Soil Quality Checkup

Well managed vineyard

WHEN we think about the important role that soil plays in the production of food and fiber for the world’s population, we realize that it is an irreplaceable resource that must be protected. Soil is the medium that supports plant growth and the source of most plant nutrients. Soil water and the soil atmosphere bathe the roots and keep the above-ground plant healthy and growing. A healthy soil environment is in everyone’s interest.

Definition of Soil Quality

Many people have attempted to define soil quality by measuring various soil characteristics and relating these to different management practices, productivity, environmental quality, or plant disease. But soil quality means different things to different people, depending on its intended use. For example, farmers generally want a soil that supports ideal crop growth year after year with a minimum of inputs. A highway builder is looking for very different soil proper- ties for a high-quality soil.

It has long been known that a major benefit of balanced crop fertilization — aside from increasing crop yields and farm profitability — is its effect on enhancing crop productivity and increasing the amount of organic matter that can be returned to the soil. Organic matter can positively influence soil properties such as structure, tilth, bulk density, and increased infiltration rates.

New Research—Mostly Good News for Soil Quality in California

A published article from the University of California reported on changes in soil quality that have occurred in the last 45 to 55 years (DeClerck, Singer, and Lindert. 2003.... click here). Soil samples collected primarily in 1945 were compared with samples collected at the same locations in 2001. These 125 sampling locations represented four major land uses throughout the state: tree crops (25 sites), row crops (44 sites), rangeland (48 sites), and vineyards (eight sites). Although these sites represent only a proportion of California agriculture, analysis of these historic samples provides an insight into changes in soil quality that have occurred throughout the state.

 Soil pH:  The average soil pH in 1945 was 6.9 compared with a value of 7.1 in 2001. This slight increase in pH is well within the acceptable range for plant growth and indicates no extreme changes towards acidification or alkalinization as a result of production practices.

Soil Salinity: The average soil salinity at the 125 sites significantly decreased during the 56-year period from 0.85 deciSiemens/meter (dS/m) in 1945 to 0.44 dS/m in 2001. The largest decrease in salinity occurred in soil used for row crops. This 48% average decrease in soil salinity likely reflects an improvement in management practices and in soil quality.

 

Soil Phosphorus (P): Concentrations of plant-available P (sodium-bicarbonate extractable) increased approximately 20% during this period, with significant increases occurring in land used for tree crops, row crops, and vineyards. The average P concentration in 1945 was 72 parts per million (ppm) and is now 85 ppm. The improved fertility status that has occurred will enhance the inherent productivity of the soil and increase the amount of crop residue that can subsequently be returned to improve the soil.

Soil Nitrogen (N) and Carbon (C):  The amount of total N and C significantly increased between 1945 and 2001— reflecting an accumulation of soil organic matter. Average soil N concentrations increased from 0.09% to 0.29% and soil C increased from 1.06 to 1.34% between 1945 and 2001. These changes in soil organic matter are typically reflected in better aggregate stability and water infiltration.

Check the soil all through the profile

Soil Texture:  The clay content of the samples consistently increased from an average of 10% to 13% for the period between 1945 and 2001. This increase in clay content may be a sign of accelerated soil erosion…which would have a negative impact on soil quality. While this increase in clay content is not great, erosion of topsoil can have very negative effects on crop production and water quality. Efforts to minimize soil loss should always be part of a farm management plan.

So What?

These results indicate that soil quality has generally been maintained or improved over the last 50 to 60 years of intensive management and cropping.  Does that mean that the status quo is fine? No, continued efforts must be made — especially to minimize soil erosion. The documented improvements in soil chemical properties and fertility reflect hard work over many years and we can’t afford to lose the advances that have been made.

However, for some soil properties, we are still losing ground. For example, a survey of soil test results for California in the 1980s revealed that between 20% and 40% of the samples were rated as medium or lower in potassium (K). This number has increased to 44 to 48% in the most recent surveys. We know that soils cannot be continually cropped and nutrients removed without depleting their native fertility and quality.

A soil scientist can evaluate suitability

Efforts to maintain high yields and soil quality are essential for long-term sustainability.  Careful management and utilization of modern technology accomplish this. The technology available in 2003 is beyond the wildest dreams of the farmers in 1945. For instance, the use of satellite-aided precision agricultural tools, computer-controlled water management, improved soil-testing techniques, rapid assessment of plant tissue samples… can all aid in protecting the quality of the precious soil resource and the environment. Let’s continue the progress that has been made and sustain our efforts to protect the soil.