The foun­da­tion of op­ti­mised plant nu­tri­tion In open land veg­etable pro­duc­tion

The key to op­ti­mised plant nu­tri­tion is to ap­ply nu­tri­ents on de­mand. De­mand-driven fer­til­i­sa­tion could mean sev­eral things, de­pend­ing on who you ask.

NZ Grower - - News -

There­fore, I want to be very clear about what I mean. When I talk about fer­til­is­ing ac­cord­ing to de­mand, I mean to meet the on­go­ing nu­tri­tional need of a crop in or­der to main­tain the de­sired growth rate and sought-af­ter com­po­si­tion of com­pounds in the crop.

The con­cept is not new, but it’s re­mark­ably un­no­ticed, con­sid­er­ing how ef­fec­tive and well sub­stan­ti­ated it is. The the­ory be­hind the method was de­vel­oped by Torsten Inges­tad and his co-work­ers at the Swedish Univer­sity of Agri­cul­tural Sci­ences sev­eral decades ago. Inges­tad was an in­tu­itive ge­nius who made sev­eral ground­break­ing dis­cov­er­ies in nu­tri­tional science. He was recog­nised for his work with the Mar­cus Wal­len­berg Prize, pre­sented by the King of Swe­den in 1989. The Wal­len­berg Prize cor­re­sponds to a No­bel Prize in Bi­ol­ogy, and is given for pi­o­neer­ing re­search in forestry.

The con­cept has many sig­nif­i­cant at­tributes: in­creased util­i­sa­tion rate of fer­tilis­ers and wa­ter re­sources, plant nu­tri­tion im­me­di­ately avail­able ac­cord­ing to crop need, and min­imised risk of nu­tri­ent leach­ing and ad­verse en­vi­ron­men­tal im­pacts. De­mand-driven fer­til­i­sa­tion also en­ables in­creased nu­tri­tional con­trol. An ad­di­tional ad­van­tage is that you can quickly and eas­ily add all nu­tri­ents to the crop, at vir­tu­ally any quan­ti­ties. The crop can ac­cess all nu­tri­ents, in­clud­ing the mi­cro nu­tri­ents. The con­cept pre­vents salt dam­age and in­hib­ited growth, which can oc­cur at high salin­i­ties in the soil so­lu­tion from large sin­gle doses of solid fer­tilis­ers. In ad­di­tion, the method ra­tio­nalises labour in-put, the use of ma­chin­ery, and the use of ex­pen­sive fer­tilis­ers. Inges­tad re­al­ized the im­por­tance of start­ing from the van­tage point of the nu­tri­ent flow from the en­vi­ron­ment into the plant. Just like hu­mans who want food when we're hun­gry, plants con­stantly need doses of nu­tri­tion in ac­cor­dance with im­me­di­ate de­mand. We can­not eat food for a whole sea­son in one sit­ting and meet the nu­tri­tional need for sev­eral months. It’s the same for plants. The nat­u­ral growth pat­tern of a plant is ex­po­nen­tial, mean­ing its weight in­creases by a

cer­tain per­cent­age each day. There­fore, nu­tri­tion should be ad­min­is­tered in doses that in­crease ex­po­nen­tially to ac­cu­rately meet the need of the crop.

The sec­ond ba­sic prin­ci­ple is about the pro­por­tions be­tween the nu­tri­ents; plants need all 14 es­sen­tial nu­tri­ents in spe­cific ra­tios. In other words, the sec­ond prin­ci­ple deals with what the com­po­si­tion of a fer­tiliser should look like in or­der for it to pro­mote max­i­mum growth. With high pre­ci­sion, Ingstad’s team could de­ter­mine how th­ese pro­por­tions look for many types of crops. Th­ese pro­por­tions are listed in the ta­ble. They stay con­stant at ex­po­nen­tial growth. Re­mark­ably, this nu­tri­tional com­po­si­tion is al­most uni­ver­sally ap­pli­ca­ble to all crops. I say al­most, as there are mi­nor ad­just­ment for species and what part of the crop is har­vested.

When plant nu­tri­tion is de­liv­ered in ac­cor­dance with the mo­men­tary need of the plant in this way, fer­til­i­sa­tion be­comes the con­trol­ling fac­tor for growth rate, the dis­tri­bu­tion be­tween root and shoot, and a range of qual­ity as­pects. Inges­tad ob­served that if nu­tri­tion is supplied to the crop in the above pro­por­tions and in ac­cor­dance with the on­go­ing need of the plant, there will be no symp­toms of de­fi­ciency.

At max­i­mum growth, all re­sources go to growth. Above all, the part of the plant above ground will grow rapidly, as the crop has ac­cess to ev­ery­thing it needs. Growth rate de­creases when re­source short­age oc­curs, for ex­am­ple due to lack of a nu­tri­ent.

This, in turn, of­ten leads to an in­crease in re­source al­lo­ca­tion to the roots to com­pen­sate for the nu­tri­tional de­fi­cien­cies.

Fur­ther­more, at max­i­mum growth, all en­ergy from pho­to­syn­the­sis goes to growth. So, by slow­ing down growth, re­sources from the pho­to­syn­the­sis are al­lo­cated to other things, such as sugar con­tent, antioxidants, chem­i­cal de­fence, com­pen­sa­tion for my­c­or­rhiza, and flow­er­ing and fruit­ing. This prin­ci­ple ap­plies to all qual­ity as­pects.

So, by ad­just­ing the nu­tri­tional in­put you can con­trol both the quan­tity and qual­ity of the plant part that you want to har­vest. If you grow pota­toes, you need to grow at max­i­mum rate, be­cause the potato is a stem botan­i­cally speak­ing, so max­i­mum biomass equals max­i­mum yield. If you grow car­rots, the nu­tri­tional sup­ply must ben­e­fit the roots, be­cause this is what you want to har­vest. If you grow toma­toes, you should slow down growth only slightly, to in­crease sugar and an­tiox­i­dant lev­els, which af­fects the taste pos­i­tively and in­creases the qual­ity of the prod­uct.

By fol­low­ing the prin­ci­ples of de­mand-driven fer­til­i­sa­tion, the nu­tri­tional util­i­sa­tion rate of the crop is max­i­mized and noth­ing is wasted. This is pos­i­tive, not only for the crop pro­duc­tion but also for the en­vi­ron­ment. If all nu­tri­ents added through fer­til­i­sa­tion are ab­sorbed by the plant and max­i­mally used, this will min­imise the risk of nu­tri­ent leach­ing dur­ing the growth sea­son. Fur­ther­more, you need to buy less fer­tiliser per kg of har­vested crop, which helps your fi­nances and means the re­spon­si­ble man­age­ment of lim­ited nat­u­ral re­sources.

I have of­ten asked my­self why this con­cept hasn’t re­ceived more at­ten­tion. I have come up with three main rea­sons. First, Inges­tad un­der­took most of his re­search within forestry, and there­fore the con­cept has not spread to other dis­ci­plines. Se­condly, I have ob­served a con­ser­va­tive attitude among agron­o­mists and hor­ti­cul­tur­ists, in terms of in­cor­po­rat­ing new ideas, es­pe­cially from dis­ci­plines other than their own. Third, Inges­tad's fer­til­i­sa­tion model re­quires con­tin­u­ous nu­tri­ent ir­ri­ga­tion (fer­ti­ga­tion) of the crop, some­times un­til har­vest. Thus the con­cept hasn’t been ac­tively spread to all grow­ers.>

My last point touches on the lim­i­ta­tions of the con­cept’s ap­pli­ca­bil­ity. With­out a sys­tem that sup­ports fer­ti­ga­tion it is more com­pli­cated to fer­tilise on de­mand. Still, for farm­ers who do not use ir­ri­ga­tion and who have to limit the num­ber of fer­tiliser doses, the prin­ci­ples still ap­ply more or less, de­pend­ing on lo­gis­tic and fi­nan­cial con­sid­er­a­tions. Con­trol re­lease fer­tilis­ers can be of good use in such cir­cum­stances.

It would ad­van­tage grow­ers who al­ready have an ir­ri­ga­tion sys­tem in place to ap­ply by fer­ti­gat­ing us­ing a de­mand-driven ap­proach.

Ramp based ir­ri­ga­tion sys­tems are the pri­mary mo­bile al­ter­na­tive with po­ten­tial for ef­fec­tive ap­pli­ca­tion in open land hor­ti­cul­ture, be­cause the sys­tem spreads the wa­ter evenly with small losses to wind, which is nec­es­sary for the fer­tilis­ers to be dis­trib­uted suf­fi­ciently evenly across the cul­ti­va­tion area.

How­ever, I’m cer­tain that de­mand­driven fer­til­i­sa­tion can fur­ther bring drip ir­ri­ga­tion to the fore in open field veg­etable pro­duc­tion. Of course, drip ir­ri­ga­tion is of­ten the most ex­pen­sive ir­ri­ga­tion sys­tem in such pro­duc­tion, which is the main rea­son for the lim­i­ta­tion in its use. At the same time, de­mand-driven fer­til­i­sa­tion through drip ir­ri­ga­tion is the most op­ti­mal way to grow crops. There is no pro­duc­tion method that is so re­source-sav­ing and at the same time al­lows for such high pre­ci­sion. Here it's not just a mat­ter of the sys­tem al­low­ing nu­tri­ent sup­ply on de­mand, it’s about giv­ing the crop ex­actly what it needs ex­actly when it needs it. This so­lu­tion is more vi­able than ever, now that there are spe­cial ma­chines that lay out and col­lect the hoses. For those who are used to it, the process is rel­a­tively quick and easy. In a rel­a­tively dry year, such a sys­tem could pro­vide a max­i­mum har­vest. So in sum­mary, de­mand-driven is to give the crop what it needs, when it needs it, and in cor­rect doses. In other words, how to sup­ply nu­tri­ents to a crop to op­ti­mise qual­ity and quan­tity at har­vest.

▴ The ta­ble il­lus­trates the pro­por­tions of nu­tri­ents needed at max­i­mum growth for most crops. The ra­tios are ex­pressed in per­cent­age of ni­tro­gen. Note that cer­tain pro­por­tions are ex­pressed in in­ter­vals. Th­ese dif­fer­ences are, in part, due to dif­fer­ences in species, but in par­tic­u­lar to which parts of the plants that are har­vested.

▴ The curve shows growth re­sponse to nu­tri­ent sup­ply. When the nu­tri­ent sup­ply reaches a cer­tain level, it can start to gen­er­ate growth in a crop. As the nu­tri­ent sup­ply in­crease, growth will in­crease to the ex­act same de­gree, un­til the crop reaches max­i­mum growth rate. There­after, in­crease if the nu­tri­ent sup­ply will not give any re­sponse, un­til it be­comes so high that tox­i­c­ity will ad­versely af­fect the crop. De­mand-driven fer­til­iza­tion en­tails keep­ing the nu­tri­ent in sup­ply on a con­stant op­ti­mum.

▴ The curve il­lus­trates how much nu­tri­tion is added to the crop from in­cep­tion to any given point dur­ing sea­son, in an ac­cu­mu­lated fash­ion, when fer­til­ized on de­mand. The dose in­creases ex­po­nen­tially as the crop grows ex­po­nen­tially. Growth will reach a max­i­mum rate, then start to de­cline due to self-shad­ing, ag­ing and ex­ter­nal fac­tors, which re­duces fer­til­izer need over time. The S-curve pro­vides a sim­pli­fied and gen­er­al­ized pic­ture of how the plant nu­tri­ents should be ap­plied dur­ing the grow­ing sea­son. De­pend­ing on the crop, har­vest can oc­cur be­fore growth is as low as sow­ing, which means a shortened curve.

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