Are yield limitations based on soil or soil management?

Friday, April 4, 2014

Both are important, but proper management can overcome many limitations, particularly with regards to moisture or fertility

by KEITH REID

Anyone who has grown a crop knows how important soil is to the ultimate yield. It is the medium that provides nutrients and water to the crop, but not every soil has the same ability to deliver. Management techniques can help overcome some limitations, but by how much? In other words, if you are looking to buy or rent land, is the soil type more important than how it has been managed?

The Canada Land Inventory, which was developed jointly by Agriculture and Agri-Food Canada (AAFC) and provincial agriculture departments during the 1970s and 1980s, classifies land into seven suitability classes for various crops, based on limitations to crop growth. Class 1 land has no limitations for the intended use, while Class 7 land is unsuitable for agriculture.  

Ten limitations were used to classify soils, plus one additional class where there were multiple limitations. Some of these are difficult (or impossible) to change, they include climate, land location, topography or bedrock. Others, like the level of natural fertility, the soil structure or amount of water, can be improved with proper management, although this could mean considerable expense. At the same time, soils with relatively little inherent limitation can be damaged by poor management.

One example of this is the Bolton plots at the Eugene Whelan Experimental Farm in Essex County. These plots, which are affiliated with AAFC's Greenhouse and Processing Crops Research Centre near Harrow, were established in 1954, and have maintained the same rotation on the plots, with both fertilized and unfertilized treatments, since 1959.

The soil type where these plots are located is a Brookston clay loam, which is poorly drained and can be slightly acidic. The plots were tile-drained to remove excess soil moisture. The highest grain corn yields in these plots have consistently been from corn that received fertilizer, and was also grown in rotation with oats and alfalfa. Continuous corn that received fertilizer had lower average yields, and corn in rotation that did not receive fertilizer was even less productive on average.

The lowest productivity by far, however, was continuous corn that did not receive any fertilizer. In these plots, the average yields declined over the first 10 years from almost four tonnes per hectare down to about one tonne per hectare, and have stayed there. It is interesting that this is almost the same pattern that appeared in the unfertilized continuous wheat plots at the Rothamsted research station in England.  

In the Bolton plots, the unfertilized continuous corn treatments have shown declines in soil organic matter, along with poorer drainage and water-holding capacity. These plots also show the greatest year-to-year variability in yield due to weather conditions. It is clear that both proper fertilization and crop rotation help to overcome the inherent limitations of the soil and insulate the crops from weather effects.

Another example can be found a little farther south, where Dr. T.Q. Zhang of AAFC tested fertilizer application through irrigation (fertigation) on sandy soils near Harrow.  These soils were Class 3 because of poor moisture-holding capacity and poor capacity to hold nutrients.

One would expect yield depression on these soils in dry years from moisture stress, and in wet years because the nutrients leached away. Dr. Zhang was able to show greater yields of tomatoes and peppers, and greater responses to nitrogen and phosphorus when both water and nutrients were supplied to the crops throughout the growing season. These yield responses did not show up on a finer-textured soil, with better nutrient-holding capacity, where broadcast fertilizer did just as well as fertigation.

The inherent limitations of the soil do not necessarily mean that a particular soil cannot be productive. Proper management can overcome many limitations, particularly with regards to moisture or fertility. Soils that are droughty, for instance, can have many advantages for crops that would suffer from root rot in a soil with good moisture-holding capacity. The key is to understand what management is going to work best to reduce the impact of a particular limitation and how expensive it is going to be. A Class 3 soil may produce the same yield as a Class 1 soil, but it will probably cost more to achieve it.

The other side of the coin is that the best soil can be damaged by poor management. The rating on a map doesn't tell you if a soil has been severely compacted, or if the fertility has been allowed to decline. Whether you are looking to rent or buy land, always follow up your research with a field walk to see what has been done in the field. BF

Keith Reid is soil scientist Agriculture and Agri-Food Canada, Guelph.