Accu­rate­ly adjust­ing soil nutrients 

Fer­til­i­sa­tion is one of the key meth­ods of gen­er­at­ing high-val­ue har­vests of a suf­fi­cient qual­i­ty and quan­ti­ty. With­out a tar­get­ed sup­ply of nutri­ents, the plants have no guar­an­teed source of food. To achieve this, soil nutri­ents must be replen­ished after being absorbed by the crop and trans­port­ed away with the har­vest. Fer­til­i­sa­tion should meet the needs of the plants and ensure the long-term fer­til­i­ty of the soil, while also pro­tect­ing the envi­ron­ment. Nutri­ents must there­fore be applied in a way that enables them to be opti­mal­ly utilised by agri­cul­tur­al crops, avoid­ing loss­es to bor­der­ing envi­ron­men­tal zones and ground­wa­ter reserves.
In addi­tion to organ­ic and min­er­al fer­tilis­ers, aux­il­iary sub­stances for plants and soils are also avail­able as soil nutri­ents. The most impor­tant macronu­tri­ents for plants are nitro­gen, potas­si­um, cal­ci­um, mag­ne­sium, phos­pho­rus and sul­phur. Some plant species also need addi­tion­al sil­i­con and sodi­um, which is impor­tant for the growth of C4 plants such as maize, for example.

In Ger­many, farm­ers use the Fer­tilis­er Appli­ca­tion Ordi­nance as a basis for spread­ing soil nutri­ents. Infor­ma­tion on sev­er­al fac­tors is need­ed in order to imple­ment the require­ments in a pro­fes­sion­al man­ner, most notably the tar­get yield. The soil nutri­ents need to be adapt­ed to the planned nutri­ent with­draw­al by the crops. To assist with this, guide val­ues can be used to deter­mine the nutri­ent require­ment at dif­fer­ent growth stages. In addi­tion to the planned fer­til­i­sa­tion mea­sures, the sub­se­quent deliv­ery poten­tial indi­cat­ed by the soil must also be tak­en into account. This infor­ma­tion on soil nutri­ents can be obtained through soil test­ing. The tests indi­cate var­i­ous lev­els of sup­ply that can be used to gen­er­ate a fer­til­i­sa­tion rec­om­men­da­tion. In the next chap­ter, we explain the char­ac­ter­is­tics that can be deter­mined from the soil.

Key soil characteristics 

Plan­ning and car­ry­ing out fer­til­i­sa­tion requires knowl­edge about the soil, its struc­ture and nutri­ent avail­abil­i­ty, which in turn is impact­ed by the soil struc­ture. As such, farm­ers need to obtain some fun­da­men­tal knowl­edge on the soil’s char­ac­ter­is­tics before apply­ing fertilisers.

The struc­ture of the soil influ­ences its heat and water bal­ances, stor­age den­si­ty, pore size, root pen­e­tra­bil­i­ty and bio­log­i­cal activ­i­ty. Soil struc­ture there­fore has a direct impact on the avail­able soil nutri­ents. The char­ac­ter­is­tics of the humus lay­er can be altered by the cul­ti­va­tion mea­sures applied. Loose soil with good root pen­e­tra­bil­i­ty, high lev­els of bio­log­i­cal activ­i­ty and a crumbly tex­ture is ben­e­fi­cial for soil nutri­ents. It enables sur­face water to be eas­i­ly absorbed and stored. Crumbly humus also enables bet­ter root pen­e­tra­tion and aer­a­tion, which ben­e­fits both plants and soil life in equal mea­sure. Poor humus can lead to water­log­ging and nitro­gen loss­es, and inhib­it root growth and micro­bial activity.

The avail­abil­i­ty of soil nutri­ents depends on sev­er­al fac­tors. The quan­ti­ty of nutri­ents is deter­mined by the total quan­ti­ty of a spe­cif­ic nutri­ent con­tained in the root-pen­e­trat­ed soil area. Only a small pro­por­tion of these nutri­ents are avail­able to the plants in the short term. The con­cen­tra­tion of a soil nutri­ent in the soil solu­tion, also known as the inten­si­ty fac­tor, is cru­cial for plant nutri­tion. The nutri­ent con­cen­tra­tion pro­vides a direct mea­sure of the nutri­ents avail­able to plants in the soil. In this con­text, it is worth not­ing that the rela­tion­ship between nutri­ent and harm­ful ele­ments is rec­i­p­ro­cal. For exam­ple, if the con­cen­tra­tion of cal­ci­um and potas­si­um is too high, less mag­ne­sium can be absorbed as a con­se­quence of the com­pe­ti­tion for ions. Sim­i­lar prob­lems emerge as a result of the com­pe­ti­tion between alu­mini­um, iron, man­ganese and zinc, which has a neg­a­tive impact of the absorp­tion of cal­ci­um and mag­ne­sium in heav­i­ly acidic soils.
There­fore, farm­ers should aim to achieve a har­mo­nious ratio of soil nutri­ents to opti­mise nutri­ent avail­abil­i­ty. The yield poten­tial of a par­tic­u­lar loca­tion can only be ful­ly exploit­ed when a bal­anced ratio and suf­fi­cient quan­ti­ty of all the ele­ments is present. The law of the min­i­mum applies here. Accord­ing to this law, one indi­vid­ual nutri­ent can restrict plant growth even when all the oth­er nutri­ents are present in suf­fi­cient quantities.

The soil usu­al­ly only con­tains a small pro­por­tion of the nutri­ents required for growth. The larg­er pro­por­tion is pro­vid­ed from reserves through min­er­al­i­sa­tion, des­orp­tion and dis­so­lu­tion process­es. This con­text is described by the kinet­ic fac­tor. The avail­abil­i­ty of soil nutri­ents to plants is influ­enced by the soil’s nutri­ent con­cen­tra­tion, water bal­ance and capac­i­ty to sub­se­quent­ly deliv­er nutri­ents. This con­text is described by the capac­i­ty fac­tor. It forms the mea­sure for the avail­able reserve of soil nutrients.

The nutri­ent con­cen­tra­tion in the soil is increased through fer­til­i­sa­tion, i.e. the tar­get­ed sup­ply of soil nutri­ents. Since arable land tends to exhib­it localised dif­fer­ences, it is gen­er­al­ly advis­able to car­ry out fer­til­i­sa­tion site-specif­i­cal­ly. In pre­ci­sion farming, fer­tilis­ers can be spread based on require­ments via appli­ca­tion maps. The Crop View com­po­nent by 365FarmNet, for exam­ple, utilis­es mul­ti-year satel­lite data from the fields to iden­ti­fy dif­fer­ent field sections.
The high­er the soil’s water con­tent, nutri­ent con­cen­tra­tion and root pen­e­tra­tion, the more nutri­ents are deliv­ered and made avail­able to the plants’ roots.
The ground water bal­ance is a major deter­min­ing fac­tor for nutri­ent avail­abil­i­ty. A lack of water in the upper soil lay­ers leads to con­di­tions such as a low nutri­ent trans­port rate and restrict­ed nutri­ent min­er­al­i­sa­tion due to a decrease in micro­bial activ­i­ty. Exces­sive water, on the oth­er hand, leads to poor­er warm­ing and aer­a­tion in the soil, and shal­low­er root pen­e­tra­tion. Too much water also has a neg­a­tive effect on microor­gan­isms. Com­pact­ed soil caus­es excess water in the humus lay­er. A loose, fine tilth is required in the soil to avoid waterlogging.

The pH lev­el in the main root area also has a con­sid­er­able impact. It indi­cates the lev­el of acid­i­ty in the soil. The term “pH” is derived from the Latin “pon­dus hydro­genii” (weight of the hydro­gen) and “poten­tia hydro­genii” (effec­tive­ness of the hydro­gen). As such, it express­es the con­cen­tra­tion of hydro­gen ions in the soil and their effec­tive­ness. This refers to the rela­tion­ship between acids and bases. The pH lev­el of the soil is easy to reg­u­late through fer­til­i­sa­tion. Once the val­ues have been deter­mined via soil test­ing, appli­ca­tion maps can also be used in this instance to bal­ance out dif­fer­ences in the field sec­tions in a tar­get­ed man­ner. The pH lev­el is also high­ly depen­dent on the ini­tial mate­r­i­al from which the soil is formed. If the mate­r­i­al con­tains a high lev­el of car­bon­ates, the soil usu­al­ly indi­cates a slight­ly acidic to slight­ly alka­line soil reac­tion. In this case, the risk of soil acid­i­fi­ca­tion is quite low. Soils con­tain­ing a high pro­por­tion of gran­ite, gneiss or sand­stone are much more at risk of acid­i­fi­ca­tion in areas with high rain­fall. A low­er pH lev­el increas­es the sol­u­bil­i­ty of phos­pho­rus fer­tilis­ers, encour­ages the release of soil nutri­ents and improves the sol­u­bil­i­ty and avail­abil­i­ty of most micronu­tri­ents. It does how­ev­er also cause an accu­mu­la­tion of heavy met­als such as cad­mi­um and lead, and can lead to increased loss­es from leaching.

On the whole, soil acid­i­fi­ca­tion is a nat­ur­al process caused by envi­ron­men­tal fac­tors. For this rea­son, reg­u­lar mon­i­tor­ing is imper­a­tive. Soil sam­pling enables soil nutri­ents to be test­ed pre­cise­ly and adjust­ed in a tar­get­ed man­ner. The test results can be inte­grat­ed into a farm man­age­ment infor­ma­tion sys­tem (FMIS) and mean­ing­ful­ly eval­u­at­ed for use as a deci­sion-mak­ing basis for cul­ti­va­tion activ­i­ties. Var­i­ous expert apps are also avail­able to help farm­ers dig­i­talise their agri­cul­tur­al process­es and make them more efficient.

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