N-P-K Explained!

With the new year comes the annual tasks of crop planning, fertility management and thinking about what your growing season will look like while also reflecting on the previous year’s successes and difficulties. As you gather your soil test results and start thinking about which crops will go where and just how much you need to grow for your community or supply chain you will want to take a look at the big 3 nutrients that will be essential throughout the season - Nitrogen, Phosphorus and Potassium! 

Biologically-based fertilizers work with soil biology to make nutrients available to plants. Mixed-analysis fertilizers can provide primary fertility and/or maintenance fertility. Analysis is stated as a percentage of total Nitrogen, Phosphorus and Potassium (N-P-K), meaning a fertilizer with an analysis of 3-3-3 would have 3% of each N, P and K per weight. This would translate to 1.5 lbs each of N, P and K per 50 lb bag. Having your soil test results handy will help you decide which analysis you will need, especially if you have requested crop specific recommendations. 

Below is an example of what you may see on the soil test results you receive. Outlined in red are the areas that show P & K values (most labs do not test for N since it is highly mobile) as well as the recommendations for how to fertilize your fields for your specific crop (in this instance, potatoes). 

Nitrogen

Nitrogen is an essential nutrient for vegetative growth both as fuel and building blocks for plants. Crop demand for N will vary significantly; it is best to look at regional production guides for the crop(s) you plan to grow to determine how much N to apply. Most vegetable crops will use between 100 and 120 lbs of N/acre (or roughly 2-3 lbs/1000 sq ft) but asparagus and beans will use much less, while beets, potatoes and corn may use more. Legume cover crops can contribute a significant amount of N, so if you've planted a cover crop remember to take this into account.   

Biologically-based N sources will take about 40 days to fully mineralize into forms of plant-available nitrogen (PAN) such as nitrate and ammonium. The percentage of total N that becomes PAN will depend on the type of fertilizer, soil temperature and moisture, and other conditions that can impact the health and activity of soil biology. Actively promoting soil biology through practices such as cover cropping, appropriate tillage and selecting biologically-based fertilizers will greatly improve the availability of N and other nutrients to plants.

Phosphorus

Phosphorus is critical for key plant functions including photosynthesis, energy transfers and reproduction. Adequate phosphorus in your soil provides good root system development, early maturation and general crop quality. Phosphorus can move from one part of the plant to another and is needed throughout the lifetime of the plant. Mixed vegetables will need between 20 and 160+ lbs of supplemental P/acre (0.5-4+ lbs/1000 sq ft) depending on what is present in the soil. Some crops may need 200-300 lbs of P/acre in certain soils. Phosphorus application rates for commercial growers should be determined by a soil test report.

Potassium Sources

Potassium is required for essential plant functions including water regulation and CO2 uptake. Potassium is needed to maintain enzyme functions, balance electrical charges in cells and moderate energy release. Plants that have access to adequate potassium tend to have strong, sturdy stems, produce large amounts of sugar and protein, mature early and can have better resistance to pests and diseases over the course of the season. Mixed vegetables will need up to 200 lbs of supplemental K/acre (up to 5 lbs/1000 sq ft) depending on what is present in the soil.  Some crops may need 300-400 lbs of K/acre in certain soils. 

It is important to consider what your pH levels are in your soil. This will determine the availability of nutrients (N-P-K as well as others) to your plants. Soil pH can be corrected with lime (raises pH) or sulfur (lowers pH). 

If you have any questions when looking at your soil tests for this upcoming growing season please feel free to send us an email or give us a call and we will be happy to help you figure out what the best solutions are for your farm! 

540-651-3228
800-540-9181
7springs@swva.net