Soil or­ganic mat­ter By Robin Boom

Or­ganic mat­ter in the soil con­sists of plant and an­i­mal mat­ter which has gone through a de­com­po­si­tion process by soil or­gan­isms to break it down into hu­mic sub­stances. These be­come sta­ble in the soil it­self and even­tu­ally reach a state where they are not

NZ Grower - - NEWS -

Soil or­ganic mat­ter con­tains just un­der 60% car­bon and its ac­cu­mu­la­tion in the soil, known as se­ques­tra­tion, helps re­duce car­bon diox­ide in the at­mos­phere and is re­garded as a so­lu­tion for global warm­ing and cli­mate change. Con­versely the loss of soil or­ganic mat­ter in­creases green house gases. In New Zealand one of the pos­i­tive at­tributes of our bet­ter class soils is their rel­a­tively high or­ganic mat­ter con­tent com­pared to many soils around the globe.

Dif­fer­ent land uses can af­fect or­ganic mat­ter losses and gains. Un­der forestry, soil or­ganic mat­ter builds up from leaf fall and from plant root ex­u­dates over time, but some of this can be lost when the for­est is har­vested. Un­der per­ma­nent pas­ture or­ganic mat­ter nor­mally builds up over time, but re­cent re­search has shown that un­der in­ten­sive dairy­ing where stock­ing rates are high and ni­tro­gen fer­tiliser use has in­creased, that there is a net an­nual loss on av­er­age of ap­prox­i­mately a tonne of car­bon on most soils ex­cept on the vol­canic an­desitic ash soils where hu­mus lev­els con­tinue to ac­cu­mu­late. Un­der ex­ten­sive sheep and beef there is on av­er­age about half a tonne of car­bon which is se­questered an­nu­ally, and un­der per­ma­nent hor­ti­cul­tural tree and vine crops there is a net ac­cu­mu­la­tion of or­ganic mat­ter. Con­tin­u­ously cropped ground can lose many tonnes of or­ganic mat­ter in a sin­gle year, de­pend­ing on the cul­ti­va­tion method used, and there are moves in­ter­na­tion­ally to re­duce the amount of car­bon loss through us­ing con­ser­va­tion tillage tech­niques such a no till, min­i­mum tillage and strip tillage.

Hav­ing a good amount of or­ganic mat­ter (hu­mus) is right at the heart of hav­ing healthy, fer­tile and pro­duc­tive ground, as it pro­vides many func­tions to do with the phys­i­cal, chem­i­cal and bi­o­log­i­cal prop­er­ties of the soil it­self. Soil or­ganic mat­ter is that dark sub­stance that is found most densely in the top­soil, and is re­spon­si­ble for giv­ing soil its tilth, mak­ing it easy to cul­ti­vate, and it also main­tains good soil struc­ture so that the soil col­loids are crumbly and hold to­gether well in nutty gran­u­lar ag­gre­gates, cre­at­ing space for wa­ter and oxy­gen to move freely through the soil pro­file. Soils low in or­ganic mat­ter be­come sticky and solid, and wa­ter just sits on the sur­face not be­ing able to per­cu­late down through the soil. With a win­ter like we have just come through, soils low in or­ganic mat­ter re­main at sat­u­ra­tion point a lot longer, and when dig­ging a spade spit of dirt, it ap­pears bound to­gether as one gluggy lump.

The re­peated cy­cle of car­bon loss from cul­ti­va­tion causes the soil to be­come more com­pacted and more prone to ero­sion and wa­ter sur­face run-off, and ever in­creas­ing amounts of fer­tiliser are re­quired to main­tain crop pro­duc­tion. Ben­e­fi­cial soil or­gan­isms such as earth­worms, fungi, bac­te­ria and acti­no­myces species which thrive on the or­ganic mat­ter de­cline as hu­mus lev­els drop and crops con­se­quently be­come more sus­cep­ti­ble to in­sect pests and soil borne dis­eases. Con­se­quently there is a greater need to use more pes­ti­cides, her­bi­cides and fungi­cides which fur­ther de­pletes the pos­i­tive soil biota

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