Plants need at least eighteen essential nutrients to grow and develop. Deficiencies of any of these essential elements can cause reduced crop yield and quality. For instance, we know that a high protein content in bread flour is essential for quality bread baking while low protein contents in flour is critical for quality cake baking. Aside from genetics, soil fertility management has a big impact on protein content and types of protein in the wheat we use for baking bread or cakes as well as all other crops. Plant proteins contain, among other elements, nitrogen and sulfur. So having adequate supplies of nitrogen and sulfur in the soil for the crop being grown will affect the development of protein in plants. Sufficient amounts of these two elements will guarantee high grain protein content and bread baking quality, while low nitrogen and sulfur availability in soil can lead to low grain protein content and cause poor bread baking quality. Often plants exhibiting nitrogen deficiencies develop yellowing on their lower leaves.

Bottom leaves of tomato turning yellow often indicate nitrogen deficiency. Photo by dmp, UConn

A common problem that we often see in our vegetable gardens is blossom end rot in tomatoes, peppers and summer squash. When conditions are right, the fruits of these plants develop quickly and each new cell that is formed requires calcium. When not enough calcium is available to plants either because there is not enough in the soil or there is not enough moisture to move the calcium from the soil into the plant roots, a calcium deficiency develops and expresses itself as blossom end rot. Also, if you have seen internal brown spot in the potatoes growing in your garden, you may also want to check the calcium level in your soil. Typically, you would do this by checking the soil pH. Since limestone (calcium carbonate) is used to raise the pH as well as supply calcium, if your soil pH is in the 6s, there should be sufficient calcium present in the soil so if you are seeing blossom end rot, it would most likely be due to insufficient watering.

Blossom end rot on tomato. Photo by dmp, UConn

Potassium is another nutrient that crops like potatoes need in high quantity. Potassium not only influences potato tuber yield and size but also potato sugar concentration, hollow heart disorder, even coloring after cooking. Sufficient availability of micronutrients, such as zinc and iron, are important for plants growth and nutrition values. These micronutrients are also essential for human health, and you can often find them in supplements.

Tomato with possible potassium deficiency. Poor locules and thick, mealy walls. Photo by dmp, UConn.

How do we know if our soil is sufficient in these nutrients? Soil testing can give you an idea of your nutrient sufficiency levels in the soil and plant tissue testing can tell you if your plant is accessing these nutrients. Some potential issues that could impact nutrient uptake by your plants include low pH, excessive amounts of another nutrient, poor soil structure and drainage, compaction, and improper watering. For macronutrients, such as phosphorus, potassium, calcium and magnesium, soil testing is sufficient while for micronutrients, such as zinc, copper, boron, manganese and iron, we recommend testing both your soil and plants if a nutrient deficiency is suspected. If you observe poor plant growth and good plant growth in the same garden or field, it’s best that you take soil and plant samples from both areas and get the samples analyzed separately so that you can compare nutrient levels in these areas. Keep in mind that many diseases have symptoms that mimic nutrient problems so it is always a good idea to send photos of the problems you are seeing to the horticulturists at the UConn Home & Garden Education Center (ladybug@uconn.edu).   

Is this a disease on cucumber or nutrient deficiency? Photo by dmp, UConn

For soil testing for garden crops, you can take soil samples at any time of the year, but fall is best. While taking samples before seeding, transplanting of annuals, and greening up of perennials is important to ensure timely application of fertilizer and soil amendment to provide your crops with sufficient nutrients during the whole growing season, keep in mind, that samples submitted in the spring take longer to process because of the higher volume. If samples are submitted in late fall rather than in early spring, recommendations are likely to be identical and if amendments such as limestone, which takes 6 to 18 months to work, are needed, they can be added in the fall so they can start working. Any fertilizer would be added in the spring before planting.

If you notice deficiency symptoms in your plants, it is important to take quick action to try and diagnose the problem and apply fertilizers, soil amendments, or change your cultural practices to deal with nutrient deficiency issues. Although your plant’s health may have declined because of nutrient deficiencies, a rescue application of nutrients can alleviate symptoms and put it on the path to good growth.

Plant tissue testing is primarily recommended for commercial growers because at the UConn Soil Nutrient Analysis Lab, there are no recommendations for home gardeners, only commercial vegetable and fruit growers. For commercial growers submitting samples, it is critical to sample the correct plant part at the right growth stage. This is because the tissue test sufficiency ranges that are use to compare your samples are established for that specific plant part and growth stage for a given crop. For example, ten uppermost recent fully developed trifoliate leaves should be sampled from green beans in summer, fifteen compound leaves adjacent to the inflorescences should be sampled during midbloom for field tomatoes, and twenty-five mature leaves from new growth should be sampled during flowering – fruiting should be sampled for peppers. Prior taking your soil and plant samples, please visit University of Connecticut Soil Nutrient Analysis Laboratory https://soiltest.uconn.edu/ for guidance on when and how the samples should be taken. Feel free to call the lab at (860) 486-4274 if you are considering submitting samples for plant analysis. UConn also has a Plant Diagnostic Lab that can culture plants for diseases.   

Another important thing to remember regarding soil and plant tissue analysis is that it is important to send your samples to your local labs. This is because different labs use different testing procedures that are calibrated for soil types specific in their region and the plants grown in these specific environments. The standard sufficiency levels established are therefore different by state and by region. For example, there are many different soil test procedures being used in the US for soil phosphorus test, however, only modified Morgan testing procedure is used for CT soils due to specific characteristics of our soils in CT.

The bottom line for home gardeners and growers is to do your best to ensure your plants receive the correct amounts of nutrients as well as water to be able to supply the nutrients to our plants. Routinely monitor your plants for insects, disease problems as well as nutrient issues. We are here to help you so feel free to contact the UConn Home & Garden Education Center (www.ladybug.uconn.edu) or UConn Soil Nutrient Analysis Lab (www.soiltest.uconn.ed) if you need help or have questions.

Haiying Tao, Ph. D. Dept of Plant Science LA, UConn