Soil pH and Buffer pH

Soil pH is a measure of the soils acidity and affects the availability of nutrients as well as the activity of soil organisms. Most plants prefer a soil pH between 5.5 and 7.5. Due to the climate and rock-types from which the soils of Massachusetts have formed, most soils tend to be naturally very acidic (4.5-5.5) and therefore liming is a frequent management practice.

Buffer pH (normal is 6.8) is a measure of the soil's capacity to resist pH change after lime has been added. Thus two soils with the same soil pH may require differing lime applications.

Cation Exchange Capacity

Is an important measure of the soil's ability to retain and to supply nutrients. There is a close positive relationship between CEC and soil organic matter (OM). Very sandy soils, low in OM, have CEC's less than 5. A CEC between 10 and 15 is typical and usually adequate. CEC is important because it represents the primary soil reservoir of available K, Ca, Mg and several micronurients. It also helps to prevent nutrient leaching.

Base Saturation

The percent base saturation of a soil is a measure of how much of the soil CEC is occupied by plant nutrients, namely the basic cations (Ca++, Mg++, K+, Na+). It excludes H+ and Al+++ because these produce acid reactions. Usual or ideal ranges for bases are Ca++ 60-85%, Mg++ 5-15%, K+ 2-5%, Na+ 1%, other bases 1%. At low soil pH more exchange sites are occupied by H+ ions.

Nitrogen

Nitrogen is absorbed as NO3- and NH4+. The soil test results shows their current levels, however, these levels change rapidly with soil and weather conditions over short periods of time. Therefore, the N results from a routine soil test should not be used for fertilizer recommendations. PSNT (pre-sidedress nitrogen test) is highly recommended.

Phosphorus

Some of the P (P2O5) is present as part of soil organic matter and becomes available to crops as the OM decomposes. Inorganic P is mainly bound to soil particles. Soil tests assess the soil's ability to supply P from bound forms during the growing season.

Potassium (Potash)

Most available K (K2O) exists as an exchangeable cation and release very slowly. Fertilization is often necessary to maintain optimum yields.

Micronutrients

Although shown on the UMass soil analysis report, results for soil micronutrients should not be used for fertilizer recommendations. These need to be accompanied by tissue analysis in most situations.

Masoud Hashemi and Stephen J. Herbert
UMass Extension and Dept. of Plant and Soil Sciences
University of Massachusetts
CDL Publ. 01-4