Soil Testing and Maintaining Soil Fertility on Organic Farms

A basic principle of organic farming is to maintain soil fertility. Organic certification encourages the organic operator to do regular soil testing and analysis to monitor levels of nutrients and to document the need to apply appropriate nutrient inputs when required. The need for environmental sustainability as well as the relatively high cost of purchased nutrient sources suitable for organic systems, means it is very important where possible to try and minimize nutrient losses from the soil, especially when using manures and compost, and build a crop rotation that optimizes the use of nitrogen that is fixed by legumes. The challenge is to close the nutrient cycle loop as much as possible.

How to take a Soil Test

The OMAFRA factsheet: Soil Sampling and Analysis for Managing Crop Nutrients http://www.omafra.gov.on.ca/english/engineer/facts/06-031.htm gives a good general perspective of how to take a soil test. You can take them yourself or there are numerous crop and soil consultants in Ontario (including some that work specifically with organic farms) that you can hire to take the samples and interpret them for you. In Ontario there are several Accredited Soil Testing Laboratories that can do the analysis. They are listed on the OMAFRA webpage at http://www.omafra.gov.on.ca/english/crops/resource/soillabs.htm

What tests need to be done?

The basic soil test includes the most important tests to be done. The basic test usually includes phosphorous, potassium, magnesium, and pH. An organic matter test is also important to have included. Nitrogen is not analyzed since it fluctuates over the year and a current soil test is not a good predictor of the nitrogen status of the soil for the next growing season. Other micronutrients, and cation exchange capacity (CEC) are also sometimes done and are optional (additional costs) to the basic test at most labs. In general most micronutrient and CEC tests are less reliable than the basic test parameters in predicting crop nutrient needs and deficiencies for the coming season.

Interpreting the Soil Test Report

Most labs will give you a report that will list the parts per million (ppm) for each nutrient tested according to the lab processes they used. They will usually give an interpretation on how much conventional fertilizer is needed but since organic farmers do not use conventional fertilizer, farmers may need some assistance to help interpret the analysis. Some labs may give organic recommendations, ask your lab if they can provide that service for your needs.

If you are using the same lab as in past years or the labs are using the same methods, you should be able to compare the numbers from one set of test reports to the next. An important part of the interpretation is to look for trends from one test to another. Most nutrients change slowly. Some changes may be due to differences in the way the soil test was taken or in the recent weather before the test was taken. Your primary goal is to maintain your soil fertility and to build the soil fertility as needed.

For organic farms, pH is the first thing to look at and if it is low (below pH 6.0) the report will usually recommend either calcitic lime or dolomitic lime. Dolomitic lime usually costs slightly more (depending on availability in your region) but it is important to use it if magnesium is low - common on sandy soils). Most conventionally available agricultural limes can be used on organic farms but check with your certification body (CB). A pH of 6.5 to 7.0 is ideal but it is impractical to reduce the soil pH when soils test higher than that. For more information see: Soil Acidity & Liming http://www.omafra.gov.on.ca/english/crops/pub811/9fertilizer.htm#soil

Next I always look at the phosphorous (P₂0₅) values. Different crops have different P₂0₅ requirements. Soils contain a lot of phosphorous but there is only a very small percentage (1-2%) that is available for crops to use. The Olsen or sodium bicarbonate lab test has been found to be the most reliable predictor of availability for plant uptake under Ontario conditions. For most crops, P₂0₅ values over 20 ppm indicate a good level of P₂0₅ in the soil and response to added phosphorous is less likely. On soils with P₂0₅ levels below 12 ppm the soil productivity is often limited by a lack of phosphorous and will likely improve with additional phosphorous being applied. Rock phosphate has sometimes been used to add phosphorus to the soil but it has very low availability and especially under higher pH may not be cost effective. Livestock manures and compost are usually the most economical means to add P₂0₅ to the soil.

Similarly potassium (K₂0) is important to look at on the soil test report. In Ontario the ammonium acetate test has been used with the most accuracy over the range of Ontario conditions. Soils with values above 120 ppm typically will have low response to added potassium, but this will vary with the crop being grown. Generally if the soil potassium values are below 80 ppm crops will respond to added potassium. Potassium can be added with naturally mined potassium sources such as potassium sulphate, sulphate of potash magnesium or potassium chloride but care should be taken to get the product that is approved for organic farms by your certification body. Livestock manures and compost are also a very economical way to add K₂0 to the soil.

Magnesium is an important macronutrient and in Ontario is usually adequate on most clay and loam soils but is often in short supply on low pH and sandy soils. If soil levels are above 60 ppm there is usually not a high level of concern unless high levels of potassium fertilizers are being added to the soil. Dolomitic limestone is the most economic source of adding magnesium. Sulphate of potassium magnesium can also be used in fields where magnesium is low but pH is above 6.5.

Calcium is usually adequate on clay and loams soils in Ontario since many of our soils have been created from high calcium bedrock. Some sandy soils, especially those with low pH are low in calcium and in those cases either calcitic or dolomitic limestone are good sources of calcium as well as to amend the soil pH.

Cation Exchange Capacity (CEC) is sometimes calculated for the report and is a reflection of the soils ability to hold cation nutrients in the soil (potassium, calcium, magnesium, ammonium). As the clay content, and organic matter content increase in soil the CEC will increase. On high pH soils, tests will sometimes over estimate the CEC due to the presence of free lime in the soil. Base saturation and cation ratios are also sometimes calculated to evaluate nutrient needs but university research in North America research has not shown these tools to be accurate predictors of which nutrients are economical to add to improve crop performance.

Maintaining good soil organic matter (SOM) levels is the best way to maintain a good CEC. Cover crops, livestock manure and crop residues all help to maintain SOM levels. Frequent tillage and soil erosion are known to deplete SOM. Organic matter levels change very slowly. Good SOM levels are in the range of 3% for sandy soils and 5% for clay and loam soils. Changes greater than 0.2% per year as shown on the report may be indications that the soil sample was taken at a different depth or some other problem in the test exists. Sandy soils will generally have lower SOM levels than clay and loam soils.

There is an array of micronutrients that can be analyzed in a soil test. For most crops there is limited research on how interpret these values to accurately predict deficiencies in the crop and on how to correct them. Using visual symptoms as an indicator of deficiency or using leaf tissue analysis is usually more reliable for most micronutrients. Farm manures and composts are a good source of most micronutrients. For specific micronutrient fertility inputs consult the Organic Standards and Permitted Substance Lists, and your input supplier or crop advisor.

NOTE: Before applying any input to certified organic farms make sure it is approved by your certifying body.

For More Information:

Soil Fertility Handbook, OMAFRA Publication 611 http://www.omafra.gov.on.ca/english/crops/pub611/p611order.htm
Managing Crop Nutrients, BMP 20 (order number BMP20E) - http://www.omafra.gov.on.ca/english/environment/bmp/crop_nut.htm

 

Table of Contents