Soil Salinity: Assessment and Remediation

Salinity challenging to irrigated growers.

Soil salinity affects conventional and alternative producers alike. Sources of soil salinity are either native or introduced. Most arable soils in Colorado have native soil salinity because ancient shallow seas occupied much of what is now Colorado and deposited large amounts of salts onto the seabed that is now our farmland. In addition, introduced salinity comes from irrigation water containing soluble salts. A third source of soil salinity is upward percolation of water and its accompanying salts from shallow water tables. Evaporation of fresh water from the soil surface and/or uptake by plants leaves salts in the soil to accumulate over time. Soil salts hold on to water very tightly making it extremely difficult for plants to take up water and nutrients and resulting in reduced plant vigor and growth.

There are three salts of concern to Colorado growers: calcium (Ca), magnesium (Mg) and sodium (Na). These salts are also plant nutrients. However, each of these salts, in and of themselves, can cause problems if the soil contains excess concentrations. Soils with excess Ca and/or Mg (known as saline soils) restrict water and nutrient uptake by plants. Soils with excess Na (known as sodic soils), along with restricting uptake, growth and vigor, also experience a breakdown of soil structure. Hydrated Na disperses soil particles, destroying the aggregation and the associated channels where water and air enter and exit in a healthy soil. In a sodic soil, water and air cannot enter soil freely, and hence plants cannot grow well.

Visual indications of salinity are stunted plant growth, poor fruit size and quality, patchy growth in pastures and/or a white crust on the soil surface when the soil dries. Soil sampling and testing is the only economical means of assessing soil salt concentrations. Two simple tests for salinity/ sodicity are pH and EC (electrical conductivity). A pH test determines
acidity or alkalinity of a soil, and the EC determines salinity. If soil pH is above 8.5, then an additional chemical analysis for salt concentrations known as sodium adsorption ratio (SAR) is required. The SAR of a soil is the amount of Na in the soil with respect to the amounts of Ca and/or Mg and can indicate whether a soil is saline, sodic or saline-sodic (see table below). It is critical to know which salt(s) are in excess in the soil in order to perform the correct remediation. You should consult a qualified soil professional PRIOR to attempting remediation. (see table)

Classification of Soils for Salinity/Sodicity.

It is critical to know which salt(s) are in excess in the soil PRIOR to attempting remediation, since remediation techniques for saline soils differ from remediation techniques for sodic soils. In order to remediate either soil problem, you must have adequate soil drainage, or there is nowhere for the salts to go. In some cases, tile drains may need to be installed to insure adequate drainage. In order to remediate a saline soil, leaching of the soil with excess irrigation water is required to ‘push’ the salts below the crop root zone. A rule of thumb for reduction of salinity is that six inches of excess irrigation water will remove 50% of the salts from the top foot of soil, and twelve inches of excess irrigation water will remove 80% of the salts from the top foot of soil. A qualified soil professional should be consulted before attempting remediation, as leaching will also remove nitrogen and other nutrients critical to good crop growth, and this must be taken into consideration.

In order to remediate sodic soil, gypsum must be added and incorporated into the soil PRIOR to leaching. If these steps are not adhered to, the problem will become much worse! The amount of gypsum required is dependent on the concentrations of the individual salts and the SAR and should be calculated by a qualified soil professional. Soils should be tested following leaching to determine if the desired effects have been achieved or if further action is required. Farmland with salinity or sodicity problems due to shallow water tables can be remediated with the installation of tile drains. Your soil professional can help you determine the amount, spacing and depth of tile drain for proper remediation.

If salinity is an ongoing problem, for example, due to the salinity of irrigation water, the addition of organic matter to the soil can help mitigate the problem to a degree, but close monitoring of soil salinity/sodicity is still a must. Another avenue to help mitigate ongoing salinity problems is to add excess irrigation water at every irrigation in order to keep salts moving below the crop root zone. The excess irrigation application is called a leaching fraction. The salt content of the irrigation water and the soil and the crop you intend to grow combine to determine the leaching fraction you’ll need to apply. A qualified soil professional can help you determine the correct leaching fraction for your particular situation. Regular soil testing for salinity, sodicity and pH is a must to insure that soil salt levels do not exceed thresholds for good crop production.

Ron Godin,
Western Colorado Research Center