Soil Compaction

STILL THE NUMBER ONE YIELD KILLER

SOIL COMPACTION IS still the number one factor that impacts negatively on crop production. It is the unexpected or unwanted outcome of a series of normal field operations. The delayed start to the spring season this year and the difficulty in establishing suitable seedbeds in finer textured soils brings this topic back to the top of the list. Soil compaction has no one identifiable feature that alerts a grower to its presence. Only after crop emergence and development do we start to see impacts on crop growth. Even then it looks like so many other factors that can alter normal crop development. 

Soil Compaction Symptoms

•    Poor crop emergence
•    Lumpy seed beds
•    Thin stands
•    Uneven growth
•    Poor crop vigour
•    More crusting
•    More replanting
•    Delayed planting
•    Delayed maturity
•    Nutrient deficiencies – even though soil tests are high
•    More seedling diseases
•    More tillage required for seedbed
•    Poor drainage
•    Drought prone
•    Water ponding
•    Lower yields
•    Restricted roots
•    More runoff
•    More surface erosion
•    Cold soils at planting time
•    Shift in weed species
•    Higher cost of production
•    More horsepower = more fuel required

CAUSE AND EFFECT
Compaction comes about essentially from performing field operations when soils are still too wet to carry the implement load. The deformation of the soil and the loss of pore space set up a series of season-long impacts which ultimately result in lower yields. The impacts of soil compaction on root development profoundly change how plants grow. 

Soil compaction can alter soil water content, temperature, and root extension which can, in turn, inhibit yield potential. A soybean plant developing roots in compacted soil early in the season will often experience a less branched, thicker, stubby root structure with fewer root hairs (a hormonal response triggered by stress). By the time the plant reaches R4 (full pod), there may be insufficient root mass to meet the high nutrient demand for nitrogen and potassium. This most likely will look like premature yellowing of the plant and early senescence. Often there is a disconnect between the cause and effect when we attempt to diagnose the issue. If it happens to be dry at the reproductive stages, we may simply say we needed a rain, which may well be true – but the soil compaction may have predisposed the stress initially at emergence.

AVOIDING THE ISSUE
Much has been written about soil compaction, both shallow and deep. There is also lots of debate over the best way to alleviate it — such as deep ripping with a chisel plough type of implement. There is equal debate on how effective that may be and there is a general consensus that avoidance is the prudent practice. This spring certainly offered its share of challenges in waiting long enough for ideal conditions.

Tile drainage pays. We continue to see more tiling done, with formerly tiled fields having tile runs split to narrower spacing.  Drainage improves soil quality by controlling the water table, enhancing plant available water, increased yields, extending the number of working days so operations can be done on a timelier basis, resuming time sensitive work sooner after rain events, and expanding crop rotation options.

However, there is another aspect of soil compaction which links all field activities — soil quality. It is often hard to define, but most farmers know it when they see it — those fields that are always high yielding, easy to make a seedbed, seldom crust, and are always productive year over year.

At the heart of this soil quality is soil organic matter (SOM) content. Crop rotation and tillage practices exert a great influence on soil and crop productivity. Excessive amounts of tillage effectively burn up the SOM, increasing oxidation. Invariably, soils that suffer compaction most often have a lower SOM content.

The return of crop residues, manures, and cover crops to the soil can have a profound effect on SOM and soil quality. Soils have a chemical, physical, and biological component. All three are impacted by field activities and returning crop residue feeds all three components. It is the residues that feed the bugs (earthworms, et al) that create the materials to make stable soil aggregate, improve structure, increase water holding capacity, lower bulk density, improve aeration, gas exchange, recycle nutrients, and provide the soil with the ability to repair its structure when bad things happen.

CROP RESIDUES ADD TO SOIL SOM
When understanding the fate of crop residues and SOM there are three main factors to consider.

1.    How much residue is produced — both above and below ground dry matter
2.    Annual SOM loss — usually a range 0.5 to 3+%
3.    Retention of crop residues — a range of 10% to 20% retained after respiration

We will assume a soil with three per cent organic matter in a corn, soybean, and winter wheat rotation with one tonne of straw sold off the field.  We will use 1.5% annual loss of SOM and both 10% and 20% retention for the net contribution to SOM. Net retention lbs/ acre = (Total x % retained – Annual loss of SOM).

Crop	Yield (bushels)	Above Ground	Below Ground	Total	Annual SOM loss lbs/acre	Net at 10% Retention	Net at 10% Retention
Corn	180	8,520	8,520	17,040	900	804	2,508
Soybeans	40	2,040	2,450	4,490	900	(451)	0
Winter Wheat	90	4,600	7,360	9,756*	900	75	1,051
Total		15,160	18,330	31,286	2,700	428	3,559
* 2,204 lbs of straw removed

A three year rotation nets 428 lbs of SOM contribution with only 10% retention and 3,559 with 20% retention. Clearly soybeans are negative on SOM contribution and corn and wheat offer more positive contributions.  Corn has equal mass below ground, soybeans 20% more than above ground, and wheat offers 60% more biomass below ground than surface residue – even more if the straw were left in the field.

Let’s now look at a popular rotation of two years of soybeans and one year of wheat:

Crop	Yield (bushels)	Above Ground	Below Ground	Total	Annual SOM loss lbs/acre	Net at 10% Retention	Net at 10% Retention
Soybeans	40	2,040	2,450	4,490	900	(451)	0
Soybeans	40	2,040	2,450	4,490	900	(451)	0
Winter Wheat	90	4,600	7,360	9,756*	900	75	1,051
Total		8,680	12,260	18,736	2,700	(827)	1,051
* 2,204 lbs of straw removed

SOM contribution: Two years of soybeans and a year of wheat are -827 lbs with 10% retention to 1,051 pounds with 20% retention. I left second year soybeans at 40 bushels because there will most likely be less yield and even lower total residue.

ACTION AND IMPACT
Crop rotation has tremendous impact on SOM contributions. Consider this soil quality fact along with untimely tillage and soil compaction can become a more regular feature in crop production.

Planting corn and soybeans with wheat in the rotation offers many benefits to the overall yield of corn and soybeans while contributing to soil quality with a prolonged period of living root mass with wheat and higher levels of crop residues.

If selling straw keeps wheat in the rotation, then grow wheat and sell the straw. The impact of a living root system in the soil for nearly 10 months a year outweighs any negatives on selling the straw. Establishing a cover crop (for many reasons) will continue to provide a positive impact on soil quality and contribute to SOM.

Reducing soil compaction is a strategy of avoidance which includes crop rotation and residue management with the smallest amount of timely tillage (if any) that leaves 30% residue uniformly distributed on the surface. l

About the author: Dale Cowan is a Senior Agronomist, CCA On, with AGRIS and Wanstead Farmers Cooperatives.

Crop advisors provide advice and council producers in their decision making process. This responsibility requires a good understanding of science, food safety, technology, economics and environment. Crop advisors combine knowledge in these disciplines with their local experience to render sound recommendations. If you would like to contact a CCA in your area or if you would like contact information for any of the above mentioned CCAs, please contact the CCA office at (519) 669-3350 or visit the website at www.ccaontario.com. •