Urban Farm Soil Test Interpretation

Soil organic matter is a measure of the carbon-containing material that comes from living and dead organisms, or what may be simply described as the living, the dead, and the very dead. Organic matter affects nearly all other soil properties, including nutrient cycling, soil structure, and water availability.

Constructed soils for urban agriculture often have a high proportion of organic matter as compared to their rural counterparts, usually in the form of large volumes of compost. In New York City, urban ag soils typically have organic matter ranging from between 8 and 15%, with some as high as 30%. There are many advantages to the high organic matter in urban ag soils including:

• Well-balanced water retention and drainage
• Low occurrence of compaction
• Improved tilth
• Steady supply of some macronutrients (especially phosphorous, calcium, and magnesium) and most micronutrients
• Slow release of nitrogen as organic matter decomposes

Compost typically provides many of the nutrients needed for plant growth, but vegetables and other crops may benefit from more potassium (K) and nitrogen (N) than compost alone can provide. For example, many urban farmers see improved yields by adding potassium sulfate at spring planting.

However, with high levels of organic matter, we may also see some challenges:

• May dry out fast, especially highly organic soils that are newly constructed or frequently tilled
• High pH related to excess nutrients
• Excess N becomes available often in late spring/early summer
• With excess nitrogen and phosphorous, there is potential for leaching, downstream eutrophication, and other water quality concerns

Nitrogen (N) is often the nutrient of primary management concern for urban and rural farmers. There are several reasons why:

• N is among the soil nutrients used in highest quantity by produce crops (up to several hundred pounds per acre per year!)
• Decomposing organic matter releases N, but rates of availability depend on soil microbial activity.
• N is highly influential in shoot production, which influences the yield potential of all fruits and vegetables, as well as the plant’s ability to make use of other soil nutrients.
• N is highly mobile and often will escape the root zone through volatilization (in the air) or leaching (down through the soil profile) before it is taken up by the crop, resulting in economic and environmental losses. 

Urban farmers face some unique challenges around N management. Since most urban ag soils are high in organic matter, there is a considerable bank of N in the soil itself. However, it is difficult to predict when soil microbes will ‘mineralize’ this N into plant available forms such as nitrate N. We often observe a flush of nitrate N released from high organic matter soils in late spring, when temperatures and moisture favor mineralization, resulting in excessive vegetative growth, leading to foliar disease and delayed maturity of fruiting vegetables. 

For this reason (and to avoid loss) we do not recommend large additions of N fertilizers to high organic matter (OM) soils. Bulk forms of N, such as feather or soy meal, are common fertilizers on rural organic farms, however the release of nitrates from these sources is similar to soils with high organic matter soil. We suggest more frequent but modest applications of soluble forms of N (such as side dressing blood meal), or where drip irrigation systems are in place, the injection of a soluble source of N at modest rates on a weekly basis to avoid excess levels and loss. Or for some urban growers, such as community gardeners, it may be acceptable to not apply any additional nitrogen. 

How much N does organic matter release?

Estimates vary, but a safe assumption is that a soil with 1% OM will release around 10 lbs of N per acre per season. For example, an acre soil with 10% organic matter may release 100 lbs of nitrogen over the course of the season. Our challenge is knowing when this will be released and balancing it with crop demand. Crop demand beyond existing soil supplies can be estimated on a weekly basis. Taking the remainder of the N needs of the crop not met by mineralization, we can divide the amount of N desired by the anticipated number of weeks the crop will be in the ground. 

Potassium (K) in another ‘macro nutrient’, and like N, crops use hundreds of pounds per acre per year. K is essential for flower development and fruit quality. When crops are low in potassium, foliage will become discolored, and the plant drops flowers and fruit. Our data reveals that this is the most common deficiency in urban vegetable crops, particularly tomatoes.

K is nitrogen’s pal, as we can calculate application rates based on anticipated levels of N. For example, tomatoes in their fruiting stage will use K at almost 3X the rate of N. For vegetative crops, like collards and kale, this ratio is closer to 1:1.

Fortunately, K is not quite as mobile as nitrogen in the soil, and we can pre-plant apply K in bulk forms based on soil tests and projected crop demand. K uptake is influenced by soil pH and the relative value of other crop nutrients in the soil. Avoiding excess levels of phosphorus, magnesium and calcium will also help achieve potassium at appropriate levels in the crop. Further, it is important to consider bulk density when deciding on K applications because lab analysis in high OM soils with low bulk density will be overestimate K levels.

Soil pH or acidity affects how available soil nutrients are for plant uptake. For most crops, the ideal pH range for nutrient availability is 6.0 - 7.0 (Figure 1). Due to human activities, the majority of urban agricultural soils in New York State exhibit a pH between 7.0 and 8.0. This elevated pH range can impact vegetable crop health and reduce yields by making certain nutrients less available for plant uptake. For example, manganese availability is reduced at high pH, and as a result it is commonly deficient for crops on urban farms with high levels of organic matter. High pH can also result in deficiencies of phosphorus, iron, copper, and boron.

Where there are many resources available on how to raise soil pH, information is more limited on strategies to lower soil pH. A common strategy for managing soils with high pH is amending with sulfur. Some less common approaches include additions of acidic peat moss or composts. Sulfur enriched products come in pelleted or powdered forms. Some people find pelleted forms easier to work with, though they may act on pH more slowly than powdered forms. Sulfur is also used as a fungicide, so make sure the product you use is labeled for soil and pH adjustment.

Our work has demonstrated that pH adjustment of urban soils by amending with elemental sulfur can lead to increased plant nutrient availability and overall crop health.

Follow these guidelines when applying sulfur products:

1. When considering when to apply sulfur amendments, growers generally apply before planting or after harvest. Fall application is recommended, as applying close to planting has a risk of burning plant roots.
2. When deciding upon application rates, the soil texture and pH need to be considered (Table 1). 

Table 1. General sulfur recommendations for different soil textures. Application rates are in pounds/acre of sulfur to lower soil pH to 6.5. To convert pounds/acre to pounds/100 ft2, divide by 436. Source: Cornell SoilNOW 

High tunnels, or soil-based greenhouses, help urban growers extend the growing season while increasing crop quality and yield. As with urban soils in general, soil pH tends to be higher in high tunnels. As these soils are covered most or all of the year, leaching events (rain or snow) are limited, which means that soluble salts coming from fertilizers, composts, and irrigation water will build up over time. These salts, which would otherwise rinse out with rainfall, can build up in the soil, further raising the pH. Additionally, high temperatures under the tunnel can accelerate this process. As salt levels increase, it becomes more difficult for plants to absorb water from the soil.

There are some management considerations and options to deal with this salt buildup. First, add a soluble salts test to your annual soil test to keep track of the salt levels. Second, to remove excess salts, increased leaching is a common and effective option. This can be done in a few ways, such as by removing the covering every three years during winter or flooding the tunnel every few years to flush out excess salts. Finally, growers may have the option of acidifying irrigation water with sulfuric or citric acid to help reduce the pH in the soil.

Contaminants

While the focus of this handout is on building soil fertility, it is important for urban growers to keep in mind that contamination by lead, other metals, or other chemicals is a concern for urban soils. There is existing guidance on soil testing for contaminants and mitigation strategies. 

Soil Lead Screening – updated fact sheet from New York Soil Health – 2024

Sources and Impacts of Contaminants in Soils – Cornell Waste Management Institute Fact Sheet – 2015

Prepared by

Lori Koenick, Extension Support Specialist, CCE Cornell Vegetable Program

Sam Anderson, Urban Agriculture Specialist, CCE Harvest New York

Jonathan Russell-Anelli, Senior Extension Associate & Senior Lecturer, Soil and Crop Sciences Section, School of Integrative Plants Sciences, Cornell University

Judson Reid, Vegetable Specialist, CCE Cornell Vegetable Program