Interpreting Soils tests to build active soils w/cover crops

Transcription

1 Interpreting Soils tests to build active soils w/cover crops Eero Ruuttila UCONN Sustainable Agriculture Specialist January 18, 2014 Getting Started in Organic Farming Conference

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3 The popular mind is still fixed on the idea that a fertilizer is the panacea. (Anon.) Organic matter functions mainly as it is decayed and destroyed. Its value lies in its dynamic nature. (W.A. Albrecht)

4 Why test your soils? Testing soils every year or two on each field is one of the best investments you can make. Soil tests provide information about the chemical properties of your soil. A soil test report indicates levels of plant nutrient elements as well as ph, buffer ph, cation exchange capacity, base saturation, & organic matter. Soil test reports provide recommendations for lime and fertilizer applications. Soil tests over the years are a useful tool to track fertility changes & will help fine tune your annual nutrient management practices. When recommendations are followed, the risk of excess nutrient runoff loading adjacent waterways will be diminished.

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6 Soil testing Most soil tests do not measure soil fertility, only mineral & organic matter levels. The accuracy of a soil test and its recommendations depend on many factors including sampling procedure, analytical process, and the laboratory philosophy in interpreting results. Soil testing choices include home test kits, university or government institution tests, or private labs. Some labs provide a wide range of analytic services however more elaborate tests can be relatively expensive. Most university testing labs make fertilizer recommendations for individual crops as a soil test service. Recommendations are backed by historical field response data, i.e. university soil lab research correlates to yield differences from the region s soil types, weather, & previous test results. A soil sample sent to a lab in another part of the country could give very different results for this reason. Recently Cornell has developed a new more comprehensive soil test that evaluates physical, biological, & chemical soil values. Detailed information about their new soil health test is provided at soiltest@cornell.edu.

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8 It s helpful to understand the chemical terms that appear on a soil test. It s helpful also to correlate numbers utilized on standard soil tests with conventional fertilizer standards. They can be confusing until you learn how they are expressed. The next few slides will try to unlock a few of these challenging chemical properties and definitions.

9 Essential Soil Elements Plants need 13 soil elements for their growth including the major elements: nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), and the minor or trace elements: iron (Fe), manganese (Mn), boron (B), zinc (Zn), molybdenum (Mo), copper (Cu), and chlorine (Cl). In addition to the soil minerals, air and water provide carbon (C), hydrogen (H), and oxygen (O), also essential for plant growth. The availability of nutrient elements for plant uptake is greatly influenced by soil ph. Soil ph is the acid status (or Hydrogen ion concentration) of a soil. Nutrient availability is also influenced by the presence of organic matter.

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11 Cation exchange capacity (CEC) Plants take up nutrients from the soil either as positively charged molecules (cations) or as negatively charged molecules (anions). Many soil nutrient elements are cations such as calcium, ammonium, magnesium, potassium, hydrogen, & aluminum. Cation exchange capacity is the measure of the soils ability to retain & supply nutrients, specifically the positive charged nutrients called cations. The amount of negative charge that exists on humus or clays, allows them to hold onto the positively charged chemicals (cations). CEC can range below 5 (milli-equivalents per 1000) in sandy soils to values over 20 in clay soils or soils high in organic matter. Many New England agricultural soils are low in clay so CEC numbers are often low. A low CEC value indicates a soil with little ability to store nutrients & one that is susceptible to nutrient loss through leaching. Organic matter is a primary contributor to CEC. Not only does organic matter improve the physical properties of soil, it also plays a vital role in soil chemistry by increasing CEC.

12 Base saturation Calcium, Magnesium, and Potassium are bases which tend to raise soil ph while Hydrogen & Aluminum are acids which often lower soil ph. In acid soils there are acid cations present and the base saturation is less than 100. Besides having sufficient quantities of Ca, Mg, & K it is also important they are in relative balance with each other because the excess of one can can suppress the uptake of another. As a general rule a Ca:Mg:K ratio of about 20:4:1 is desirable. When expressed as per cent base saturation, desired levels are: Ca 65-80%; Mg 5-15%; and K 2-5%.

13 Soil ph and liming ph is a measure of soil acidity. 7 is neutral, less than 7 is acid & greater than 7 is alkaline. Lime, or limestone is a mineral that can neutralize acids and is commonly applied to acid soils. Soil ph is important because it affects the availability of nutrient elements for plant uptake. When the soil is acid, the availability of N, P, and K is reduced. There are usually low amounts of calcium and magnesium in an acid soil and there may be toxic levels of iron, manganese, and aluminum present. Under alkaline conditions, most trace elements are less available. Beside raising soil ph, lime is the most economical source of Ca and Mg for crop nutrition. Select liming materials based on Ca & Mg content with the intent of achieving base saturation ratios. If Mg is low, a lime high in Mg should be used (dolomite). Lime high in calcium (hi Cal) is recommended if Mg is high and Ca is low. Lime will react most rapidly if it is thoroughly mixed when applied. Ideally the soil should be relatively dry. Lime tends to cake when soils are wet and it will not mix well with soil molecules.

14 Buffer ph Buffer ph is the measure of reserve acidity in soils and is used by soil labs to estimate lime requirements. In order to raise soil ph we must neutralize both active and reserve acidity. The change in ph of the buffering solution is a measure of the soil s capacity to resist ph after lime has been added. Soils that have a high CEC can be expected to have a high reserve acidity. Low buffer ph readings indicate high amounts of reserve acidity and therefore high amounts of lime will be required. The extent to which buffer ph is lower than 6.8 is directly related to the amount of limestone needed. The soil ph should always be lower than the buffer ph except in alkaline soils.

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20 Organic matter and its influence on soil biology The types of crops you grow, the amounts of roots they have, their yield, the portion of crop harvested, and how you treat the crop residues will affect organic matter. Soil fertility itself will influence the amount of organic residues returned, since more fertile soils grow higher yielding crops i.e. they yield more residues. The more residues your crops leave in your field, the greater the populations of soil organisms. Conventional tillage systems are aggressive and will decrease earthworm populations as well as other soil organisms. When rotations are more complex and include green manure crops you will increase soil biological diversity (& activity).

21 Maintaining a diverse environment Diversity is important in maintaining a well functioning and stable ecological system. Where many different types of organisms coexist in the same area, there are fewer disease, insect, weed, and nematode problems. Don t forget diversity below the soil surface is as important above ground. Growing cover crops and using crop rotations help maintain the diversity below ground. Adding manures and composts and making sure crop residues are returned to the soil are also important to promoting soil organism diversity. The universe of soil microorganisms include bacteria, fungi, algae, protozoa, nematodes and earthworms.

22 Biological process Mineralization is a process by which soil organisms change organic elements into the mineral or inorganic form as they decompose organic matter. In oxidation carbon combines w/oxygen resulting in a release of energy. The biological process of respiration allows living organisms to use the energy stored in carbon-containing chemicals. This is a plant driven process of converting sugars, starches, and other compounds into a directly usable form of energy. How does energy get stored inside organic residues in the first place? Green plants use the energy of sunlight to link carbon atoms together into larger molecules, i.e. the process of photosynthesis. Solar energy is a driving force to assist plant growth & respiration.

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24 Earthworms are the workhorses of active soils Consider this, 1 acre of soil 6 inches deep weighs about 1000 tons. Relative to the number of earthworms in a soil, they can move between.006 to I inch of soil a year or about tons an acre. With the proper conditions there can be as many as 1,000,000 earthworms to an acre. They fragment and mix fresh organic matter as it passes through their digestive systems w/minerals, bacteria and enzymes. Their worm casting make available plant nutrients such as nitrogen, calcium, magnesium, and phosphorus. They also mine thousands of channels for conducting oxygen and water into the soil.

25 The special relationship of legumes to atmospheric nitrogen Some nitrogen-fixing bacteria form symbiotic or mutually beneficial relationships w/plants. Rhizobia bacteria live on nodules formed on the roots of legumes. These bacteria provide nitrogen in a form that leguminous plants can use while the legumes provide the bacteria w/sugars for energy. Rhizobia bacteria can fix hundreds of pounds of available nitrogen from the atmosphere/acre/year. Available fixed nitrogen varies among legume species. Some vetches and alfalfa can fix as many as a couple of hundred pounds of nitrogen/acre whereas beans and peas will fix about pounds of nitrogen a year.

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32 Direct seeded spring greens following field pea/oats fallow previous year

33 Late August early September sowing of hairy vetch with winter rye

34 Overwintered stand of rye/hairy vetch; rye killed by mowing at pollen stage

35 Late August transplanted broccoli, undersown with winter rye mid-september

36 Larger plants late-september (rye well established)

37 Post-harvest December broccoli plants with well-established winter rye from mid-september under-sowing

38 Quick early spring cover crop of rye (April 1 st ) prior to winter squash transplants (tillage May 22 nd )

39 Field ready for laying of plastic mulch following minimal tillage (June 7 th )

40 Cutting strips in overwintered rye/hairy vetch to reduce field tillage & to provide straw mulch for direct seeded winter squash

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43 Under sowing of medium red clover into 10-foot center spacing of winter squash

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49 Medium red clover 2 nd year, early June

50 Overwintered medium red clover utilized as living mulch strips in tomatoes

51 Sudan grass for building soil organic matter & for suppressing annual summer weeds

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53 Mowing stimulates root mass & keeps Sudan grass residue more manageable for tillage

54 Under sowing medium red clover prior to last mowing of Sudan grass

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56 Buckwheat s tap roots pull P from subsoil; flowering plant rapidly breakdowns following tillage

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61 The End

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