Alan's suc­cess with… fer­tiliser

Con­fused by all the dif­fer­ent fer­tilis­ers on of­fer? Alan ex­plains which plants you should feed, when to do it and how of­ten When we grow plants in pots, tubs and troughs, we re­ally do need to ‘play God’

Gardeners' World - - Contents -

Would you ever re-use a tea bag? No, I thought not. So why do we of­ten as­sume that the help­ing of fer­tiliser we gave our plants last sea­son is still work­ing? It’s easy to for­get that cul­ti­vated plants can’t sur­vive on rain alone – and in fact rain only makes mat­ters worse by wash­ing or ‘leach­ing’ valu­able nu­tri­ents out of the soil. As a re­sult, many gar­den plants are far from dis­play­ing their ful l po­ten­tial, with weak growth, poor colour, and a lack of f low­ers and fruit . Kept wel l sup­plied with the three main nu­tri­ents that are present in what we call a ‘gen­eral’ fer­tiliser – ni­tro­gen (N), phos­pho­rus (P) and potas­sium (K) – they are given ev­ery chance not only to f lour­ish, but also to bet­ter with­stand at­tacks by pests and dis­eases. There are some cir­cum­stances in which feed­ing plants is even more vi­tal than when they’re grow­ing in the gar­den with a free and un­re­stricted root run. When we grow plants in con­tain­ers – pots, tubs and troughs – we re­ally do need to ‘play God’, as we’re then to­tally in charge of the amount of food and wa­ter made avail­able to them. What’s more, mois­ture and food are likely to run out more rapidly than in open ground, as the plants can’t send their roots fur­ther in search of ex­tra sus­te­nance. You’ll know when your plant is hun­gry, as op­posed to thirsty: a lack of wa­ter re­sults in the fo­liage wilt­ing and soft, sappy stems. Lack of food isn’t quite as ob­vi­ous at first. It man­i­fests it­self in a grad­ual diminu­tion of growth, a lack of lus­tre in the fo­liage, cou­pled with pal­lor. The plant will be­come stunted – it may have one fi­nal go at pro­duc­ing a few tiny f low­ers, but this doesn’t mean it’s happy; it means it’s mak­ing one brave, last- ditch at­tempt to set seed and pro­duce prog­eny to carry on its good name. In short, a starv­ing plant wi l l look ‘of f colour’, all on ac­count of nutri­ent star­va­tion. The three main plant foods each have their own part to play in plant health and well-be­ing. Ni­tro­gen pro­motes healthy leaf and shoot growth, phos­pho­rus helps root de­vel­op­ment, and potas­sium en­cour­ages flower and fruit pro­duc­tion. A gen­eral fer­tiliser of­fers a bal­ance of all three ma­jor nu­tri­ents, plus lots of mi­nor ones and trace el­e­ments too – ev­ery­thing from mag­ne­sium and iron to man­ganese and molyb­de­num. The avail­abil­ity of some of these nu­tri­ents is gov­erned by the very na­ture of the soil in which the roots grow. We know that rhodo­den­drons and aza­leas, camel­lias and pieris, for ex­am­ple, don’t grow well on chalky or lime­stone soils. This is be­cause the al­ka­line na­ture of the

Slow-re­lease or con­trolled-re­lease fer­tilis­ers are handy for busy gar­den­ers who are of­ten ab­sent, as they sup­ply con­tin­u­ous nu­tri­tion over a pe­riod of time

soil ‘locks up’ iron and mag­ne­sium, mak­ing it dif­fi­cult for these ‘lime-haters’ to ex­tract what they need. As a re­sult, their leaves turn yel­low and their growth be­comes stunted. Grown in eri­ca­ceous ( lime-free) com­post, they are per­fectly happy. The nu­tri­ents that we give plants can be sup­plied in or­ganic or in­or­ganic forms. Or­ganic fer­tilis­ers are plant- and an­i­malderived prod­ucts, such as hoof and horn, bone­meal, sea­weed ex­tract, and blood, fish and bone. These usually need to be bro­ken down in the soil be­fore the el­e­ments they con­tain can be ab­sorbed in so­lu­tion by the plants’ roots. This process makes soil bac­te­ria ‘work’ to break down the or­ganic ma­te­rial into sim­pler con­stituents, to which the plants have di­rect ac­cess. In­or­ganic fer­tilis­ers, on the other hand, are de­rived from min­er­als or are syn­thetic prod­ucts, which, once dis­solved in wa­ter, are im­me­di­ately avai lable to plants. Ex­am­ples in­clude sul­phate of am­mo­nia, sul­phate of potash, su­per­phos­phate, Grow­more, Mir­a­cle-Gro and Phostro­gen. Their ad­van­tage is their rel­a­tively rapid ac­tion (al­though it can be short-lived), as they can be ab­sorbed the mo­ment they’re dis­solved in wa­ter. How­ever, be­cause they don’t need soil bac­te­ria to break them down into a read­ily ab­sorbable form, such bac­te­ria are made re­dun­dant. In­or­ganic fer­tilis­ers, while pro­vid­ing in­stant nu­tri­tion, do noth­ing for the long-term health of the soil. Some in­or­ganic fer­tilis­ers are mar­keted as slow re­lease or con­trolled re­lease, Os­mo­cote be­ing an ex­am­ple of the lat­ter. Here, the ef fects of the fer­tiliser are de­liv­ered over a longer pe­riod. These prod­ucts are handy for busy gar­den­ers who are of­ten ab­sent, as they sup­ply con­tin­u­ous nu­tri­tion over a pe­riod of time. Ap­plied in spring, they can last well into sum­mer. Re­mem­ber that soil bac­te­ria aside, even or­ganic fer­tilis­ers do lit­tle to im­prove soil struc­ture – only bulky or­ganic mat­ter (gar­den com­post and ma­nure) can do that, and both are vi­tal in­gre­di­ents of good earth.

When and how

The next is­sue is tim­ing. Plants slip into a semi-dor­mant or com­pletely dor­mant state in win­ter, so it’s a waste to of­fer them any kind of food dur­ing this pe­riod. They’re most in need of nu­tri­tion just as growth is about to start, so I usually give plants a good help­ing of gen­eral fer­tiliser in March, so it can be read­ily ab­sorbed by the time the strong growth spurt be­gins in April or May. They then get an­other dose in June or July to see them through the sum­mer. Don’t feed plants in cold, frosty weather, or in hot, dry sum­mers. The com­post or soil must al­ways be moist at the time of feed­ing, whether with a liq­uid or solid fer­tiliser. Pow­dered or gran­u­lar fer­tilis­ers are best sprin­kled on to the sur­face of the soil and gen­tly hoed in, to al­low rain, or wa­ter­ing,

to dis­solve them and take them down to the roots. A mulch laid over the soil af­ter ap­pli­ca­tion will seal in mois­ture and help to keep the fer­tiliser work­ing. Liq­uid feeds (in which the fer­tiliser is dis­solved and ap­plied in so­lu­tion) are wa­tered on and there­fore act more quickly, but they – and fo­liar feeds, which are dis­solved and sprayed on to leaves – pro­vide ben­e­fits for a shorter pe­riod. Liq­uid feeds can be given weekly or fort­nightly dur­ing the grow­ing sea­son and are es­pe­cially use­ful for plants in con­tain­ers. It is pos­si­ble to over­feed plants, so al­ways fol­low the in­struc­tions on the packet. Over­feed­ing can lead to fo­liage scorch and a ‘burnt’ ap­pear­ance. In se­vere cases, the plant may even col­lapse, due to a process called ex­os­mo­sis – when the sur­round­ing soil wa­ter is richer than the sap, which then flows out­wards from the plant. Mod­er­a­tion in all things, then.

Feed­ing fre­quency

Pot­ting and seed com­posts al­ready con­tain a cer­tain amount of fer­tiliser to ‘see the plant on its way’. This is known as a base dress­ing, but will run out af­ter a month or six weeks, so you need to pro­vide ad­di­tional fer­tiliser from then on­wards. Peren­nial plants that spend all their lives in a con­tainer need to be ‘pot­ted on’ (given a larger con­tainer and fresh com­post) in spring, ei­ther ev­ery year or when their size de­mands it. Af­ter a few years though, such a move be­comes im­prac­ti­cal and then the top few cen­time­tres of com­post can be re­moved in spring and re­placed with fresh – a process known as top dress­ing. A mod­est amount of fer­tiliser can be mixed in with the new com­post to of­fer a greater boost. Most plants will en­joy an an­nual dose of gen­eral fer­tiliser, while leafy veg­eta­bles will ap­pre­ci­ate a boost of ni­tro­gen-rich feed. Lawns are gen­er­ally fed in spring with a high-ni­tro­gen fer­tiliser and in au­tumn with a more bal­anced dress­ing that won’t en­cour­age lush, frost-ten­der top growth, but wi l l pro­mote root growth. These prod­ucts al­ways give clear in­struc­tions on their time of use on the packet. Ap­plied cor­rectly, fer­tilis­ers can make a big dif­fer­ence to your plants’ per­for­mance. To get started, just use my quick guide on p88 and you’re sure to reap the ben­e­fits.

Di­lute liq­uid fer­tiliser in a wa­ter­ing can be­fore ap­ply­ing Add slow-re­lease pel­lets when plant­ing up con­tain­ers Scat­ter rose feed on the soil around your shrubs to im­prove flow­er­ing

gar­den­er­ Check the in­struc­tions care­fully and don’t be tempted to over­feed – more isn’t al­ways bet­ter! April 2018

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