The vast majority of agricultural soils have been in production for generations – however, ongoing nutrient management is crucial to maintaining economical production going forward. Nutrient management relies on thorough pre-plant soil analysis and in-season tissue analysis. These involve three steps: sample collection, testing and recommendations.
Let’s examine these areas in more detail:
Samples should be representative and collected as intensively as is practical to manage. For example, a sample representing 10 ha may be adequate if a flat-rate application is intended. However, if three different soil types exist across the 10 ha – and you have the ability to manage each independently – then it would be more advantageous to collect a sample from each. By creating management zones that separate areas of variability, you can also introduce variable rate application.
Due to the wide variety of soils that exist, soil test methods are not universal. Regional methods must be developed to best determine the precise amount of extractable nutrients in the soil, which is largely influenced by the soil’s pH level.
Test results are typically rated based on local yield response trials. For instance, a ”low” soil test indicates that the soil would benefit from additional nutrients. Conversely, a ”high” soil test indicates that there is an adequate supply, and that no benefit would be gained from adding more.
There are different recommendation strategies, and it is worth highlighting that the rate should be crop specific.
With the sufficiency approach, for instance, you apply the exact amount of nutrient(s) that would generate the most economic yield response. This is best utilized as a short term strategy, or when input costs are high. However, this does not consider the drawdown effect of crop removal.
In contrast, the build and maintain approach is a long term strategy. In this case, the goal is to apply nutrients at a higher rate than the crops are removing them, meaning fertility is maximized to a point that is non-limiting, and nutrients are maintained at this level.
There has been an increased focus on environmentally sustainable practices in recent years, meaning it is important to apply nutrients responsibly without the risk of pollution. However, in order to determine and document the appropriate rate of nutrient application, you must first begin with a soil test.
Soil Health Benchmarking
Where practices are changing in a bid to enhance soil health, biological and physical tests should be performed to establish an initial baseline for the sake of future comparison. Carbon dioxide respiration, active carbon, labile amino-nitrogen, and potentially mineralizable nitrate (PMN) all provide indicators of biological activity. Bulk density, aggregate stability, and permeability all provide measurements of physical structure. It is worth noting that organic carbon is the common contributor to improving all the above soil components.
Plant Tissue Analysis
Although soil testing indicates whether nutrient levels are adequate, it does not predict crop utilization. Many factors contribute to the uptake of nutrients by plant roots, and a fertile soil is not necessarily productive.
Testing the plant tissue allows you to monitor nutrient efficiency, which can indicate a hidden hunger before deficiency symptoms even appear and allow you to diagnose visual symptoms of nutrient deficiency. In high production systems, such as irrigated vegetables, the soil supply cannot maintain the crop demand, and additional nutrients can then be foliar-applied or fertigated based on tissue testing results. Labor efficiency can also be realized, as evenly grown crops require fewer manual harvest passes and less sorting.
Due to the varying mobility of plant nutrients, it is important to sample the portion of the plant that best represents its content – this is usually the newest fully-developed leaf on a growing plant. Make sure to collect samples from many plants to have an adequate sampling quantity (200 grams or so). Most tissue reports will often have ”normal ranges” or ”critical minimums” to help interpret results, but sampling good and poor areas in the same field can reveal stark differences.
Analyzing tissue is a precise process. Leaves must be dried, ground, ashed and digested before measuring on a dry weight basis. It is standard practice to report the macro and secondary nutrients in percent, and the micronutrient in parts per million (ppm).
Understanding the results is often a straightforward exercise – for instance, you might see that a pale crop is showing a deficiency in N. You may eliminate the deficiency simply by applying more nutrients. However, results can often lead to a deeper investigation into existing processes and wider management improvements, such as work to improve the drainage, compaction, or seed bed preparation.
Most field activities and yield-reducing impacts are influenced by weather conditions, which are obviously beyond human control. It is crucial that we focus on managing the things that are within our control, such as fertility. Soil and tissue analysis provides precise measurements that can enhance this management– so they are of paramount importance.
SGS is a global leader in soil fertility monitoring for the agricultural sector. Our experts and global network of laboratories can help to optimize crop development, increase growth and product quality and maintain soil sustainability.
About The Author
Jack Legg is the Branch Manager of SGS’s Guelph crop science laboratory in Canada, where he also serves as the staff agronomist. Jack joined the laboratory in 1996, managing field services for the lab and providing GPS sample collection. He has been a Certified Crop Advisor since 1999. In 2016, Jack was proud to be among the first to earn the 4R Nutrient Management Specialist designation, which focuses on responsible nutrient use.
Jack is also active in the industry, serving on numerous committees. He has been Past Chair of the Ontario Soil Management Research and Services Committee and was elected Director of Certified Crop Advisors Ontario. He also serves as Chair of CCA’s Standards and Ethics committee and is a member of the CCA Mentorship committee, the Canadian Standards Association’s Bioretention Soil Technical committee and the Ontario Agri Business Association’s Soil and Research Committee.
For further information contact:
Branch Manager and Agronomist
t: +1 519 837 1600
SGS is the world's leading inspection, verification, testing, and certification company. SGS is recognized as the global benchmark for quality and integrity. With more than 89,000 employees, SGS operates a network of over 2,600 offices and laboratories around the world.