The Albrecht system for soil nutrient testing

The concept of the base-cation saturation ratio system is synonymous with American soil scientist William Albrecht. Find out the origins of this system and its use in the UK.

William A. Albrecht (1888–1974) was a pioneering soil scientist in the USA.

Albrecht’s early work at the University of Missouri was on nitrogen fixation by soybeans, during which he discovered the importance of calcium in maintaining nitrogen fixation by legumes.

Calcium was important to both the microbial partner and the plant partner in this symbiotic relationship, particularly in soils that were acidic.

By the 1950s, Albrecht had concluded that when lime is applied to acid soils, it is the supply of calcium that is more important than the change in pH.

This led to an interest in the role of calcium in animal and human nutrition, which he explored during his retirement, travelling widely to promote his ideas on balanced nutrition of plants and animals based on balanced ratios of exchangeable cations¹.

The concept of ideal ratios of nutrients such as calcium and magnesium in soil has been around for over 100 years, but the evidence directly relating specific cation ratios to crop productivity is inconsistent ^2,3

In the 1940s, scientists⁴ defined what they thought of as the ideal soil in terms of the balance of exchangeable calcium (Ca), magnesium (Mg), potassium (K) and hydrogen (H) ions.

This notion, which emphasised the importance of the ratio of cationic nutrients rather than the total amounts present, ended up being formalised as the base cation saturation ratio system, and became associated with the name of Professor Albrecht.

Research indicates that plants do well over a broad range of cation ratios, as long as there are sufficient supplies of potassium (K), calcium (Ca) and magnesium (Mg).

These positively charged cations are held in soil on negatively charged sites on the surfaces of clay minerals and organic matter.

Soils containing more clay will have higher amounts of exchangeable cations and have the potential to hold greater reserves of nutrients such as Ca and Mg. But on light sandy soils, which have fewer cation exchange sites, more frequent, smaller additions of fertilisers will be required to maintain yield.

Standard soil tests for cations in the UK measure the exchangeable K and Mg, and express this as a Soil Index (England and Wales) or Soil Status (Scotland).

In some situations the ratios of cations are important. For example, The Nutrient Management Guide (RB209) highlights the importance of K:Mg ratios for fruit, vines and hops. The recommendation is that the ratio of exchangeable K to exchangeable Mg should not be greater than 3:1 to avoid the risk of Mg deficiency.

Calcium should not be limiting if Ca-containing liming materials are used to correct soil acidity. However, over time calcium is progressively leached from the soil by rainfall and irrigation water.

In certain parts of the country, dolomitic limestone – which has a high Mg content – has been used, resulting in high soil Mg levels.

There is no evidence that the balance between cations has any significant impact on soil biology, such as earthworm activity or on weeds.

¹ Marshall, C. E. (1977) William A. Albrecht. Plant and Soil, 48 (1), 1–4.

² Kopittke, P. M., & Menzies, N. W. (2007) A review of the use of the basic cation saturation ratio and the “ideal” soil. Soil Science Society of America Journal, 71(2), 259–265.

³ Johnston, J. (2011) Assessing soil fertility: the importance of soil analysis and its interpretation. Potash Development Association.

⁴ Bear, F. E., & Toth, S. J. (1948) Influence of calcium on availability of other soil cations. Soil Science, 65(1), 6–74.