Soil sampling is an important aspect of crop management. It is essential for managing nutrients, especially phosphorus (P) and potassium (K), and adjusting the pH of the soil. It can also be an important tool for diagnosing soilborne pest issues such as plant parasitic nematodes and fungal pathogens. Many lavender growers are new to farming and/or are too small to work with farm supply companies for custom sampling. Improper sampling methodology can lead to skewed results that can lead to improper fertilizer application rates. Here are the basics of soil sampling in lavender:

Soil Nutrient Sampling:

The very first step in growing lavender should be soil sampling for nutrients, pH and potentially soil texture (to understand soil type and potential drainage issues). Soil sampling is primarily used for pH, P and K management, but can also be useful for several other nutrients (e.g., magnesium, manganese, zinc) and salt and organic matter content. It is best to sample for nutrients every 2 to 3 years to track changes in nutrients over time and ensure your fertility program is working for correcting deficiencies.

Tools

The best tool for collecting a soil sample is a stainless-steel soil probe. These are readily available online or from local farm supply stores. It is best to get one with a foot pedal to give more leverage. If you have stony or heavier soil, you may want to get an auger type instead. These will ensure you get an even distribution of all levels of the soil profile compared to a shovel that would give you more soil from the surface compared to lower down. A narrow trowel could potentially be used instead. You will also need a bucket for collecting the sample but avoid galvanized steel as it may introduce micronutrients into the sample and skew the results.

Area

Generally, for field crops, a single soil sample should not represent more than 25 acres of land, but for lavender consider collecting separate samples from much smaller areas given the higher value of the crop. Each distinct area of the field should be sampled separately. Distinct areas could include low or high areas, areas with a different cropping history, areas with noted differences in current crop performance, or obvious differences in soil type or drainage.

Depth

Soil samples for nutrients should be collected to 15 cm depth, which is the zone in which most of the roots will be located. For a soil texture analysis, you may want a separate sample of deeper soil layers to adequately assess drainage potential.

Number of Cores

A single soil sample should consist of 15-20 soil cores collected in a random pattern over the area to be sampled. The more cores you collect, the more representative the sample will be. An X or W pattern through the area to be sampled is often recommended. For example, with an X pattern you would start in one corner of the block and collect several cores in a roughly diagonal line to the opposite corner of the block, and then do the same in the other direction.  A W pattern is more of a zigzag across the block. Once all the cores are collected in a bucket, break up any clumps of soil and thoroughly mix the sample. Remove any clumps of organic debris, earthworms or vegetation as these may skew the results. The labs usually need about two cups worth of soil, and you can discard the rest. If you are sampling in an existing lavender field, collect cores from the lavender root zone at the edge of the rows. If you have been fertilizing in the drip irrigation system (fertigation), you may need some cores from directly next to the plants to get accurate results, which may mean slicing through the plastic in a few places.

Time of Year

Nutrient samples can be collected at any time of year. Most growers sample in the fall so fertilizers can be purchased over the winter and applied when appropriate in the spring.

Analysis

Soil samples for nutrients should be submitted to an OMAFRA accredited laboratory. It is best to use the same laboratory consistently over time. A listing of accredited labs is available here: https://www.ontario.ca/page/soil-leaf-and-petiole-tissue-and-forages-and-feed-testing-labs. Soil sampling is not useful for all nutrients. For many micronutrients, plant tissue sampling is the better approach. OMAFRA only accredits labs for testing pH, buffer pH, P, K, Mg, Mn index, Zn index and Nitrate-N in Ontario soils. Nitrate-N sampling is not very useful for lavender and would require slightly different sample collection procedures.

Clean Up

Between samples, clean your probe and bucket of all soil to prevent contaminating the next sample.

Sampling for Pests:

Soil sampling can be useful for diagnosing issues that cannot be explained by nutrient testing or an examination of the tops of the plant. It can also give you a baseline scan of a field to know if there may be issues that need to be corrected before planting the crop.

Plant Parasitic Nematodes

Nematodes are microscopic worms. Certain species can attack plant roots and cause considerable damage that weakens the plant, which can resemble nutrient deficiencies or other diseases. They are often very patchy in the field. Nematode samples are collected the same way as nutrient samples but with three important differences:

1. Collect cores to 20 cm depth and remove the top 3 cm of each core before placing in the bucket. Nematodes avoid the top layer of soil because it is often two hot and/or dry. More cores or samples per area may be necessary to get an accurate count.
2. Collect in spring or fall (May or October) when soil temperatures are moderate. Nematodes move down the soil profile in the fall before winter and then back up in the spring. During the summer they enter plant roots. If you sample when it is too cold, you will not capture the nematodes. If you sample in the summer, you may miss them in the soil sample.
3. Submit to the lab shortly after collection. While soil samples for nutrients can be dried or frozen and stored indefinitely before submitting to the lab, nematodes are living organisms and may die if the sample is not carefully handled and submitted soon after collection. Ideally, collect samples, keep them in a cooler and submit to the lab on the same day. Avoid submitting on a Friday to ensure the lab can process them right away. Only a few labs conduct nematode analysis, so do your research in advance.

Fungal Analysis

Some labs can conduct a genetic scan of the soil for common species of pathogenic soil fungi and water moulds (oomycetes). This can include known pathogens of lavender like Phytophthora nicotianae and several Fusarium and Pythium species. It is generally not worthwhile to test fields ahead of planting unless you know of a specific issue in the preceding crop. Sampling for fungi is useful when you have a patch of dying or dead plants in an area of the field that cannot be explained by flooding or winter weather. A plant sample that includes the root system is often best for diagnosing the problem if the plant is still partially alive, but occasionally a soil sample may be more appropriate (e.g., if all the plants in an area are dead or you had a problem in an area that currently has no lavender crop, and you want to know if it is still occurring). In these cases, collect a sample the same as you would a nutrient sample, from within the problem area. It is best to compare this to a separate sample from a healthy area of the same field. Unlike nematodes, it is not critical to submit the sample the same day.