Physico-chemical processes in greens profiles I Salt build-up

Salt accumulation can be explained by mass balance calculations between different analytical results of saturated extract and irrigation water analysis.

Soil is a very complex material that is constantly evolving. The physico-chemical processes occurring in soil can be difficult to explain.

The irrigation water used in our installations has certain physical and chemical characteristics. It is logical to think that these characteristics are transferred to the profile of the soil it irrigates. From then on this water will no longer be the original irrigation water, but will be called  pore water of that soil profile.

The pore water will modify its physico-chemical characteristics:

• Due to the evapotranspiration with the consequent accumulation of salts
• Acidification due to the dissolution of CO2 edáfico (from both the breathing root and organic decomposition).
• Cation exchange mainly with the surfaces of clays, metal oxides and hydroxides and organic matter. Process that exchanges cations and anions between the solid and liquid phase.
• Dissolution and precipitation of minerals. A reversible chemical process by which water dissolves or precipitates minerals present in water.

  

All these processes mean that the nature of the pore water is substantially modified from the irrigation water original. Depending on the intensity of these processes and their physico-chemical relationships between soil and water, the more intense the change will be.

In the following we will explain an approach for the calculation of the thandle evapotranspiration that we can calculate for our sports surfaces.

In order to relate irrigation water to the accumulation of salts in the soil profile, the chloride ion (Cl) present in the pore water (in this case we will study the result from the saturated extract) and that of the irrigation water must be studied. The chloride ion is a conservative solute (it does not precipitate, oxidise, reduce or exchange), if the only source of Cl for the soil is the irrigation water, the increase of chloride in the soil profile with respect to that of the irrigation water from which it comes is due solely to the effect of evapotranspiration. (Appelo and Postma, 2005; Horero et al., 2016). It is important to remember that chlorine can only evaporate if it is in the gaseous phase, i.e. as dichlorine gas (Cl2), a situation that only occurs in nature with certain compounds such as sodium hypochlorite (common disinfectant) in an acid medium.

In order to be able to apply the chloride balance method, certain conditions must be reasonably fulfilled:

1. In the solid phase of the soil there should be no Cl-bearing minerals. This is true for USGA greens soils, which consist of siliceous sands. Chloride fertilisers are therefore to be avoided for this approach.
2. The contribution of Cl from rainfall should be negligible compared to the contribution from irrigation, a hypothesis that can be applied to greens in arid climate zones.
3. The Cl balance between soil and vegetation should be in equilibrium (the amount of Cl that disappears from the pore water as it is incorporated into the plant tissue should be the same as the amount that is incorporated back into the soil as the greens are mowed and decomposed). This hypothesis is not likely to be fulfilled on greens, as the greens are mowed every day and most of it is removed. However, for a first approximation the error is considered to be small.

The mass balance to be carried out is as follows:

Cl |Saturated extract| = Cl |Irrigation water| * * Cl |Saturated extract| = Cl |Irrigation water| * * Cl |Saturated extract| = Cl |Irrigation waterfc

fc is a concentration factor due to evapotranspiration, therefore:

fc = Cl |saturated extract|/Cl |irrigation water|

The value of fc will give us information on the accumulation of salts on our sports surfaces and will be modifiable by our management with the washes of salts.

There are different instruments available on the market to measure the concentration of salts in water and soil. We offer you:

• Direct soil and water EC meter

• The TDR 350

• The ph-and-ec-in-liquids-meter

From Tiloom we can help you to manage correctly the washing of your surface, contact us.