Drip It Like It’s Hot: A Technical Guide to Using Calcium Thiosulfate in Processing Tomatoes

By Dan Cook, Agronomist

In the pursuit of maximizing yield and fruit quality in processing tomato production, nutrient management remains a cornerstone of agronomic success. Among the various inputs available to growers, calcium thiosulfate (CaTs®) has been a valuable tool for decades—particularly when applied through drip irrigation systems. CaTs delivers essential calcium when and where you need it.  

Drip irrigation offers a precise and efficient method for nutrient delivery, particularly in high-value crops like processing tomatoes. When CaTs is applied through drip systems, it ensures that calcium and sulfur are delivered directly to the root zone, where uptake is most efficient. This method minimizes nutrient losses and allows for timed, incremental applications that align with crop demand. 

2. Improved Fruit Set and Quality 

Calcium strengthens the cell walls of plant cells, including in the fruit. Firmer fruit can mean less damage during harvest and transport. Calcium is also important in stimulating root growth, enzyme activity and proper fruit development. Additionally, the soluble calcium from CaTs, has been shown to displace potassium from the soil cation exchange sites. So, along with a calcium response, sometimes a potassium response can be expected.

3. Improved Soil Structure 

Applications of CaTs improve the soil physical properties. Soluble calcium opens the soil pores for improved water infiltration and movement through the soil. This can help to prevent harmful salts from accumulating in the rootzone, but it also promotes air movement into the soil for improved root health and to support beneficial microbial activity.   

4. Thiosulfate Sulfur – The Silent Partner 

While calcium is usually the focus of CaTs applications, sulfur is quietly working behind the scenes. Sulfur can improve nitrogen use efficiency, making better use of the nitrogen you’re already applying. Thiosulfate sulfur, in particular, is a nitrogen stabilizer, and can help to reduce nitrogen losses from the soil. Additionally, thiosulfate has been shown to improve the solubility and uptake of other nutrients in the soil like phosphorus, iron and manganese. 

Application Tips from the Field 

Alright, let’s get practical. Here’s how to make CaTs work for you: 

• Timing: Applications can be started early in the season during early vegetative growth. However, the key timing for prevention of BER is at flowering, and continues through fruit set and early fruit development. When it comes to improving water infiltration, CaTs can be applied immediately, throughout the growing season, whenever ponding or poor water infiltration is noticed.  

• Rate: Typical rates range from 5 to 10 gallons per acre per application, depending on soil tests and crop needs. Always consult your crop advisor, agronomist, or a Tessenderlo Kerley representative for tailored advice. 

• Compatibility: CaTs plays well in blends with most nitrogen fertilizers, including UAN solutions, urea, and liquid calcium ammonium nitrate (CAN-17). Calcium can sometimes be tricky in liquid blends, so be sure to jar test to ensure compatibility, and definitely avoid mixing it directly with phosphate fertilizers to avoid a potential mess.