Market Vegetable Farms: Soil Characteristics & Testing

Department of Land Resources and Environmental Sciences Montana State University

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MARKET VEGETABLE FARMS:

Soil Characteristics & Testing

This is one of three MSU Extension bulletins available to help market vegetable farmers manage soil fertility and health. This bulletin presents soil properties important to soil fertility management and how soil tests quantify these properties. Market Vegetable Farms: Soil Nitrogen & Sulfur and Market Vegetable Farms: Soil Phosphorus, Potassium, & Micronutrients present crop rotation, cover crop selection, tillage, and amendment source, rate, timing and placement to optimize nutrient availability while protecting the environment. We use the term amendment to include organic materials, fertilizers that provide specific plant nutrients, and other products, like lime, that can improve soil properties. In this bulletin, organic material refers to plant- or animal-based sources (e.g., plant residue or feather meal), not whether the material is allowed by national or state organic standards.

Overview        

Knowing soil nutrient content and characteristics is necessary to make good fertilizer and irrigation management decisions. Soil amendment rates should be based on soil tests, and for nitrogen (N), an estimate of N that becomes available during the growing season is also needed. Plant deficiency symptoms and tissue tests can help complement soil tests to identify nutrient deficiencies.

Unguided fertilization without the knowledge of what is already in the soil can lead to under-fertilization and reduced vegetable yields. Conversely, over-fertilization uses limited resources possibly better used elsewhere (the amendment, money, and labor), and potentially contaminates surface and ground water. Soil organic matter (SOM), pH, salts, cation exchange capacity (CEC), and texture are additional soil characteristics that affect nutrient and water management.

Soil organic matter is an important source of nutrients and needed for good soil aggregation (soil particles bound together in clusters), water infiltration and drainage, ease of cultivation, minimal crusting, high nutrient holding capacity, and resistance to pH change. However, there can be too much of a good thing. High soil organic matter levels can produce more plant nutrients than the crop needs, cause nutrient imbalance, and reduce plant growth.

Most vegetables prefer nearly neutral pH, yet most Montana soils are above pH 7. Soil pH is difficult to reduce quickly and economically at a large scale. Soil organic matter and clay help keep soil near neutral pH.

Knowledge of soil texture and CEC can guide watering and fertilizing schedules. Low CEC/coarse soils best receive water and nutrients in low, frequent doses, while high CEC/ clay soils should have long, slow, and less frequent irrigation, and nutrient amendments should be applied less frequently and incorporated.

Soil Health       

Healthy soils absorb and store water, and are a living system teeming with bacteria, fungi and other organisms which decompose plant material to cycle nutrients, produce enzymes which form soil aggregates, and suppress disease. They provide farms with resilience to uncontrollable factors such as weather and input costs. They support root and plant growth, which, in turn, feed soil microbial activity, further increasing soil health and productivity. Fruits and vegetables grown in healthy soils can have higher phytochemical richness, thus be healthier for their consumers (1). Many market vegetable farm soils can benefit from just a couple of management adjustments or inputs, others might be on soils depleted of soil organic matter and nutrients and take more inputs and time to become healthy.

Soil Testing       

Soil testing is best done annually in the early spring, or late fall if fields are expected to be too wet to sample in early spring. To track soil test levels over years, sampling is ideally done around the same time each year. This reduces variation in nutrient levels due to things such as soil temperature, microbial activity, and amount of time plant residue has had to decompose or leach [e.g., potassium (K)]. Soil samples should be 6-inches deep. Sometimes, if just soil nitrate-N is being tested, samples are done 12-inches deep, which is why Table 1 has desirable N levels for 6-inch and 12-inch samples. Areas that have dissimilar soils (e.g., upland, well-drained vs. lowland with high water content) and nutrient management history (e.g., with or without manure application) should be sampled separately. Traditional soil tests assess soil N, phosphorous (P), K, SOM, pH, CEC, and sometimes salts. In addition to soil texture, these all affect soil nutrient and irrigation management. See Interpretation of Soil Test Reports for Agriculture, Home Garden Soil Testing & Fertilizer Guidelines and MSU Extension Soil Fertility Soil Sampling for more on soil sampling and lab selection. Most do-it- yourself, home soil test kits are not reliable, other than a few high-quality, expensive ones which may be worth sharing among market vegetable farmers. See https://landresources.montana.edu/soilfertility/html/PR_SoilTestKits.html  for a review of test kits.

NUTRIENTS

Based on soil samples from market vegetable farms across Montana (results provided by 3, Western Ag Research Center, and five regional commercial testing labs), N was the one nutrient commonly in low supply. Conversely, there were very high levels of N, P, and K in some soils. Additional N, P or K on these latter soils would increase the risk for environmental contamination, could hurt crop growth, and/or be an unnecessary expense.

Soil test values are the basis for determining which nutrients, and how much, to add. To calculate N amendment rates, soil test values are combined with estimates of N that will become plant available from organic material during the growing season. Estimating in-season plant available N is challenging, because it depends on soil conditions and type and quantity of organic material present. In contrast, soil P and K levels are relatively stable over time, if there has not been a large amendment application (Table 1). Less information is available on soil nutrient levels for vegetable production for boron (B), chloride (Cl), copper (Cu), Iron (Fe), magnesium (Mg), manganese (Mn), nickel (Ni) and zinc (Zn). These eight micronutrients are needed in very small amounts. Soil high in organic matter generally provides sufficient levels, although over half of the Montana vegetable farm soil tested had B levels below sufficiency guidelines and available copper can be low in soils with high SOM (Market Vegetable Farms: Soil Phosphorus, Potassium, & Micronutrients).

Table 1: Desirable soil nutrient and property levels for market vegetable farms (Home Garden Soil Testing & Fertilizer Guidelines unless otherwise noted).

Plants take up N as either ammonium (NH₄₊) or nitrate (NO_3-). Standard lab tests measure nitrate-N available in the soil at the time the soil sample was analyzed. Ammonium is often much lower than nitrate in tilled soil, and generally not analyzed. A traditional soil nitrate test in the late winter/early spring is valid for early plant growth needs (3).

The in-season N quick test (with a nitrate strip) takes practice but can be valuable to adjust N rates within the growing season (Soil Nitrate Testing Supports Nitrogen Management in Irrigated Annual Crops, and Soil Nitrate Testing for Willamette Valley Vegetable Production). Market Vegetable Farms: Soil Nitrogen & Sulfur offers ways to estimate N which will become available during the growing season from cover crops and other organic material and explains how to adjust N amendment rates based on that information.

Soil nitrate-N tests do not measure potentially mineralizable N (PMN), which is the N amount that can become plant available N during the growing season through decomposition of organic material. Soil health tests such as Haney (H3A) and Cornell Comprehensive Assessment of Soil Health offer estimates of PMN. However, they can overestimate actual PMN in the field, and lab PMN in Montana market vegetable farm soils was not a good indicator of crop-N uptake, thus of how much more N should be added to the soil (3).

Microbial activity is a good indicator of soil health, but not of PMN as the microbes can be very active decomposing high C organic material while tying up (immobilizing) N, leaving less for plants (3). Soil health tests offer estimates of microbial activity by measuring soil respiration (the amount of CO2 released), microbial biomass, and enzymes left in the soil by microbial activity. These tests can be tracked over time to evaluate soil management practices’ effect on soil health. As soil health on a given field increases over time, there is a good probability that crop yields will also increase. But there is little correlation between soil health test values and potential crop yield when evaluated in a single year (4, 5).

At this time, soil health test values cannot be used to calculate N amendment rates because the relationship between soil health values and amount of amendment required to meet plant demands has not been established. See Evaluating Soil Quality and Health for information on measuring and monitoring soil health.

The most common lab tests used for soil P use the Olsen, Bray, or Mehlich extraction process. Montana soil fertility guidelines (Home Garden Soil Testing & Fertilizer Guidelines and Fertilizer Guidelines for Montana Crops) are based on Olsen P test results; Olsen P is best suited for most Montana soils which are high in calcium carbonate (CaCO₃) and above pH 6. Acidic soils can be analyzed with the Mehlich-3 P test, and results converted to Olsen P values to calculate approximate P amendment rates.

Bray P test values cannot be converted to Olsen P values or used with Montana fertilizer guidelines and should be avoided because they can be highly inaccurate on high pH soils. Phosphorus and K are generally reported on soil tests in ppm (parts per million) and corresponding amendment recommendations in pounds P₂O₅ and K₂O per 1000 ft² or acre.

The ‘base-cation saturation ratio’ (BCSR) is proposed as a method to make soil amendment recommendations by comparing a soil’s ratio of calcium (Ca), Mg, K, and other cations (nutrients with a positive charge, e.g., sodium) to an ideal ratio that varies with soil type. However, the cation ratio does not indicate total available cation amount. A sandy soil can have the ideal base cation ratio but in insufficient amounts. Also, extensive field trials have found no correlation between cation ratios and yield when cations were above critical sufficiency levels, and not in excess (7). Within cation ratios commonly found in soils, soil physical properties (e.g., aggregation), fertility, and microbial activity are generally not influenced by the changes in the ratio of Ca, Mg, and K (7). The BCSR concept was not developed to provide N, P, S, or micronutrients fertilization rate recommendations and is not recommended for use in our region.

Soil sulfur (S) tests may determine potentially deficient soils (less than 5 ppm SO4-S in top 6 inches), but a soil test level does not help determine how much S is needed or whether yields will increase with the addition of S. Soil organic matter is a major source of S for plants; the S content and its release during the growing season is highly variable and hard to predict. Sandy, shallow soils, with low organic matter, in areas with high rainfall or irrigation from low S waters are at high risk for S deficiency. Most ground and stream water in Montana is low in S, but the Yellowstone and Missouri Rivers are high in S. See Secondary Macronutrients: Cycling, Testing, and Fertilizer Recommendations on how to measure S in irrigation water and calculate the amount of S applied per growing season.

Some crops may take up and remove more S than others; for example, brassica harvest can remove twice as much S as other crops (8). Sulfur is necessary for plants to take up N and build protein. If plants are N deficient, but soils are high in N, then S could be the limiting factor.

Visual symptoms can help determine nutrient deficiencies. For example, S deficient plants have uniform yellow upper leaves, in contrast to N deficient plants which have uniform yellow lower leaves. If visual symptoms are caught early, then nutrient deficiencies can be corrected before yields have been compromised. See MSU Extensions Soil Fertility Nutrient Deficiency and Toxicity for a guide to visual deficiency symptoms. For information on tissue testing see Plant Tissue Analysis and Interpretation for Vegetable Crops in Florida. Deficiency symptoms and tissue testing can help identify what nutrient is missing in the plant but will not explain why. For example, Fe deficiency can be due to cold or high pH soil, or root rot, each requiring a different solution to the deficiency.

Fertilizer rate calculations are explained in Market Vegetable Farms: Soil Nitrogen & Sulfur and Market Vegetable Farms: Soil Phosphorus, Potassium & Micronutrients as well as other MSU Extension bulletins for other crops and in home gardens.

SOM

Soil organic matter is made up of dead and decomposed plant and animal material and living and dead soil microbes (e.g., bacteria, fungi, nematodes). Plant residue serves primarily to feed the microbial community. Fully decomposed microbes and plant residue form the humus (also called stable organic matter) portion, which makes up around half of SOM.

Soil organic matter is an important source of many nutrients, but often does not supply enough early season N for optimum plant growth. It is an important source of nutrients, and for soil aggregation, water infiltration and drainage, low compaction, minimal crusting, nutrient holding capacity, and resistance to pH change. Microbial activity in soil organic matter produces dissolved compounds (chelates) that bind with metals (e.g., Fe, Zn), increasing metal availability to plants. Adding organic material benefits soils low in SOM (less than 3.5%) more than ones with higher SOM (9).

Maintaining SOM levels between 5 and 8% is a good goal. Based on dryland crop systems, building SOM requires an annual average of more than 80 lb above ground plant residue (on dry matter basis) per 1000 ft² (10, 11). In soils with more than 13- to 16-inch annual water, high microbial activity, or high clay content, this critical level is likely higher, and could be hard to meet with just vegetable harvest residue since corn and sunflowers produce around 40 to 50 lb dry residue per 1000 ft² (12, Slovakia). Rotate among high- and low-residue producing crops and integrate cover crops with high biomass production (e.g., fall planted grass with legume) to maintain SOM over years (Table 2). It is difficult to increase SOM in soils already high in SOM (9).

Table 2. Relative amount of shoot residue left behind after vegetable harvest (12, Slovakia; 13, California).

Roots are slow to decompose and improve soil aggregation, water absorbing and holding capacity, and suitability for cultivation. Use cover crops to increase the time live roots are in the soil and consider undercutting rather than tilling cover crops or pulling weeds. Composts decompose more slowly than fresh material; the quickly decomposed portion disappeared in the compost pile.

Soil organic matter levels above 8% can produce more plant nutrients than the crop needs. Nelson (14, OR) found 94% of 33 residential food garden soils had greater than 6% SOM. The soils contained higher, in some cases much higher, nutrient levels than recommended, other than N and B. Excess nutrients can contaminate surface and ground water and lead to nutrient imbalance and reduce plant growth. See Market Vegetable Farms: Soil Phosphorus, Potassium & Micronutrients for the effects of P and K accumulation through manure application.

SOIL pH, CEC, AND TEXTURE

Soil pH, CEC, and texture all relate to soil fertility. Soil pH and CEC are often included in soil tests. Soil pH can be accurately tested with a pH probe, which is less subjective to interpretation than using color strips. Lab analysis of soil texture is relatively expensive; texture can instead be determined using the jar or ribbon tests (https://landresources.montana.edu/soilfertility/soil-sampling-methods.html). 

Most vegetables prefer pH between 6 and 7.5, a few grow well above or below this range (Table 3). Crop production has challenges when soil pH is acidic (less than 6) or greater than 8 (Table 4). Soil pH is difficult to change quickly and economically at a large scale. Elemental S acidifies soil as it oxidizes. The effect is not sufficient to lower pH in highly buffered soils (high SOM and calcium) but can create acidic conditions in sandy soil with repeated use. In a calcareous loam soil, 230 lb S/1000 ft² supplied as elemental S only brought soil pH 8 down to pH 7.5 (21). This is costly (around $375/1000 ft²) and could lead to salt and sulfate toxicity. Ammonium-based fertilizer acidifies soil from 0.04 to 0.14 pH units per 2.25 lb N/1000 ft² (22), but do not add N fertilizer above crop needs to acidify soil because it can create N toxicity and will likely leach N into groundwater.

Increasing pH is easier and less expensive. Liming with calcium sources that contain some form of oxygen (carbonate, hydroxide, oxide) increases soil pH. Gypsum (calcium sulfate) does not increase soil pH. It contains calcium, but no oxygen, and is used to replace sodium in high sodium soils to improve soil aggregation. See Soil Acidification: Management for liming guidelines.

Soil organic matter helps stabilize against changes in soil pH, keeping soils neutral. Mulches increased soil pH of an acidic soil in the top 4-inches in a year, from pH 6.0 to an average pH 6.3, whether the mulches were acidic (coarse bark pH 5.2), or basic (horse manure pH 8.8; 23, England). Using manure, pine needles/bark, or any form of compost is not a viable option to lower soil pH. Manure and compost are generally neutral pH. Acidic bark or leaves will briefly produce acidic compounds in early decomposition, but as decomposition continues the acids are neutralized. It would take thousands of pounds of acidic bark per 1000 ft2 to lower soil pH from 7.5 to pH 6.5, and the effect would be short-lived (24).

Table 3. Crops that also grow well in soils outside the typical optimum range of pH 6 to 7.5.1.

Table 4. Soil pH and impact on production.

The ideal soil texture is loam to clay loam, which have similar parts sand, silt and clay. Finer textured soils hold more water, but have higher risk for ponding, crusting, and cracking from shrink/swell. Texture is difficult to change other than in the top few inches by adding sand or clay. Some challenges due to very coarse or fine texture, such as poor water absorption and holding capacity and difficult cultivation, can be improved by adding organic material.

Cation exchange capacity is the parking space for positively-charged nutrients in the soil. A soil with CEC greater than 15 meq/100 g (milliequivalents per 100 grams) has the capacity to attract and hold cations such as K⁺, Zn²⁺, NH⁴⁺, Mg²+, and Fe³⁺. Loamy sand has CEC around 10 meq/100 g and clay around 40 meq/100 g. Cation exchange capacity can be increased with the addition of SOM (CEC around 200 meq/100 g). Soil texture and CEC can guide water and nutrient management.

Understanding soil properties and soil test values helps farmers know which properties can be reasonably changed towards healthier, more fertile soils.

Acknowledgements    

We greatly appreciate the time and expertise of our reviewers Wes Cawood, MSU Towne’s Harvest Garden; Max Smith, Missoula Grain & Vegetable Co.; Zach Miller, Superintendent MSU Western Ag Research Center/ Associate Professor; and MSU Extension Communications in producing this bulletin.

References       

1. Reeve, J., L. Hoagland, J. Villalba, P. Carr, A. Atucha, C. Cambardella, D. Davis, and K. Delate. 2016. Organic farming, soil health, and food quality: Considering possible links. Adv. Agron. 137:319-367. doi:10.1016/bs.agron.2015.12.003
2. Abreu, C., B. Raij, M. Abreu, and A. Gonzalez. 2005. Routine soil testing to monitor heavy metals and boron. Soils Plant Nutr. 62. doi:10.1590/S0103- 90162005000600009
3. Neff. K. 2014. The Ecology of Nutrition: Managing Soil Organic Matter to Supply Soil Nutrients, Increase Soil Biotic Activity and Increase Crop Nutritive Value. PhD Dissertation. Montana State University. https://scholarworks.montana.edu/xmlui/handle/1/9361 
4. Crookston, B., M. Yost, M. Bowman, K. Veum, and G. Cardon. 2021. How Often do Soil Health Assessment Scores Relate to Crop Yield? Soil Health Innovations Conference. March 2021. NCAT-ATTRA.
5. Jones, C., and D. Boss. 2014. Montana State University unpublished data.
6. Dari, B., C. Rogers, A. Leytem, and K. Schroeder. 2019. Evaluation of soil test phosphorus extractants in Idaho soils. Soil Sci. Soc. Am. J. 83:817–824. doi:10.2136/sssaj2018.08.0314
7. Kopittke, P., and N. Menzies 2007. A review of the use of the basic cation saturation ratio and the “ ideal” soil. Soil Sci. Soc. Am. J. 71:259–265. doi:10.2136/ sssaj2006.0186
8. Ernest, E. 2018. Sulfur and Vegetable Crops. UD Cooperative Extension. https://sites.udel.edu/weeklycropupdate/?p=12050
9. McConkey, B., M. St. Luce, B. Grant, W. Smith, A. Anderson, G. Padbury, K. Brandt, and D. Cerkowniak. 2020. Prairie Soil Carbon Balance Project: Monitoring SOC Change Across Saskatchewan Farms from 1996 to 2018. Saskatchewan Soil Conservation Association.
10. Engel, R., P. Miller, B. McConkey, and R. Wallander. 2017. Soil organic carbon changes to increasing cropping intensity and no-till in a semiarid climate. Soil Sci. Soc. Am. J. 81:404–413. doi:10.2136/sssaj2016.06.0194
11. Shrestha, B., B. McConkey, W. Smith, R. Desjardins, C. Campbell, B. Grant, and P. Miller. 2013. Effects of crop rotation, crop type and tillage on soil organic carbon in a semiarid climate. Can. J. Soil Sci. 93:137-146. doi:10.4141/CJSS2012-078
12. Torma, S., J. Vilcek, T. Losak, S. Kuzel, and A. Martensson. 2017. Residual plant nutrients in crop residues – an important resource. Acta Agriculturae Scandinavica. 68:358-366. doi:10.1080/09064710.2017.1406134
13. Mitchell, J., T. Hartz, S. Pettygrove, D. Munk, D. May, F. Menezes, J. Diener, and T. O’Neill. 1999. Organic matter recycling varies with crops grown. California Agriculture. 53:37-40. doi:10.3733/ ca.v053n04p37
14. Nelson, M., G. Mhuireach, and G. Langellotto. 2022. Excess fertility in residential-scale urban agriculture soils in two western Oregon cities, USA. Urban Agric. Region. Food Syst. 2022;7:e20027. doi:10.1002/ uar2.20027
15. Almanac. 2021. Soil pH levels for plants. https://www.almanac.com/plant-ph
16. Government of Western Australia. 2014. Soil pH and plant health. Department of Primary Industries and Regional Development. https://www.agric.wa.gov.au/soil-ph-and-plant-health?nopaging=1
17. Labroski, C., and J. Peters. 2012. Nutrient Application Guidelines for Field, Vegetable, and Fruit Crops in Wisconsin. UW Extension A2809. https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A2809.pdf
18. Master Gardener Handbook. MSU Extension. EB0185. https://store.msuextension.org/Products/Montana-Master-Gardener-Handbook-EB0185__EB0185.aspx
19. Smith, E. 2000. The Vegetable Gardeners Bible. Storey Books, VT. 309 pp.
20. Western Fertilizer Handbook: Horticulture Edition. 2012. Waveland Press, Inc. 400 pp.
21. AgVise Laboratories. Unpublished data.
22. Jones, C. Montana State University unpublished data.
23. Pickering, J., and A. Shepherd. 2000. Evaluation of organic landscape mulches: Composition and nutrient release characteristics. Arboric. J. 24:175-187. doi:10.108 0/03071375.2000.9747271
24. Mickelbart, M, K. Stanton, S. Hawkins, and J. Camberato. 2012. Lowering soil pH for horticulture crops. Purdue Extension HO-241-W. https://www.extension.purdue.edu/extmedia/ho/ho-241-w.pdf

Resources Cited     

MSU EXTENSION BULLETINS:

Montana State University Extension bulletins are available at https://store.msuextension.org/ or by contacting the Distribution Center at 406-994-3273, orderpubs@montana.edu. 

MSU EXTENSION SOIL FERTILITY SOIL SCOOPS: 

https://landresources.montana.edu/soilfertility/soilscoop/index.html 

ADDITIONAL RESOURCES