Soil Analysis 101

After writing about the different methods of field irrigation used by West African farmers (see here), the Tomato Jos team realized we would need to dig deeper (literally!) to learn more about what *really* makes or breaks a tomato harvest.  So we turned to the soil. 

Tomatoes prefer mineral soil types that are deep, free from salt water and have a sandy loam soil composition with good aeration and water retention properties. Soil acidity is also an important consideration. Tomatoes are moderately tolerant to a wide variety of pH (level of acidity), but they tend to grow best in soils with a pH of 5.5-6.8. Tomato roots can stretch 15 to 20cm (6 to 8 inches), so a few inches of topsoil is not adequate – ideally, topsoil in the field should extend at least 25cm (10 inches) deep to provide a happy home for a healthy tomato crop. 

The Tomato Jos team has started sampling soil in all of our prospective farming locations to test for acidity, organic content, mineral content, and soil structure. This analysis will tell us whether the soil is conducive for tomato growth, what specific nutrients we need to introduce, and how likely we are to encounter soil borne diseases.

Soil borne disease occurs when three factors – the pathogen, the host, and a favourable environment – come together. The pathogen is typically a microorganism that survives and moves about in the soil. Microorganisms such as nematodes can lie dormant in the soil up to seven years! However, they need a host in order to become active. Many microorganisms cannot feed on crops such as corn or soy – they often need a tomato plant or another more sensitive vegetable to attack. Finally, there must be favourable environmental conditions in place for the pathogen to be able to infect the host. Some pathogens favor damp conditions, some like certain soil pH levels, while others target tender, succulent growth.  

The results from the soil analysis enable the farmer to make a data driven and informed decision of how to anticipate (and hopefully prevent!) soil disease. In the US, soil maps, such as the UC Davis “soilweb database”, provide a detailed description of soil types and farming potential for certain regions. 

There are no public soil maps in West Africa so we conducted our own primary analysis, guided by the principles and techniques of composite soil sampling. With composite sampling, several small subsamples are taken randomly throughout the field and mixed together into one large representative sample that is sent to a laboratory for analysis. We chose a zigzag pattern when walking through the field and took 5 to 6 subsamples to get an appropriate representation of each field that we assessed. Areas that had abnormal looking crop growth or soil were recorded and sampled separately. Following, best practices, we did not sample field entrances, edges, depressions, hilltops, or areas where animals congregate. We also made sure to remove all non-soil particles from the sample, such as grass, thatch, leaves, and rocks. Watch below as Mira explains the process of soil analysis on the video. 

Smallholder farmers in West Africa are unable to analyse the soil where they grow their crops, because there are very few public labs that can conduct soil analysis, and the farmers generally lack the resources to access them.  As a result, farmers struggle to contain soil disease, which is a common occurrence. As we grow deeper relationships with the farmers in our network, Tomato Jos plans to extend soil analysis services to our smallholder partners so that they can continue to learn and improve their farming practices.