MG and some neighboring states; and Embrapa Solos, which covers the rest of the country, especially the Cerrado, Southeast, North and Northeast regions. Approximately 220 laboratories participate in these programs, and the granting of “seals” that attest to their affiliation to the proficiency program represents a great incentive for the reliability of the laboratories (DA SILVA et al., 2009).

From now on, we will use as a basis the work of Prezotti and Martins (2013), which explains the parameters analyzed in a chemical analysis of the soil in an educational way. The quality of the research allows us to calibrate and interpret the results of the analysis, based on which recommendations for correctives and fertilizers are made.

Soil chemical analysis consists of two main stages: extraction and quantification. In the extraction stage, chemical solutions called “extractants” are used to simulate the absorption of nutrients by plants. In this stage, a certain volume of the extractant is stirred with a defined volume of soil, displacing the nutrients from the solid phase to the liquid phase (equilibrium solution).

In the quantification stage, the levels of the elements in the equilibrium solution are determined using devices such as the atomic absorption spectrophotometer, which allows the quantification of the elements: K, Na, Ca, Mg, Zn, Cu, Fe, Mn, Pb, Cd, Cr, Ni, etc. The quantification of P, B and S is performed using a UV/Visible spectrophotometer (colorimeter), Al by titrimetry and the pH is determined using a potentiometer.

The nutrient levels determined by chemical analysis of the soil are compared with reference values, presented in interpretation tables, thus allowing the classification of the soil fertility level and the indication of the quantity of correctives and fertilizers to be applied for maximum crop efficiency.

The main parameters analyzed in soil analysis are:

pH

pH measures the active acidity of the soil which is the activity of H⁺ present in the soil solution. The pH varies over time, changing its value according to soil management, successive crops and fertilization. Plants, when absorbing positively charged nutrients (K⁺, Mg⁺⁺, Ca⁺⁺ etc.), release H⁺ from the roots to the soil solution, which reduces the pH. In the reaction of nitrogen fertilizers with the soil, specifically in nitrification (change of ammonium to nitrate), there is also the release of H⁺. In addition to these, other factors contribute to increased soil acidity, such as rainfall, irrigation, among others.

Aluminum (Al³⁺)

Indicates the aluminum content in the ionic form Al³⁺ (also called exchangeable acidity) which is the form that is toxic to plants. All soils contain aluminum in various forms or compounds, and their total content is practically constant. What varies are the forms in which the aluminum is found.

Over time, the leaching of bases from the soil, caused by rain, the absorption of bases by plants in successive crops and the application of fertilizers, mainly ammoniacal nitrogen fertilizers, acidify the soil again, reducing its pH and thus increasing the solubility of aluminum, which changes from the form Al(OH)₃⁰ for Al³⁺ (and other intermediate forms), causing toxicity to plants again.

In acidic soils, the solubility of Al³⁺ is very high, causing damage to plant roots. As the pH rises, the solubility of Al decreases.³⁺, up to pH 5.5, with no more presence of the toxic form, with aluminum predominating in the form Al(OH)³, which is an inert precipitate.

The Al³⁺ causes thickening of the roots, reduces their growth and prevents the formation of root hairs, impairing the absorption of water and nutrients. However, there are plant species with high tolerance to Al³⁺, like several species of the genus Eucalyptus.

H+Al

Also called “potential acidity” or “total acidity”. The interpretation classes for potential acidity (H+Al) estimated by correlation with pH SMP These interpretation classes are generic and of little practical application, since the main objective of determining H+Al is to calculate the Total CEC of the soil (T). Generally, H+Al values are higher in soils rich in organic matter, especially if these have low pH values.

Sum of Bases (SB)

It represents the sum of the bases present in the soil, that is, the elements K⁺, In⁺ Here²⁺ and Mg²⁺. It is also called S, but this representation should be avoided so as not to confuse it with sulfur, whose symbol is also represented by the letter S.

The interpretation classes for the sum of bases (SB) are generic and have no practical application, being estimated to assist in the calculations of Total CTC, effective CTC and base saturation (V).

Total CTC

It is the soil's cation exchange capacity, measured at pH 7, also represented by the letter T. It is one of the most important variables for interpreting the soil's productive potential. It indicates the total amount of negative charges that the soil could present if its pH were 7. These charges are able to adsorb (retain) positively charged nutrients (K⁺, Ca²⁺ and Mg²⁺), added to the soil via liming or fertilization, and other elements such as Al³⁺, H⁺, In⁺ etc.

AT is a characteristic of the soil and has a practically constant value (it can only be altered with the application of high doses of organic matter or as a result of an intense erosion process, when there is loss of the surface layer). Thus, since the total amount of negative charges in the soil is practically constant, the greater the amount of Al³⁺, H⁺ and Na+ in the soil the smaller the amount of negative charges available to adsorb the K bases⁺ Here²⁺, Mg²⁺. When the amount of cationic nutrients added via fertilization is greater than the soil CEC, these nutrients (K⁺, Ca²⁺, Mg²⁺) can be lost through leaching.

• Clay soils and/or soils with a high organic matter content generally have a high T, that is, they can adsorb large quantities of cationic nutrients.
• Sandy soils have low T and, even with small additions of bases, they are susceptible to losses through leaching.
• Soils from temperate climate regions, less weathered, generally have a higher T than soils from tropical regions, due to the mineralogy and higher organic matter content due to the lower mineralization rate provided by low temperatures.

Effective CTC (t)

Indicates the amount of negative charges occupied by exchangeable cations. In this case, H is not considered.⁺.

Base saturation (V)

Indicates the percentage of total negative charges occupied by bases (K⁺ + In⁺ +Ca²⁺+ Mg²⁺).

The unit used to express base saturation is the percentage (%), which is accepted by the International System of Units as it is a calculated index and not a concentration or level.

Liming aims to increase the soil's base saturation to values appropriate to the crop's requirements, which generally range from 50 to 80%. By increasing the soil's base saturation with liming, there is a proportional reduction in H+Al, thus reducing soil acidity.

Aluminum saturation (m)

Al is the only element whose proportion is determined based on t, since T is estimated by considering all negative charges occupied by bases, at pH 7. For the other elements (K⁺, Ca²⁺, Mg²⁺ and In⁺), the proportion is calculated in relation to T. For adequate growth and development of plants, ideally there should be no presence of Al³⁺, that is, the pH is greater than 5.5, at which point om is equal to zero. Therefore, soils with the same Al content³⁺ can present different values of m, as long as they have different values of t.

Na and ISNa

Na is the available (exchangeable) sodium content and ISNa is the soil sodium saturation index, also called Percentage of Exchangeable Sodium (PST).

When present in high concentrations in the soil, Na can have a depressive effect on crop productivity by hindering the absorption of water and nutrients by the plant or by its dispersing effect on clays, causing soil destructuring and reducing water infiltration, gas exchange and hindering root penetration.

Information on the available Na content of the soil alone is not sufficient to assess adverse effects on plant growth and development. It is also important to know the proportion in relation to other soil cations, such as K.⁺, Ca²⁺ and Mg^2+.

Soil organic matter (OM)

Soil organic matter (OM) is formed by residues from the aerial and root parts of plants, microorganisms and root exudates. It is basically composed of C, H, O, N, S and P. The proportion of these elements is around 58% of C, 6% of H, 33% of O and 3% of N, S and P.

The soil's organic matter content is an indication of its productive potential, as soils with a higher OM content have higher T values and a greater capacity to supply nutrients to plants, when compared to soils with lower OM contents.

In tropical soils, OM is primarily responsible for the generation of negative soil charges, contributing up to 80% of soil negative charges.

With the mineralization of OM in the soil, there is a release of bases that were immobilized in the carbon chains of plant tissues, which promote an increase in pH and nutrient availability. Al complexation also occurs.³⁺ from the soil by the organic molecules released, which helps to reduce the toxicity of this element and raise the pH.

In soil analysis, the N content is generally not determined due to its complex dynamics in the soil, with changes in its form depending on environmental conditions, such as humidity, temperature, pH, microorganism activity, etc. Although there are methods for determining the different forms of N in the soil, a method that integrates such a large number of factors and provides an index of N availability has not yet been developed.

One way to estimate the potential for soil N supply is to quantify its availability through soil organic matter.

Available phosphorus (P)

The “available” phosphorus (P) content for plants is a relative measure of the amount of the element in the soil. Specific extractors are used to determine it, the most common being Mehlich1 and Resina. The amounts of P recovered by these extractors are different. However, for any of them, the indication of high levels means that in that soil there is a low probability of crop response to the application of P. Otherwise, if the values determined in the analysis are low, it means that there is a need to apply P for adequate plant growth and for them to be able to achieve the desired productivity.

Remaining phosphorus (P-rem)

It measures the soil's P adsorption capacity, i.e., how much of the applied P is retained by the soil's clays. The more clayey the soil, the greater the adsorption of P by the clays and the lower the amount of P in the equilibrium solution, since part of the P in the solution will be retained by the clays. After a certain contact time, the P is quantified in the equilibrium solution (hence the name "remaining phosphorus"). The final concentration of P in the solution indicates the soil's adsorption capacity and allows us to infer its texture, whether clayey, medium or sandy.

Available potassium (K)

Indicates the potassium content available in the soil. It is extracted by the Mehlich-1 extractor or Cation Exchange Resin.

With the weathering of minerals, part of the structural K passes into exchangeable and solution forms. However, this is a slow process and, in most cases, insufficient to supply commercial crops with higher productivity, especially those with a short cycle.

The soil's greater or lesser capacity to replenish K in solution depends on the amount of structural K, which varies with the quantity and quality of soil minerals. For this reason, crops behave differently depending on the type of soil. An example is banana cultivation, which grows best in soils with high K levels and a high capacity for replenishment by minerals.

Calcium (Ca) and Magnesium (Mg)

They indicate the amount of calcium and magnesium in the soil in exchangeable form (Ca²⁺ and Mg²⁺), that is, capable of absorption by plants.

Ca levels²⁺ and Mg²⁺ are directly related to soil acidity. Acidic soils generally have low Ca levels.²⁺ and Mg²⁺ and soils with good fertility, higher Ca contents²⁺ and Mg²⁺. These are the elements that most influence V due to their higher occupation rate of T.

In acidic soils, their levels are increased with the application of limestone, which, in turn, increases the soil's base saturation, increases the pH and reduces Al toxicity.

Soils with low T and low Ca and Mg contents (characteristics of sandy soils) may present medium to high V. This is because V is a relative value and may give a false indication of high fertility.

Sulfur (S)

The S content in the soil is easily altered by soil management or rainfall, as it is easily leached in the form of SO₄^2-. Generally, its content is higher in lower layers, such as, for example, 20 to 40 cm.

In fertilization recommendations, S is usually relegated to the background because it is supplied via fertilizers such as ammonium sulfate, simple superphosphate or potassium sulfate. Another reason is that relatively low doses (40 to 80 kg/ha) are sufficient to meet the demand of most crops.

However, when fertilizers that do not contain S in their composition are continuously used, such as high-concentration formulas, which are mainly made up of urea, triple superphosphate, MAP and potassium chloride (Example: 25:05:20) and in high-productivity crops, S deficiency may occur.

The species that require the most S belong to the cruciferous (cabbage, cauliflower, etc.) and liliaceae (garlic, onion, etc.) families, with average demands of 70 to 80 kg/ha of S. Legumes, cereals and forages have lower requirements (15 to 50 kg/ha). In general, legumes require higher amounts of S than grasses, due to their higher protein content.

Micronutrients

Micronutrient analysis has some limitations that make it difficult to assess their real availability in the soil. The low levels extracted, especially of B, Cu and Zn, the pH, clay content and organic matter in the soil are variables that can influence the interpretation of micronutrient availability, in addition to the different levels of crop demand.

It is necessary to know the characteristics of each micronutrient, its dynamics in the soil and in the plant, so that preventive measures can be taken, thus avoiding future deficiencies in crops.

B: Easily leached in sandy soils with low organic matter content. High rainfall and excessive irrigation levels increase leaching losses. Deficiency symptoms occur in dry periods and tend to disappear when adequate soil moisture returns. This occurs due to reduced mineralization of organic matter, an important source of B for the soil. Drought also reduces B transport in the soil and root growth, thus reducing its absorption.

Zn: Deficiencies are more common in clayey soils with high pH. Like P, it is retained with high energy by the clays in the soil, which makes it difficult for plants to absorb it. Liming reduces the availability of Zn due to the increase in pH. High doses of phosphate fertilizers also reduce the availability of Zn.

Ass: In organic soils, there is a greater probability of Cu deficiency due to the formation of stable complexes, which makes it difficult for plants to absorb Cu. Sandy soils are more deficient in Cu than clay soils due to the ease of leaching.

Faith: Generally abundant in tropical soils. Its availability is greatly reduced with increasing soil pH. For this reason, liming is an efficient practice to reduce Fe toxicity in crops sensitive to this element. Fe deficiency can be caused by excess P, high pH and low temperatures.

 Mn: Like Fe, it is generally abundant in tropical soils. Its availability also decreases with increasing soil pH. In organic soils, complexes are formed that reduce the availability of Mn to plants. Symptoms of deficiency are more common in sandy soils, with low T and in dry seasons and high temperatures.

Cl: Although it is one of the most mobile ions in the soil, being easily leached, it is generally available to plants. Its availability increases with liming. It is found in higher levels in soils close to the sea or in those treated with saline water, such as those from dairy farms.

Mo: Deficiencies occur in sandy soils and acidic soils, and liming increases their availability. It is important in the fixation of atmospheric N by legumes. It is required in small quantities by plants, with 40 to 50 g/ha generally meeting their needs.

Ni: It became an essential micronutrient for plants after studies proving its function as a component of urease, an enzyme that catalyzes the reaction of urea, transforming it into ammonia and carbon dioxide. There are no studies that confirm Ni deficiency in plants. Care should be taken with toxicity caused by applications of industrial waste and sewage sludge.

There is a wide variety of extractants used to determine micronutrients in soil. However, in routine analyses, the most commonly used is Mehlich-1, due to the ease of preparing the solution and because the extract in which P and K were determined is already available. The most commonly used extractant for B is hot water.

In view of this, we can understand the importance of carrying out chemical analysis of the soil, as it indicates parameters that serve as a guide for decision-making in choosing the best formulas and products in addition to the correct quantities, which leads to an economic return in addition to a balance in the production environment.

References

SILVA, Fabio Cesar et al. Manual of chemical analysis of soils, plants and fertilizers. Brasilia, DF: Embrapa Information Technology; Rio de Janeiro: Embrapa Soils, 2009.

PREZOTTI, LC; MARTINS, AG Guide to the interpretation of soil and leaf analysis. (p. 1-104). Vitoria: Capixaba Institute of Research, Technical Assistance and Rural Extension, 2013.

Authors

Agr Eng. Dr. Angélica Schmitz Heinzen

Agricultural Eng. Msc. Thiago Stella de Freitas