Sustainable Organic Options 34

“Soil is a of productivity and to minimise the is a dynamic substance and a process. It is negative environmental impacts of mixture that reflects the balance a mixture of materials – farming activities. between additions of new organic matter and losses of organic matter and remains and What is soil organic matter? already in the soil. Soil organic products of decay processes matter exists in various forms that have been going on for Soil organic matter is the fraction which differ in their biodegradability months or years.” of the soil that includes: plant and or resistance to . It is F. E. Broadbent animal residues at various stages generally divided into three pools: of decomposition; plant roots; cells active, intermediate or slow, and Introduction and tissues of soil ; recalcitrant or resistant. The active and substances synthesised pool includes microbial The biggest difference between by the soil population. Fresh or and labile organic compounds that soil and rock is the presence of undecomposed plant materials make up less than 5 percent of the organic matter and the associated such as litter and straw and animal soil organic . The slow pool biological activity that takes place dung lying on the soil surface usually makes up 20 to 40 percent in the former. Soil organic matter are not included in this definition These three pools have different is at the heart of healthy and although upon decomposition, rates of turnover. The active pool productive . Although organic these will eventually become part ranges from months to years, the matter typically makes up a small of the soil organic matter. Organic slow pool in decades, and the percentage of most mineral soils matter is largely made up of recalcitrant pool in hundreds to (less than 10 percent by weight), the elements carbon, , thousands of years. it is very important because it , and and also positively influences or modifies the and sulphur. It is The active and part of the effects of almost all soil properties. expressed as a percentage intermediate pools are involved in Understanding the role of organic of the soil mass less than 2 mm supply and in the binding matter in maintaining a healthy soil in diameter. of small soil particles together is essential to ensure a high level to form larger structural units called aggregates. Aggregation is important for , The formation of soil organic matter aeration and drainage, and reduces the soil’s susceptibility to . On the other hand, the recalcitrant pool or possesses a large quantity of negative charges and contributes largely to the nutrient- holding capacity (cation exchange Cow dung capacity) of the soil. It also imparts a dark colour to . Soil rich in organic matter Why soil organic matter ? Grass Organic matter is critical for Partially decomposed maintaining . The level of organic matter is highest in the topsoil and, with few exceptions, Soil which has little declines dramatically with depth. organic matter New Zealand farmers regard Litter the top 6 inches as their most productive soil. Topsoil is therefore Decomposition of plant residues and animal added to the soil produces organic matter that is a precious resource primarily due beneficial to soil health and plant growth to the high level of organic matter 34 Land Management Soil Organic Matter

and it contains relative to subsurface soil. Once depleted, organic matter takes many years to replace. With this in mind, adding plant and animal residues and minimising topsoil loss through erosion and other forms of soil disturbance should be an important goal of every land manager. Organic matter serves several functions, the most important of which are as a soil conditioner and as a source of plant nutrients. Organic matter adds body to sandy soils and increases its moisture- and nutrient-holding capacity. It promotes granulation in clayey Topsoil is a precious resource primarily due to the high level of organic matter and nutrients it soils which helps in plant root penetration and entry of water of sequestration of carbon from lots of negative charges. These and air into the soil. It makes the atmosphere. This is currently negative charges contribute to the cultivation easier, resulting in better an active area of research nutrient retention capacity of soils seedbeds, and reduces surface by scientists concerned with by attracting positively charged crusting that can adversely affect increasing the carbon content of ions (cations) in soil such as the emergence of seedlings. The soils to mitigate the adverse effects calcium, magnesium, potassium, functions of soil organic matter are of climate change and to potentially ammonium,etc. These would broadly categorised into biological, utilise it in emission trading otherwise leach and get lost in the physical and chemical but they schemes. soil profile. It should be noted that actually overlap and interact with negatively charged ions (anions) each other. Physical such as nitrate and sulphate are Biological One of the major effects of organic not held by the negative charges Organic matter is a food and matter is to improve . of organic matter and are therefore energy source for soil organisms Plant roots, , , subject to loss. and a source of plant nutrients. fungi and other micro-organisms Organic matter acts as a chemical Organic matter decomposition release organic compounds which buffer by resisting rapid change is a microbiological process help bind soil particles together to in pH. On the positive side, this which releases inorganic forms produce stable aggregates. This mechanism delays of nutrients such as nitrogen (N) improves aeration and increases particularly in soils subjected to phosphorus (P) and sulphur (S) permeability which in turn makes long-term fertilisation by and which slowly becomes available the soil less susceptible to erosion. ammonium-containing fertilisers. for plant use. The humus which Stable aggregates also resist On the negative side, one may develops as an intermediate compaction caused by ploughing, need to apply larger quantities of product of this decomposition and vehicle and animal traffic. material in order to raise the also acts as a store for nutrients. Organic substances have been pH of an already acidic soil to a Soil organic carbon is generally shown to hold up to five times desirable level. highly correlated with total their own weight of water. This nitrogen. Therefore, the amount of Organic matter complexes cations, contributes to improving the particularly micronutrient cations N mineralisation (i.e. conversion available water-holding capacity of organic N compounds to such as iron and zinc, through of soil, making growing less a process known as chelation. ammonium-N) increases as soil prone to short-term . organic carbon increases. The formation of iron-organic The dark colour imparted by complexes makes the insoluble Organic matter provides active organic matter in the topsoil iron more available for plant use. absorption sites for the deactivation increases the absorption of solar of organic chemicals such as radiation. This may facilitate pesticides, particularly herbicides. How is soil organic soil warming in spring and matter measured? Micro-organisms associated with consequently seed germination soil organic matter may also rapidly and plant growth. In the laboratory, organic matter decompose soil-applied organic is actually measured as total Chemical chemicals. carbon by high temperature Adding organic matter to the Organic matter possesses a combustion analysers which raise soil contributes a certain level high surface area and contains the soil sample to a temperature

Sustainable Options Land Management 34 Soil Organic Matter

of about 1,300°C and the total Soil organic matter serves several functions carbon is measured as carbon dioxide. Total carbon is expressed as a percentage of the soil weight. Physical Biological The measurement includes both Stabilises soil structure Source of energy for organic and inorganic (carbonates) biological processes carbon. However, since most New Improves retention Supplies plant nutrients Zealand soils contain very little (N, P, S) inorganic carbon, total carbon is Improves drainage and aeration Deactivates organic a good measure of organic matter chemicals (e.g. herbicides) content. To obtain the percentage Reduces susceptibility Contributes to of organic matter, total carbon is of soil to erosion Functions sequestering multiplied by a conversion factor Imparts dark of from the of 1.72. This assumes that organic colour to soil organic atmosphere matter is 58 percent carbon. In matter reality, the carbon content of organic matter varies and so it is better not to convert and just report the measured total carbon as is. Chemical Ratings of total carbon for New Zealand soils are as follows: Contributes negative charges to reduce leaching of nutrient cations (Ca++, Mg++, K+, NH4+) Level Total carbon (%) Resists rapid change in soil pH Very low Less than 2 (delays soil acidification) Complexes cations Low 2-4

Medium 4-10

High 10-20 What factors influence decomposing organic matter. The Very high Above 20 organic matter levels optimum soil temperature range for bacterial activity is between 21 to Depending on but in soils? 38°C, but some activity may occur regardless of , desirable Soil organic matter represents at temperatures as low as 5°C, provisional target values of total the largest terrestrial carbon pool although at greatly reduced rates. carbon for New Zealand mineral being almost three times that of soils set by Landcare Research Oxygen plant and animal biomass. The are: above 2 percent (Semi-arid, amount of carbon stored in a Most soil micro-organisms are Pumice and Recent soils), above soil is determined by a dynamic aerobic and require oxygen 3 percent (Allophanic soils), and equilibrium between carbon and water for their respiration. above 2.5 percent (all other soils). additions from plant and animal When the soil is saturated The quality of soil organic matter biomass production and carbon with water or gets compacted, can be described by the so-called losses by mineralisation or microbial respiration slows down carbon to nitrogen (C/N) ratio microbial respiration and removal significantly which then reduces which is the total carbon content by processes the decomposition of soil organic (C) of the soil divided by its total through time. matter. Partially or continuously nitrogen content (N). A C/N ratio of saturated soil condition has If additions are greater than losses, less than 10 means that organic protected organic () soils from soil organic matter accumulates. If matter is easily mineralised which losing organic matter through additions are less than losses, soil releases soluble inorganic forms oxidation. organic matter becomes depleted. of N (ammonium and nitrate) into When the system is in balance Rainfall the soil. On the other hand, a and additions equal losses, the C/N ratio greater than 10 means Rainfall (or irrigation) influences quantity of soil organic matter does that organic matter is less easily content which in turn not change appreciably over time. mineralised. A high C/N ratio may determines plant growth and the In general, any factor that affects indicate possible N deficiency but resulting input of plant biomass into soil microbial activity also affects different land uses will vary in their the soil. Rainfall also influences breakdown of soil organic matter. expected responses. In pastures, soil moisture content by affecting soil microbial activity. Organic the optimum C/N ratio ranges from Temperature 8 to 12. matter decomposition is faster in Soil temperature influences the moist soil than in dry soil. As the activity of soil micro-organisms soil becomes wet or saturated,

Sustainable Options 34 Land Management Soil Organic Matter

microbial activity declines, as does that retention or maintenance of Recommended practices for raising decomposition. soil organic matter level will be an or maintaining soil organic matter important challenge on intensively are as follows: Soil reaction or pH used pastures. Under acidic soil conditions, bacterial activity which is Increasing inputs of organic materials Reducing organic matter losses responsible for most of the decomposition of organic matter Apply organic materials to soil Minimise soil erosion is greatly reduced. However, the Apply manure, farm dairy effluent and Erosion removes organic matter contained biosolids in pastures. Add plant residues, in topsoil. Maintaining good vegetative or activity of soil fungi is less affected , green , prunings ground cover to protect soil from erosion by low pH. or mulch in arable and horticultural will ensure that valuable topsoil and situations. organic matter are conserved. Management practices Increase pasture/crop growth Minimise soil degradation to raise or maintain soil Increase plant growth by optimising by farm machinery and organic matter levels and good /crop pugging by animal treading reduces plant management practices (e.g. irrigation) productivity which reduces organic matter Raising or maintaining soil organic to maximise the capture of carbon from inputs to soil. matter to desirable levels is crucial the atmosphere. to sustainable land management as it retains nutrients for plant Retain crop residues Reduce operations use, reduces the runoff rate All crop residues such as straw and Because excessive tillage accelerates stalks should be incorporated into the soil organic matter decomposition and makes and the hazard of erosion, and or left on the soil surface to decompose soils susceptible to erosion, the adoption improves the physical condition whenever possible. of zero or minimum tillage becomes of the soil. Broadly speaking, soil important in reducing organic matter loss. organic matter can be increased by increasing inputs of organic Grow cover crops Avoid materials and/or decreasing Growing cover crops rather than leaving Overgrazing leads to reduction of pasture losses. However, most organic the land fallow over winter adds carbon biomass and therefore productivity. Lower to the soil. If grazed in situ, a significant pasture biomass means low input of matter added to the soil will rapidly proportion of crop carbon will be returned organic matter to the soil. Overgrazing decompose to carbon dioxide gas to the soil in manure. also increases the area of bare ground through the process of microbial making the surface soil more prone respiration and will not become to erosion. part of the soil organic matter. In general, only about 5 to 15 percent Include a pasture phase in Manage decomposition rates of applied carbon to soil eventually arable cropping Encourage soil organisms (e.g. worms, Grasses and legumes are regarded as beetles) to enhance the burial and becomes soil organic carbon. soil builders because their root residues incorporation of into soil Therefore, very large amounts of add active organic matter to the soils. aggregates to protect organic matter from organic matter need to be added to They will help return the paddock to either loss by decomposition. soil to increase its organic carbon long-term or short-term pasture depending Living shelterbelts with deep roots will on the degree of soil degradation. content in the long term. Periodic capture and sequester carbon at deeper application of organic matter is soil layers. required to maintain desirable levels. The key to organic matter management is to strike a balance Additions and losses between additions and losses so determine the level of that a significant decline is avoided. soil organic matter Most pastoral soils in New Zealand are generally considered to be Additions rich in organic carbon (range: ▪▪ Plant residues 3.5 percent to 15 percent by ▪▪ Manures Bay of Plenty Regional Council weight) so that large increases ▪▪ Compost and vermicompost in productivity are not expected ▪▪ Biosolids 5 Quay Street, Whakatāne by adding more organic matter. 1125 Arawa Street, Rotorua However, recent research has Rata Street, Mount Maunganui shown that in intensive lowland Soil organic matter content livestock systems (e.g. dairying), Post: PO Box 364, Whakatāne 3158 soils have lost organic carbon by Phone: 0800 884 880 an average of 1 tonne carbon/ha/ Losses yr over the last 20-30 years while ▪▪ Mineralisation (microbial Fax: 0800 884 882 in hilly land soils, organic carbon respiration) levels have increased. This implies ▪▪ Soil erosion Pollution Hotline: 0800 884 883 Email: [email protected] Website: www.boprc.govt.nz

Sustainable Options