Set 2nd feild value when select is used

I have a select in my form that chooses a field
<ion-item *ngIf="!">
Field Name
<ion-select [(ngModel)]=“job.fieldname"
name=“fieldname” required (ionChange)=“onSelectField(fieldId)”>
<ion-option *ngFor=“let fieldname of fieldnames” value=”{{fieldname.description}}">{{ fieldname.description }}

this is the data that comes back from the query to the database, the description is in the value -
“fieldnames”: [
“id”: “7e2fd11a-8449-47ea-8a24-824ee75f7db5”,
“companyid”: “1”,
“description”: “North of driveway”,
“farmername”: “Orlin Ensz”,
“fieldacres”: 60,
“latitude”: “38.327446”,
“longitude”: “-97.242594”

how do I put the fieldacres in the next form item

Acres to Swath
<ion-input type=“number” [(ngModel)]="job.acres"
name=“acres” required>