Better Landscaping Today Newsletter
Volume 3, Issue 7 July 2000
The Effect of the
"Law of the Minimum" on Landscape Maintenance during Periods of
Drought and Irrigation Curtailment
In 1840, a German soil scientist, Justin Von Liebig, introduced to the world a concept that has become to be known as Liebig's "Law of the Minimum."
Simply stated, Liebig's law means that the plant growth factor in the greatest limitation will have the controlling effect on plant growth. An analogy is the "weakest link in a chain" concept where the chain fails when the weakest link breaks. Improvement in other growth limiting factors will not enhance the plant.
Liebig's "Law of the Minimum" would predict that there is no compensation for water deficient and no possible way to ameliorate the problem.
Over the past several decades, research studies on many factors which control plant growth and on the yields and nutritional content of the plants, have shown to us that Liebig's "Law of the Minimum" is not correct in most landscape and farm situations. We published a new concept in the fall of 1984 in BETTER CROPS and a detailed series of papers in a dedicated issue of the JOURNAL OF PLANT NUTRITION in 1990 (Volume 13, numbers 3 and 4) entitled "Special Issue on Interactions of Limiting Factors in Crop Production." The concept has been coined "Dr. Wallaces' Law of the Maximum."
The "Law of the Maximum" states that factors which control plant growth are interactive meaning that some factors can be corrected to compensate for other uncorrected factors even if one may be the most limiting factor and result in an increase in growth. The result is synergism in which the increase is more than what is expected from the individual improvement of various factors. Factors affecting growth can be quantified and the growth response can be predicted from an evaluation of all factors.
The "Law of the Maximum" predicts, within limits, that plants may be successfully irrigated with "deficit irrigation" (less application of water than which a plant would normally use). This has been shown to be true and can be a major method for conserving the use of water.
Conversely, other problems may occur to prevent this, especially during drought. Severe problems can bring the Liebig's "Law of the Minimum" into action. Not only do rains supply us with water, but they also cleanse the soil of excess salts which accumulate from irrigation with ground water or well water. In drought periods soil salinity will increase unless anticipated and managed well. Excessive salinity can cause plants to wilt at moisture levels which would normally be sufficient for plants. This is due to the fact that salts can prevent the absorption of water because of osmolarity effects. Salinity can have a negative synergistic effect. Reclaimed water may have potentially worse problems than simple salinity effects. Additional potential problems are bicarbonates, biological oxygen demand, excessive levels of trace metals, etc.
One major factor limiting the value of deficit irrigation is soil compaction. Compacted soils have higher salinity levels than non-compacted soils due to poor porosity. Compacted soils cause shallow rooting of plants and wilting of plants occurs even if the soil moisture is high deeper in the soil. Loose, friable soils act like mulches and decrease evaporative water loss as well as preventing wasteful water run-off.