Basic Aspects of High Tunnel Soil Fertility Management

Basic Aspects of High Tunnel Soil Fertility Management(Petrus Langenhoven, plangenh@purdue.edu, 765-496-7955) – Spring has arrived! Every high tunnel grower is now thinking of planting summer vegetable crops in high tunnels or has already planted. Whichever scenario applies to you, I hope that you have submitted soil samples or are in the process of submitting samples to your closest laboratory. Have you analyzed your irrigation water? It will be a good idea to send a water sample along too. There is a lot of important information locked up in your water and soil test results. The results will help you to plan and manage your high tunnel fertility program. Remember, growing in a high tunnel is like growing crops in an irrigated desert. Natural rainfall is unavailable inside your high tunnel and therefore all your plants water needs are satisfied through an irrigation system. Fertilizer needs could be addressed by adding water-soluble fertilizer to irrigation water (fertigating at times throughout the growing season), or by adding compost and other soil amendments prior to planting and during the growing season. High tunnels have some unique problems, which include temperature extremes, possible nutrient deficiencies due to faster crop growth under warmer conditions, salt accumulation in the soil, and faster release of nutrients from organic materials. In anticipation of these issues, growers can plan to manage soil fertility and moisture issues timely. In this article we will be focusing on water quality and the interpretation of your soil test report.

Importance of Water Quality Testing – The effect of using of high alkalinity irrigation water in greenhouse bench crop production is well known. Over time, the water can cause a rise in soil or media pH, resulting in deficiencies of zinc, manganese and iron in specific crops. High tunnel growers experience this problem too. Total alkalinity and pH of your irrigation water should be in the range of 0 to 100 ppm CaCO3 and 5.5 to 6.5, respectively. Ideally, it should be about 80 ppm CaCO3 with a pH of 5.8. An additional negative effect of alkalinity levels higher than 150 ppm CaCO3 is an increase in the incidence of clogged drippers. Remember, water pH and alkalinity are not strictly related. Therefore, it is important to test for both. There are two options to deal with high alkalinity and pH. Acidification and a reduction in alkalinity of the irrigation water could be achieved with the addition of an acid (i.e. citric, sulfuric, phosphoric or nitric). Rates could be calculated with this very handy ALKCALC resource. Amending your water alkalinity and pH while you irrigate is perhaps the fastest and most cost-effective method to manage soil pH. If you are interested to acidify your soil slowly with an organic amendment, apply elemental sulfur at 15 lb/1000 sq. ft. of bed area for each 0.5 pH unit drop needed (Sideman, 2018). Also make sure that iron, manganese and sulfate is within acceptable ranges. Iron levels higher than 5 ppm could be toxic to the plant and could result in iron precipitates forming at the emitter, plugging your irrigation system. Similarly, manganese levels higher than 1.5 ppm and sulfate levels higher than 240 ppm could cause emitter blockage. It is also important to keep track of the sodium and chloride levels in the water. High levels (>50 ppm Na and >70 ppm Cl) can lead to an increase in soil salinity, especially in the top 2-4 inches, and therefore has to be managed carefully. Particularly in view of the fact that the high tunnel cover prevents natural rainfall from washing or leaching excess soluble salts from the soil and that the elevated soil temperatures inside the tunnel increases soil microorganism activity, releasing nutrients from organic materials such as manures and composts faster into the root zone.

Understanding your Soil Test Report – The optimum pH varies by crop but it is generally accepted that the ideal pH range for organic and mineral soils are 5.3 to 5.8 and 6.0 to 7.0, respectively. In mineral soils nitrogen, phosphorus, potassium, calcium, magnesium, boron and molybdenum are most available when the pH is between 6.0 and 7.0. With a soil pH below 6.5, zinc, manganese, iron and copper tend to be most available. It is therefore desirable to maintain mineral soil pH between 6.0 and 6.5. The available aluminum increases as the mineral soil pH decrease, especially below 5.5. The increasing aluminum concentration can further contribute to soil acidification and aluminum toxicity, which inhibits root growth. The target pH range for organic soils are between 5.3 to 5.8. The lower pH range is acceptable because aluminum levels are very low in organic soils (Warncke et al., 2004). The capacity of a soil to hold exchangeable cations is measured and reported as the cation exchange capacity (CEC) of the soil. This value is a good indicator of soil fertility. A good soil has a CEC between 5 and 35 meq/100g soil. Generally sandy soils have a low CEC and soils with a high CEC is more likely to have a high clay or organic matter content. Two tests are performed to determine the phosphorus levels, P1 (weak Bray) and P2 (strong Bray). The P1 test is an indication of the phosphorus that is readily available to plants (20 to 50 ppm is adequate) and the P2 test confirms the level of phosphorus that is available and part of the active reserve in the soil (40 to 60 ppm is a desirable level). Potassium should be in the range of 150 to 300 ppm, calcium 1000 to 2500 ppm, and magnesium >50 ppm. Soluble salts results are presented as the electrical conductivity (EC) of the soil and is measured in mmhos/cm. An EC <1.0 (<640 ppm salt) is considered good and and EC >2.5 (>1600 ppm salt) is unsuitable for crops. Percent base (cation) saturation gives you an indication of what proportion of the CEC is occupied by cations such as Ca2+, Mg2+ and K+. Optimum ranges for Ca2+, Mg2+ and K+ are 40-80%, 10-40%, and 1-5%, respectively. Micronutrient ranges for vegetable crops are between 1 to 3 ppm Zn, 1 to 5 ppm Mn, 11 to 16 ppm Fe, 0.5 to 1.5 ppm Cu, 0.7 to 1.0 ppm B, and 0.11 to 0.20 ppm Mo.

Considerations for the Plan – Now you are able to initiate the development of a soil fertility management strategy. Nitrogen is an important element to promote vegetative growth. Under warm and high light intensity conditions, over fertilization of nitrogen can lead to excessive growth and deficiencies, especially in fruiting vegetable crops. The nitrogen form applied is also an important consideration. Applying most nitrogen as ammoniacal or urea will result in a soil pH decrease in the root zone. The conversion of these nitrogen forms to nitrate nitrogen is temperature dependent, converting at a faster rate when the soil temperatures are higher. Excess ammonium nitrogen can lead to deficiencies, especially calcium, magnesium and potassium. It is a smaller molecule and are therefore more readily taken up by plants. On the other hand, the uptake of nitrate nitrogen can lead to a slight increase in pH in the root zone. Phosphorus is most available to plants at a pH between 6.2 and 7.2. It is therefore important to keep the soil pH in check for maximum availability of phosphorous. Apart from soil amendments, you can adjust your irrigation water pH and nitrogen form to help manage soil pH in the root zone. Potassium is an essential element for plant growth and the production of a high quality crop. Higher soil moisture conditions means that more potassium is available in the soil solution and therefore enhances the availability for uptake by plant roots. However, be careful. Excessive soil moisture reduce root respiration, limits root activity and therefore decrease the uptake of potassium. The uptake of both potassium and phosphorus increase with an increase in soil temperature. The optimum soil temperature for uptake is 60 to 80°F.

Remember that recommendations are usually given as pounds per acre of P2O5 and K2O, because fertilizer grades are expressed as percent N-P2O5-K2O. To convert from P to P2O5 multiply the P value by a factor of 2.3. To convert from P2O5 to P multiply by a factor of 0.43478. Similarly, to convert from K to K2O multiply the K value by a factor of 1.2. To convert from K2O to K multiply by a factor of 0.83333. If you would like to know how much of P or K is available in one acre one foot deep, multiply the ppm value in the soil test report by a factor of 3.6.

Use guides such as the Midwest Vegetable Production Guide for Commercial Growers, the Indiana High Tunnel Handbook, the Nutrient recommendation for Vegetable Crops in Michigan, and the Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota to help you plan and manage your fertility program.

Resources:

Sideman, B. 2018. High Tunnel Soil Management Update. UMass Extension. Vegetable Notes, Vol. 30 No. 2. https://ag.umass.edu/sites/ag.umass.edu/files/newsletters/february_15_2018_vegetable_notes.pdf

Warncke, D., J. Dahl and B. Zandstra. 2004. Nutrient recommendation for Vegetable Crops in Michigan. Michigan State University, Extension Bulletin E2934. http://msue.anr.msu.edu/resources/nutrient_recommendations_for_vegetable_crops_in_michigan_e2934

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