One amazing advantage of working for Utah State University, the land-grant University for the State of Utah, is my access to the broad and deep expertise of our specialists housed on-campus.
Dr. Jennifer Reeve is an Associate Professor of Organic and Sustainable Agriculture for USU. Although she does not have an Extension appointment, I will sometimes ask her advice on client questions that exceed my own knowledge. Last month I got a question from a client and asked her for help answering it. I felt Jennifer’s reply would be helpful to a broader audience so here it is – many thanks Jennifer for your willingness to share your nuggets of organic gardening wisdom with all of us!
Q: Thanks for helping me break down the soil reports today- it’s always helpful to bounce results off another person. As we discussed, my main concerns are the salinity and phosphorus levels. While it seems like the salinity will be less of an issue as we should be able to plant more tolerant varieties and leach the soil, the phosphorus levels are extremely elevated. Could this pose a major problem with growing next season? Are we running the risk of contamination/runoff?
A: Elevated salinity, phosphorus (P) and potassium (K) are common problems associated with excessive compost use. You are correct that much of the salt (sodium) can be leached out with good quality irrigation water. And this should solve any plant growth problems you may have been seeing. Excess P and K are much harder to deal with as I’m sure you know. While P and K are not toxic to plants per se, luxury consumption by plants can interfere with the uptake of other essential elements which can negatively impact produce quality (nutritional and storage quality). For example, luxury consumption of P can interfere with trace element uptake, particularly zinc (Zn), and luxury consumption of K can reduce plant uptake of calcium (Ca) and magnesium (Mg).
K is benign to the environment but you are right to be concerned about the potential for contamination of rivers and lakes with excess P runoff and leaching. Nitrate is also very prone to leaching and runoff. While nitrate is the most common plant limiting nutrient the levels in your soil are also quite high, especially for organic production.
The risk of runoff to the environment will depend of on your location. Plant buffer strips of grass and even build stone retaining walls and catchment areas to help soak up or stop any water in danger of reaching streams, paved areas and storm water drains.
If these soil tests were taken in the fall, you might find that much of the salt leaches naturally below the root zone over the winter months. You could take another soil test in the spring to make sure. To address the high P and K I suggest that you stop using compost until your P and K levels have fallen to a more reasonable level. This could take several years. Build soil organic matter using cover crops and fix as much nitrogen (N) with legumes as you can.
Fixing adequate N with legumes can be challenging in Utah’s short growing season because it is difficult to generate sufficient biomass in the short windows you have between cash crops. If you have the space, take out an entire season to grow cover crops. At the USU Student Organic Farm we have achieved excellent results with the following rotation: 2/3rd mix of hairy vetch or Austrian winter pea + 1/3rd winter wheat or rye planted after vegetables (early to mid-October) and allowed to overwinter and grow until flowering, followed by buckwheat, followed by a second earlier planted (early-September) hairy vetch + winter wheat cover crop, followed by more vegetables. If you don’t have the space for this you may be able to get creative with timing. For example, plant late season (June-planted) vegetables after a late fall planted cover crop and early season vegetables after early planted fall cover crops. This will help you maximize the window for cover crop biomass production.
Depending on the biomass you can grow, you may also find that you need to supplement additional N with a P and K free organic fertilizer. Feather meal (cheapest) and blood meal are pretty much your only options.
Unfortunately, many books and websites available on organic farming and gardening indicate that when it comes to compost, you really can’t overdo it. It certainly is a quick and easy way to build soil organic matter and soil fertility. Most of these recommendations have been written by people in wetter climates than we have, so natural leaching of salts occurs to prevent problems for the plants, although certainly not for the environment! We have a real need for better recommendations for organic growers in dry climates. In dry climates the best results are often achieved with a mix of cover crops combined with very small amounts of compost. Usually no more than a 1/8th inch or even just a sprinkling of compost once a year is needed depending on the fertility of your soil. Feather meal (basically steamed pelleted feathers) has also become popular with organic growers in the West. We are in the process of hiring a new urban agriculture extension specialist so hopefully locally generated recommendations will become more widely available soon.
For more information on organic and sustainable agriculture, check-out this awesome video featuring Dr. Reeve and the USU Student Organic Farm in Logan, Utah.