5. Soil, Plant Tissue and Manure Analysis Profitable crop production depends on applying enough nutrients to each Soil analysis field to meet the requirements of Handling and preparation the crop while taking full advantage When samples arrive for testing, of the nutrients already present in the laboratory: the soil. Since soils vary widely in their fertility levels, and crops in • checks submission forms and their nutrient demand, so does the samples to make sure they match amount of nutrients required. • ensures client name, sample IDs and requests are clear Soil and plant analysis are tools used • attaches the ID to the samples and to predict the optimum nutrient submission forms application rates for a specific crop in • prepares samples for the drying a specific field. oven by opening the boxes or bags and placing them on drying racks Soil tests help: • places samples in the oven at • determine fertilizer requirements 35°C until dry (1–5 days) (nitrate • determine soil pH and samples should be analyzed lime requirements without drying) • diagnose crop production problems • grinds dry samples to pass through • determine suitability for a 2 mm sieve, removing stones and biosolids application crop residue • determine suitability for • moves samples to the lab where specific herbicides sub-samples are analyzed Plant tissue tests help: What’s reported in a soil test Commercial soil-testing laboratories • determine fertilizer requirements offer different soil testing/analytical for perennial fruit crops packages. How the laboratory reports • diagnose nutrient deficiencies the results will also differ between • diagnose nutrient toxicities labs. It is important to select an • validate fertilizer programs analytical package that meets your requirements. Analyses common to almost all soil test packages include: pH, buffer pH, phosphorus, potassium and magnesium. 89 Soil pH is included in almost all soil Soils with suspected excess salts tests. Although it is not a nutrient, can be analyzed for electrical soil acidity or alkalinity has a great conductivity. influence on the availability of nutrients and on the growth of crops. See the next page for more detail The buffer pH will also be reported on what is included in a typical soil for acid soils to determine the test report. lime requirement. How the numbers are reported The main nutrient analyses reported Soil test results are expressed in are phosphorus, potassium and many ways, particularly when dealing magnesium. These represent the with labs from outside Ontario. nutrients, aside from nitrogen, most Most Ontario labs express results as commonly applied as fertilizer. Some milligrams per litre of soil (mg/L): that labs include an analysis for calcium. is, the weight of nutrient extracted from a volume of soil. This is close Nitrate-nitrogen analysis is performed in value to the weight-by-weight on a separate soil sample taken to a measure of milligrams per kilogram greater depth. of soil (mg/kg), which is equivalent to parts per million (ppm). Micronutrient tests are not performed as frequently but are Some labs, particularly in the United becoming more popular. Zinc and States, express soil test results as manganese have tests that are well pounds per acre of available nutrient, calibrated with crop requirements, which is confusing since the soil and these are performed almost test results don’t reflect a physical routinely. Other micronutrients quantity. An acre-furrow slice weighs (copper, iron, boron) are not about 2 million pounds. The results well calibrated, but some reports can be converted back to parts per include them. million by dividing by 2. For example, if soil test phosphorus is 120 lb/acre, Sulphur tests are becoming more divide by 2 to get 60 ppm. common in Ontario as atmospheric depositions of sulphur from air Quebec results are expressed as pollution decrease, but they kg/ha. Use the following formula to are not well calibrated with convert these results to ppm: crop requirements. kg/ha x 0.455 = ppm More labs are routinely analyzing for organic matter. It is often used as an It is also important to know which indicator of soil quality and also tied extractants have been used to to herbicide recommendations. perform the soil test. 90 Soil Fertility Handbook Information found on a soil test report General Information • Sample number — This is provided by the grower relating the sample results to a particular field. • Lab number — This is assigned by the lab, to track the sample through the various analytical steps. Analytical Values • Soil pH — Every report should include soil pH, measured in a soil-water paste. • Buffer pH — Buffer pH is only measured on acid soils (normally where soil pH < 6.0). • Phosphorus (ppm) — Ontario accredited soil tests must include the results from the sodium bicarbonate extraction (Olsen method). Some labs will also include results from Mehlich or Bray extractions. The method and the units should always be shown. • Potassium, magnesium (calcium, sodium) (ppm) — The cations are measured in an ammonium acetate extract, with the results reported as mg/L of soil or ppm. Calcium and sodium are sometimes also reported. • Nitrate-N (ppm) — This is not part of a regular soil test, since the interpretation of results is only valid for a deeper sample taken at planting time or before side-dressing. • Sulphur (S) or sulphate (SO4-S) (ppm) — This optional test has not been calibrated. It should be used on deeper samples, similar to nitrate. • Micronutrients (ppm) — Mn and Zn are the only micronutrients with an Ontario accredited test. These are reported as a manganese index and a zinc index (see “Derived Values” below). Values may be reported for other micronutrients, but Ontario research has not shown reliable correlation to plant availability. • Organic matter (%) — This is an optional test. Note carefully whether the result reported is for organic matter or organic carbon. • Electrical conductivity (EC) (millisiemens/cm) — This optional test indicates the presence of excessive salts in the soil. Derived Values • Zinc and manganese index — These are calculated from the analytical result and the soil pH. • Cation exchange capacity (CEC) and base saturation % — These numbers are calculated from the soil pH and analytical results for K, Mg and Ca. They may be skewed in high pH soils by the presence of free lime. Ontario fertilizer recommendations are not affected by CEC or base saturation. Nutrient Recommendations • Fertilizer and lime recommendations — These will only be printed if information about the crop to be grown has been provided. The analytical results can be used to determine nutrient requirements for specific crops from tables in the appropriate production recommendations. Some labs will give Ontario recommendations where requested. Often the labs will provide their own recommendations. • Adjustments to fertilizer recommendations — Adjustments for manure application or for a previous legume crop will be included in the fertilizer recommendations if the information is provided. • Notes and warnings — Some reports will include additional information based on the crop and soil test data. Note: Ratings for soil test values are based on the soil test result and the crop to be grown. Chapter 5. Soil, Plant Tissue and Manure Analysis 91 Extractants However, because we are estimating Analyzing soils to determine fertilizer only the available portion of the requirements is complicated because nutrient in the soil, the first step we are trying to estimate how much differs from a normal chemical nutrient is available from a specific analysis. In the case of soil tests, soil to a wide variety of crop plants the soil is first treated with an throughout the entire growing extractant to remove a portion of season. This would be simple if soils the nutrient that is related to the had uniform nutrient distribution, all amount available to plants. This the nutrients were wholly available extract is then analyzed to determine for plant uptake and there were the amount of nutrient that only one method by which plants was extracted. took up nutrients. However, soil is an extremely complex medium with Choosing an extractant a wide variety of physical, chemical To be useful in predicting crop and biological interactions occurring needs, an extractant must provide simultaneously. The interactions at the best possible estimate of the the soil-root interface are even more amount of additional nutrient complex and less well understood. needed for optimum crop yields. This is complicated to measure, so An example of this complexity is the assessment of extractants is phosphorus. The most common more commonly made by measuring chemical form of phosphorus in how well the extractant estimates the soil is phosphate. In neutral-to- the nutrients available to plants in alkaline soils, phosphate will combine the range of soils tested in the lab with calcium. In neutral-to-acid soils, or in a region. The extractant must it will bind to iron or aluminum. also be relatively inexpensive and Phosphate also reacts with various easy to use, involve as few toxic or clay minerals or organic compounds corrosive chemicals as possible and to form complex combinations, and use procedures that are reproducible it may be present in the organic from lab to lab. fraction of the soil or the soil biomass. All these forms are available No extractant pulls out the exact to a greater or lesser degree to plants fraction available to plants. Each has through a variety of processes, which strengths and weaknesses specific we try to measure with a single, rapid to various soils. The choice of an chemical test. extractant should be governed by how appropriate it is to the soils in Every chemical analysis has two question and by the availability of steps. First, the compound being data relating it to crop response. See analyzed is converted to a form that Chapter 7 for more details.