- Author: Mark Bolda
Great article here by Dr. Robert Mikkelson from the International Plant Nutrition Institute concerning the why and how of liming a soil which is acidic.
Here's the short of it:
Several factors contribute to the acidity of a soil, one being the geologic composition of the base material, another being lots of rain which leaches out calcium and magnesium. Another contributing factor to soil acidity in agricultural soils is the continual use of nitrogen fertilizers, especially in the same spot over a longer period of time. Both urea and ammonium, when converted to nitrate by soil micro-organisms, release hydrogen ions, the higher the concentration of which raise the acidity of the soil.
A common grower solution to acidic soils is the addition of ground limestone. However, while limestone neutralizes acidity, adds calcium and enhances the solubility of phosphate (and consequently availability of phosphorous to the the plant) does not dissolve well at all at pH's above 6.5 (common for soils of the Central Coast) and one would be hard put to realize the aforementioned benefits at this pH. So if one needs to add calcium to a soil with a pH above 6.5, the better choice would be gypsum, which while it also does not dissolve well in a neutral pH soil, does supply more soluble calcium.
- Author: Steve Grattan
For those of you who attended Steve Grattan's presentation at UCCE Monterey last week on managing salinity in vegetable production, you may recall he mentioned that strawberries in gypsiferous soils can tolerate a higher EC reading than the salt tolerance guidelines allow. I wasn't quite clear about this, so I emailed about it and his answer concerning this is as follows:
"Crop salt tolerance is based on crops response to the electrical conductivity of the saturated soil paste (ECe). But in actuality, crops respond to the salinity in the soil water. These are different.
The field soil water content for many berries and vegetable crops is slightly above or below the field capacity. This means that about 1/2 of the pore volume in the soil is water and the other half is air. To make a saturated paste, distilled water (pure water without salts) is added to fill the extra pore space. Now if the salts in the soil water are largely sodium and chloride (very soluble), then the ECe would be about half the EC of the soil water in the field...which is what the crop is truly responding to. But if the soil contains a lot of gypsum (CaSO4), then by adding distilled water, more salts will become dissolved so that the ECe would be higher.
As an example, the literature indicates that strawberries can tolerate a maximum ECe of 1 dS/m, beyond which yields decline 33% for every 1 EC unit is increased beyond that. This was based on a chloride dominated water. Therefore, stawberries can tolerate about an EC of 2 in the field water. Gypsum has a maximum solubility of about 20 meq/l which is an EC of about 2 dS/m but this is just a generalization and can vary depending on soil chemistry. But for simplicity, if the salts were all gypsum (no sodium chloride) than if the soils had excess gypsum, the EC of the field water would be about 2 (no yield reduction). But to make a saturated paste, distilled water is added and more gypsum is dissolved so the resulting ECe is not 1 but remains at 2. Therefore an ECe of 2 dS/m is not growth limiting in this case.
The rule of thumb is that plants can typically tolerate a 1-2.5 dS/m higher ECe than the salt tolerance guidelines indicate in gypsiferous soils because of this relationship."
