Questioning the Value of Soil Quality for the Irrigated Arid West

August 21, 2014
By Andrew McGuire

Grant County potato yield Sign“The Nation’s Leading Potato Producing County” states a sign on I-90 at the Grant County border1. In 2010, Washington potato yields averaged 33 tons per acre, compared to Nebraska at 20.7, Wisconsin at 19.8, and Maine at 14.5 tons per acre (Idaho’s main potato producing counties averages 27.2 tons per acre). And it is not just potatoes; the Columbia Basin produces high yields of corn, dry beans, onions, and many other crops. However, the productive soils in the Columbia Basin often have soil organic matter levels less than 1%, much less than the level considered as adequate for proper functioning, and certainly not high enough to be considered high quality soils. How can such “low quality” soils produce high crop yields, yields higher than other regions with higher soil quality? This paradox highlights a problem with the concept of soil quality; that it does not take into account the soil management practices that farmers employ to overcome problems in so-called “low quality soils” and therefore does not reflect real production capacity of soils, especially in the West.

Soil quality (or soil health2), is currently receiving renewed appreciation among farmers, Extension, NRCS and Conservation Districts across the nation. While I commend this interest in soils, pursuing a vaguely defined soil quality or soil health can obscure the specific problems and their solutions.

For instance, across much of the arid West, low levels of organic matter can cause problems like crusting, low infiltration rates, low water holding capacity, poor horizontal water movement, low nutrient storage and cycling, restricted air movement, and compaction. Add to this the intensive tillage that often accompanies production of crops like potatoes, and the resulting bare soil is prone to wind erosion, increased evaporative losses of water and hot soil temperatures. Nevertheless, yields have been maintained, and even improved, on these “low quality” soils. If soil quality is a valuable concept, why doesn’t it reflect the high yields produced on these soils? The missing component is management.

For the most part, farmers have overcome problems in low organic matter soils by focusing on solving the problems, not by focusing on increasing soil quality. The former is specific and requires specific management, while the latter is nebulous and spawns all sorts of confusion, not the least of which are the many products being pushed to improve, restore, balance, synergize, or stimulate the soil, assumed to be “dead.” With no problem identified besides low soil quality, and with increased soil quality as the goal, we run the risk of applying placebo practices, where farmers who have caught the soil quality fever (I have caught it several times myself; soil quality promoters are very passionate and persuasive), believe they have done something good and so see results in line with their thinking. But because soil quality assessments are still in flux, filled with conflicting opinions, and do not often include crop yield (“divorced from utilitarian soil function” as one critic puts it (Sojka et al., 2003)), we have no idea if anything that matters really improved.

The ambiguity of assessing soil quality is demonstrated well by one commonly included measure in soil quality assessments: soil bulk density. Soil bulk density is the mass of soil in a specific volume: high bulk density indicates “dense” soil with low pore space while low bulk density is desirable for crop production. While organic matter may stabilize bulk density, it is still possible have compacted (high bulk density) high organic matter soils, and to loosen low organic matter soils; bulk density is subject to management in the form of tillage, where the main goal is often to reduce bulk density. This loosening of the soil (lower bulk density) through tillage is often short-term, but as long as it allows the crop to get off to a good start, the growing crop roots can stabilize the gains through harvest. Since organic matter levels are already low, this tillage does not necessarily reduce sustainability, especially if other soil management practices are incorporated in a crop rotation. These practices could include high residue farming to leave residue cover that controls wind erosion, minimizing passes through the field to minimize compaction, or even timely cultivation to re-loosen the soil while killing weeds. This management factor, largely ignored in soil quality assessment, means that high yields can be maintained in soils that do not support low bulk densities in the long-term, and so would be termed “low quality.”

Some will point out that irrigation accounts for the main yield advantage in irrigated regions of the West over other regions of rainfed agriculture. Granted that irrigation is a big advantage, but the fact that it seems to eclipse the advantages of high soil quality also points out that soil quality is a relative term, perhaps having more importance in rainfed regions where increased soil water holding capacity translates directly to improved yields and environmental benefits.

Others will point out that high yields in the arid West’s irrigated agriculture requires high levels of external nutrient inputs and that higher quality soils require less external inputs. This ignores the fact that in any soil, what comes off in yield must be replaced with external inputs, otherwise you are just mining your soil, or more specifically, your soil’s organic matter. If Nebraska produced 33 ton per acre potato yields, they would have to apply the same amount of external nutrients as Washington farmers to sustain that level of production – they have to come from somewhere.

By Krazytea (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Tall grass prairie under which developed high organic matter soils (photo: Krazytea)

Equating high soil organic matter levels with high soil quality is the result of the soil quality concept being developed in the Midwest and eastern parts of the country. In those areas, mainly rainfed, organic matter levels, built up under prairie and forests, decline under agriculture along with the benefits that came with higher organic matter levels. The focus on soil quality is a lament for lost organic matter with calls to halt or reverse the decline.

By Famartin (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Sagebrush steppe, the native vegetation under which our Columbia Basin soils developed. (photo: Famartin)

In contrast, soils in arid regions of the West started with low levels of organic matter, and so with “poor” soil quality. With irrigation and management producing high yielding crops, soil organic matter levels in many areas have actually increased. Farmers here are actually in the position of being able to improve their soils (in regard to organic matter levels) with agriculture, a very different situation than that facing Midwest farmers.

In his review of the relationship between soil physical properties and crop production, Letey (1985) concludes that “because management and weather are integral factors, no meaningful correlation between texture, bulk density, or structure and crop productivity is possible.” In the irrigated, arid West, a “high quality soil” will not prevent crop failure due to poor management, nor will a “low quality soil” prevent high yields, if managed well. Soil is important to sustaining crop production, and so soil management is important, but soil quality lacks the specificity needed to solve soil problems through management (but not micro-management), especially in the irrigated West. Instead of focusing on a fuzzy vision of soil quality, the challenge is to adapt the concept to address specific problems that limit or prevent us from sustaining production (and mitigating unwanted impacts such as nitrate leaching) and focus on the specific management tools that can solve those problems; management is the key.

 

References

Letey, J. (1985). Relationship between soil physical properties and crop production. In Advances in soil science (pp. 277–294). Springer.

Sojka, R. E., Upchurch, D. R., & Borlaug, N. E. (2003). Quality soil management or soil quality management : performance versus semantics. In Advances in Agronomy (Vol. Volume 79, pp. 1–68). Academic Press.

1 As I understand it, this is sometimes true, but in some years Benton or Franklin counties may produce more potatoes by weight.

2 In scientific terminology, there is a difference between soil health, the term preferred by farmers, and soil quality.

Leave a Comment

Your email address will not be published. Required fields are marked *


*

*