Spatial Variability and Nutrient Management
By Ivan O’Halloran, Ridgetown College, University of Guelph

This article is summarized from the "Integrated Crop Management Symposium – Optimizing Inputs’" March 6 at the University of Guelph.

Soil productivity and spatial variability in crop growth and yield, have always been realities of farming life. Newer technologies, such as yield monitors, differential global positioning systems (GPS) and variable rate applicators, have been widely promoted as the answer to managing spatial variability within fields. However, questions pertaining to the type of variability present and the potential management opportunities that each field offers must be addressed before the economic benefit of this technology will be realized by farmers.

Standard soil testing and fertilizer recommendation practices, which encompass an average soil test value and constant rate of fertilization for an entire field, will result in certain areas of the field being either over or under fertilized. Site-specific crop management systems, which attempt to manage different areas within a field to their optimum, may improve economic returns and reduce environmental contamination due to a more judicial application of nutrients and better utilization of the soil’s resources.

Criteria for Profitable Site-Specific Management

Successful implementation of a site-specific nutrient management (SSNM) system requires these four components: · ability to accurately locate yourself within the field,

• ability to vary the input application rate, while in motion,
• ability to identify correctly, the different management units within the field, and
• in-field variability which provides a (profitable) management opportunity.

To properly vary the application of a nutrient within the field, a reasonable understanding is needed of either how that nutrient – or the crop response to that nutrient – varies within the field. The extent to which this is accomplished by current techniques varies from site to site, and with the method used to collect and interpret the data. If SSNM is to be profitable, a certain scale and level of variability is required to make the extra investment worthwhile. There is little or no hope of an economic return in situations where: regardless of the variability, the lowest level observed is more than adequate for crop growth; the within-field variability is small, meaning there is little change in the actual amount of nutrient recommended; or the variability is large, but occurs at such a small scale (i.e. large and/or many variations within very short distances) within the field so as not to be manageable. Producers also need to determine if other factors such as soil structure, weed interference and moisture availability could be limiting crop growth and the potential response to nutrient application.

The Variable Rate Soil Map

A crucial component of site-specific management is the creation of the ‘expert map’ which indicates how to alter the rate of nutrient(s) applied at different locations within the field. This is not a simple task, particularly for nutrients like P and K which often reflect past management practices.

The usefulness of soil sampling in a grid-pattern to create a soil fertility map can vary depending on the scale of variability in the field, the sampling scale (grid-size), and even the mathematical model used to compute and prepare the map. Although there are considerable differences of opinions, most research has demonstrated marginal benefit to simple 2.5-acre grid sampling, particularly for making recommendations for variably applying P and K. Development of a field-specific sampling scheme which accounts for observed variations in crop yields, topography and past management practices (and other previously mentioned influencing factors) may greatly improve the quality of information obtained from soil sampling. Incorporating this information into geographical information systems (GIS) may allow better visualization of the inter-relationships between the various components, for example between elevation and pH, crop yield, etc. However, there is a cost for collecting and maintaining this information database, which must be weighed against the usefulness of the data.

Recommendations

SSNM is a developing area of technology and the financial return to farmers is still unclear. Intrinsic in site-specific management is the fact that potential benefits will vary from site to site. A good basic knowledge of soil test levels, crop response to nutrient applications, overall crop yields and other yield influencing factors can go a long way in determining the potential benefits of site-specific technology. Yield monitors, used increasingly for on-farm management comparisons, such as nutrient applications, seeding rates, cultivar/hybrid comparisons, etc., can provide helpful insight for management decisions. When establishing such comparisons, it is important – essential in fact – that a ‘check’ or ‘standard practice’ treatment be included, and that the different treatments be in strips along side one another. The ‘delta-yield’ approach, whereby a yield map of one or more check strips is compared to adjacent treated areas of the field is an increasingly common example. Comparing yields from two different years, two different fields, or even two halves of the same field will not give an adequate comparison because of other variability factors present.

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