AgFACT no. 99, June 1996 ISSN 1172-2088 Soil characteristics important to management Farmers must be familiar with several soil characteristics if   they   are   to   manage   their   soil   effectively:   texture, structure,   organic   matter   content,   porosity   and   water- holding  capacity,  and  some  aspects  of  chemical  fertility and   fertiliser   management,   as   well   as   plant   nutrient requirements. Texture Soil texture refers to the size of the particles that make up the soil. Mineral soils are mixtures of coarse sand particles, finer silt, and very fine  clay, and can be classified on the proportions of each of these they contain. Texture can be assessed  in  the  field  by the behaviour of moist soil when worked in the hand (Table 1). Table 1: Field texture assessment of moistened soil Feel and sound Cohesion Field soil class and plasticity texture Gritty and   Cannot be moulded   SAND rasping sound into a ball   Will almost mould into      LOAMY   a ball but disintegrates          SAND   when pressed flat Slight grittiness,      Moulds into a cohesive         SANDY faint rasping ball that fissures  LOAM sound   when pressed flat Smooth soapy   Moulds into a cohesive        SILT feel, no grittiness    ball that fissures  LOAM when pressed flat Very smooth,   Plastic, moulds into a  CLAY slightly sticky cohesive ball that  LOAM to sticky deforms without fissuring Very smooth,   Very plastic, moulds  CLAY sticky to into a cohesive ball that very sticky that deforms without fissuring Structure Soil structure describes the way in which the soil particles come together to form aggregates.   Unstructured  soils  are  either  single-grained,  like  recently deposited  sand,  or  massive,  like  compressed  clay.  Soil structure is developed in nature by wetting and drying and freezing and thawing cycles. Plant roots are also important, breaking  up  large  soil  aggregates,  and  binding  together small    aggregates.    Organic    matter    provides    adhesive materials    to    stabilise    aggregates,    and    prevent    them breaking   down   when   they   are   wet.   This   is   why   soil structure is usually improved under pasture. Soil structure can  also  be  improved  by  appropriate  cultivation  at  the correct moisture content, but can be destroyed by excessive cultivation when the soil is either too dry or too wet, or by compaction. A  well-structured  soil  will  provide  a  favourable  medium for root growth, and a good balance of air and moisture in the  soil.  It  will  also  allow  water  to  enter  the  soil  readily, reducing run-off and erosion during heavy rain. Organic matter New   Zealand   mineral   soils   contain   3   to   20%   organic matter, mainly formed from dead plant material. Peats and other organic soils contain much more. The major effect of soil organic matter is to stabilise soil structure, but it also supplies plant nutrients and stores moisture. Organic matter deactivates   many   organic   compounds   (e.g.,   pesticides). Application rates of soil-acting agricultural chemicals must be increased in soils high in organic matter. The organic matter level in soil remains fairly constant if the   vegetation   cover   remains   unchanged.   Clearing   and cultivation,   however,   can   lead   to   a   decline.   Including pasture  in  a  crop  rotation,  adding  farmyard  manure  and compost,  ploughing  in  crop  residues  and  green  manure crops, and establishing crops by minimum cultivation and direct drilling, can all increase organic matter levels in the soil. Porosity and water-holding capacity Texture  and  structure  both  affect  a  soil’s  water-holding properties.  Sands  have  large  pores  between  the  particles, and    these    drain    quickly    after    rain    or    irrigation. Unstructured   clays   have   many   very   small   pores   and, although their total pore space is greater than that of sands, water  can  flow  through  them  only  very  slowly.  A  well- structured soil has both  large and small pores. The larger drain  quickly,  and  provide  air  for  plant  roots  and  soil micro-organisms;   the   smaller      drain   more   slowly,   and supply water to plant roots. After heavy rain or irrigation, water drains rapidly from the large  pores.  As  the  soil  water  content  falls,  progressively smaller and smaller pores drain, and drainage slows. After 2 or 3 days the drainage rate is insignificant compared with the rate at which water is being removed by the vegetation growing  in  the  soil.  The  moisture  content  at  this  point  is called field capacity. The soil moisture content continues to  fall  as  vegetation  takes  up  water.  The  drier  the  soil becomes, the more tightly the water is held. Eventually it is